Lunulae - definition of lunulae by The Free Dictionary Lunulae - definition of lunulae by The Free Dictionary (lo͞on′yə-lə) n. pl. lu·nu·lae (-lē′) A small crescent-shaped structure or marking, especially the white area at the base of a fingernail that resembles a half-moon. [New Latin lūnula, from Latin, crescent-shaped ornament, diminutive of lūna, moon; see leuk- in the Appendix of Indo-European roots.] lunula n, pl -nulae (-njʊˌliː) or -nules (Anatomy) the white crescent-shaped area at the base of the human fingernail. Nontechnical name: half-moon [C16: from Latin: small moon, from lūna] lu•nu•la n., pl. -nu•lae (-nyəˌli) also -nules. something shaped like a narrow crescent.

[1565–75; < Latin lūnula, derivative of lūn(a) moon] ThesaurusAntonymsRelated WordsSynonymsLegend: region , area - a part of an animal that has a special function or is supplied by a given artery or nerve; "in the abdominal region" nail - horny plate covering and protecting part of the dorsal surface of the digits 2. decoration , ornament , ornamentation - something used to beautify Translations lunula n pl <-e> → Lunula f (spec), → Nagelmöndchen nt Want to thank TFD for its existence? Tell a friend about us , add a link to this page, or visit the webmaster's page for free fun content . Link to this page: Ten asteroids for Sphere The exploitation of parentheses is not Cummings's invention and has been the topic of a breathtakingly vast and thorough research by John Lennard, who makes the distinction between parenthesis the mark of punctuation and parenthesis the grammatical category by referring to the punctuation marks as lunulae ("little moons").

Copyright © 2003-2017 Farlex, Inc Disclaimer All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. Nail Anatomy: Overview, Gross Anatomy, Nail Growth Gross Anatomy Overview Overview The nail organ is an integral component of the digital tip. It is a highly versatile tool that protects the fingertip, contributes to tactile sensation by acting as a counterforce to the fingertip pad, and aids in peripheral thermoregulation via glomus bodies in the nail bed and matrix. [ 1 , 2 ] Because of its form and functionality, abnormalities of the nail unit result in functional and cosmetic issues. The structures that define and produce the nail (nail plate) include the matrix (sterile and germinal), the proximal nail fold, the eponychium, the paronychium, and the hyponychium (see the images below).

Collectively, the nail bed (sterile matrix), nail fold, eponychium, paronychium, and hyponychium are referred to as the perionychium. Nail surface anatomy. See 15 Fingernail Abnormalities: Nail the Diagnosis , a Critical Images slideshow, to help identify conditions associated with various nail abnormalities. Next: Gross Anatomy The nail plate emerges from the proximal nail fold and is bordered on either side by the lateral nail folds (paronychium). The nail plate is composed of hard, keratinized, squamous cells that are loosely adherent to germinal matrix but strongly attached to the sterile matrix. [ 2 ] The nail fold, the most proximal aspect of the perionychium, is composed of a dorsal roof and a ventral floor. It is found approximately 15 mm distal to the distal interphalangeal joint (DIP). [ 3 , 4 ] The dorsal roof rests above the forming nail, and the ventral floor lies beneath the nail, immediately distal to the insertion of the extensor tendons.

The ventral floor is the site of the germinal matrix and is responsible for 90% of nail production. The dorsal roof of the nail fold plays a role in housing cells that impart shine to the nail. The skin proximal to the nail that covers the nail fold is the eponychium. The tissue distal to the eponychium in contact with the nail represents the cuticle. Extending from proximal to distal on the nail is a half-moon shaped white arc known as the lunula (see the image below). The lunula is the distal extent of the germinal matrix. [ 4 ] This characteristic color change is due to the persistence of nail cell nuclei in the germinal matrix; distal to this location, nuclei are absent, and the nail is transparent. [ 2 ] The area of the nail bed distal to the lunula is the sterile matrix. This is a secondary site of nail production and is tightly adherent to the nail plate and the periosteum of the distal phalanx. Nail surface anatomy. Next: Nail Growth Development of the nail matrix begins in the ninth embryonic week from the nail anlagen. By week 16, the fetal nail is identifiable.

[ 1 ] Nearly 90% of the nail plate is produced by the proximal half of the matrix, more specifically, the germinal matrix. As a result, more of the nail plate substance is produced proximally, leading to a natural convex curvature of the nail from proximal to distal. Ulnar and radial projections of the matrix extend proximally to form points or horns of the matrix (see the image below). [ 3 ] These lateral horns are attached to the dorsal expansion of the lateral ligament of the distal interphalangeal joint (DIP). [ 1 ] Nail matrix horns. Contributor Information and Disclosures Author Benjamin Z Phillips, MD, MPH Hand and Microsurgery Fellow, Department of Plastic and Reconstructive Surgery, Washington University Disclosure: Nothing to disclose. Coauthor(s) Scott T Schmidt, MD Clinical Assistant Professor, Department of Surgery, The Warren Alpert Medical School of Brown University; Director of Hand Surgery, Assistant Program Director, Director of Microsurgical Resident Training Lab, Department of Plastic Surgery, Rhode Island Hospital Disclosure: Nothing to disclose.

Reena A Bhatt, MD Attending Physician, Clinical Assistant Professor of Surgery, Department of Plastic Surgery, The Warren Alpert Medical School of Brown University Disclosure: Nothing to disclose. Chief Editor Thomas R Gest, PhD Professor of Anatomy, Department of Medical Education, Texas Tech University Health Sciences Center, Paul L Foster School of Medicine Disclosure: Nothing to disclose. Definition of Lunula Definition of Lunula Lunula: 1. The crescent-shaped area at the base of a fingernail or toenail. 2. Any small crescent- or moon-shaped area or structure. Lunula is the diminuitive of the Latin luna, the moon. Also called the lunule . Last Editorial Review: 6/9/2016 Integumentary System Integumentary System Home > Integumentary System Integumentary System The integumentary system is an organ system consisting of the skin, hair, nails, and exocrine glands. The skin is only a few millimeters thick yet is by far the largest organ in the body. The average person’s skin weighs 10 pounds and has a surface area of almost 20 square feet.

Skin forms the body’s outer covering and forms a barrier to protect the body from chemicals, disease, UV light, and physical damage. Hair and nails extend from the skin to reinforce the skin and protect it from environmental... Move up/down/left/right: Click compass arrows Rotate image: Click and drag in any direction, anywhere in the frame Identify objects: Click on them in the image 2D Interactive 3D Rotate & Zoom Change Anatomical System Female Full Integumentary System Description [Continued from above] . . . damage. The exocrine glands of the integumentary system produce sweat, oil, and wax to cool, protect, and moisturize the skin’s surface. Anatomy of the Integumentary System Epidermis The epidermis is the most superficial layer of the skin that covers almost the entire body surface. The epidermis rests upon and protects the deeper and thicker dermis layer of the skin. Structurally, the epidermis is only about a tenth of a millimeter thick but is made of 40 to 50 rows of stacked squamous epithelial cells. The epidermis is an avascular region of the body, meaning that it does not contain any blood or blood vessels.

The cells of the epidermis receive all of their nutrients via diffusion of fluids from the dermis . The epidermis is made of several specialized types of cells. Almost 90% of the epidermis is made of cells known as keratinocytes. Keratinocytes develop from stem cells at the base of the epidermis and begin to produce and store the protein keratin. Keratin makes the keratinocytes very tough, scaly and water-resistant. At about 8% of epidermal cells, melanocytes form the second most numerous cell type in the epidermis. Melanocytes produce the pigment melanin to protect the skin from ultraviolet radiation and sunburn. Langerhans cells are the third most common cells in the epidermis and make up just over 1% of all epidermal cells. Langerhans cells’ role is to detect and fight pathogens that attempt to enter the body through the skin. Finally, Merkel cells make up less than 1% of all epidermal cells but have the important function of sensing touch. Merkel cells form a disk along the deepest edge of the epidermis where they connect to nerve endings in the dermis to sense light touch.

The epidermis in most of the body is arranged into 4 distinct layers. In the palmar surface of the hands and plantar surface of the feet, the skin is thicker than in the rest of the body and there is a fifth layer of epidermis. The deepest region of the epidermis is the stratum basale, which contains the stem cells that reproduce to form all of the other cells of the epidermis. The cells of the stratum basale include cuboidal keratinocytes, melanocytes, and Merkel cells. Superficial to stratum basale is the stratum spinosum layer where Langerhans cells are found along with many rows of spiny keratinocytes. The spines found here are cellular projections called desmosomes that form between keratinocytes to hold them together and resist friction. Just superficial to the stratum spinosum is the stratum granulosum, where keratinocytes begin to produce waxy lamellar granules to waterproof the skin. The keratinocytes in the stratum granulosum are so far removed from the dermis that they begin to die from lack of nutrients.

In the thick skin of the hands and feet, there is a layer of skin superficial to the stratum granulosum known as the stratum lucidum. The stratum lucidum is made of several rows of clear, dead keratinocytes that protect the underlying layers. The outermost layer of skin is the stratum corneum. The stratum corneum is made of many rows of flattened, dead keratinocytes that protect the underlying layers. Dead keratinocytes are constantly being shed from the surface of the stratum corneum and being replaced by cells arriving from the deeper layers. Dermis The dermis is the deep layer of the skin found under the epidermis. The dermis is mostly made of dense irregular connective tissue along with nervous tissue, blood, and blood vessels. The dermis is much thicker than the epidermis and gives the skin its strength and elasticity. Within the dermis there are two distinct regions: the papillary layer and the reticular layer. The papillary layer is the superficial layer of the dermis that borders on the epidermis. The papillary layer contains many finger-like extensions called dermal papillae that protrude superficially towards the epidermis.

The dermal papillae increase the surface area of the dermis and contain many nerves and blood vessels that are projected toward the surface of the skin. Blood flowing through the dermal papillae provide nutrients and oxygen for the cells of the epidermis. The nerves of the dermal papillae are used to feel touch, pain, and temperature through the cells of the epidermis. The deeper layer of the dermis, the reticular layer, is the thicker and tougher part of the dermis. The reticular layer is made of dense irregular connective tissue that contains many tough collagen and stretchy elastin fibers running in all directions to provide strength and elasticity to the skin. The reticular layer also contains blood vessels to support the skin cells and nerve tissue to sense pressure and pain in the skin. Hypodermis Deep to the dermis is a layer of loose connective tissues known as the hypodermis, subcutis, or subcutaneous tissue. The hypodermis serves as the flexible connection between the skin and the underlying muscles and bones as well as a fat storage area.

Areolar connective tissue in the hypodermis contains elastin and collagen fibers loosely arranged to allow the skin to stretch and move independently of its underlying structures. Fatty adipose tissue in the hypodermis stores energy in the form of triglycerides. Adipose also helps to insulate the body by trapping body heat produced by the underlying muscles. Hair Hair is an accessory organ of the skin made of columns of tightly packed dead keratinocytes found in most regions of the body. The few hairless parts of the body include the palmar surface of the hands, plantar surface of the feet, lips , labia minora , and glans penis . Hair helps to protect the body from UV radiation by preventing sunlight from striking the skin. Hair also insulates the body by trapping warm air around the skin. The structure of hair can be broken down into 3 major parts: the follicle, root, and shaft. The hair follicle is a depression of epidermal cells deep into the dermis. Stem cells in the follicle reproduce to form the keratinocytes that eventually form the hair while melanocytes produce pigment that gives the hair its color.

Within the follicle is the hair root, the portion of the hair below the skin’s surface. As the follicle produces new hair, the cells in the root push up to the surface until they exit the skin. The hair shaft consists of the part of the hair that is found outside of the skin. The hair shaft and root are made of 3 distinct layers of cells: the cuticle, cortex, and medulla. The cuticle is the outermost layer made of keratinocytes. The keratinocytes of the cuticle are stacked on top of each other like shingles so that the outer tip of each cell points away from the body. Under the cuticle are the cells of the cortex that form the majority of the hair’s width. The spindle-shaped and tightly packed cortex cells contain pigments that give the hair its color. The innermost layer of the hair, the medulla, is not present in all hairs. When present, the medulla usually contains highly pigmented cells full of keratin. When the medulla is absent, the cortex continues through the middle of the hair. Nails Nails are accessory organs of the skin made of sheets of hardened keratinocytes and found on the distal ends of the fingers and toes.

Fingernails and toenails reinforce and protect the end of the digits and are used for scraping and manipulating small objects. There are 3 main parts of a nail: the root, body, and free edge. The nail root is the portion of the nail found under the surface of the skin. The nail body is the visible external portion of the nail. The free edge is the distal end portion of the nail that has grown beyond the end of the finger or toe. Nails grow from a deep layer of epidermal tissue known as the nail matrix, which surrounds the nail root. The stem cells of the nail matrix reproduce to form keratinocytes, which in turn produce keratin protein and pack into tough sheets of hardened cells. The sheets of keratinocytes form the hard nail root that slowly grows out of the skin and forms the nail body as it reaches the skin’s surface. The cells of the nail root and nail body are pushed toward the distal end of the finger or toe by new cells being formed in the nail matrix. Under the nail body is a layer of epidermis and dermis known as the nail bed.

The nail bed is pink in color due to the presence of capillaries that support the cells of the nail body. The proximal end of the nail near the root forms a whitish crescent shape known as the lunula where a small amount of nail matrix is visible through the nail body. Around the proximal and lateral edges of the nail is the eponychium , a layer of epithelium that overlaps and covers the edge of the nail body. The eponychium helps to seal the edges of the nail to prevent infection of the underlying tissues. Sudoriferous Glands Sudoriferous glands are exocrine glands found in the dermis of the skin and commonly known as sweat glands. There are 2 major types of sudoriferous glands: eccrine sweat glands and apocrine sweat glands. Eccrine sweat glands are found in almost every region of the skin and produce a secretion of water and sodium chloride. Eccrine sweat is delivered via a duct to the surface of the skin and is used to lower the body’s temperature through evaporative cooling. Apocrine sweat glands are found in mainly in the axillary and pubic regions of the body.

The ducts of apocrine sweat glands extend into the follicles of hairs so that the sweat produced by these glands exits the body along the surface of the hair shaft. Apocrine sweat glands are inactive until puberty, at which point they produce a thick, oily liquid that is consumed by bacteria living on the skin. The digestion of apocrine sweat by bacteria produces body odor. Sebaceous Glands Sebaceous glands are exocrine glands found in the dermis of the skin that produce an oily secretion known as sebum. Sebaceous glands are found in every part of the skin except for the thick skin of the palms of the hands and soles of the feet. Sebum is produced in the sebaceous glands and carried through ducts to the surface of the skin or to hair follicles. Sebum acts to waterproof and increase the elasticity of the skin. Sebum also lubricates and protects the cuticles of hairs as they pass through the follicles to the exterior of the body. Ceruminous Glands Ceruminous glands are special exocrine glands found only in the dermis of the ear canals.

Ceruminous glands produce a waxy secretion known as cerumen to protect the ear canals and lubricate the eardrum . Cerumen protects the ears by trapping foreign material such as dust and airborne pathogens that enter the ear canal . Cerumen is made continuously and slowly pushes older cerumen outward toward the exterior of the ear canal where it falls out of the ear or is manually removed. Physiology of the Integumentary System Keratinization Keratinization, also known as cornification, is the process of keratin accumulating within keratinocytes. Keratinocytes begin their life as offspring of the stem cells of the stratum basale. Young keratinocytes have a cuboidal shape and contain almost no keratin protein at all. As the stem cells multiply, they push older keratinocytes towards the surface of the skin and into the superficial layers of the epidermis. By the time keratinocytes reach the stratum spinosum, they have begun to accumulate a significant amount of keratin and have become harder, flatter, and more water resistant. As the keratinocytes reach the stratum granulosum, they have become much flatter and are almost completely filled with keratin.

At this point the cells are so far removed from the nutrients that diffuse from the blood vessels in the dermis that the cells go through the process of apoptosis. Apoptosis is programmed cell death where the cell digests its own nucleus and organelles, leaving only a tough, keratin-filled shell behind. Dead keratinocytes moving into the stratum lucidum and stratum corneum are very flat, hard, and tightly packed so as to form a keratin barrier to protect the underlying tissues. Temperature Homeostasis Being the body’s outermost organ, the skin is able to regulate the body’s temperature by controlling how the body interacts with its environment. In the case of the body entering a state of hyperthermia, the skin is able to reduce body temperature through sweating and vasodilation. Sweat produced by sudoriferous glands delivers water to the surface of the body where it begins to evaporate. The evaporation of sweat absorbs heat and cools the body’s surface. Vasodilation is the process through which smooth muscle lining the blood vessels in the dermis relax and allow more blood to enter the skin.

Blood transports heat through the body, pulling heat away from the body’s core and depositing it in the skin where it can radiate out of the body and into the external environment. In the case of the body entering a state of hypothermia, the skin is able to raise body temperature through the contraction of arrector pili muscles and through vasoconstriction. The follicles of hairs have small bundles of smooth muscle attached to their base called arrector pili muscles. The arrector pili form goose bumps by contracting to move the hair follicle and lifting the hair shaft upright from the surface of the skin. This movement results in more air being trapped under the hairs to insulate the surface of the body. Vasoconstriction is the process of smooth muscles in the walls of blood vessels in the dermis contracting to reduce the flood of blood to the skin. Vasoconstriction permits the skin to cool while blood stays in the body’s core to maintain heat and circulation in the vital organs. Vitamin D Synthesis Vitamin D, an essential vitamin necessary for the absorption of calcium from food, is produced by ultraviolet (UV) light striking the skin.

The stratum basale and stratum spinosum layers of the epidermis contain a sterol molecule known as 7-dehydrocholesterol. When UV light present in sunlight or tanning bed lights strikes the skin, it penetrates through the outer layers of the epidermis and strikes some of the molecules of 7-dehydrocholesterol, converting it into vitamin D3. Vitamin D3 is converted in the kidneys into calcitriol, the active form of vitamin D. Protection The skin provides protection to its underlying tissues from pathogens, mechanical damage, and UV light. Pathogens, such as viruses and bacteria, are unable to enter the body through unbroken skin due to the outermost layers of epidermis containing an unending supply of tough, dead keratinocytes. This protection explains the necessity of cleaning and covering cuts and scrapes with bandages to prevent infection. Minor mechanical damage from rough or sharp objects is mostly absorbed by the skin before it can damage the underlying tissues. Epidermal cells reproduce constantly to quickly repair any damage to the skin. Melanocytes in the epidermis produce the pigment melanin, which absorbs UV light before it can pass through the skin.

UV light can cause cells to become cancerous if not blocked from entering the body. Skin Color Human skin color is controlled by the interaction of 3 pigments: melanin, carotene, and hemoglobin. Melanin is a brown or black pigment produced by melanocytes to protect the skin from UV radiation. Melanin gives skin its tan or brown coloration and provides the color of brown or black hair. Melanin production increases as the skin is exposed to higher levels of UV light resulting in tanning of the skin. Carotene is another pigment present in the skin that produces a yellow or orange cast to the skin and is most noticeable in people with low levels of melanin. Hemoglobin is another pigment most noticeable in people with little melanin. Hemoglobin is the red pigment found in red blood cells, but can be seen through the layers of the skin as a light red or pink color. Hemoglobin is most noticeable in skin coloration during times of vasodilation when the capillaries of the dermis are open to carry more blood to the skin’s surface. Cutaneous Sensation The skin allows the body to sense its external environment by picking up signals for touch, pressure, vibration, temperature, and pain.

Merkel disks in the epidermis connect to nerve cells in the dermis to detect shapes and textures of objects contacting the skin. Corpuscles of touch are structures found in the dermal papillae of the dermis that also detect touch by objects contacting the skin. Lamellar corpuscles found deep in the dermis sense pressure and vibration of the skin. Throughout the dermis there are many free nerve endings that are simply neurons with their dendrites spread throughout the dermis. Free nerve endings may be sensitive to pain, warmth, or cold. The density of these sensory receptors in the skin varies throughout the body, resulting in some regions of the body being more sensitive to touch, temperature, or pain than other regions. Excretion In addition to secreting sweat to cool the body, eccrine sudoriferous glands of the skin also excrete waste products out of the body. Sweat produced by eccrine sudoriferous glands normally contains mostly water with many electrolytes and a few other trace chemicals. The most common electrolytes found in sweat are sodium and chloride, but potassium, calcium, and magnesium ions may be excreted as well.

When these electrolytes reach high levels in the blood, their presence in sweat also increases, helping to reduce their presence within the body. In addition to electrolytes, sweat contains and helps to excrete small amounts of metabolic waste products such as lactic acid, urea, uric acid, and ammonia. Finally, eccrine sudoriferous glands can help to excrete alcohol from the body of someone who has been drinking alcoholic beverages. Alcohol causes vasodilation in the dermis, leading to increased perspiration as more blood reaches sweat glands. The alcohol in the blood is absorbed by the cells of the sweat glands, causing it to be excreted along with the other components of sweat. Prepared by Tim Taylor, Anatomy and Physiology Instructor Human Physiology/Integumentary System - Wikibooks, open books for an open world Human Physiology/Integumentary System Introduction[ edit ] The integumentary system consists of the skin, hair, nails, the subcutaneous tissue below the skin,and assorted glands.The most obvious function of the integumentary system is the protection that the skin gives to underlying tissues. The skin not only keeps most harmful substances out, but also prevents the loss of fluids.

A major function of the subcutaneous tissue is to connect the skin to underlying tissues such as muscles. Hair on the scalp provides insulation from cold for the head. The hair of eyelashes and eyebrows helps keep dust and perspiration out of the eyes, and the hair in our nostrils helps keep dust out of the nasal cavities. Any other hair on our bodies no longer serves a function, but is an evolutionary remnant. Nails protect the tips of fingers and toes from mechanical injury. Fingernails give the fingers greater ability to pick up small objects. There are four types of glands in the integumentary system: Sudoriferous glands, Sebaceous glands, Ceruminous glands, and Mammary glands. Sudoriferous glands are sweat producing glands. These are important to help maintain body temperature. Sebaceous glands are oil producing glands which help inhibit bacteria, keep us waterproof and prevent our hair and skin from drying out. Ceruminous glands produce earwax which keeps the outer surface of the eardrum pliable and prevents drying. Mammary glands produce milk.

Skin[ edit ] In zoology and dermatology, skin is an organ of the integumentary system made up of a layer of tissues that guard underlying muscles and organs. As the interface with the surroundings, it plays the most important role in protecting against pathogens. Its other main functions are insulation and temperature regulation, sensation and vitamin D and B synthesis. Skin is considered one of the most important parts of the body. Skin has pigmentation, melanin, provided by melanocytes, which absorbs some of the potentially dangerous radiation in sunlight. It also contains DNA repair enzymes which reverse UV damage, and people who lack the genes for these enzymes suffer high rates of skin cancer. One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has sometimes led to the classification of people(s) on the basis of skin color. Damaged skin will try to heal by forming scar tissue, often giving rise to discoloration and depigmentation of the skin. The skin is often known as "the largest organ in the human body".

This applies to exterior surface, as it covers the body, appearing to have the largest surface area of all the organs. Moreover, it applies to weight, as it weighs more than any single internal organ, accounting for about 15 percent of body weight. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters, most of it is between 2-3 mm thick. The average square inch of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than a thousand nerve endings. The use of natural or synthetic cosmetics to treat the appearance of the face and condition of the skin (such as pore control and black head cleansing) is common among many cultures. Layers[ edit ] The skin has two major layers which are made of different tissues and have very different functions. Diagram of the layers of human skin Skin is composed of the epidermis and the dermis. Below these layers lies the hypodermis or subcutaneous adipose layer, which is not usually classified as a layer of skin. The outermost epidermis consists of stratified squamous keratinizing epithelium with an underlying basement membrane.

It contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum, granulosum, spinosum, basale. Cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate, inducing expression of new types of keratin genes. They eventually reach the corneum and become sloughed off (desquamation). This process is called keratinization and takes place within about 30 days. This layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out. Blood capillaries are found beneath the dermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips. The dermis lies below the epidermis and contains a number of structures including blood vessels, nerves, hair follicles, smooth muscle, glands and lymphatic tissue.

It consists of loose connective tissue otherwise called areolar connective tissue - collagen, elastin and reticular fibers are present. Erector muscles, attached between the hair papilla and epidermis, can contract, resulting in the hair fiber pulled upright and consequentially goose bumps. The main cell types are fibroblasts, adipocytes (fat storage) and macrophages. Sebaceous glands are exocrine glands which produce, a mixture of lipids and waxy substances: lubrication, water-proofing, softening and antibactericidal actions are among the many functions of sebum. Sweat glands open up via a duct onto the skin by a pore. The dermis is made of an irregular type of fibrous connective tissue consisting of collagen and elastin fibers. It can be split into the papillary and reticular layers. The papillary layer is outermost and extends into the epidermis to supply it with vessels. It is composed of loosely arranged fibers. Papillary ridges make up the lines of the hands giving us fingerprints. The reticular layer is more dense and is continuous with the hypodermis. It contains the bulk of the structures (such as sweat glands).

The reticular layer is composed of irregularly arranged fibers and resists stretching. The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (the hypodermis contains 95% of body fat). Fat serves as padding and insulation for the body. Clinical Application: The patch drug delivery system. The transdermal patch is an increasingly popular drug delivery system. These patches are designed so that the drug molecules diffuse through the epidermis to the blood vessels in the dermis layer. A typical patch works well for small lipid-soluble molecules (for example, estrogen, nitroglycerin, and nicotine) that can make their way between epidermal cells.

Functions[ edit ] Protection: Skin gives an anatomical barrier between the internal and external environment in bodily defense; Langerhans cells in the skin are part of the immune system Sensation: Skin contains a variety of nerve endings that react to heat, cold, touch, pressure, vibration, and tissue injury; see somatosensory system and touch . Heat regulation: The skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss while constricted vessels greatly reduce cutaneous blood flow and conserve heat. Erector pili muscles are significant in animals. Wikipedia has related information at Hair Types of hair[ edit ] Humans have three different types of hair: Lanugo, the fine, unpigmented hair that covers nearly the entire body of a fetus, although most has been replaced with vellus by the time of the baby's birth Vellus hair, the short, downy, "peach fuzz" body hair (also unpigmented) that grows in most places on the human body.

While it occurs in both sexes, and makes up much of the hair in children, men have a much smaller percentage (around 10%) vellus whereas 2/3 of a female's hair is vellus. Terminal hair, the fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair, and often is found in regions such as the axillary, male beard, and pubic. Pathological impacts on hair[ edit ] Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones. Other diseases and traumas can cause temporary or permanent loss of hair, either generally or in patches. The hair shafts may also store certain poisons for years, even decades, after death. In the case of Col. Lafayette Baker, who died July 3, 1868, use of an atomic absorption spectrophotometer showed the man was killed by white arsenic. The prime suspect was Wallace Pollock, Baker's brother-in-law. According to Dr. Ray A.

Neff, Pollack had laced Baker's beer with it over a period of months, and a century or so later minute traces of arsenic showed up in the dead man's hair. Mrs. Baker's diary seems to confirm that it was indeed arsenic, as she writes of how she found some vials of it inside her brother's suit coat one day. Parts of the fingernail[ edit ] The parts of a finger nail The fingernail is an important structure made of keratin. The fingernail generally serve two purposes. It serves as a protective plate and enhances sensation of the fingertip. The protection function of the fingernail is commonly known, but the sensation function is equally important. The fingertip has many nerve endings in it allowing us to receive volumes of information about objects we touch. The nail acts as a counterforce to the fingertip providing even more sensory input when an object is touched. Nail Structure[ edit ] The structure we know of as the nail is divided into six specific parts - the root, nail bed, nail plate, eponychium (cuticle), perionychium, and hyponychium. Root The root of the fingernail is also known as the germinal matrix.

This portion of the nail is actually beneath the skin behind the fingernail and extends several millimeters into the finger. The fingernail root produces most of the volume of the nail and the nail bed. This portion of the nail does not have any melanocytes, or melanin producing cells. The edge of the germinal matrix is seen as a white, crescent shaped structure called the lunula. Nail Bed The nail bed is part of the nail matrix called the sterile matrix. It extends from the edge of the germinal matrix, or lunula, to the hyponychium. The nail bed contains the blood vessels, nerves, and melanocytes, or melanin-producing cells. As the nail is produced by the root, it streams down along the nail bed, which adds material to the undersurface of the nail making it thicker. It is important for normal nail growth that the nail bed be smooth. If it is not, the nail may split or develop grooves that can be cosmetically unappealing. Nail Plate The nail plate is the actual fingernail, made of translucent keratin. The pink appearance of the nail comes from the blood vessels underneath the nail.

The underneath surface of the nail plate has grooves along the length of the nail that help anchor it to the nail bed. eponychium The cuticle of the fingernail is also called the eponychium. The cuticle is situated between the skin of the finger and the nail plate fusing these structures together and providing a waterproof barrier. Perionychium The perioncyhium is the skin that overlies the nail plate on its sides. It is also known as the paronychial edge. The perionychium is the site of hangnails, ingrown nails, and an infection of the skin called paronychia. Hyponychium The hyponychium is the area between the nail plate and the fingertip. It is the junction between the free edge of the nail and the skin of the fingertip, also providing a waterproof barrier. Nails: left hand, adult human male Nail Diseases[ edit ] Nail diseases are in a separate category from diseases of the skin. Although nails are a skin appendage, they have their own signs and symptoms which may relate to other medical conditions. Nail conditions that show signs of infection or inflammation require medical assistance and cannot be treated at a beauty parlor.

Deformity or disease of the nails may be referred to as onychosis. There are many disease that can occur with the fingernails and toenails. The most common of these diseases are ingrown nails and fungal infections. Ingrown Nails[ edit ] Onychocryptosis, commonly known as "ingrown nails" (unguis incarnatus), can affect either the fingers or the toes. In this condition, the nail cuts into one or both sides of the nail bed, resulting in inflammation and possibly infection. The relative rarity of this condition in the fingers suggests that pressure from the ground or shoe against the toe is a prime factor. The movements involved in walking or other physical disturbances can contribute to the problem. Mild onychocryptosis, particularly in the absence of infection, can be treated by trimming and rounding the nail. More advanced cases, which usually include infection, are treated by surgically excising the ingrowing portion of the nail down to its bony origin and cauterizing the matrix, or 'root', to prevent recurrence. This surgery is called matricectomy. The best results are achieved by cauterizing the matrix with phenol.

Another method, which is much less effective, is excision of the matrix, sometimes called a 'cold steel procedure' Nail Fungus[ edit ] An infection of nail fungus (onychomycosis) occurs when fungi infect one or more of your nails. Onychomycosis generally begins as a white or yellow spot under the tip of the fingernail or toenail. As the nail fungus spreads deeper into the nail, it may cause the nail to discolor, thicken and develop crumbling edges — an unsightly and potentially painful problem. Infections of nail fungus account for about half of all nail disorders. These infections usually develop on nails continually exposed to warm, moist environments, such as sweaty shoes or shower floors. Nail fungus isn't the same as athlete's foot, which primarily affects the skin of the feet, but at times the two may coexist and can be caused by the same type of fungus. An infection with nail fungus may be difficult to treat, and infections may recur. But medications are available to help clear up nail fungus permanently.

Clinical Application[ edit ] Nail inspection can give a great deal of information about the internal working of the body as well, and like tongue or iris inspection, has a long history of diagnostic use in cantraditional medical practices such as Chinese medicine. Pliability: Brittleness is associated with iron deficiency, thyroid problems, impaired kidney function, circulation problems[2], and biotin deficiency[3] Splitting and fraying are associated with psoriasis, folic acid, protein and/or Vitamin C deficiency. Unusual thickness is associated with circulation problems. Thinning nails and itchy skin are associated with lichen planus[4]. Shape and texture: Clubbing, or nails that curve down around the fingertips with nail beds that bulge is associated with oxygen deprivation and lung, heart, or liver disease. Spooning, or nails that grow upwards is associated with iron or B12 deficiency. Flatness can indicate a B12 vitamin deficiency[5] or Raynaud's disease[6] Pitting of the nails is associated with Psoriasis. Horizontal ridges indicate stress, and Beau's lines are associated with many serious conditions. Vertical ridges are associated with arthritis[7].

Vertical grooves are associated with kidney disorders, aging, and iron deficiency[8]. Beading is associated with rheumatoid arthritis[9]. Nails that resemble hammered brass are associated with (or portend) hair loss[10]. Short small beds are associated with heart disease[11]. Coloration of the nail bed: Mee's lines are associated with arsenic or thallium poisoning, and renal failure. White lines across the nail are associated with heart disease, liver disease, or a history of a recent high fever[12]. Opaque white nails with a dark band at the fingertip are associated with cancer, cirrhosis, congestive heart failure, diabetes and aging[13]. Paleness or whitening is associated with liver or kidney disease and anemia[14]. Yellowing of the nail bed is associated with chronic bronchitis, lymphatic problems, diabetes, and liver disorders. Brown or copper nail beds are associated with arsenic or copper poisoning, and local fungal infection. Grey nail beds are associated with arthritis, edema, malnutrition, post-operative effects, glaucoma and cardio-pulmonary disease[15]. redness is associated with heart conditions. dark nails are associated with B12 deficiency.

Stains of the nail plate (not the nail bed) are associated with nail polish[16], smoking, and henna use. Markings: Pink and white nails are associated with kidney disease[17]. Parallel white lines in the nails are associated with hypoalbuminemia. red skin at the base of the nail is associated with connective tissue disorders[18]. blue lunulae are associated with silver poisoning or lung disorder[19]. blue nail beds are (much like blue skin) associated with poor oxygenation of the blood (asthma, emphysema, etc)[20]. small white patches are associated with zinc or calcium deficiency or malabsorption, parasites, or local injury[21]. receded lunulae (fewer than 8) are associated with poor circulation[22], shallow breathing habits or thyroid mysfunction[23]. large lunulae (more than 25% of the thumb nail) is associated with high blood pressure. Sweat Glands[ edit ] A diagrammatic sectional view of the skin (magnified). Sweat gland labeled as "sudoriferous gland" at center right.

In humans, there are two kinds of sweat glands which differ greatly in both the composition of the sweat and its purpose: Also "click" here "How our body Sweats" to see a short movie on sweat glands. Eccrine (a.k.a. merocrine)[ edit ] Eccrine sweat glands are exocrine glands distributed over the entire body surface but are particularly abundant on the palms of hands, soles of feet, and on the forehead. These produce sweat that is composed chiefly of water (99%) with various salts. The primary function is body temperature regulation. Eccrine sweat glands are coiled tubular glands derived leading directly to the most superficial layer of the epidermis (out layer of skin) but extending into the inner layer of the skin (dermis layer). They are distributed over almost the entire surface of the body in humans and many other species, but are lacking in some marine and fur-bearing species. The sweat glands are controlled by sympathetic cholinergic nerves which are controlled by a center in the hypothalamus. The hypothalamus senses core temperature directly, and also has input from temperature receptors in the skin and modifies the sweat output, along with other thermoregulatory processes.

Human eccrine sweat is composed chiefly of water with various salts and organic compounds in solution. It contains minute amounts of fatty materials, urea, and other wastes. The concentration of sodium varies from 35–65 mmol/l and is lower in people acclimatised to a hot environment. The sweat of other species generally differ in composition. Apocrine[ edit ] Apocrine sweat glands only develop during early- to mid-puberty (approximately age 15) and release more than normal amounts of sweat for approximately a month and subsequently regulate and release normal amounts of sweat after a certain period of time. Apocrine sweat glands produce sweat that contains fatty materials. These glands are mainly present in the armpits and around the genital area and their activity is the main cause of sweat odor, due to the bacteria that break down the organic compounds in the sweat from these glands. Emotional stress increases the production of sweat from the apocrine glands, or more precisely: the sweat already present in the tubule is squeezed out. Apocrine sweat glands essentially serve as scent glands. In some areas of the body, these sweat glands are modified to produce wholly different secretions, including the cerumen ("wax") of the outer ear.

Other glands, such as Mammary glands, are greatly enlarged and modified to produce milk. Sebaceous Glands[ edit ] Schematic view of a hair follicle with sebaceous gland. The sebaceous glands are glands found in the skin of mammals. They secrete an oily substance called sebum (Latin, meaning fat or tallow) that is made of fat (lipids) and the debris of dead fat-producing cells. These glands exist in humans throughout the skin except in the palms of the hands and soles of the feet. Sebum acts to protect and waterproof hair and skin, and keep them from becoming dry, brittle, and cracked. It can also inhibit the growth of microorganisms on skin. Sebaceous glands can usually be found in hair-covered areas where they are connected to hair follicles to deposit sebum on the hairs, and bring it to the skin surface along the hair shaft. The structure consisting of hair, hair follicle and sebaceous gland is also known as pilosebaceous unit. Sebaceous glands are also found in non haired areas of lips, eyelids, penis, labia minora and nipples; here the sebum reaches the surface through ducts.

In the glands, sebum is produced within specialized cells and is released as these cells burst; sebaceous glands are thus classified as holocrine glands. Sebum is odorless, but its bacterial breakdown can produce odors. Sebum is the cause of some people experiencing "oily" hair if it is not washed for several days. Earwax is partly sebum, as is mucopurulent discharge, the dry substance accumulating in the corners of the eye after sleeping. A hair follicle with associated structures. The composition of sebum varies from species to species; in humans, the lipid content consists of about 25% wax monoesters, 41% triglycerides, 16% free fatty acids, and 12% squalene. The activity of the sebaceous glands increases during puberty because of heightened levels of androgens. Sebaceous glands are involved in skin problems such as acne and keratosis pilaris. A blocked sebaceous gland can result in a sebaceous cyst. The prescription drug isotretinoin significantly reduces the amount of sebum produced by the sebaceous glands, and is used to treat acne.

The extreme use (up to 10 times doctor prescribed amounts) of anabolic steroids by bodybuilders to prevent weight loss tend to stimulate the sebaceous glands which can cause acne. The sebaceous glands of a human fetus in utero secrete a substance called Vernix caseosa, a "waxy" or "cheesy" white substance coating the skin of newborns. The preputial glands of mice and rats are large modified sebaceous glands that produce pheromones. Ceruminous glands[ edit ] Wet-type human earwax on a cotton swab. Earwax, also known by the medical term cerumen, is a yellowish, waxy substance secreted in the ear canal of humans and many other mammals. It plays a vital role in the human ear canal, assisting in cleaning and lubrication, and also provides some protection from bacteria, fungus, and insects. A comprehensive review of the physiology and pathophysiology of cerumen can be found in Roeser and Ballachanda. Excess or impacted cerumen can press against the eardrum and/or occlude the external auditory canal and impair hearing.

Production, composition, and different types[ edit ] Cerumen is produced in the outer third of the cartilaginous portion of the human ear canal. It is a mixture of viscous secretions from sebaceous glands and less-viscous ones from modified apocrine sweat glands. Two distinct genetically determined types of earwax are distinguished -- the wet-type which is dominant, and the dry type which is recessive. Asians and Native Americans are more likely to have the dry type of cerumen (grey and flaky), whereas Caucasians and Africans are more likely to have the wet type (honey-brown to dark-brown and moist). Cerumen type has been used by anthropologists to track human migratory patterns, such as those of the Inuit. The difference in cerumen type has been tracked to a single base change (an single nucleotide polymorphism) in a gene known as "ATP-binding cassette C11 gene". In addition to affecting cerumen type, this mutation also reduces sweat production. The researchers conjecture that the reduction in sweat was beneficial to the ancestors of East Asians and Native Americans who are thought to have lived in cold climates.

Function[ edit ] Wet-type earwax fluoresces weakly under ultraviolet light. Cleaning. Cleaning of the ear canal occurs as a result of the "conveyor belt" process of epithelial migration, aided by jaw movement. Cells formed in the center of the tympanic membrane migrate outwards from the umbo (at a rate equivalent to that of fingernail growth) to the walls of the ear canal, and accelerate towards the entrance of the ear canal. The cerumen in the canal is also carried outwards, taking with it any dirt, dust, and particulate matter that may have gathered in the canal. Jaw movement assists this process by dislodging debris attached to the walls of the ear canal, increasing the likelihood of its extrusion. Lubrication. Lubrication prevents desiccation and itching of the skin within the ear canal (known as asteatosis). The lubricative properties arise from the high lipid content of the sebum produced by the sebaceous glands. In wet-type cerumen at least, these lipids include cholesterol, squalene, and many long-chain fatty acids and alcohols. Antibacterial and antifungal roles.

While studies conducted up until the 1960s found little evidence supporting an antibacterial role for cerumen, more recent studies have found that cerumen provides some bactericidal protection against some strains of bacteria. Cerumen has been found to be effective in reducing the viability of a wide range of bacteria (sometimes by up to 99%), including Haemophilus influenzae, Staphylococcus aureus, and many variants of Escherichia coli. The growth of two fungi commonly present in otomycosis was also significantly inhibited by human cerumen. These antimicrobial properties are due principally to the presence of saturated fatty acids, lysozyme and, especially, to the relatively low pH of cerumen (typically around 6.1 in normal individuals). Mammary Glands[ edit ] Cross section of the breast of a human female. Mammary glands are the organs that, in the female mammal, produce milk for the sustenance of the young. These exocrine glands are enlarged and modified sweat glands and are the characteristic of mammals which gave the class its name.

Structure[ edit ] The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting epithelial cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells can contract, similar to muscle cells, and thereby push the milk from the alveoli through the lactiferous ducts towards the nipple, where it collects in widenings (sinuses) of the ducts. A suckling baby essentially squeezes the milk out of these sinuses. Dissection of a lactating breast. 1 - Fat 5 - Sinus of lactiferous duct 6 - Lactiferous duct One distinguishes between a simple mammary gland, which consists of all the milk-secreting tissue leading to a single lactiferous duct, and a complex mammary gland, which consists of all the simple mammary glands serving one nipple. Humans normally have two complex mammary glands, one in each breast, and each complex mammary gland consists of 10-20 simple glands.

(The presence of more than two nipples is known as polythelia and the presence of more than two complex mammary glands as polymastia.) Also, "click" this; "Breast tissue" , to this a movie visual of the breast. Development and hormonal control[ edit ] The development of mammary glands is controlled by hormones. The mammary glands exist in both sexes, but they are rudimentary until puberty when in response to ovarian hormones, they begin to develop in the female. Click this [1] to see what breast tissue does in a female during menustration. Estrogen promotes formation, while testosterone inhibits it. At the time of birth, the baby has lactiferous ducts but no alveoli. Little branching occurs before puberty when ovarian estrogens stimulate branching differentiation of the ducts into spherical masses of cells that will become alveoli. True secretory alveoli only develop in pregnancy, where rising levels of estrogen and progesterone cause further branching and differentiation of the duct cells, together with an increase in adipose tissue and a richer blood flow. Colostrum is secreted in late pregnancy and for the first few days after giving birth.

True milk secretion (lactation) begins a few days later due to a reduction in circulating progesterone and the presence of the hormone prolactin. The suckling of the baby causes the release of the hormone oxytocin which stimulates contraction of the myoepithelial cells. Breast cancer[ edit ] As described above, the cells of mammary glands can easily be induced to grow and multiply by hormones. If this growth runs out of control, cancer results. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands. Types of breast cancer Homeostasis[ edit ] As a whole, the integumentary system plays a big part in maintaining homeostasis. The integumentary system is the outermost organ system of the body and many of its functions are related to this location. The skin protects the body against pathogens and chemicals, minimizes loss or entry of water, and blocks the harmful effects of sunlight. Sensory receptors in the skin provide information about the external environment, helping the skin regulate body temperature in response to environmental changes and helping the body react to pain and other tactile stimuli. The large surface area of the skin makes it ideal for temperature regulation.

The rate of heat loss can be regulated by the amount of blood flowing through the blood vessels in the dermis close to the surface of the skin. When the body temperature rises, as for example during exercise, sympathetic tone is reduced and this brings about dilation of the blood vessels supplying the skin. The increase in skin blood flow allows heat to be lost more rapidly so that body temperature does not rise above the normal homeostatic range. The rate of heat loss can also be boosted by the production of sweat, which takes up additional heat as it evaporates. Conversely, if heat production is less than required, the dermal vessels constrict, sweating stops, and heat is conserved by the body. Glossary[ edit ] Areolar Areolar connective tissue is a pliable, mesh-like tissue with a fluid matrix and functions to cushion and protect body organs. It acts as a packaging tissue holding the internal organs together and in correct placement. Skin, Hair, and Nails Skin, Hair, and Nails La piel, el cabello y las uñas Your skin is your largest organ.

If the skin of a typical 150-pound (68-kilogram) adult male were stretched out flat, it would cover about 2 square yards (1.7 square meters) and weigh about 9 pounds (4 kilograms). Skin protects the network of muscles, bones , nerves, blood vessels , and everything else inside our bodies. Eyelids have the thinnest skin, the soles of our feet the thickest. Hair is actually a modified type of skin. Hair grows everywhere on the human body except the palms of the hands, soles of the feet, and lips. Hair grows more quickly in summer than winter, and more slowly at night than during the day. Like hair, nails are a type of modified skin — and they're not just for beauty. Nails protect the sensitive tips of our fingers and toes. Human nails are not necessary for living, but they do provide support for the tips of the fingers and toes, protect them from injury, and aid in picking up small objects. Without them, we'd have a hard time scratching an itch or untying a knot. Nails can be an indicator of a person's general health, and illness often affects their growth.

Skin Basics Skin is essential to a person's survival. It forms a barrier that prevents harmful substances and microorganisms from entering the body. It protects body tissues against injury. Our skin also controls the loss of life-sustaining fluids like blood and water, helps us regulate body temperature through perspiration, and protects us from the sun's damaging ultraviolet rays. Without the nerve cells in our skin, we couldn't feel warmth, cold, or other sensations. Our skin can also respond to situations and emotions: Muscles in the skin called erector pili contract to make the hairs on our skin stand up straight (goosebumps) when we are cold or frightened — for insulation and protection. Every square inch of skin contains thousands of cells and hundreds of sweat glands, oil glands, nerve endings, and blood vessels. Skin is made up of three layers: the epidermis (pronounced: ep-ih-DUR-mis), dermis, and the subcutaneous (pronounced: sub-kyoo-TAY-nee-us) tissue. continue Skin Cells and Layers The upper layer of our skin, the epidermis, is the tough, protective outer layer. It is about as thick as a sheet of paper over most parts of the body.

The epidermis has four layers of cells that are constantly flaking off and being renewed. In these four layers are three special types of cells: Melanocytes (pronounced: meh-LAH-nuh-sites) produce melanin, the pigment that gives skin its color. All people have roughly the same number of melanocytes; the more melanin that is produced, the darker the skin. Exposure to sunlight increases the production of melanin, which is why people get suntanned or freckled. Keratinocytes (pronounced: ker-uh-TIH-no-sites) produce keratin, a type of protein that is a basic component of hair and nails. Keratin is also found in skin cells in the skin's outer layer, where it helps create a protective barrier. Langerhans (pronounced: LAHNG-ur-hanz) cells help protect the body against infection. Because the cells in the epidermis are completely replaced about every 28 days, cuts and scrapes heal quickly. Below the epidermis is the next layer of our skin, the dermis, which is made up of blood vessels, nerve endings, and connective tissue. The dermis nourishes the epidermis.

Without certain molecules in the dermis, our skin wouldn't stretch when we bend or reposition itself when we straighten up. These two types of fibers in the dermis, collagen and elastin, help the skin stretch and reposition itself when we move. Collagen is strong and hard to stretch and elastin, as its name suggests, is elastic. In older people, some of the elastin-containing fibers degenerate, which is one reason why the skin looks wrinkled (most wrinkles are caused by sun exposure, though!). The dermis also contains a person's sebaceous glands. These glands, which surround and empty into our hair follicles and pores, produce an oil called sebum (pronounced: SEE-bum) that lubricates the skin and hair. Sebaceous glands are found mostly in the skin on the face, upper back, shoulders, and chest. Most of the time, the sebaceous glands make the right amount of sebum. As a person's body begins to mature and develop during the teenage years, though, hormones stimulate the sebaceous glands to make more sebum. This can lead to acne when pores become clogged by too much sebum and too many dead skin cells.

Later in life, these glands produce less sebum, which contributes to dry skin in older people. The bottom layer of our skin, the subcutaneous tissue, is made up of connective tissue, sweat glands, blood vessels, and cells that store fat. This layer helps protect the body from blows and other injuries and helps it hold in body heat. There are two types of sweat-producing glands. The eccrine (pronounced: EH-krun) glands are found everywhere in our bodies, although they are mostly in the forehead, palms, and soles of the feet. By producing sweat, these glands help regulate body temperature, and waste products are excreted through them. The other type of sweat-producing gland, the apocrine glands, develop at puberty and are concentrated in the armpits and pubic region. The sweat from the apocrine glands is thicker than that produced by the eccrine glands. Although this sweat doesn't smell, when it mixes with bacteria on the skin's surface, it can cause body odor. A normal, healthy adult secretes about 1 pint (about half a liter) of sweat daily, but this may be increased by physical activity, fever, or a hot environment.

previous continue Hair Basics The hair on our heads isn't just there for looks. It keeps us warm by preserving heat. The hair in our nose, ears, and around our eyes protects these sensitive areas of the body from dust and other small particles. Eyebrows and eyelashes protect our eyes by decreasing the amount of light and particles that go into them. The fine hair that covers our bodies provides warmth and protects our skin. Hair also cushions the body against injury. Human hair consists of the hair shaft, which projects from the skin's surface, and the root, a soft thickened bulb at the base of the hair embedded in the skin. The root ends in the hair bulb. The hair bulb sits in a sac-like pit in the skin called the follicle, from which the hair grows. At the bottom of the follicle is the papilla, where hair growth actually takes place. The papilla contains an artery that nourishes the root of the hair. As cells multiply and produce keratin to harden the structure, they are pushed up the follicle and through the skin's surface as a shaft of hair.

Each hair has three layers: the medulla at the center, which is soft; the cortex, which surrounds the medulla and is the main part of the hair; and the cuticle, the hard outer layer that protects the shaft. Hair grows by forming new cells at the base of the root. These cells multiply to form a rod of tissue in the skin. The rods of cells move upward through the skin as new cells form beneath them. As they move up, they are cut off from their supply of nourishment and start to form a hard protein called keratin in a process called keratinization (pronounced: ker-uh-tuh-nuh-ZAY-shun). As this process occurs, the hair cells die. The dead cells and keratin form the shaft of the hair. Each hair grows about ¼ inch (about 6 millimeters) every month and keeps on growing for up to 6 years. The hair then falls out and another grows in its place. The length of a person's hair depends on the length of the growing phase of the follicle. Follicles are active for 2 to 6 years; they rest for about 3 months after that.

A person becomes bald if the scalp follicles become inactive and no longer produce new hair. Thick hair grows out of large follicles; narrow follicles produce thin hair. The color of a person's hair is determined by the amount and distribution of melanin in the cortex of each hair (the same melanin that's found in the epidermis). Hair also contains a yellow-red pigment; people who have blonde or red hair have only a small amount of melanin in their hair. Hair becomes gray when people age because pigment no longer forms. previous continue All About Nails Nails grow out of deep folds in the skin of the fingers and toes. As epidermal cells below the nail root move up to the surface of the skin, they increase in number, and those closest to the nail root become flattened and pressed tightly together. Each cell is transformed into a thin plate; these plates are piled in layers to form the nail. As with hair, nails are formed by keratinization. When the nail cells accumulate, the nail is pushed forward. The skin below the nail is called the matrix. The larger part of the nail, the nail plate, looks pink because of the network of tiny blood vessels in the underlying dermis.

The whitish crescent-shaped area at the base of the nail is called the lunula. Fingernails grow about three or four times as quickly as toenails. Like hair, nails grow more rapidly in summer than in winter. If a nail is torn off, it will regrow if the matrix is not severely injured. White spots on the nail are sometimes due to temporary changes in growth rate. Skin Problems Some of the things that can affect the skin, nails, and hair are described below. Dermatitis Medical experts use the term dermatitis (pronounced: dur-mah-TY-tus) to refer to any inflammation that might be associated with swelling, itching, and redness of the skin. There are many types of dermatitis, including: Atopic dermatitis is also called eczema . It's a common, hereditary dermatitis that causes an itchy rash primarily on the face, trunk, arms, and legs. It commonly develops in infancy, but can also appear in early childhood. It may be associated with allergic diseases such as asthma or food, seasonal, or environmental allergies. Contact dermatitis occurs when the skin comes into contact with an irritating substance or a substance that a person is allergic to.

The best-known cause of contact dermatitis is poison ivy. But lots of other things cause contact dermatitis, including chemicals found in laundry detergent, cosmetics, and perfumes, and metals like jewelry, nickel plating on a belt buckle, or the back of the buttons on your jeans. Seborrheic dermatitis, an oily rash on the scalp, face, chest, and back, is related to an overproduction of sebum from the sebaceous glands. This condition is common in teens. Bacterial Skin Infections Impetigo . Impetigo (pronounced: im-puh-TY-go) is a bacterial infection that results in a honey-colored, crusty rash, often on the face near the mouth and nose. Cellulitis . Cellulitis (pronounced: sell-yuh-LY-tus) is an infection of the skin and subcutaneous tissue that typically occurs when bacteria are introduced through a puncture, bite, or other break in the skin. The affected area is usually warm and tender and has some redness. Streptococcal and staphylococcal infections. These two kinds of bacteria are the main causes of cellulitis and impetigo.

Certain types of these bacteria are also responsible for distinctive rashes on the skin, including the rashes associated with scarlet fever and toxic shock syndrome . Fungal Infections Fungal Infections of the Skin and Nails Candidal dermatitis. A warm, moist environment, such as that found in the folds of the skin in the diaper area of infants, is perfect for growth of the yeast Candida. Yeast infections of the skin in older children, teens, and adults are less common. Tinea infection ( ringworm ). Ringworm, which isn't a worm at all, is a fungus infection that can affect the skin, nails, or scalp. Tinea (pronounced: TIH-nee-uh) fungi can infect the skin and related tissues of the body. The medical name for ringworm of the scalp is tinea capitis; ringworm of the body is called tinea corporis; and ringworm of the nails is called tinea unguium. With tinea corporis, the fungi can cause scaly, ring-like lesions anywhere on the body. Tinea pedis ( athlete's foot ). This infection of the feet is caused by the same types of fungi (called dermatophytes) that cause ringworm.

Athlete's foot is commonly found in adolescents and is more likely to happen during warm weather. Other Skin Problems Parasitic infestations. Parasites (usually tiny insects or worms) can feed on or burrow into the skin, often resulting in an itchy rash. Scabies and lice are examples of parasitic infestations. Both are contagious — meaning they can be easily caught from other people. Viral infections. Many viruses cause characteristic rashes on the skin, including varicella (pronounced: var-ih-SEL-uh), the virus that causes chicken pox and shingles; herpes simplex, which causes cold sores; human papillomavirus (HPV), the virus that causes warts ; and a host of others. Acne (acne vulgaris). Acne is the single most common skin condition in teens. Some degree of acne is seen in 85% of adolescents, and nearly all teens have the occasional pimple, blackhead, or whitehead. Skin cancer.

Skin cancer is rare in children and teens, but good sun protection habits established during these years can help prevent skin cancers like melanoma (pronounced: mel-uh-NO-ma, a serious form of skin cancer that can spread to other parts of the body) later in life, especially among fair-skinned people who sunburn easily. In addition to these diseases and conditions, the skin can be injured in a number of ways. Minor scrapes, cuts, and bruises heal quickly on their own, but other injuries — severe cuts and burns, for example — require medical treatment. Disorders of the Scalp and Hair Tinea capitis, a type of ringworm, is a fungal infection that forms a scaly, ring-like lesion in the scalp. It's contagious and common among school-age children. Alopecia (pronounced: ah-luh-PEE-sha) is an area of hair loss. Ringworm is a common cause of temporary alopecia in children. Alopecia can also be caused by tight braiding that pulls on the hair roots (this condition is called traction alopecia).

Alopecia areata (where a person's hair falls out in round or oval patches on the scalp) is a less common condition that can sometimes affect teens. Fingernail & Toenail Anatomy - Nail Structure & Growth Our nails are constantly growing, but the growth rate slows down due to poor circulation and with age . Fingernails grow faster than toenails at a rate of 3 millimeters per month. It takes 6 months for a fingernail to grow from the root to the free edge. Toenails grow much slower, at just 1 millimeter per month. It takes a toenail 12 to 18 months to grow from root to tip. Nail Structure The structure of the nail is divided into six parts: the root, nail bed, nail plate, eponychium, paronychium, and hyponychium. Each of these six components has a specific function, and if a component is disrupted it can result in the abnormal appearance of the nail. Nail Root: The root of the fingernail is also known as the germinal matrix. Its edge appears as a white crescent, known as the lunula.

The root portion of the nail lies below the skin, underneath the fingernail, and it extends several millimeters into the finger. It produces most of the volume of the nail and the nail bed. Nail Bed: The nail bed is also referred to as the sterile matrix. It extends from the edge of the nail root, or lunula, to the hyponychium. The nail bed contains blood vessels, nerves, and melanocytes that produce melanin. As the root grows the nail, the nail streams down along the nail bed and adds material to the underside of the nail to make it thicker. When the nail grows properly, the nail bed is smooth, but if it doesn't grow correctly, the nail may split or develop ridges that aren't cosmetically attractive. Nail Plate: The nail plate is the actual fingernail, and it's made of translucent keratin. The pinkish appearance of the nail comes from the blood vessels underneath it. The underside of the nail plate has grooves that run along the length of the nail and help anchor it to the nail bed. Eponychium: The eponychium is more commonly known as the cuticle.

The cuticle is situated between the skin of the finger and the nail plate. It fuses these structures together and provides a waterproof barrier. Perionychium: The paronychium is the skin that overlaps onto the sides of the nail plate, also known as the paronychial edge. The paronychium is the site of hangnails, ingrown nails , and paronychia , a skin infection. Hyponychium: The hyponychium is the area between the nail plate and the fingertip. It is the junction between the free edge of the nail and the skin of the fingertip. It also provides a waterproof barrier. Continue Reading

Question: Where on the human body is the lunula?
Answer: Fingernail