As a branched kind of acinar gland, the sebaceous glands are found in people all over the skin except in the palms of the hands and soles of the feet.
Sebaceous glands are often found in hair-covered areas, where they are connected near the top of hair follicles ("see image at top"). The glands deposit sebum on each hair, and bring it to the skin surface along the hair shaft. The structure consisting of hair, hair follicle, arrector pili muscle, and sebaceous gland is known as a pilosebaceous unit.
Sebaceous glands are also found in non-haired areas (glabrous skin) of the eyelids, nose, penis, labia minora, and nipples. Here, the sebum travels along ducts which end inside "sweat pores" near the surface of the skin.
At the rim of the eyelids, meibomian glands are a specialized form of sebaceous gland. They secrete a form of sebum (known as meibum) onto the eye, which slows the drying of tears.
Sebaceous glands produce the oily, waxy substance called "sebum" (Latin, meaning "fat" or "tallow") that is made of fat (lipids), wax, and the leftover dead fat-producing cells. 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 has no smell, but its bacterial breakdown can produce a bad smell. Sebum is the cause of some people experiencing "oily" hair,
as in hot weather or if not washed for several days. Earwax is partly composed of sebum. Sebum can be washed using plain detergent, to dissolve the waxy material in the skin. Also, water for washing should be over to keep the sebum molten.
Excess sebum has been linked to eating red meats, fried and oily foods and some other type of foods but different research shows multiple theories about it.
It is commonly believed that sebum acts to save skin from drying or to waterproof hair and skin. But some scientists have contended that "low levels of sebaceous gland activity are not correlated with dry skin", and it may serve little or no purpose in modern humans.
The substances in sebum vary between species. In people, the lipid content is as follows:
Also, sapienic acid is a sebum fatty acid that is found only in people, not in animals.
The following medicines have been shown to reduce the flow of sebum from skin:
The sebaceous glands of a human fetus "in utero" secrete a substance called Vernix caseosa, a "waxy" or "cheesy" white substance covering the skin of newborn babies.
The activity of the sebaceous glands increases during puberty because of heightened levels of androgens. In males, sebaceous glands begin to appear predominantly on the penis, on the shaft and around the rim of the penile head during and after puberty. This is however normal, not to be confused with an STD.
In females, they appear predominantly in the labia minora.
Sebaceous glands are involved in skin problems such as oily skin, acne, and keratosis pilaris. In the skin pores, sebum and keratin can create a hyperkeratotic plug called a "microcomedo" which can block a pore and cause pimples. The prescription medicine 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, for muscle gain can also cause acne. The sebaceous gland is stimulated due to some steroid conversion into dihydrotestosterone. This event may cause serious acne on the face, neck, chest, back and shoulders.
A blocked sebaceous gland can result in a sebaceous cyst.
A condition involving enlarged sebaceous glands is known as sebaceous hyperplasia.
Sebaceous gland carcinoma is a very uncommon and aggressive form of cancer involving the sebaceous glands; sebaceous adenoma is a more benign neoplasm of the sebaceous glands.
Sebum can also build up around body piercings.
Certain species of Demodex mites feed on sebum and are commonly found in the sebaceous glands of mammals, including those of humans.
The preputial glands of mice and rats are large modified sebaceous glands that produce pheromones.
Microorganism
A micro-organism or "microbe" is an organism which is microscopic, which means so small that people cannot see them with naked eye. The study of microorganisms is called microbiology.
Micro-organisms include bacteria, fungi, archaea, protists and viruses, and are among the earliest known life forms. The first of these four types of micro-organisms may either be free-living or parasitic. Viruses can only be parasitic, since they always reproduce inside other living things.
Most micro-organisms are unicellular organism with only one cell, but there are unicellular protists that are visible to the human eye, and some multicellular species are microscopic.
Micro-organisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the earth's crust. Such habitats are lived in by extremophiles.
Micro-organisms are critical to nutrient recycling in ecosystems, because they act as decomposers. Because some micro-organisms can also take nitrogen out of the air, they are an important part of the nitrogen cycle. Pathogenic, or harmful, microbes can invade other organisms and cause disease.
Free-living microbes get their energy in many different ways. Some use photosynthesis, like plants do. Some break down natural chemicals in their environment. Others feed on things that were once living, such as fallen leaves and dead animals, causing them to breakdown or decay. Some fungi and bacteria cause food to decay. Moldy bread or fruit, sour milk, and rotten meat are examples of decayed food. In nature, decayed materials mix with soil, providing essential nutrients for plants to use. Without this process, the nutrients in the soil would run out. These types of organisms are called decomposers. They are the natural recyclers of living things on our planet.
Microbes also help us make some of our foods, such as bread, cheese, yogurt, beer, and wine. They feed on the sugar in grain, fruit, or milk, giving these foods a special texture and taste.
Some microbes, often called germs, cause illness or disease. They are parasites which live by invading living things. Chickenpox, mumps, and measles are all caused by viruses. Some bacteria are also germs. They cause many infectious diseases including tuberculosis and tetanus. Certain bacteria cause tooth decay. It is possible to protect humans against some harmful microbes by storing and preparing food properly, cleaning the teeth, washing hands, and by avoiding close contact with ill people.
All animals seem to have bacteria and protozoa living in or on them without doing much harm. Sometimes, as with herbivores, the microorganisms are vital to the digestion of food. The human gut has more organisms living inside it than there are cells in the human body.
Amoeba
Amoeba (plural = amoebae) is a well known genus of unicellular organism, a protist. One of its most common species, the Amoeba Proteus, is about 0.2 to 0.3 mm large. The amoeba was first discovered by August Von Rosenhof in 1757. It is a genus of protozoa that moves with "false feet", called pseudopodia.
The amoeba is a member of a whole group of amoeboid eukaryotic protists called Amoebozoa. They are heterotrophs, eating bacteria and other protists.
The pseudopodia (false feet) are extensions of the organism's cell membrane. They are used by the amoeba for phagocytosis (active food/nutrient intake) and motility (self-propelled movement).
Amoebae are often found within freshwater, typically on vegetation in decay in still or slow moving water, or in the benthic zone of some lakes. However, they are common organisms of study because it is easy to keep them in a laboratory. They are used to study "protozoa" and to demonstrate cell structure and function.
Laboratory