# Science: An Elementary Teacher’s Guide/Tissues in the Body
### Introduction

The body is composed of approximately 75 trillion cells. These cells are organized into only four categories called tissues. A tissue is a group of cells performing similar functions. Tissues vary in structure, function, and the content of their extracellular matrix a substance produced by the cells of a specific tissue and can contain protein fibers, salts, H2O, and dissolved macromolecules located outside of cells
In biology, tissue is a cellular organizational level intermediate between cells and a complete organ. A tissue is an ensemble of similar cells from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

### Animal tissues

-The structure of the cell varies according to its function. The tissues are different and are classified into four types:
Animal tissues are grouped into four basic types: connective, muscle, nervous, and epithelial. Collections of tissues joined in structural units to serve a common function compose organs. While all animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals.
The epithelium in all birds and animals is derived from the echinoderm and endoderm with a small contribution from the mesoderm, forming the endothelium, a specialized type of epithelium that composes the vasculature. By contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier. This tissue covers all organism surfaces that come in contact with the external environment such as the skin, the airways, and the digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina.
Epithelial Tissue
* Sheet-like layer of cells
* Cover surfaces or line cavities
* Often has glandular cells for fluid secretion
* Endothelium
* Epithelium that lines heart, blood vessels and lymphatic vessels
Connective Tissue Tissue that connects
* Most diverse, abundant, widely distributed, and structurally varied of all four tissue types
* Function is to “connect” one structure to another structure
* CT is the “glue” and “filler” of the body
* Examples of CT are: tendons, ligaments, body fat, bones, and cartilage
* Loose connective tissue
-adipose (fat)
* Dense connective tissue
-cartilage
-Bone
* Vascular tissue
-Erythrocytes: Red Blood Cells – carry oxygen
-Leucocytes: White Blood Cells – part of the immune system
-Platelets - clotting
Functions of Connective Tissue
* Physical protection
* Support and structural framework
* Binding of structures
* Storage
* Transport
* Immune protection
Muscle Tissue Types
Comprised of cells called fibers. When stimulated by the nervous system, fibers shorten or contract. The result of contraction is movement (i.e., movement of bones, blood, food, sperm)
* Smooth Muscle (=Involuntary Muscle)
Ex. Small intestine
Found in walls of most internal organs stomach, intestines, urinary bladder
Cells are:relatively short, wide in the middle, and tapered at the ends (fusiform)involuntary and non-striated
* Skeletal Muscle (=Voluntary Muscle)
Ex. Large muscles of body
* Cardiac Muscle
Nervous Tissue Function is communication and control of body functions.
* Includes Nerves, Spinal Cord, Brain
* Cells are called neurons

## Plant tissue

- A plant tissue can be defined as a cell or a group of cells dividing, to give rise to large number of cell, which is collectively referred as tissues. They are structurally and functionally similar to these cells.
Plant tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.
Plant tissues can also be divided differently into two types:

## Skin

It is thin and flexible the skin is the body outer security system it glands, nerves, blood vessels, and even fatty tissues perform functions that are necessary to the preservation of our lives. External protection, it provides waterproof covering for the body, keeping inside moisture in and outside moisture out a service that is necessary for life. It protects the body from the invasion of unwelcome moisture the skin protects it from being invaded by a horde of potentially harmful bacteria.Its complex nervous system provides the sense of touch and warns the body of danger also enables it to experience important input from the outside world. The skin is 3 millimeters thick and it covers the entire body also it consists of two layers the epidermis and the dermis. The epidermis is a paper thin outer layer with a protective covering of dead skin cells. The new epidermal cells are produced the older ones are pushed up become flat, hard, and dry it flake off.The epidermis is completely replaced approximately once a month. There are no blood vessels in the epidermis live cells are nourished by diffusion from below. The callus is a protective thickened layer of epidermal tissue formed in response to a rubbing or wearing action on the surface of the skin. The strong elastic connective tissue that lies just beneath the epidermis is the dermis the layer is much thicker than the epidermis it contains an elaborate collection of nerves, blood vessels, glands, and cells that produce hair. When is a hot day dermal blood vessels dilate bringing more blood to the interior of the body and conserving heat. Sweet glands assist the dilated blood vessels in cooling the body surface secreting moisture when it is needed. Hundreds of glands are contained in each square inch of dermal tissue 2 million on the surface of the body. The role of sweat glands extends beyond the air conditioning system they also help to take out the garbage and they continue to function even on cool days. The pigment melanin is produced by cells in the skin the color of eyes, hair , and skin.