Tissues

Cells, the basic units of structure and function within the human organism, are organized into groups and layers called tissues. Each type of tissue is composed of similar cells that are specialized to carry on particular functions. For example, epithelial tissues form protective coverings and function in secretion and absorption; connective tissues pro­vide support for softer body parts and bind structures together; muscle tis­sues are responsible for producing body movements; and nerve tissue is specialized to conduct impulses that help control and coordinate body activ­ities.

Epithelial tissues are widespread throughout the body. They cover all body surfaces—inside and out—and are the major tissues of glands.

Since epithelium covers organs, forms the inner

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lining of body cavities, and lines hollow organs, it always has a free surface—one that is exposed to the outside or to an open space internally.

As a rule, epithelial tissues lack blood vessels. However, they are nourished by substances that diffuse from underlying connective tissues, which are well sup­plied with blood vessels.

Epithelial cells are tightly packed, and there is little intercellular material between them. Often they are attached to one another by desmosomes. Consequently, these cells provide effective protective barriers in such structures as the outer layer of the skin and the inner lining of the mouth . Other epithelial functions include secretion, absorption, excretion, and sensory reception.

Connective tissues occur throughout the body and rep­resent the most abundant type of tissue by weight. They bind structures together, provide support and protec­tion, serve as frameworks, fill spaces, store fat, produce blood cells, provide protection against infections, and help repair tissue damage.

Connective tissue cells are usually further apart than epithelial cells, and they have an abundance of in­tercellular material, or matrix, between them. This matrix consists of fibers and a ground substance whose consistency varies from fluid to semisolid to solid.

Connective tissue cells are able to reproduce. In most cases, they have good blood supplies and are well nourished. Although some connective tissues, such as bone and cartilage, are quite rigid, loose connective tissue,

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adipose connective tissue, and fibrous connec­tive tissue are more flexible.

Muscle tissues are contractile; that is their elongated cells, or muscle fibers, can change shape by becoming shorter and thicker. As they contract, the muscle fibers pull at their attached ends and cause body parts to move. The three types of muscle tissue are skeletal muscle, smooth muscle, and cardiac muscle.

Nerve tissue is found in the brain, spinal cord, and pe­ripheral nerves. The basic cells of this tissue are called nerve cells, or neurons, and they are among the more highly specialized body cells. Neurons are sensitive to certain types of changes in their surroundings. They respond by transmitting nerve impulses along cyto-plasmic extensions to other neurons or to muscles or glands.

As a result of the extremely complex patterns by which neurons are connected with each other and with various body parts, they are able to coordinate and reg­ulate many body functions.

In addition to neurons, nerve tissue contains neuroglial cells. These cells support and bind the compo­nents of nerve tissue together, carry on phagocytosis, and help supply nutrients to neurons by connecting them to blood vessels.

Answer the questions:

1 What is a tissue?

2 List the four major types of tissue.

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3 What are the general characteristics of epithelial tissue?

4 List the general characteristics of connective tissue.

5 What is matrix?

6 What are the three types of muscle tissue?

7 Describe the general characteristics of nerve tissue.

8 Distingwish between neurons and neuroglial cells.

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