Animal Tissues

The development of a fertilized egg into a newborn child requires an average of 41 rounds of mitosis (241 = 2.2 x 1012). During this period, the cells produced by mitosis enter different pathways of differentiation; some becoming blood cells, some muscle cells, and so on.

There are more than 100 visibly-distinguishable kinds of differentiated cells in the vertebrate animal. These are organized into tissues; the tissues into organs. Groups of organs make up the various systems - digestive, excretory, etc. - of the body.

The actual number of differentiated cell types is surely much larger than 100.
  • All lymphocytes, for example, look alike but actually represent a variety of different functional types, e.g., B cells, T cells of various subsets.
  • The neurons of the central nervous system must exist in hundreds of different functional types, each representing the result of a particular pathway of differentiation.

This page will give a brief introduction to the major types of animal tissues. The links along the left side of the figure will take you directly to the individual paragraphs indicated.

Epithelial
Muscle
Connective
Nerve
Blood

1. Epithelial

Epithelial tissue is made of closely-packed cells arranged in flat sheets. Epithelia line the various cavities and tubes of the body. They also form the surface of the skin.

The apical surface of epithelial cells is exposed to the "external environment", lumen of the organ or the air.

The basolateral surface is exposed to the internal environment (ECF). The entire sheet of epithelial cells is attached to a layer of extracellular matrix that is called the basement membrane (but it is not a membrane in the biological sense) or, better, the basal laminar.

View showing relationship between the apical and basolateral surfaces of epithelial cells and how they maintain their distinction.

The function of epithelia always reflects the fact that they are boundaries between masses of cells and a cavity or space. Some examples:

2. Muscle

Three kinds of muscle are found in vertebrates:
Link to page devoted to the structure and properties of the three kinds of muscles.

3. Connective

The cells of connective tissue are embedded in a great amount of extracellular material. This matrix is secreted by the cells. It consists of protein fibers embedded in an amorphous mixture of protein-polysaccharide ("proteoglycan") molecules.

Supporting connective tissue

Gives strength, support, and protection to the soft parts of the body.

Binding connective tissue

It binds body parts together.

Fibrous connective tissue

It is distributed throughout the body. It serves as a packing and binding material for most of our organs. Collagen, elastin, and other proteins are found in the matrix.

Fascia is fibrous connective tissue that binds muscle together and binds the skin to the underlying structures.

Adipose tissue is fibrous connective tissue in which the cells have become almost filled with oil. The cells of adipose tissue, called adipocytes, secrete the hormone leptin. All forms of connective tissue are derived from cells called fibroblasts, which secrete the extracellular matrix.

4. Nerve

Nerve tissue is composed principally of nerve cells called neurons. They are specialized for the conduction of nerve impulses. A typical neuron consists of The nerve impulse is conducted along the axon. The tips of axons meet:
Link to a page describing the properties of synapses.
Link here to a page describing how neurons work.
Link here to a page describing the neuromuscular junction.
Link here to a page describing the types and organization of neurons in the peripheral nervous system.

5. Blood

The bone marrow is the source of all the cells of the blood. These include:
Link here to a page describing the blood cells in detail.
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3 August 1999