Animal Tissues

Index to this page

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 a thousand or more 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 form the surface of the skin, line the various cavities and tubes of the body, and cover the internal organs.

Subsets of Epithelia

The basolateral surface of all epithelia 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 or, better (because it is not a membrane in the biological sense), the basal lamina. [View example]

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.

Dense connective tissue

Often called fibrous connective tissue.

Loose connective tissue

It is distributed throughout the body. It serves as a packing and binding material for most of our organs. Sheets of loose connective tissue that bind muscles and other structures together are called fascia. Collagen, elastin, and other proteins are found in the matrix of loose connective tissue.

Both dense and loose connective tissue are derived from cells called fibroblasts [View], which secrete the extracellular matrix.

Adipose tissue

Adipose tissue is "fat". There are two kinds found in mammals:

White adipose tissue and brown adipose tissue differ in function as well as cellular structure. These differences are described on a separate page. Link to it.

New adipocytes in white adipose tissue are formed throughout life from a pool of precursor cells. These are needed to replace those that die (after an average life span of 10 years). Whether the total number of these adipocytes increases in humans becoming fatter as adults is still uncertain. If not, why do so many of us get fatter as we age? Because of the increased size of individual adipocytes as they become filled with oil.

The adipocytes of white adipose tissue secrete several hormones, including leptin and adiponectin.

4. Nerve

Nerve tissue is composed of

Neurons

Neurons are specialized for the conduction of nerve impulses. A typical neuron consists of

The nerve impulse is conducted along the axon.

Link to a page devoted to neuron structure.

The tips of axons meet:

Link here to a page describing how neurons work.
Link here to a page describing the types and organization of neurons in the peripheral nervous system.

Glia

Glial cells surround neurons. Once thought to be simply support for neurons (glia = glue), they turn out to serve several important functions.

There are three types:

In addition, the central nervous system contains many microglia — mobile cells (macrophages) that respond to damage (e.g., from an infection) by

Microglia are also active in the healthy brain, at least in young mice where, like astrocytes, they engulf synapses thus reducing the number of synapses in the developing brain.

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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24 December 2013