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.

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 and line the various cavities and tubes of the body. The epithelia that form the inner lining of blood and lymph vessels are called endothelia.

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 or, better (because it is not a membrane in the biological sense), the basal lamina. [View example]

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

• The epithelium of the skin protects the underlying tissues from
  • mechanical damage
  • ultraviolet light
  • dehydration
  • invasion by bacteria
• The columnar epithelium of the intestine
  • secretes digestive enzymes into the intestine;
  • absorbs the products of digestion from it.
• An epithelium also lines our air passages and the alveoli of the lungs. It secretes mucus which keeps it from drying out and traps inhaled dust particles. Most of its cells have cilia on their apical surface that propel the mucus with its load of foreign matter back up to the throat.

2. Muscle

• Skeletal muscle is made of long fibers whose contraction provides the force of locomotion and other voluntary body movements.
• Smooth muscle lines the walls of the hollow structures of the body, such as the intestine, urinary bladder, uterus, and blood vessels. Its contraction, which is involuntary, reduces the size of these hollow organs.
• The heart is made of cardiac muscle.

3. Connective

Supporting connective tissue

• cartilage. Example: the outer ear
• bone. The matrix of bone contains collagen fibers and mineral deposits. The most abundant mineral is calcium phosphate, although magnesium, carbonate, and fluoride ions are also present. [More on bone]

Binding connective tissue

• Tendons connect muscle to bone. [View] The matrix is principally collagen, and the fibers are all oriented parallel to each other. Tendons are strong but not elastic.
• Ligaments attach one bone to another. They contain both collagen and also the protein elastin. Elastin permits ligaments to be stretched.

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 several hormones, including leptin and adiponectin.

All forms of connective tissue are derived from cells called fibroblasts [View], which secrete the extracellular matrix.

4. Nerve

• nerve cells called neurons and
• glial cells.

Neurons

• a cell body which contains the nucleus;
• a number of short fibers — dendrites — extending from the cell body
• a single long fiber, the axon.

The nerve impulse is conducted along the axon.

The tips of axons meet:

• other neurons at junctions called synapses

  Link to a page describing the properties of synapses.

• muscles (called neuromuscular junctions)

  Link here to a page describing the neuromuscular junction.

• glands

Glia

There are three types:

• Schwann cells. These produce the myelin sheath that surrounds many axons in the peripheral nervous system.
• Oligodendrocytes. These produce the myelin sheath that surrounds many axons in the central nervous system (brain and spinal cord).
• Astrocytes. These — often star-shaped — cells are clustered around synapses and the nodes of Ranvier where they perform a variety of functions:
  • stimulating the formation of new synapses;
  • modulating the activity of neurons;
  • repairing damage;
  • supplying neurons with materials secured from the blood. (It is primarily the metabolic activity of astrocytes that is being measured in brain imaging by positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI).

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

• engulfing cell debris
• secreting inflammatory cytokines like tumor necrosis factor (TNF-α) and interleukin-1 (IL-1)

5. Blood

• red blood cells (RBCs or erythrocytes)
• five kinds of white blood cells (WBCs or leukocytes)
• platelets (or thrombocytes)