Animal Tissues: Epithelial, Muscle, and Nervous Tissues Explained

Animal Tissues

Epithelial Tissue

Epithelial tissue consists of sheets of densely packed and connected cells that cover internal and external body surfaces.

Simple Epithelia

Simple epithelia are formed by a single layer of cells that can be:

• Flattened, like the endothelial walls of capillaries and the lining of blood vessels, which protect yet allow the exchange of substances.
• Polyhedral (cubic and cylindrical), such as the epithelial lining inside the intestine. Their cells possess microvilli or fingerings on the surface that faces the lumen of the tube.

Stratified Epithelia

Stratified epithelia consist of several layers of cells. Cells in the deepest layer are constantly dividing, and new cells push the older ones upward. The most superficial cells, laden with keratin, are dead and are shed continuously.

Glandular Epithelia

Glandular epithelia are specialized secretory cells interspersed among the lining cells. These cells are involved in the production and secretion of different substances. The cells forming glands can be found associated with:

• Endocrine glands (ductless): Produce substances that are discharged directly into the blood.
• Exocrine glands (external secretion): Produce substances discharged to the exterior.
• Mixed glands: For example, the pancreas.

Muscle Tissue

Muscle tissue is responsible for movement. It is made up of elongated cells or fibers specializing in contraction.

Striated Muscle Fiber

Striated muscle fiber shows transverse striations under a microscope. Each muscle fiber contains contractile elements in its cytoplasm called myofibrils. Each myofibril has two filament types: actin and myosin.

Skeletal Muscle Fibers

Skeletal muscle fibers are responsible for the movement of the skeleton. They are cylindrical, very long, and multinucleated cells. Their contraction is voluntary.

Cardiac Muscle Fibers

Cardiac muscle fibers are shorter, uninucleate cells closely connected to each other in a network. Their contraction is involuntary.

Smooth Muscle

Smooth muscle cells do not have transverse striations. They are uninucleate and have a long, spindle shape. They are the most common type in vertebrates and invertebrates. They form sheets lining the blood vessels and surrounding internal organs. Their contraction occurs without conscious control.

Nervous Tissue

Nervous tissue is specialized in receiving stimuli and conducting them from one body part to another.

The Neuron

The neuron is the basic functional unit of the nervous system. From the cell body, or soma, extend one or more extensions called dendrites, which are capable of receiving stimuli. These stimuli may be captured by receptors or come from other nerve cells. The received stimuli are transformed into nerve impulses and transported through a long extension called the axon, from the cell body to another nerve cell or an effector organ. The functional connection between two neurons or a neuron and an effector organ is called a synapse.

Neuroglia

Neuroglia are a variety of non-nerve cells that provide metabolic support and protection to neurons. An example is Schwann cells, which form the myelin sheath.