Animal Tissue - An Introduction

In animals the organs are composed of a number of different tissue types.

The four major categories of tissues are: Epithelial, Connective, Muscle and Nervous.

Epithelial Tissue:

It covers body surfaces and lines body organs and cavities. The functions include lining; protecting organisms from microorganisms, injuries and fluid loss; and forming glands. Epithelial tissues are very closely packed structure which serves as barriers between the external environment and the organ it covers. These types of tissues are specialized to function in secretion and absorption.

Connective tissue:

Cells of connective tissue are separated by non-living material. As the name suggests, connective tissues serve "connecting" functions, while giving shapes to organs and holding them in place. It serves many purposes in our body i.e., binding and supporting other tissues; forming protective layer over muscle, nerves, and blood vessels; storing fats, filling space etc. These tissues are fibrous tissues, separated from one another by a non-cellular matrix, called extracellular matrix. Blood and Bones are examples of connective tissue. Connective tissue typically has cells scattered throughout an extracellular matrix.

Muscle tissue:

These types of tissues facilitate movement of the body by contraction of individual muscle cells. These tissues function to produce force and cause motion, either locomotion or movement within internal organs.
The three major types of muscle tissue are:
Cardiac Muscle -  found in the heart, allowing heart to contract and pump blood throughout an organism. It is striated and branched
Skeletal Muscle - It is voluntary, found attached to bone providing for gross movement. They are multinucleate, and striated.
Visceral (Smooth) Muscle: It is involuntary; found in arteries, bladder, digestive tract and inner linings of organs. The ends of the cells are tapered.

Nervous tissue:
The human nervous system is anatomically divided into:
Central Nervous System (CNS), brain and spinal cord. and
Peripheral Nervous System (PNS), nerve fibers, aggregates of nerve cells and glia and ganglia.
Nervous tissue is dedicated to react to stimuli and conduct impulses to various organs in the body. Nervous tissue consists of two groups of cell types: Nerve cells (Neurons) and Neuroglia.

-- Neurons are easily stimulated and transmit impulses very rapidly. Most neurons have three main parts: Dendrites, Perikarya (cell bodies) and Axon

-- Glia or Neuroglia cells are in direct contact with neurons and often surround them. The four basic types of neuroglia, based on morphology and function are: Astrocytes, Oligodendrocytes, Microglia, Ependymal cells

Bone marrow is the source of all the cells of the blood:

-- Red blood cells (RBCs or Erythrocytes)

-- Five kinds of white blood cells (WBCs or Leukocytes)

-- Platelets (or Thrombocytes)