What are the main types of animal tissues?

There are four main types of animal tissues – epithelial, connective, muscular, and nervous tissue. These tissues vary significantly in their structure, origin, and function. 

Epithelial Tissues

Epithelial tissues are located on the outer or inner surfaces of organs. They are made up of tightly packed cells that may be arranged in a single layer (simple epithelium) or in multiple layers stacked atop each other (stratified epithelium). 

Found in the skin, and the lining of the mouth, nose, and digestive system, the main functions of epithelial tissues include protection, absorption, and secretion. 

Epithelial tissues have the power to regenerate but they do not contain blood vessels. They derive essential nutrients from other connective tissues.   

Muscle Tissues

Muscle tissues are composed of cells that contain contractile filaments, which enable the cells to change in size. Three distinct components make up the structure of muscle tissue – the sarcoplasm (cytoplasm in the muscle fibers), the sarcoplasmic reticulum (a membrane network), and the sarcolemma (the surrounding membrane).  

There are three types of muscle tissues:

1. Smooth muscle tissue, which is found in the inner linings of organs
2. Skeletal muscle tissue, which is attached to the bone and facilitates movement
3. Cardiac muscle tissue, which is found only in the heart  

Muscle tissues are primarily responsible for movement and locomotion. They also support the bones and other bodily structures and are responsible for initiating peristalsis and parturition.   

Nervous Tissues

Nervous tissues are composed of neurons or nerve cells that make up the central and peripheral nervous system. In the central nervous system, they form the brain and the spinal cord. In the peripheral nervous system, they form the cranial and spinal nerves. 

The primary function of nervous tissues is transmitting and carrying nerve impulses to and from various parts of the body. 

The three main components of nervous tissue include: 

1. Neurons, which form the structural and functional unit of the nervous system
2. Neuroglia found in the brain and spinal cord, which provide support to the neurons and fibers 
3. Neurosecretory cells, which function as endocrine organs, releasing chemical from the axons directly into the bloodstream

Connective Tissues

Connective tissue is composed of several different types of cells including plasma cells, fibroblasts, mast cells, and adipocytes. It is found in abundance inside the body and may be densely or loosely packed. Blood, bone, cartilage, and fat are all connective tissues. 

Connective tissues perform multiple functions in the body. They attach organs and tissues together, prevent organs from mechanical shocks, help to repair tissues, and store fat in the form of adipose tissues.  All cells except blood are involved in collagen secretion.