Animal Tissues

Introduction

Multicellular organisms show division of labour. Cells that specialise in one function are often grouped together in the body. This group of cells are similar in structure and function and are called a tissue. Plants and animals have different type of tissues based on differences in their structure and required functions.

Animal Tissues:

Based on the functions they perform, different types of animal tissues are epithelial tissue, connective tissue, muscular tissue, and nervous tissue. Both blood and muscles are examples of tissues found in our body. Muscle forms muscular tissue, and blood is a type of connective tissue.  

Epithelial tissue:

1. Epithelial tissues are tissues that cover or protect the animal’s body. They cover most organs and body cavities. 
2. It forms a barrier that keeps different body systems separate.
3. The skin, lung alveoli, the lining of the blood vessels, the lining of the mouth, and kidney tubules consist of epithelial tissue. 
4. Epithelial tissue cells are tightly packed tissues that form a continuous sheet. They have a small amount of material filled between them and almost no intercellular spaces. 
5. Anything that enters or leaves the body must cross at least one layer of epithelium.
6. The permeability of the cells of different epithelia plays an important role in the regulation of  material exchange between the body and the external environment and also between different body parts. 
7. Different epithelia have different structures that correlate with their unique functions:
8. Simple squamous: These are extremely thin and flat in shape which forms a delicate lining. For example, the oesophagus and the lining of the mouth are covered with squamous epithelium. 
9. Stratified squamous: The epithelial cells of the skin, which protects the body, are arranged in many layers to prevent wear and tear. For example, skin is also made of squamous epithelium.
10. Columnar epithelium: Tall epithelial cells are present where absorption and secretion occur, like the inner lining of the intestine. This columnar (meaning ‘pillar-like’) epithelium  aids the movement across the epithelial barrier. 
11. Ciliated columnar epithelium: In the respiratory tract, the columnar epithelial tissue also has cilia, which are hair-like protrusions on the outer surfaces of epithelial cells. These cilia can move and push the mucus forward to clear it. 
12. Cuboidal epithelium: Cuboidal epithelium (cube-shaped cells) provides mechanical support by forming the lining of kidney tubules and ducts of salivary glands.
13. Glandular epithelium: Epithelial cells often acquire an additional specialisation than glandular cells, which allows them to secrete substances on the epithelial surface.

Connective tissue: The cells of connective tissue are loosely spaced. They are embedded in an intercellular matrix. The matrix may be jelly-like, fluid, dense, or rigid in nature. 

The different types of connective tissues in our body consist of areolar tissue, adipose tissue, bone, tendon, ligament, cartilage, and blood.

1. Blood: Blood flows and transports gases, digested food, hormones, and waste materials to different parts of the body. 
2. It consists of a fluid matrix called plasma, that consist of red blood corpuscles (RBCs), white blood corpuscles (WBCs), and platelets that are suspended. 
3. The plasma contains proteins, salts, and hormones.
4. Bone: It forms the framework that supports the body, anchors the muscles, and supports the main organs of the body. 
5. It is a strong and nonflexible tissue.
6. Bone cells are embedded in a hard matrix. The matrix is composed of phosphorus and calcium compounds. 
7. Two bones are inter-connected by another type of connective tissue called the ligament.
   • Ligaments are very elastic and have considerable strength
   • Ligaments contain minimal matrix and connect bones with bones 
8. Tendons are another type of connective tissue that connect muscles to bones. They are fibrous in nature and possess great strength but limited flexibility. 
9. Cartilage: It has cells that are widely spaced. Cartilage smooths the bone surface at joints. It is present in the nose, ear, trachea, and larynx in the body
10. Areolar: Found between the skin and muscles, around blood vessels and nerves, and in the bone marrow, it also fills the space within the organs, supports internal organs, and helps in repairing tissues. 
11. Adipose tissue: Adipose tissue is a fat storing tissue found below the skin and between internal organs. This tissue’s cells are filled with fat globules. Storage of fats allows it to act as an insulator.

Muscular tissue:

Muscular tissue, also called muscle fibres, consists of elongated cells. This tissue is responsible for the movement in our body. Muscles contain special proteins called contractile proteins. These proteins relax and contract which causes movement.

Voluntary and Involuntary Muscles

1. Voluntary muscles: These muscles can be moved according to our will. For example, the muscles present in our limbs.
2. These muscles are also called skeletal muscles. They are mostly attached to bones and facilitate body movement. 
3. Under the microscope, when stained appropriately, these muscles show alternate light and dark bands or striations. As a result, they are also known as striated muscles. 
4. The cells of this tissue are long, unbranched, cylindrical, and multinucleate (having many nuclei). 
5. Smooth or involuntary muscles: Smooth or involuntary muscles control such movements which cannot be regulated by the conscious will. For example, the contraction and relaxation of blood vessels or movement of food in the alimentary canal.
6. The cells are long and have pointed ends (spindle-shaped) and are uninucleate (have only one nucleus). 
7. They are also called unstriated muscles. 
8. They are also found in the bronchi of the lungs, iris of the eye, and in ureters.
9. The heart muscles show a lifelong rhythmic contraction and relaxation. These involuntary muscles are called cardiac muscles. They are cylindrical, branched, and uninucleate in nature.

Nervous tissue:

All cells have the ability to respond to stimuli but nervous tissue cells are highly specialised to be stimulated and to transmit the stimulus very quickly through the body. 

1. The brain, spinal cord, and nerves are all made up of the nervous tissue. 
2. The cells in this tissue are called nerve cells or neurons.
3. A neuron consists of a cell body with a nucleus and cytoplasm, from which long,  hair-like parts emerge. 
4. Usually, every neuron has only one long part called the axon, and many short, branched parts called dendrites. 
5. A single nerve cell can be up to a metre long. Many nerve fibres bound together by connective tissue form a nerve. 
6. A nerve impulse is the signal that passes along the nerve fibre. Nerve impulses allow us to move our muscles according to our will. 
7. The functional combination of muscle tissue and the nerve is fundamental to most animals. This combination allows animals to move rapidly in response to stimuli.

Conclusion

Animal tissues are made up of animal cells that have been grouped together. The structure, function, and origin of these tissues are all different. Epithelial, connective, muscular, and nervous tissues are the four types of tissues found in animals.