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Introduction
The word ‘tissue’ comes from the French word ‘tissu’, the past participle of the verb tisser, which means ‘to weave.’
A tissue is a collection of cells which share common origin, similar basic structures and perform the same function in an organism.
Histology is the branch of Biology that deals with the study of tissues. In the case of disease or histopathology also tissues are studied. In 1801, Xavier Bichat introduced the term ‘tissue’.He is also known as ‘The Father of Histology.’
Types of Animals on the Basis of Body Organisation
The animal kingdom has two types of organisms:
- Parazoa
- Eumetazoa
The more straightforward form of organisms, the parazoa, do not have tissue or tissue system. They possess specialized cells which perform specific tasks, but these cells do not get organized into tissues that do the same.
Eumetazoa, on the other hand, have increasingly complex cell structures that give rise to true tissues which culminate in organs and organ systems.
Types of Animal Tissue
There are four main categories of tissues generally present in animals-
- Epithelial tissues
- Nerve tissue
- Connective tissue
- Muscle tissue
Even though most animals have the above four categories of tissues they may differ among the different taxa. This is because the cells forming the tissues have different developmental stages.
Let us study the different kinds of animal tissues found commonly in the animal kingdom.
Epithelial Tissue
The epithelial tissue is one of the types of tissue in animals. This tissue forms the lining of the body, both inside and outside. It makes up the skin, lining and covering the organs. There are different kinds of epithelial tissues depending on the shape of the cells. Moreover, epithelial tissues can also be classified by the number of layers in the lining formed by the tissue. If a single layer of cells includes the lining, then the tissue is called simple epithelia. If there are multiple layers of cells, the tissue is called stratified epithelia. Following are the kinds of epithelial tissues:
- Squamous epithelia: The cells are-
○ Usually flat, somewhat round in shape.
○ Form the linings or coverings.
○ The nucleus is prominent and centrally placed in the cell.
○ The cell shape can be irregular since the cells fit together to form the tissue.
○ When the tissue has a single layer of cells (simple epithelia), it helps with the diffusion of substances, such as the gases in the lungs.
- Cuboidal epithelia: The cells are-
○ Cube-shaped.
○ Usually found in the lining of glands.
○ Help in the secretion of glandular material.
○ They are also found in tubules and kidneys.
- Columnar epithelia: These cells are-
○ Longer than they are broad.
○ Mostly form single-layer structures (simple epithelia)
○ Found in the lining of the digestive tract where the nuclei of the cells are located near the bottom of the cell, away from the lumen end.
○ The cells in the digestive tract absorb the nutrients from the lumen of the lot.
○ They are also found in the respiratory tract, where they are stratified but not. The base of each cell is attached to the basal membrane, so the structure is that of superficial epithelial tissue. This is called a pseudostratified column.
○ In the respiratory tract, the cells have tiny projections called cilia. These structures cause the mucous and the other particles out of the respiratory tract, thus helping in removing harmful microorganisms.
- Transitional epithelia: Here the cells are:
○ These cells are found only in the urinary system and are known as uroepithelial cells.
○ The central location of the cells is in the urinary tract and the bladder.
○ When the bladder is relaxed and empty, the cells are piled on top of each other, forming a stratified layer.
○ As the bladder fills, the tissue cells shift, expanding the lining.
○ In this process, the lining becomes thinner and thinner, and the tissue ‘transitions’ to simple epithelia. Hence the name.
Connective Tissue
These are other kinds of tissue common in the animal body .This kind of tissue has two components – the cells and the ground substance. Together, these two components form the connective tissue matrix, which gives the tissue its density. Usually, a protein combines with water or a mineral to create the ground substance, and the primary cell of connective tissue is the fibroblast. These cells can move on their energy, undergo mitosis, and make any connective tissue. Connective tissues are categorized into the following:
- Loose or areolar connective tissue: The structure of this kind of tissue has characteristics in all types of connective tissue. There are fibroblasts present along with collagen and elastin fibers. The collagen fibers are widely placed and are thicker than the elastic fibers. Other cells are adipocytes, macrophages,plasma cells and mast cells.The jelly-like ground substance fills the space.Fibroblasts form fibres, macrophages scavenge foreign material of the tissue, mast cells are responsible for immune response and plasma cells produce antibodies.
- Fibrous connective tissue: These are made up of many collagen fibers, very few cells, and matrix material. Usually found in body areas that have to undergo stress from all directions like the skin.
- Cartilage: The main component of this tissue is the flexible matrix. The cells of this tissue are called chondrocytes (cartilage cells). These cells and cartilage matrix are formed from chondroblasts which have filopodia that contain actin filaments . The cartilage cells are present in groups of two to eight in lacunae.The cartilage is usually found at the end of long bones, reducing the friction between the ends of the bones in the articulation.
- Bone: Also known as the bony tissue. This tissue has different kinds of matrix materials. The collagen in the matrix gives the tissue flexibility, and the mineral, normal calcium, provides the tissue its hardness and rigidity.
- Adipose tissue: This is an exceptional connective tissue because it does not have any matrix and few fibers. This tissue comprises cells called adipocytes, which are the fat-storing cells in vertebrates.
- Blood: Blood is also a connective tissue because of its matrix. The living part of the tissue is the different cells (erythrocytes, leukocytes), and the matrix is the plasma.
Muscle Tissue
The muscle tissue is differentiated based on striations or bands, the location of the nucleus, voluntary or involuntary nature, and their function, which is often the cause of their site. So there are three types of muscles in the body:
Smooth muscle
○ No striations
○ Involuntary
○ The single nucleus is centrally located.
○ Located in the walls of blood vessels, digestive system, tubes of the reproductive system.
Skeletal muscle
○ This is the most abundant tissue in the human body
○ Striated
○ Cells are long and have many nuclei
○ Voluntary
○ Found attached to bones because they move the bones.
Cardiac muscle
○ Specialised for the functioning of the heart
○ Striated
○ Not voluntary, but the heartbeat can become fast or slow depending on the signals sent by the autonomic nervous system.
Nervous tissue
The cells that make up this tissue are called neurons. Each nerve cell has following parts-
(i) Cell body (Cyton)
(ii) Dendrites
(iii) Axon
Cell body is irregular in shape and is filled with cytoplasm called neuroplasm which has different cell organelles just like any eukaryotic cell.
From the cell body or cyton numerous small projections arise which are called dendrites.
One long branch arises from the cell body which is called axon and it terminates into fines branches called nerve endings.
These cells are specialised to send and receive electrical impulses. The nervous tissue forms all the nerves, the spinal cord, and the brain.
Conclusion
Histology is the study of tissues. Advancements in technology like electron microscopy, immunofluorescence, and improvements in methods like using frozen sections of tissues have contributed to a better understanding of how these structures function. This is good because studying tissues and their operation has helped a tremendous scientific effort to improve medical procedures and treatments.
