A tissue is the group of cells organized in a specific pattern to assist the body activities. Our body is composed of a variety of structures differ in the length, size, appearance, complexity and the distribution and this is possible only when the multitude of tissues being interlinked in different ways. The human tissues are classified into 4 major types, epithelium, connective tissues, muscular tissues, and nervous tissues. Among all, the epithelium is the most abundant tissue found in the human body as it is a part of almost every organs in our body. The epithelial tissues, as the name suggests(Epi-outer) cover the outer surfaces of many organs, the lumen of blood vessels as well as our skin. It also helps in protecting the visceral layers of internal organs including the surface of mucus membranes across various body systems. Epithelial layers lack the direct blood supply from the blood vessels hence they receive nourishment through the diffusion of substances from their underlying connective tissues. Let us explore the types of epithelium, their distribution, structure and functions.
Classification of epithelium
The epithelium is classified based on their shape, location and functions. Accordingly, the 2 main types of epithelium are the simple epithelia and stratified epithelia. Simple epithelium is made up of a single layer of cells whereas the stratified epithelium allows a bundle of layers attached together in a complex pattern. One of the most unusual types of epithelium is called pseudostratified type, pseudo means false because a single layer of cells is presented in a varying pattern which gives the appearance of real stratified epithelium. Based on the shape of the epithelium, they are classified into 4 types; squamous epithelium, cuboidal epithelium, columnar epithelium and the ciliated columnar epithelium. Squamous epithelium contains closely embedded cell layers hence they appear flat. Ciliated epithelium have ciliary hair-like projections whereas cuboidal epithelium is characterized by their presence of cube-like cells with a long and cylindrical shape.Furthermore,ciliated columnar epithelium contains cilia at the top their long and cube-like cells. The paragraphs below explain the detailed structure and functions of the most common types of epithelium.
1. Simple squamous epithelia and Stratified squamous epithelium
2. Simple cuboidal epithelium and Stratified cuboidal epithelium
3. Simple columnar epithelium and Stratified columnar epithelium
4. Pseudo-stratified and transitional epithelium.
Simple and compound Epithelium
1. Simple squamous epithelium
The cells in simple squamous epithelium looks-like thin scales, single-layered and the nuclei tend to be flat and spread horizontally. There are many varieties of the simple epithelium and one of the variants, endothelium lines the inner walls of blood vessels as well as the lymphatic vessels. Since the cells of the simple epithelium are thin and flat, they can easily expand and contract when needed. They are found in most areas of internal organs, mainly in the alveoli of lungs, blood vessels and lymphatic vessels, and the lining of the heart among many others. They are also seen on some layers of the kidney and the inner lining of a few glands. Another variant of the squamous epithelium is the mesothelium that covers the outer layers of the serous membrane lining the body cavities and internal organs.
2. Simple columnar epithelium
As the name suggests, simple columnar epithelium composed of long, slender and column-like cells that helps in the active absorption and the secretory functions. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract as well. The ciliated columnar epithelium is another variant of columnar epithelium characterized by the presence of many ciliary projections at the top of the columnar epithelial cells. The ciliated columnar epithelium is found on the upper respiratory tract, the fallopian tubes, the uterus, and the central part of the spinal cord. The major functions of simple columnar epithelium is the secretion of lubricating substance, diffusion in the lungs and filtration in the kidneys
3. Simple cuboidal epithelium
Simple cuboidal cells are perfect cube-like cells piled in a systematic fashion. They are found on the surface of ovaries, the lining of nephrons, the walls of the renal tubules, and some surfaces of the eye and thyroid gland. They perform the secretion and absorption functions.
4. Stratified epithelium
Stratified Epithelium is formed by several stacked layers of cells hence they are the combination of multiple layers of simple epithelium. Stratified epithelium helps to block the physical and chemical barriers from outside. It is again classified as stratified squamous epithelium, stratified columnar tissues, and stratified ciliary columnar epithelium. Stratified epithelium is also present on the apical layers of certain spaces. Another form of stratified epithelium is the transitional epithelium. It is a transitional epithelium found in the bladder, ureters and the urethra.
5. Stratified squamous epithelium
A stratified squamous epithelium is a type of squamous epithelium comprises of flattened epithelial cells. Thee cells are arranged in multiple layers attached to their basal membrane, therefore, only one layer comes in contact with the basement membrane whereas the top layers adhere to one another which helps in maintaining the structural integrity. In a typical stratified epithelium, intercellular spaces are absent and they are appropriate in those areas that are constantly under the influence of abrasion. Witht he help of stratified epithelium, thickest layers can be sequentially sloughed off and replaced before the basement membrane is exposed. It forms the outermost layer of the skin and the inner lining of the mouth, oesophagus and vagina.
6. Stratified columnar tissues
It is one of the unusual type of epithelium found in a very few organs. It has a column-shaped cells arranged in multiple layers. They are found in the conjunctival layer of the eye( white coloured area), few parts of the pharynx, anus, the uterus, and urethra of male, vas deferens in the testicle areas. It is also found in the lobar ducts in salivary glands. The primary function of this epithelium is the secretion and protection.
Common functions of Epithelial Tissues
1. Secretory function: Epithelium inside the glands and mucus membrane helps in secretory function
2. Protective function: As a skin ad mucus membrane, epithelium creates a barrier against sunlight, infections and other foreign material.
3. Movement: Helps in respiratory movement, expansion and constriction of the urinary bladder.
- Define tissues and classify them.
- Which is the most common tissue-type in our body?
- What is the key difference between simple epithelium and compound epithelium?
- What is the specialty of ciliated columnar epithelium?
- Name any 4 organs where epithelial cells are found
- What is the shape of cells in squamous epithelium?
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