Epithelial tissues are classified into several categories on the basis of the cell shape and number of cell layers. Mainly there have the fallowing three types of epithelial tissue:
1) Simple epithelium
2) Compound epithelium
3) Transitional epithelium
Simple epithelium as one of the types of Epithelial Tissue
Simple epithelium consists of a single layer of cells resting upon a basement membrane. It occurs mainly on secretory and absorptive surfaces and seldom covers surfaces exposed to mechanical or chemical abrasions since it cannot effectively protect underlying tissues. The following five types of simple epithelia are acknowledged on the basis of the shape and structure of component cells.
This types of Epithelial Tissue consists of a layer of thin, flat, scale-like cells with prominent nuclei. These cells are closely fitted together like the tiles in a floor. Inter-digitations and other junctions between adjacent cells keep the epithelium intact as a sheet. This epithelium easily exchange materials by diffusion across it.
The cuboidal epithelium consists of cells which appear cuboidal in vertical section. These cells possess usually rounded nuclei, and participate in secretion, excretion and .absorption. Cuboidal epithelium forms the lining of salivary and pancreatic ducts and thyroid vesicles, and so also of ciliary body, choroid and iris of eye.
The cells of columnar epithelium appear rectangular in vertical section and polygonal in surface view. In keeping with the shape of the cell, the nuclei are also frequently elongated. The surface of the columnar epithelium is sometimes covered with microvilli, which are arranged regularly or irregularly. The cells of the columnar epithelium have a secretory or absorptive function. Secretory columnar cells are scattered in the mucosa of the stomach and intestine.
These types of Epithelial Tissue which called Ciliated epithelium consists of columnar or cubical cells bearing cilia on their free surfaces. The function of the cilia is to move particles or mucus in a specific direction over the epithelial surface. Ciliated columnar epithelium lines most of the respiratory tract, uterus, and uterine tubes. It is also found in the efferent duct of testis, parts of the middle ear and auditory tube, and in the lining of neurocoel cavity of the spinal cord and brain.
These types of Epithelial Tissue are invariably made up of a single layer of columnar cells. But it appears more than one cell layer as some cells are shorter than the others and their nuclei are present at different levels. Some cells are broader near the base, whereas others near the apex. As the nuclei occur in the broader part of the cell, they do not form a layer. To distinguish it from a true stratified epithelium, it is called pseudo-stratified epithelium.
Compound epithelium as one of the types of Epithelial Tissue
Compound epithelium consists of more than one layer of cells. The cells of the innermost layer rest on the basement membrane. Being multi-layered, compound epithelium has little role in secretion or absorption, but it provides protection to underlying tissues against mechanical, chemical, thermal or osmotic stress. The following two types of compound epithelium have been acknowledged.
It consists of many layers of epithelial cells; the innermost layer consists of cuboidal cells. On the basis of the morphology of superficial layers, the stratified epithelium is divided into two sub-types.
i) Stratified squamous epithelium: It consists of several superficial layers of living squamous cells and deeper layers of interlinked polygonal cells. Most of the stratified epithelium in the adult body is of this type and these cells contain a highly insoluble fibrous protein, called keratin, which forms a non-living covering over the epithelium.
ii) Stratified cubical epithelium: In these types of epithelial tissue, the superficial cells are cuboidal. The conjunctiva of eyes and the lining of the ducts of sweat glands, female urethra and some parts of the anal canal have this type of epithelium.
Transitional epithelium as one of the types of Epithelial Tissue
Transitional epithelium is comparatively thin and more stretchable than stratified epithelium. It is 4-6 cell layers thick with extensive inter-digitations. The innermost layer consists of columnar or cuboidal cells, the 2 or 3 middle layers are made up of polyhedral or pear-shaped cells, and the surface layer has large, broad, rectangular or oval cells. Transitional epithelium occurs in the renal pelvis, ureter, urinary bladder and part of the urethra.