The term epithelium was introduced by a Dutch Scientist Ruysch in the 18th century. Epithelial tissues are sheets of cells that cover the body surfaces. They form the covering of all body surfaces, line body cavities, and hollow organs, and are the major tissue in glands.
Functions of Epithelial Tissue
Epithelial tissue protects the tissues from dehydration, mechanical and chemical injuries, and entry of germs.
Epithelial tissues secrete various substances like mucus, gastric juices, and some hormones that are necessary for normal metabolic activities.
The epithelial lining of the intestine helps in the absorption of digested food.
The epithelial lining of uriniferous tubules in the kidney eliminates nitrogenous wastes.
Sensory epithelium in sense organs sends nerve impulses from the environment to the Central Nervous System (CNS).
The germinal epithelium of sex organs produces gametes.
The epithelial lining of the lung alveoli brings about the gaseous exchange between air and blood.
Types of Epithelium
There are two types of epithelium tissues simple and compound epithelium.
The simple epithelium consists of a single layer of cells resisting a basement membrane and functions as a lining for body cavities ducts and tubes. It occurs basically on absorptive surfaces. On the basis of shape and component, there are five types of simple cells.
The squamous epithelium is made up of a single thin layer of flattened cells with irregular boundaries. These cells are closely packed like the tiles on the floor. This epithelium exchanges materials easily by diffusion across it.
The cuboidal epithelium consists of a single layer of cuboidal cells. These cells possess usually a round nucleus and participate in excretion, secretion, and absorption. It forms the lining of salivary and pancreatic ducts and thyroid vesicles.
The columnar epithelium appears rectangular in vertical section and polygonal in surface view. The nuclei is elongated. The surface of the columnar epithelium is covered with microvilli, which are arranged regularly or irregularly. These cells have a secretory or absorptive function.
The ciliated epithelium consists of columnar cells bearing cilia on their free surface. The function of cilia is to move particles in a specific direction over the epithelial surface. Ciliated columnar epithelium lines the uterus, respiratory tracts, and uterine tubes, testis.
The pseudostratified epithelium is 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. Pseudostratified epithelium may be ciliated or non-ciliated.
The compound epithelium consists of more than one layer of cells. Being multilayered, the compound epithelium has little role in secretion or absorption, but it provides protection to underlying tissues against mechanical, chemical, thermal, and osmotic stress. There are two types of compound epithelium:
It consists of many layers o epithelial cells. The innermost consists of od cuboidal cells. On the basis of morphology compound, the stratified epithelium can be divided into two groups:
Stratified Squamous Epithelium
It consists of oof several superficial layers of living squamous cells and deeper cells of interlinked polygonal cells. Most of the stratified epithelium in the adult body is of this type.
Stratified Cubical Epithelium
In this type, the superficial cells are cuboidal. The conjunctiva of the eyes and the lining of the sweat glands, female urethra, and some parts of the anal canal.
The transitional epithelium is comparatively thin and more stretchable than the stratified epithelium. It is 4-6 cell layers thick with extensive interdigitations. Transitional epithelium occurs in the renal pelvis, ureter, urinary bladder, and part of the urethra.