Epithelial tissue is formed by juxtaposed cells, that is, which are intimately joined together through intercellular junctions or integral membrane proteins.
The main function of epithelial tissue is to coat the outer surface of the body, the inner body cavities and the organs. It also features secretory function.
The following are functions of epithelial tissue:
The close union between their cells makes epithelial tissue an efficient barrier against invasive agents and loss of body fluids.
According to their function, there are two types of epithelial tissue : lining and glandular. However, there may be cells with secretory function in the lining epithelium.Types of epithelial tissue
Epithelia are made up of one or more layers of cells of different shapes, with little or no interstitial fluid (substance between cells) and vessels between them.
However, all epithelium is located on a glycoprotein mesh called basal lamina , which has the function of promoting nutrient exchange between epithelial tissue and adjacent connective tissue .
According to cell layers , epithelia can be classified into:
The epithelial tissue of human skin has closely joined cells, which is a stratified epithelium .
This is because the function of the skin is to prevent foreign bodies from entering the body, acting as a kind of protective barrier, as well as protecting against friction, sunlight and chemicals.
The epithelial tissue that covers the organs is simple , because the tissue cannot be so thick due to the need for substance changes.
The epithelia are also classified as to the shape of the cells:
Glandular epithelial tissue cells have the same characteristics as the lining epithelium, however, unlike them, they are rarely found in layers.
Therefore, their cells are very tight and usually arranged in a single layer .
The glandular epithelia are tissues with secretory function , which constitute specialized organs called glands .Formation of epithelial tissue
Secretory epithelial cells are capable of synthesizing molecules from, or modifying, smaller precursor molecules.
Secretion cells may also be isolated between or forming the epithelial lining cells. For example, lining the stomach cavity or part of the respiratory tract.
Most glands in the human body are formed from the glandular epithelium. They can be of two types: exocrine or endocrine.
In endocrine glands binding to the lining epithelium no longer exists, cells rearrange into follicles (thyroid) or cords (adrenal, parathyroid, Langerhans islets).
The exocrine glands are formed of two parts: a secretory part (formed by the secretion cells) and an excretory duct (composed of lining epithelial cells).
The duct releases secretions into internal cavities ( salivary glands ) or to the outside of the body ( sweat and sebaceous glands).