Epithelial tissue (or epithelium) is formed by tightly adherent cells.
It lines the surface of the skin, the inner surface of the respiratory, digestive, and genitourinary systems; forms glands.
Thus, the epithelium occupies a borderline position, therefore it participates in metabolic processes. Epithelial tissue also has a protective function, protecting other tissues from damage.
Example: gas exchange occurs through the epithelium of the alveoli of the lungs.
The cilia on the surface of the ciliated epithelium of the respiratory tract move mucus outward, helping the body to get rid of microorganisms, dust, allergens, etc. that have entered along with the inhaled air.
Different types of epithelium have structural features associated with the functions they perform.
The outer layer of the skin and mucous membranes are represented by the integumentary epithelium.
Example: the skin epithelium performs mainly protective and barrier functions, delimiting the human body from the external environment. Therefore, the skin epithelium is stratified and may have a stratum corneum.
The glandular epithelium consists of cells that secrete active substances (secret, hormone). It is characteristic of the glands (salivary, pancreas, etc.).
Glands are formations of the epithelium, the cells of which produce and secrete special substances – secrets (or hormones).
Example: the intestinal epithelium performs the function of exchange, absorbing nutrients into the blood and lymph. It is single-layered and has microvilli that increase the suction surface.
The intestinal epithelium is also glandular, since it secretes a special secret for tissue protection and chemical treatment of the absorbed substances.
Some types of epithelial tissue have intercellular gaps or openings (for example, the renal epithelium) that facilitate filtration and absorption.
All types of epithelium have common features:
- epithelial cells form a continuous layer located on the membrane that feeds the cells;
- the cells of the epithelium are characterized by the polarity of the structure (the structure of the part of the cell located near the membrane differs from the structure of the opposite part of the same cell);
- epithelial tissue has a high capacity for recovery (regeneration).
Connective tissue cells are different in shape and are surrounded by a developed intercellular substance. It can be in the form of fibers, bone plates, cartilage, fluid.
These structural features allow connective tissue to perform various functions:
- mechanical (supporting) function is performed by bone and cartilage tissue, as well as fibrous connective tissue of tendons;
- the protective function is performed by adipose tissue;
- the transport function is performed by liquid connective tissues: blood and lymph.
Blood provides the transport of oxygen and carbon dioxide, nutrients, metabolic products.
Thus, the connective tissue connects the parts of the body to each other.