Characteristics of epithelial tissue

When there is a problem with the human skin, it is often in the epithelial tissue. The characteristics of epithelial tissue are that there are a lot of cells and they are very tight. So when the skin is dehydrated, the cells will become loose, causing wrinkles to appear on the skin. The main function of epithelial cells is to help the skin absorb, secrete, protect and excrete. Any problem in any aspect will be reflected in the skin.

Epithelial tissue is composed of a large number of cells with regular shapes and very little intercellular matrix, and is referred to as epithelium. The characteristics of epithelial tissue are: a large number of cells and relatively few stroma; epithelial cells are bipolar, one pole is called the free surface, facing the body surface or the cavity surface of hollow organs, and the other pole is called the basal surface, facing the deep connective tissue; there are no blood vessels in the epithelial tissue, and the required nutrients depend on the blood vessels in the connective tissue to be supplied through the basement membrane. Epithelial tissue is usually rich in nerve endings that can sense various stimuli.

The structural features are

1. The cells are tightly connected and there is little intercellular matrix. It usually has the functions of protection, absorption, secretion and excretion.

2. The cells are tightly bound together, and there is a layer of cell coat outside the cells. The cell coat is the exposed sugar chains of glycoproteins that make up the cell membrane, which has a strong adhesion effect. The calcium ions in the intercellular space and the special intercellular connections between cells also play an important role in the adhesion of cells.

3. Obvious polarity. The two ends of the cell are different in structure and function. One end faces the body surface or the cavity of the tube, cavity, or sac inside the body, which is called the free surface; the other end opposite to the free surface is called the basal surface.

The cell free surfaces of epithelial tissues distributed in different parts often have different specialized structures to adapt to their specific functions. For example, the intestinal epithelium has dense microvilli that expand the absorption surface; the respiratory epithelium has vibrating cilia that can expel invading dust and other foreign matter. The basal surface is generally connected to its deeper connective tissue by a basement membrane.

The basal surface of some epithelial cells also has structures such as cell membrane infolds that expand the basal surface area of ​​the cells or strengthen the connection with the basement membrane. regeneration

Epithelial tissue has a strong regenerative capacity. There are two different types of regeneration, namely physiological regeneration and pathological regeneration.

1. Physiological regeneration

Physiological regeneration refers to the renewal of epithelial tissue under normal physiological conditions. Under normal physiological conditions, surface cells of epithelial tissue often die and fall off, especially the epidermis and gastrointestinal epithelium. Due to frequent wear and tear and shedding, the epithelial cells are replaced by less differentiated cells within the epithelium through mitosis to maintain the constancy and integrity of the epithelial cell number. The epithelial renewal rates of each organ vary. For example, it only takes 2 to 5 days for the villi of the small intestine to be completely renewed, while it takes about 50 days for the pancreatic epithelium.

2. Pathological regeneration

Pathological regeneration refers to the repair of epithelial tissue after damage caused by pathological factors such as inflammation and trauma. For example, after the skin epidermis is damaged and lost, the epithelial basal cells at the edge of the wound or the epithelial cells of the accessory gland ducts divide and proliferate, move to the exposed area of ​​connective tissue, become a single layer of flat cells, and cover the wound surface. Later, as the number of migrated cells increases, the number of cell layers of the regenerated epithelium gradually increases; after the digestive tract epithelium is damaged and falls off, normal epithelial cells in the adjacent area or the epithelial cells in the neck of the gland divide and proliferate, starting out as a cube, and then gradually increasing in height to become a single-layer columnar epithelium. It is generally believed that the regenerative ability of the glandular epithelium is weaker than that of the covering epithelium.