Where is the stratified squamous epithelium?

The term epithelial tissue is not unfamiliar to most people, because they have come across it in biology classes at school, but not in great depth. In fact, epithelial tissue has a very obvious characteristic, that is, there are a lot of cells and relatively few stroma. Epithelial tissue is just a general term, and it is divided into many specific types. So, where is the stratified squamous epithelium?

1. Simple squamous epithelium: also known as simple squamous epithelium, it is composed of only one layer of flat cells. From the surface, the cells are irregular or polygonal, and the cell edges are serrated or wavy and interlocking. From the vertical section of the epithelium, the cells are flat and thin, with little cytoplasm, and only the part containing the nucleus is slightly thick. The nucleus is oval and located in the center of the cell. The epithelium covering the heart, chest cavity, abdominal cavity, blood vessels and lymphatic vessels is called endothelium. Its smooth surface facilitates the flow of blood and lymph. The epithelium covering the pleural cavity, peritoneal cavity and pericardial cavity is called mesothelium. It can secrete a small amount of serous fluid to maintain friction between organs, which is beneficial to the flow of blood and lymph and the permeability of substances.

2. Simple cuboidal epithelium: composed of a layer of cubic epithelial cells. When observed from the epithelial surface, the cells are hexagonal or polygonal; in vertical sections, the cells are cubic with round, centrally centered nuclei. For example, it is distributed in the epithelium of the thyroid gland and renal tubules, and has both secretory and absorptive functions.

3. Simple columnar epithelium: composed of a layer of columnar epithelial cells. Observed from the surface, the cells are hexagonal or polygonal; in vertical sections, the cells are columnar, with oblong nuclei, often located near the base of the cell, and their long axis is consistent with the long axis of the cell. For example, the epithelium lining the gastrointestinal tract, gall bladder, and uterine cavity has functions such as secretion and absorption. The free surfaces of the columnar epithelial cells of the small intestine have many tiny protrusions called microvilli. Microvilli increase the surface area of ​​cells, which helps the small intestine absorb nutrients.

4. Pseudostratified ciliated columnar epithelium: composed of columnar cells, spindle cells, cone cells and goblet cells, among which columnar cells are the most numerous. The cells of this epithelium vary in height, and the positions of the cell nuclei also vary in height on vertical sections. Epithelial tissue appears to be a stratified layer, but the base of each cell is located on the basement membrane, so it is actually a single layer. Its free surface has many cilia, which are thicker and longer than villi. The cilia can swing rhythmically in one direction, and with the help of this swinging, some secretions or foreign matter such as dust and bacteria attached to the surface can be removed. This epithelium is mainly distributed on the luminal surface of the respiratory tract and has both protective and secretory functions.

5. Stratified squamous epithelium: also known as stratified squamous epithelium, composed of more than ten or dozens of layers of cells. In vertical sections of the epithelium, cells have varying shapes. The basal cells close to the basement membrane are short columnar and are stem cells with the ability to proliferate and differentiate. Some of their daughter cells move to the shallow layer. Above the basal layer are several layers of polygonal cells, and above that are several layers of spindle-shaped or flat cells. Only a few layers of cells near the surface are flat. The basal layer cells can continuously divide and proliferate to replenish the aging, damaged or shed cells in the surface layer. The connective tissue deep in the stratified squamous epithelium is rich in capillaries, which is beneficial to the nutrition of the stratified squamous epithelium. This type of epithelium is distributed on the surface of the skin, the oral cavity, esophagus, vagina and other organs. It has protective functions such as resistance to friction and prevention of foreign body invasion. After being damaged, the epithelium has a strong repair ability.

6. Stratified columnar epithelium: It is composed of several layers of cells, the deep part is one or several layers of polygonal cells, and the shallow part is a layer of neatly arranged short columnar cells. This epithelium is mainly distributed in the conjunctiva, male urethra and large ducts of some glands.

7. Transitional epithelium: also known as transitional epithelium, is another type of stratified epithelium that lines the cavity surface of the urinary duct. It can be divided into surface cells, intermediate cells and basal cells. Its surface cells are large and thick and are called cap cells. One cap cell may cover several middle layer cells. Because the volume of the urinary tract often changes, the number and shape of epithelial cells also change accordingly, causing the epithelial area to expand and shrink. For example, when the bladder is empty and shrinks, the epithelium becomes thicker and has more cell layers. When the bladder is full and expands, the epithelium becomes thinner, the number of cell layers decreases, and the cells become flatter in shape.