Observed from the surface, the cells are irregular polygonal, with a smooth surface, jagged cell edges, adjacent cells chimeric to each other, and an oblate nucleus located in the center of the cell. Observed from the vertical section, the cells are very thin and only the part containing the nucleus is slightly thicker.

Endothelium: The single layer of flat epithelium that lines the inner surface of the heart, blood vessels and lymphatic vessels is usually called endothelium. It has a smooth surface and is conducive to blood and lymph flow and material transport.

Mesothelium: The single layer of flat epithelium distributed on the surface of the pleura, peritoneum and pericardium is called mesothelium, which can reduce friction between organs and facilitate organ movement.

When viewed from the surface, cells appear hexagonal or polygonal; Observed from the vertical section, the cells are cuboidal in shape with a round nucleus located in the center.

It is mainly distributed in renal tubules, thyroid follicles, etc., and has functions such as absorption and secretion.

When viewed from the surface, cells appear hexagonal or polygonal; In the vertical section, the cells are columnar and the nuclei are oval, mostly located at the base of the cells.

Distributed in the cavities of the stomach, intestines, gallbladder, fallopian tubes, uterus and other organs, it mainly has the functions of absorption and secretion.

It is generally composed of columnar cells (mostly), spindle cells, cone cells, and goblet cells. Observed from the vertical section, because the cells are of different heights and the nuclei are not on the same plane, they appear to be stratified under a light microscope, but the basal surface of all cells is attached to the basement membrane; there are a large number of cilia on the free surface.

This kind of epithelium is mainly distributed in the pharynx, larynx, trachea, bronchi, Eustachian tube, tympanic cavity and other cavity surfaces, and mainly plays a protective role.

The surface cells are flat and scaly. When viewed in a vertical section, the layer close to the basement membrane is called the basal layer. It is a stem cell with the ability to proliferate and differentiate. The middle layer is multi-layered polygonal cells, and the surface layer is multi-layered flat cells.

The connection between the epithelium and the deep connective tissue is uneven, which not only ensures the supply of nutrients to the epithelial tissue but also makes the connection stronger.

The epithelial cells distributed on the surface of the skin have no nuclei and contain a large amount of keratin. The cells are dry and hard and can fall off. They are called keratinized stratified squamous epithelium.

The superficial cells distributed in the oral cavity, esophagus and vaginal cavity are living cells with nucleation, containing less keratin, and are called non-keratinized stratified squamous epithelial cells.

It is composed of multiple layers of cells, with a superficial layer of neatly arranged columnar cells and a deep layer of one or several layers of polygonal cells.

This type of epithelium is mainly found in the palpebral conjunctiva, the male urethra and the large ducts of some glands.

Mainly distributed in the renal calyces, renal pelvis, ureters, bladder and other cavities.

Divided into surface layer, middle layer and basal layer. The superficial cap cells are large and thick, in larger round and oval shapes, and some have binucleates.

The characteristic is that the cell layer and morphology can change with the functional status of the organ.

Under a light microscope, the free surface of the cap cells is strongly eosinophilic due to the concentration of cytoplasm, and is called the shell.

It is a tiny finger-like protrusion that extends from the cell membrane and cytoplasm to the free surface of the cell.

The striated border of the columnar epithelium of the small intestine and the brush border of the renal tubules seen under light microscopy are both composed of microvilli.

Under an electron microscope, microvilli are about 0.1 μm in diameter, with a cell membrane on their surface, and their axis contains many longitudinal microfilaments.

The outer surface of the cell membrane of microvilli is often covered with a thick layer of cell coat. The cell coat has the functions of adhesion, protection and material exchange. In addition, it is also related to cell surface antigenicity and cell recognition.

Microvilli can expand the surface area of ​​cells and facilitate the cell's absorptive function.

It is a long protrusion extending from the free surface of epithelial cells and has the function of rhythmic directional swing.

There are two microtubules arranged parallel to its long axis in the center of the cilium, surrounded by 9 groups of doublet microtubules. There is a dense particle at the root called the matrix.

The swing of cilia can push particles, foreign matter, mucus, etc. adhered to the epithelial surface in a direction, so cilia have the function of removing foreign matter and transporting substances.

Structure: Mostly located at the top; point-like fusion; mesh-like structure

Function: Seals the intercellular space, which not only prevents extracellular macromolecular substances from entering deep tissues through the intercellular spaces, but also prevents the overflow and loss of material components in deep tissues.

Structure: Located below the tight junction; there is a gap of 15~20nm, and filaments are connected in the middle; there are dense substances attached to the inner surface of the cell membrane and are connected to the terminal mesh filaments

Function: inter-cell adhesion, information transmission, maintaining cell function and shape, and transmitting cell contractility, etc.

Structure: There are gaps between the patchy membranes, with sticky substances and filaments inside, and a dense midline; there are attachment plates and tension filaments on the cytoplasmic surface.

Function: Mechanical connection and information transmission between cells (the strongest connection)

Structure: Surrounded by 6 connexin molecules, with a lumen of 2 nm in diameter in the center

Function: Exchange small molecule substances and ions; facilitate the conduction of electrical impulses (resistance here is very low)

Two or more of the four types of cell connections exist at the same time

Placode (composed of mucopolysaccharide substances secreted by epithelial cells)

Reticulum (matrix and reticular fibers secreted by fibroblasts)

They are many cell folds of varying lengths formed by the cell membrane on the basal surface of epithelial cells folding into the cytoplasm and distributed vertically to the basal surface of the cells.

Effectively expands the surface area of ​​the basal surface of cells; facilitates the rapid transport of water, electrolytes and other substances

Located at the contact point between the basal surface of epithelial cells and the basement membrane. Its structure is half of a desmosome

Functions to strengthen the connection between epithelial cells and basement membrane