Epithelium of the skin
The skin (epidermis) is separated from the outside by a multi-layered cornified squamous epithelium. This provides mechanical protection, prevents the penetration of bacteria and prevents the body from drying out. It is called squamous epithelium because the uppermost cell layer consists of flat cells.
Since these cells die constantly, turn into horny scales and are sheared off, it is called cornified. The uppermost layer is therefore the horny layer (stratum corneum), in whose cells no nuclei can be found, because the cells have already died. The stratum lucidum, the stratum granulosum and the stratum spinosum are connected to the inside of the body.
The latter is characterized by a strong interlocking of the cells among themselves. The regeneration of the cells takes place in the stratum basale. The basal cells located there sit on the basal membrane, can divide and thus new cells are created for the skin layers above.
Epithelium of the lung
The different sections of the lung are lined by different epithelia. The upper airways, i.e. the trachea and the larger bronchi, are covered by a multi-rowed ciliated epithelium. Since this affects a large part of the airways, this is also called the respiratory epithelium.
Multi-row means that all cells of the epithelium are in contact with the basement membrane, but not all of them reach the surface. It is called ciliated epithelium because the cells that reach the surface have numerous small protuberances that move during breathing and are mainly used for absorption. The smaller bronchial tubes are lined by a single-row ciliated epithelium, which is no longer cylindrical but cubic, i.e. flatter.
In the pulmonary alveoli themselves, there is a flat epithelium that allows the exchange of gases between the air we breathe and the blood without any problems. The epithelial cells of the alveoli can be divided into two types. The alveolar epithelial cells type 1, which are responsible for the gas exchange and the alveolar epithelial cells type 2, which produce surfactant. The surfactant is responsible for preventing the alveoli from collapsing.
