Epithelial cells can originate from any of the embryonic germ layers. Initially, they form continuous sheets that are only one cell thick, covering the external surface of the embryo (ectoderm) and its internal cavities (mesoderm or endoderm). Subsequently, during embryonic development, cells in these epithelia differentiate to give rise to various types of lining epithelia and exocrine or endocrine glands.
Epithelial cells are generally short-lived because they are often subjected to various physical or chemical aggressors, as well as the effects of their high metabolic activity. Therefore, it is common to find damaged or dead epithelial cells that shed from the epithelia.
The death of epithelial cells can also be a phase of their life cycle. For example, the cornification of the epidermis in many vertebrates involves the progressive terminal differentiation of keratinocytes, leading them to form the scales of the stratum corneum. Similarly, in holocrine glands, the secretion process includes the death of glandular cells.
Therefore, most epithelia are constantly renewing, involving processes of mitosis and cell differentiation. Thus, in the tissues of adult individuals, it is common to find epithelial cells in mitosis in the epithelia. These mitotic cells are undifferentiated progenitor cells, which, after dividing, give rise to another progenitor cell and a cell that will differentiate to replace another.
In simple lining epithelia, mitotic progenitor cells are located at the same level as differentiated cells. However, depending on the type of epithelium, they can be localized in specific regions. For example, in the Lieberkühn crypts (tubular exocrine glands in the small intestine), progenitor cells are located in the region near the base, which is why most mitoses are observed at that level.
The localization of progenitor cells in specific areas of epithelia is due to multiple humoral and physical factors that create a suitable microenvironment. For example, one of these factors is the interaction with mesenchymal-origin cells and their products. Thus, the progenitor cells responsible for hair formation and growth are localized in a precise region of the hair bulb, which is in proximity to the dermal papilla, where mitoses that will give rise to hair epithelial cells are observed.
In stratified epithelia, progenitor cells are located in the germinative layer in contact with the basement membrane, where mitoses are observed, from which one of the daughter cells will differentiate into a keratinocyte and ascend to the upper layers.