The stratum basal It is the innermost layer of the epidermis and is made up of cells with germinative characteristics. This cell layer is responsible for skin renewal, a process that occurs cyclically. The cells of the basal stratum undergo mitosis, thus effecting cell renewal. It is also known as the basal layer.
As the cells divide, they are pushed towards the surface and progressively keratinized until they reach the stratum corneum where they are shed. The basal layer is made up of living cells, while the stratum corneum contains dead cells and functions as a barrier against the environment..
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The stratum basalis is made up of a layer of cells that is only one cell thick. It is characterized by being the germ layer of the epidermis, since it contains stem cells that give rise to keratinocytes. The latter are the constituents of the thorny, granular, lucid and corneal layers. In addition to stem cells, melanocytes and keratinocytes are found in this stratum.
The cells of the stratum basalis are small in size and cubic or cylindrical in shape. They have little cytoplasm, so the nuclei are close to each other. The basal stratum presents basophilia, tending to be easily stained by any basic stain.
The melanin that is contained mainly in the melanocytes is capable of migrating from these to the other cells of the stratum, forming variable amounts in the cytoplasm of said cells.
Structures called desmosomes hold basal cells together and with keratinocytes. These are attached to the dermis through a basement membrane.
Stem cells divide and differentiate into keratinocytes. These then migrate to the following layers and reach the outer layer of the epidermis, where they undergo keratinization and form the horny layer of the skin..
In the basal layer there are two classes of germ cells: stem cells and progenitor cells of the basal layer or transient amplification.
Stem cells are pluripotent and are found in the apical area of the hair follicles or hair bulb. These move to any region of the epidermis and act in the regeneration and reconstruction of tissue in the event of an injury. They have a slow division, making replacements three to four times a year, and have a long life.
Transient amplification cells are derived from stem cells and are found in the junction area between the dermis and the epidermis.
These cells are unipotent and can perform cell division (mitosis) more rapidly, up to once a week. They have a shorter life, since after a limited number of divisions, they undergo a terminal differentiation towards keratinocytes.
The stem cells of the basal stratum must be renewed to maintain homeostasis in that stratum. These can be divided symmetrically or asymmetrically.
By dividing asymmetrically, two cells are produced, one with the original phenotype and the other that differentiates into some other cell type. This ensures that the stem cell pool remains constant..
When the two daughter cells divide symmetrically, they have a differentiated phenotype. In embryonic development, stratum basal cells differentiate mostly symmetrically and parallel to the axis of the stratum. In this way, the uniform growth of the surface of the embryo is ensured, leaving the epithelium in a single layer.
By stratifying the epithelium in several layers, the divisions of the basal cells are mostly asymmetric (around 70% of the divisions), which ensures that supra-basal cells develop, causing the establishment of a skin barrier when the epidermis is formed and maintain your homeostasis into adulthood.
The cells of the stratum basalis play an essential role in epidermal repair and renewal. In fish, during development, this layer has the function of producing collagen and interacting with the mesenchyme for the formation of scales. Possibly it also acts on the deposit of epidermal substances in the scales.
This basal or germ layer produces new cells from stem cells. These differentiate and migrate towards the upper layers until they reach the surface of the skin where they keratinize, lose their nuclei and peel.
This constant cell turnover allows permanent skin renewal, maintaining cutaneous homeostasis (constant number of cells).
Despite the fact that in each region of the stratum the stem cells are responsible for replacing the differentiated cells that die, it is possible that they can migrate to other regions and participate in their repair, in case the stem cells from these regions are defective.
There is evidence that progenitor cells of the stratum basalis are more efficiently responsible for maintaining cutaneous homeostasis. On the other hand, stem cells are responsible for the repair and healing of the epidermis, activating when an attack or injury is detected.
The cells of the stratum basalis also act in the healing process when there is tissue damage. The mechanisms of action of stem cells in the face of damage vary according to the region where the damage occurred..
In interfollicular tissue repair, for example, a hoarding of stem cells occurs near the wound area. These cells divide and their clones travel from the periphery of the wound to the center of it, staying in that area for a long time..
On the other hand, the progenitor cells of the basal layer differentiate and migrate towards the damaged area, in a much smaller number than stem cells and they remain there for a very short time..
Stem cells from the base of the follicle and infundibulum have the ability to migrate to the outer layers of the epidermis, working to repair that area. When migrating to the epidermis, the markers for hair follicles of these cells become inactive, being able to adopt a phenotype similar to that of interfollicular stem cells..
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