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Skin renewal is a highly organized systemic process that involves coordinated control of cell proliferation, migration and tissue modification by growth factors and hormones. The cell renewal process is completed after about 28 days, but the process decelerates with age. During the period, the old scales, containing dead cells, are naturally peeled off, while new ones are formed. The natural wearing process of the skin as one age is known as desquamation.
The layer of the skin that undergoes continual renewal is the epidermis. The major cell type in the epidermis is the keratinocytes, and a programmed form of cell death is called cornification (Lanza, 2009). The most inner layer of the epidermis contains stem cells, whose sole purpose is to divide and maintain the reservoir of epidermal cells and provide the differentiated cells, which make up the skin. New cells are formed in the stratum germinativum. From here they migrate to the skin surface. Once they reach the uppermost layer, the stratum conium, they are normally dead. These dead cells tightly pack themselves on the stratum and continuously slough away, as new cells pave their way to the surface for replacement.
Just like any other cell in the body, the skin cells grow and divide through mitosis from an original cell that is formed through meiosis after fusion of a sperm and egg. The single cell divide into two by mitosis and advances to form more complex body structures like the skin. In meiosis, a single cell divides twice forming four daughter cells that are modified to become sperm and eggs. Each daughter cell contains twenty-three chromosomes that are half of the original forty-six and which are later restored when fertilization takes place.
The skin cells located within the skin cells actively provide the differentiating cell that forms the body surface or other skin constituents. The stem cells are capable of rejuvenating themselves and continually replace the ever-dying cells of the upper skin layers. The cell starts to form in the lower skin layers, known as the dermis and move upwards to form the epidermis that is the outermost layer. The skin cells develop from two embryonic germs. The dermal ectoderm forms the epithelial integument and subjects the layers of connective tissues that arise from the dermatomes. These cells become larger with time, and in about two months they form a second layer of cells. They progressively form new different layers as time passes by, until the epithelium becomes multilayered. In its external layers keratinization occurs, especially in palms and soles. On the third of the prenatal period, the skin epithelial constituents of glands, hair, and fingernails emerge. Formation of fibers and dense layer of blood vessels later follows this.
The desquamation of the skin cells is naturally compensated by the renewal of the epidermis. The process is undertaken by the keratinocytes, and referred to as keratinization. This cell possess has two advantageous properties: ability to divide and differentiate actively. The keratinocytes divide in the mitotic layer of the skin to form daughter cells. The mitotic layer is joined to the dermis by the desmosomes. One of the daughter cells remains static and the other migrates to the differentiation layer. This is the layer where the keratinocytes grow and flatten. The keratin progressively fills the cells and flattens them as their nucleus starts to degenerate. They later secrete cement made of lipids, cholesterol, free fatty, saturated fatty acids and ceramides within the intercellular spaces that compact the layer, making it an effective barrier (Shamban, 2011). In the corneum, they form corneocytes, which are having neither the nucleus nor organelles. They are thus biologically dead, but remain active: they contain the content of the result of the keratinocyte differentiation. The corneocytes contain two distinct layers, whereby the outer one undergoes natural desquamation. The corneocytes in the inner layer are linked together with corneal desmosomes.