The skin (cutis) is composed of two layers, the epidermis and the dermis (corium). The outermost layer of the skin is the horny layer or stratum corneum. It is composed of approximately twenty layers of brick-like, densely packed horny cells which are interlocked with their neighboring cells. They are formed by dead cells which are transformed into keratin (protein). These cells do not contain any cell organelles.
The horny layer has the function to protect the living skin layers through these numerous layers of flat, dead (horny) cells, which flake away at the skin's surface. This process of sloughing off cells on the skin's surface is called desquamation.
The horny cells, which, in this layer, have a length of approximately 30 µm and a thickness of 0.5 µm are baked together with the horny substances (keratin) to form plates. These are finally sloughed off as horny flakes. During the migration to the skin's surface the cells change their appearance. This process is also called differentiation. In our illustration, this process of differentiation is shown in various phases (see 1-4).
The epidermis consists of four cell layers. The cells of the epidermis migrate through these layers within a life-time of about four weeks. The cells are built by cell division in the lowest layer of the subcutis, the basal layer (1). The cells change their appearance while they migrate to the skin's surface. They flatten as they rise through the layers (2). In the stratum spinosum or prickle-cell layer above the basal layer, large cells with "prickle-like" extensions are found. In the stratum granulosum (or granular layer), which is located above the prickle-cell layer, the cells become smaller (3), finally turn into dead cells and are shed (4).
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