Initially, the embryo is covered by a single layer of ectodermal cells (Fig. 18.1 A). In the beginning of the second month, this epithelium divides, and a layer of flattened cells, the periderm, or epitrichium, is laid down on the surface (Fig. 18.1 B). With further proliferation of cells in the basal layer, a third, intermediate zone is formed (Fig. 18.1 C). Finally, at the end of the fourth month, the epidermis acquires its definitive arrangement, and four layers can be distinguished (Fig. 18.1 D): The basal layer, or germinative layer, is responsible for production of new cells. This layer later forms ridges and hollows, which are reflected on the surface of the skin in the fingerprint.
A thick spinous layer consists of large polyhedral cells containing fine tonofibrils.
The granular layer contains small keratohyalin granules in its cells. The horny layer, forming the tough scalelike surface of the epidermis, is made up of closely packed dead cells containing keratin. Cells of the periderm are usually cast off during the second part of intrauterine life and can be found in the amniotic fluid.
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