Macerating Action Amniotic Fluid

Figure 18.1 Formation of the skin at various stages of development. A. 5 weeks. B. 7 weeks. C. 4 months. D. Birth.

During the first 3 months of development, the epidermis is invaded by cells arising from the neural crest. These cells synthesize melanin pigment, which can be transferred to other cells of the epidermis by way of dendritic processes. After birth, these melanocytes cause pigmentation of the skin (Fig. 18.1 D).

CLINICAL CORRELATES Fingerprints

The epidermal ridges that produce typical patterns on the surface of the fingertips, palms of the hand, and soles of the feet are genetically determined. They form the basis for many studies in medical genetics and criminal investigations (dermatoglyphics). In children with chromosomal abnormalities, the epidermal pattern on the hand and fingers is sometimes used as a diagnostic tool.

The dermis is derived from lateral plate mesoderm and the dermatomes from somites. During the third and fourth months, this tissue, the corium (Fig. 18.1 D), forms many irregular papillary structures, the dermal papillae, which project upward into the epidermis. Most of these papillae contain a small capillary or sensory nerve end organ. The deeper layer of the dermis, the sub-corium, contains large amounts of fatty tissue.

At birth, the skin is covered by a whitish paste, the vernix caseosa, formed by secretions from sebaceous glands and degenerated epidermal cells and hairs. It protects the skin against the macerating action of amniotic fluid.

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