Alkaline phosphatase (AP) appears early in fetal life in the mesenchymal cells of the presumptive dermis, especially in the anlagen of the papillae of hair follicles and in the endothelial cells of the blood vessels. It appears later in the eccrine and apocrine sweat glands from which it later mostly disappears. AP is inconstantly present in the cells of the bulb of hair follicles, and it is never present in the cells of the sebaceous glands and the epidermis. In the tips of the fingers and toes it is concentrated in the nail wall, around the nail matrix and nail plate, and in the sole plate. Towards the end of fetal life, AP is only moderately active or absent in the tips of the fingers and toes.
The physiological role of AP in the skin may be related to morphogenesis and particularly to the differentiation of the pilary system, and to keratinization of the epidermis and the nail plate. In the endothelium of small vessels its function could be that of aiding the transfer of materials to and from the vessels (phosphorylation and dephosphorylation). AP disappears from the endothelium of vessels as soon as the tunicae are formed; it remains in the arterial limbs of capillaries.
SERRI F, MONTAGNA W, HUBER WM. Studies of Skin of Fetus and the Child The Distribution of Alkaline Phosphatase in the Skin of the Fetus. Arch Dermatol. 1963;87(2):234–245. doi:10.1001/archderm.1963.01590140096016
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