The function of alkaline phosphatase activity in the skin, as in many other tissues, is not clear. It is generally agreed that this enzyme occurs in the vascular endothelium of afferent vessels 30,am or less in diameter. It is absent from veins and venules. Furthermore, blood vessels in man and many other species normally display strong adenosine triphosphatase (ATPase) activity (Moretti, 1968) and 5'-nucleotidase activity (Hardonk, 1968). Because of this, the histochemical demonstration of these three enzymes has been employed for examining the vascular architecture of various tissues in different species. During the embryonic development of skin in the chick and in man, alkaline phosphatase activity is present in the ectodermal cells for a short period only (McKay, Adams, Hertig&Danziger, 1956). Theenzyme generally appears in postnatal life in the dermis (Moretti, 1968). The presence of ectodermal alkaline phosphatase activity in developing rat skin has been reported by Hashimoto & Ogawa (1963), but at a time which would place it in the pre-embryonic period, that is, when a bilaminar embryonic area, rather than a formed embryo, is present. It was therefore decided to investigate anew the development of rat skin, with particular reference to the presence in it of alkaline phosphatase, ATPase and 5'-nucleotidase activities.
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