A histochemical study on the vitamin D synthesis into the epidermis.

Using a suitable histochemical method vitamins D and 7-dehydrocholesterol could be shown into the epidermis of several mammal species. As the histochemical method used is able to discriminate vitamins D and 7-dehydrocholesterol from cholesterol and its esters, the sites where these vitamins were synthesized within the epidermis layers could be established. Vitamins D and 7-dehydrocholesterol were found into the epidermis in the same sites where cholesterol and its esters take place, such as: the keratinizing cell thick membrane and the stratum spinosum and stratum granulosum keratinocytes cytoplasm. Inside the keratinocyte cytoplasm vitamin D shows a granular pattern and appears weakly bound to proteins. The reactivity of such granules seems to be partially blocked as could be shown by an hydrolysis accomplished previously. After the hydrolysis reactive vitamin D was also found inside the epidermis intercellular space. The results suggest that vitamin D is synthesized into the cytoplasm of stratum spinosum and stratum granulosum keratinocytes, where it appears weakly bound to proteins. Afterwards it reaches the intercellular space, where its synthesis is accomplished and it becomes firmly protein-bound losing its histochemical reactivity. However, after a suitable hydrolysis the histochemical reactivity could be recovered. From the intercellular spaces vitamin D could take 2 fates: It was partially incorporated on the keratinizing cell thick membrane out surface and eliminated by means of the epidermis exfoliation. It was partially absorbed after passing across the basement membrane. On the other hand, the vitamin D placed inside the stratum spinosum and stratum granulosum keratinocytes cytoplasm become incorporated on the inner surface of the keratinizing cell thick membrane. The relationship between vitamin D biosynthesis and the epidermis lamellar bodies was discussed.