Study of epidermis from fetal rats from 17 days until birth includes the whole keratinization process. In 18-day fetal skin, so-called ‘compound’ granules appear. These are different from keratohyalin granules. The layer containing these granules undergoes a flattening in 20-day fetal skin but the individual granules remain visible. On the 18th day of fetal life, the synthesis of keratohyalin granules starts. They seem to originate by the clumping of ribosomes. While growing larger (19-day fetal skin) they become patchy. Many of the granules are in contact with fibrillar material, in particular when the cells become filled with this fibrillar material (21-day skin). Whether this contact happens by accident or whether this material becomes incorporated in these granules is not yet resolved. In 20-day fetal skin a stratum corneum is present. In 17-day fetal skin cultured in vitro, differentiation to morphologically adult skin proceeds faster than in vivo. Already after 2 days in culture a stratum corneum is present which shows most of the ultrastructural characteristics present in vivo. However, at that time the scantiness of keratohyalin granules and of fibrillar material is striking. During the 3rd and 4th day in culture, the amount of fibrillar material increases, as do the number of keratohyalin granules. Our findings on keratohyalin granules are discussed in relation to the current ideas about the role of these keratohyalin granules.

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