Fine structure of epidermal differentiation in the mouse

The structure of the epidermis of mice from nine days in utero to adult has been studied with the electron microscope. During fetal development the epidermis increases in thickness and four structurally distinct regions are present by 15–17 days in utero . Fetal epidermis differs from adult epidermis by the presence of large, tripartite keratohyalin granules in the superficial layers. Such structures are absent in adult epidermis. Furthermore, there are fewer tonofilament bundles in fetal epidermis, and no cornified layer containing 100 Å keratin fibers is found in the fetal stages. A peridermal layer is also present in the fetus.