Epidermis

During weeks 4 to 5, the surface ectoderm consists of a single cell layer. The mesenchyme beneath consists of star-shaped cells loosely spread in an extracellular matrix. The basal lamina lies between the ectoderm (epidermis) and mesenchyme (dermis). Towards the end of week 5, the surface ectoderm forms two layers: the basal germinative layer (stratum germinativum), which gives rise to the definitive postnatal epidermis, and the temporary periderm. The periderm is independent of the germinative layer and increases through mitotic activity of its own cells. It contributes to the formation of the amniotic fluid. From week 20, periderm cells undergo keratinization and initially form a protective layer for the epidermis underneath. Periderm squames cast off into the amniotic fluid. They contribute to the vernix caseosa ("cheesy varnish"), a greasy layer of cell debris that covers the skin of the newborn.