Developmen of the skin and its appendages.

The skin consists of the superficial layer, the epidermis, and the deep layer, the dermis. The epidermis is derived from the ectoderm. In the fifth week, the skin of the embryo is covered by simple cuboidal epithelium. By the seventh week, the skin contains single squamous layer (periderm or epitrichium), and a basal layer. During the fourth month, an intermediate layer, containing several cell layers, is interposed between the basal cells and periderm. During the early fetal period the epidermis is invaded by melanoblasts, cells of the neural crest origin. The dermis is derived from the mesenchyme of somatopleura and the dermatomes.

Hair begins to develop during the third month as an epidermal proliferation into the underlying dermis. The epithelial cells of the hair bud give rise to the hair, and the epithelial root sheath. Surrounding mesenchymal cells differentiate into dermal root sheath. Peripheral cells of the epithelial root sheath proliferate to form a sebaceous gland but. Sweat glands develop as downgrowths of epithelial cords into the underlying dermis.

Nails begin to develop at about ten weeks of gestation as thickened areas of the epidermis at the tip of the digits. Later, these nail fields extend to the dorsal surface and become surrounded by the nail folds. Cells from the proximal nail fold grow over the nail field and form keratinized nail plate, the primordium of the nail.

The mammary glands begin to develop during the sixth week as thickened strips on the ectoderm (mammary ridges) that extend from the axillary to the inguinal regions. They regress in most locations except in the area of the pectoral muscle, where they proliferate. The downgrowth of epithelial tissue continues to proliferate into sixteen to twenty-four solid outbudding which dive rise to the lactiferous ducts. Fibrous connective tissue fat of the mammary gland develops from the surrounding mesenchyme. The lactiferous ducts at first open into a small mammary pit.

Не знайшли потрібну інформацію? Скористайтесь пошуком google:

Генерація сторінки за: 0.003 сек.