Skin and Adnex

• The development of the dermis involves the dorsal dermis, which arises from the dermatome of the somites, the ventral and lateral dermis, and the dermis of limbs.
• The developmental stages of the epidermis include surface embryonic ectoderm, epidermal growth, and a primordium of epidermis.
• The lateral plate mesoderm, cranial neural crest (ectoderm), and mesenchymal cells have begun to produce collagenous and elastic connective tissue fibers by 11 weeks.
• Sensory nerves grow into the dermis and the epidermis.
• As the epidermal ridges form, the dermis projects into the epidermis, forming dermal ridges.
• In the face, the dermis arises from much of the cranial skin, the anterior neck, and dermal cells.
• Ectodermal (epidermal)/mesenchymal (dermal) interactions involve mutual inductive mechanisms.
• Capillary loops develop in some of these ridges and provide nourishment for the epidermis.
• Central cells of the primordial ducts degenerate during the development of the skin.
• The primordia of nails appear as thickened areas or nail fields of epidermis at the tip during the development of the skin.
• Apocrine sweat glands begin to secrete during puberty during the development of the skin.
• Ichtyosis is an excessive keratinization disorder during the development of the skin.
• The vernix protects the developing skin from constant exposure to amniotic fluid, with its high urine content, during the fetal period.
• At first, the developing nail is covered by a narrow band of epidermis, eponychium during the development of the skin.
• Ducts of these glands open, not onto the skin surface, but into upper part of hair follicles superficial to openings of the sebaceous glands during the development of the skin.
• Albinism is a disorder where melanocytes fail to produce melanin because of lack of tyrosinase; skin, hairs and retina lack pigment during the development of the skin.
• Fingernails reach fingertips by 32 weeks, toenails reach toetips by 36 weeks during the development of the skin.
• Hypertrichosis is a disorder with supernumerary hair follicles during the development of the skin.
• Eccrine sweat glands begin to function shortly after birth during the development of the skin.
• Development of the skin involves the formation of the vernix caseosa, a whitish, greasy coating of the skin, which protects the epidermis.
• Peripheral cells of the secretory part of the gland differentiate to myoepithelial and secretory cells during the development of the skin.
• Mammary glands are a modified and highly specialized type of sweat gland.
• Mammary buds have several secondary buds lactiferous ducts and their branches during the development of the skin.
• Apocrine sweat glands develop from downgrowths of the stratum germinativum of the epidermis during the development of the skin.
• Absence of Skin is a rare disorder during the development of the skin.
• Mammary buds begin to develop during the sixth week downgrowth of the epidermis into mesenchyme during the development of the skin.
• Mammary crests develop along each side of the ventral surface of the embryo at week 4 during the development of the skin.
• Later, these nail fields migrate onto the dorsal surface and are surrounded laterally and proximally by folds of epidermis, the nail folds during the development of the skin.
• Mammary crests in the 5th week produce primary mammary buds during the development of the skin.
• Cells from the proximal nail fold grow over the nail field and become keratinized to form the nail plate during the development of the skin.
• The greasy vernix facilitates birth of the fetus during the development of the skin.
• Later, the eponychium degenerates and a cuticle is formed during the development of the skin.
• Hair buds soon become club shaped, forming hair bulbs (primordia of hair roots).
• Sebaceous glands develop in a similar manner to buds from the epidermis.
• Sebaceous glands develop as buds from the sides of developing hair follicles, glandular buds grow into surrounding connective tissue and branch to form primordia of several alveoli and their associated ducts, and central cells of the alveoli break down, forming an oily secretion, sebum.
• Eccrine sweat glands develop as epidermal downgrowths (cellular buds) into the underlying mesenchyme, as buds elongate, their ends coil to form the primordium of the secretory part of gland, and epithelial attachment of developing gland to epidermis forms primordium of sweat duct.
• Skin ectoderm and mesenchyme can respond to each others inductive signals.
• Dermis determines ectodermal differentiation.
• Arrector muscles of hairs differentiate from mesenchyme surrounding the hair follicle and attach to the dermal root sheath of hair follicles and papillary layer of dermis.
• Hair bulbs are invaginated by small mesenchymal hair papillae.