The skin

The integumentary system includes the skin and its specialized derivatives, including the hairs, nails, sweat and sebaceous glands. The mammary glands and teeth are also considered as components of the integumentary system. The system develops from surface ectoderm, mesoderm and neural crest cells.

By the end of the first month the single layer of ectodermal cells divide, forming a superficial layer of cells known as the periderm and a basal layer. The basal layer becomes the stratum germinativum which produces all the definitive layers of the epidermis (Fig. 4.1A, B). The cells of the periderm become keratinized, and are continuously lost in the amniotic fluid during fetal life. In a newborn infant, the sloughed cells of the periderm mix with the skin secretions to form the vernix caseosa that covers the entire skin. Later in fetal life, the peridermal cells are replaced by the stratum corneum and the melanocytes derived from the neural crest migrate into the epidermis (Fig. 4.1B, C).

Fig. 4.1 Development of the skin at (A) 4 weeks, (B) 12 weeks and (C) in the newborn.

The dermis is derived from two sources: the somatic layer of the lateral plate of mesoderm and the dermatomes of the somites. The capillary network develops in the dermis to nourish the epidermis, and by 9 weeks, the sensory nerve endings grow into the dermis and epidermis. By 12 weeks, the outer layer of dermis projects into the overlying epidermis as dermal papillae (Fig. 4.1B).

The hairs and glands of the skin

Hairs develop during the fetal period as proliferations of the stratum germinativum of the epidermis growing into the underlying dermis. The tip of the hair bud becomes a hair bulb and is soon invaginated by the mesenchymal hair papilla in which the vessels and nerve endings develop (Fig. 4.2A, B). The epidermal cells in the centre of the hair bud become keratinized to form the hair shaft, and the surrounding mesenchymal cells differentiate into the dermal root sheath (Fig. 4.2C). Small bundles of smooth muscle fibres called arrector pili muscles develop in the mesenchyme and are attached to the dermal sheath (Fig. 4.2C). Most sebaceous glands develop as buds from the side of the epithelial root sheath growing into the dermis; these glands produce an oily secretion that lubricates the hair and skin. The sweat glands develop as epidermal buds into the underlying dermis which become coiled to form the secretory part of the glands (Fig. 4.2C).

Fig. 4.2 Development of the hair, arrector pili muscle, sebaceous and sweat glands at (A) 14 weeks, (B) 24 weeks and (C) in the newborn.

Mammary glands and teeth

The mammary glands arise from a pair of epidermal thickenings called mammary ridges which appear in both sexes during the fourth week from the axilla to the inguinal region (Fig. 4.3A). Normally these ridges disappear except in the pectoral region where breasts develop, but part of the mammary ridges may persist to form an extra breast (polymastia) or supernumary breasts and nipples (Fig. 4.3A). The breast develops from a primary mammary bud which gives rise to several secondary buds that form the lactiferous ducts and their branches. The buds branch throughout the fetal period, become canalized and open into a small depression called the mammary pit of the nipple (Fig. 4.3B, C).

Fig. 4.3 Development of the mammary glands in (A) showing the location of the mammary ridges and common formation sites of supernumerary breasts and nipples at (B) 7 weeks and (C) 8 months.

The teeth develop as tooth buds from the epithelial lining of the oral cavity along a U-shaped epidermal ridge called the dental lamina along the curves of the upper and lower jaws. The ectodermal tooth bud grows into the neural crest-derived mesenchyme, and passes through a ‘cap’ and a ‘bell’ stage. The enamel is produced by ameloblasts which are derived from the ectoderm; the underlying dentin and other tissues are derived from the mesenchyme and neural crest cells.

Development of the axial skeleton

The axial skeleton is composed of the skull, vertebral column, sternum and ribs. This part of the skeleton is derived from the paraxial mesoderm, which is soon organized into the somites. The first somites appear on day 20 in the cranial region, and by 30 days approximately 37 pairs are formed. Somites appear as rounded elevations under the surface ectoderm on the dorsal aspect of the embryo from the base of the skull to the tail region. Each somite subdivides into two parts: the sclerotome and the dermomyotome (Fig. 4.4). The cells of the sclerotome give rise to the vertebrae and ribs, and those of the dermomyotome form muscle and the dermis of the skin.

Fig. 4.4 Differentiating somites in a 4-week embryo.