Anatomy and physiology of the skin and appendages

The integumentary system includes the skin, mucous membrane and the appendages, namely the nails, hair and sweat glands. The organ has a variety of embryological origins, which accounts for the different types of structures that are found within it. The developing embryo contains several layers; significant to this topic are the ectoderm and mesenchyme layers that give rise to the epidermis and dermis, respectively. Between the fouth to seventh gestational weeks, the ectodermal layer develops from the previous single layer of cells and divides into the periderm (epitrichium) and a basal layer. The epitrichium covers the surface of the embryo and undergoes a continuous cycle of keratinization and desquamation for 21 weeks.
The basal layer (stratum germinativum) acts as the germinal layer that continuously replicates to replace superficial cells lost during desquamation. It should be noted that the exfoliated cells, along with sebaceous secretions and shed lanugo hair, goes on to form the vernix caseosa (a white, cheesy coating found on neonates). Subsequent differentiation of the stratum germinativum results in the formation of an intermediate layer around week 11. The intermediate layer further differentiates into two distinct layers called the stratum spinosum and the stratum granulosum. The former contains polyhedral cells that are joined by tonofibrils (associated with the macula adherens that anchors cellular cytoskeleton), while the latter has the characteristic keratohyalin granules (initial proteinaceous compound responsible for keratinization of cells).
During the 3rd month, neural crest cells migrate into the epidermis and differentiate into melanoblasts that mature to form melanocytes (cells that produce melanin). Melanocytes in individuals of all races produce melanin. The difference is that in darker skinned individuals, the melanin is stored in larger melanosomes (melanin granules) than those vacuoles in lighter skinned individuals. By the end of the fourth month, there are four distinct epidermal layers that are present in the adult skin. These are (from superficial to deep) the; stratum corneum, stratum granulosum, stratum spinosum and the stratum germinativum or s. basale.
The dermis has tripartite embryology inclusive of the lateral and paraxial mesoderm as well as neural crest cells. Close to the end of the third trimester, the dermis (corium) differentiates into dermal papillae (papillary layer), which contain sensory nerve endings and capillary loops and extend into the epidermal layer. The subcorium (reticular layer) is the fatty region of the dermis that also contains large quantities of elastin and collagen.
Hair follicles develop between 2-4 months (8-16 weeks) first in the cranial region. They start by the formation of arrector pili muscle Hair follicle bulge germinal matrix and then the Inner and outer root sheaths. Babies are born with first generation of fine un-pigmented hairs called, lanugo. Further complex developmental changes result into the mature hair follicle.
There are also, other types of glands formed by the downward growth of the epidermis. The main ones include; the sebaceous glands, the sweat glands, the apocrine glands and the mammary glands. These are widely spread over the body and of these, only the sweat and sebaceous glands occur in the head and neck. The mammary glands later become located in the chest. The other structure usually regarded as part of the skin in their development is the teeth.