The development of the dermis is induced by the epidermis. The connective tissue of the dermis develops from mesenchyme, which is derived from two sources: (1) somatic layer of the lateral plate mesoderm, and (2) dermatomes of the somites. However, most of the dermal connective tissue is
derived from the first source, ie, so- (trisomy 21), in which the skin creases
matic mesoderm. By the 11th week of of the palm and the epidermal ridges of
development, the mesenchymal cells the fingers have unusual pattern,
differentiate into fibroblasts, which pro- -. . . - o. . .
duce the collagen and elastic fibers of Development OT SKIH
the dermis. The outer part of the de- AppendaCjeS veloping dermis forms many irregular
papillary protrusions called dermal pa- Development Of Hair pillae. The papillae protrude into the The hairs begin to develop dur-overlying epidermis. The intervening jng the third month of development projections of the epidermis into the (week 9 to 12) but do not become eas-dermis are called epidermal ridges. \\y recognizable until about the 20th The superficial region of the dermis week. A hair makes its appearance as bearing the dermal papillae is called a hair bud, which is a cylindrical the papillary layer, whereas the thick downgrowth of the stratum germi-underlying layer of dense, irregular nativum of epidermis into the underiy-connective tissue is known as the re- jng mesenchyme (ie, the developing Ocular layer. The dermis differentiates dermis). The terminal part of the hair into its definitive form in the later fetal bud expands to form the hair bulb. period. However, it is still thin at birth The hair bulb is the primordium of a and thickens progressively during in- hair follicle. The hair bulb becomes infancy and childhood. vaginated from below by a conden-Formatipn of dermal papillae sation of the mesenchyme, which is and epidermal ridges results in pro- called hair papilla. Soon the central duction of grooves and ridges on the cells of the hair bud become spindle-external surface of skin. The pattern of shaped and keratinized, and fuse to-these ridges and grooves varies in gether to form the hair shaft. The peri-different regions of the body. The skin pheral cells of the hair bud become cu-of the palms, soles and digits carries a boidal to form the epithelial root very distinctive pattern of whorls and sheath. The mesenchyme surrounding loops. The patterns of the epidermal the hair bud condenses to form the • ridges are genetically determined and connective tissue root sheath. that is why the fingerprints of one per- Growth of the hair results from son do not match with those of any continued multiplication of those cells other person. The study of the patterns Of the hair bud that are located around of the skin ridges is called dermato- the hair papilla (these cells are collec-glyphics. The knowledge of dermato- tively called germinal matrix). glyphics is used in the diagnosis of cer- Melanocytes appear in the hair tain diseases, which are due to chro- roots during the 4th month of intra-mosal disorders, eg, Down syndrome uterine life. The melanin produced by
these cells is transferred to the cells of the germinal matrix. The relative content of the melanin pigment accounts for different hair colours.
Though the hairs begin to develop during early fetal life, the first generation of fetal hairs appears during the 12th week of development. These hairs are very fine, soft and lightly pig-mented and are called lanugo. The lanugo form a dense covering over the fetal body especially on the back and limbs. The lanugo hairs are short-lived and all are shed off before or just after birth. The lanugo are replaced by coarser hair called velus, which mostly develop from new hair follicles. In the postnatal life the velus hair persist over most of the body. However, at puberty the velus hair of the axillary and pubic regions become replaced by • coarser and darker hair, called terminal hair. In the adult males the terminal hair also appear on the chest and face (beard area).
The old hairs are shed and new ones are produced periodically throughout life. At the end of any growth cycle, the hair is carried upward by its shortening, regressive follicle. Ultimately, the hair is shed off from the skin. After sometime, the follicle reorganizes and begins to produce a new hair, which soon appears on the surface of the skin.