By: Dr. Mujahid Khan

 The skin is a membranous protective covering of the body  Is a complex organ system  It consists of two layers derived from two different germ layers  Ectoderm and mesoderm

 The epidermis is a superficial epithelial tissue  It is derived from surface ectoderm  The dermis is a deeper layer composed of dense irregularly arranged connective tissue  It is derived from mesoderm  The meshwork of embryonic connective tissue derived from mesoderm  It forms the connective tissues in the dermis

 Skin structures vary from one part of the body to another  The skin of eyelids is thin and soft and has fine hairs  The skin of eyebrows is thick and has coarse hairs  The embryonic skin at 4-5 weeks consists of a single layer of surface ectoderm overlying the mesenchyme

 During the first and second trimester, epidermal growth occurs in stages and result in an increase in epidermal thickness  Primordial of the epidermis is the layer of surface ectodermal cells  These cells proliferate and form a layer of squamous epithelium called periderm and a basal germinative layer

 The cells of periderm continually undergo keratinization and desquamation and are replaced by cells arising from basal layer  The exfoliated peridermal cells form part of the white greasy substance called vernix caseosa which covers the fetal skin  Vernix caseosa protects the developing skin from constant exposure to amniotic fluid with its urine content during the fetal period  Vernix also facilitates birth of the fetus because of its slippery nature

 The basal layer of the epidermis becomes the stratum germinativum  It produces new cells that are displaced into the layer superficial to it  By 11 weeks, the cells from stratum germinativum have formed an intermediate layer  Replacement of peridermal cells continues until about 21 weeks

 Thereafter the periderm disappears and the stratum corneum forms  Proliferation of cells in the stratum germinativum also forms epidermal ridges which extend into the developing dermis  Epidermal ridges produce grooves on the surface of the palms and soles including digits  The type of pattern is determined genetically and constitute the basis for examining fingerprints

 Abnormal chromosome complements affect the development of ridge patterns  Infants with Down syndrome have distinctive patterns on their hands and feet that are of diagnostic value

 Late in the embryonic period the neural crest cells migrate into the mesenchyme of the developing dermis and differentiate into melanoblasts  Later these cells migrate to the dermoepidermal junction and differentiate into melanocytes  The differentiation of melanoblasts into melanocytes involves the formation of pigment granules

 Melanocytes appear in the developing skin at 40 – 50 days, immediately after the migration of neural crest cells  In white races, the cell bodies of melanocytes are usually confined to basal layers of the epidermis  Melanocytes begin producing melanin before birth and distribute it to the epidermal cells

 Pigment formation can be observed prenatally in the epidermis of dark-skinned races  There is a little evidence of such activity in light- skinned fetuses  Increased amount of melanin are produced in response to ultraviolet light  The relative content of melanin in the melanocytes accounts for the different colors of skin

 It covers the palms and soles and lacks  Hair follicles  Arrector muscles  Sebaceous glands  But it has sweat glands

 It covers most of the rest of the body and contains  Hair follicles  Arrector muscles of hairs  Sebaceous glands  Sweat glands

 The dermis develops from mesenchyme, derived from the mesoderm underlying the surface ectoderm  Most of the mesenchyme that differentiates into the connective tissue of the dermis originates from the somatic layer of lateral mesoderm  Some of it is derived from the dermatomes of the somites

 By 11 weeks the mesenchymal cells have begun to produce collagenous and elastic connective tissue fibers  As the epidermal ridges form, the dermis projects into the epidermis forming dermal ridges that interdigitate with the epidermal ridges  Capillary loops develop in some of these ridges and provide nourishment for the epidermis

 Sensory nerve endings form in other ridges  The developing afferent nerve fibers play an important role in the spatial and temporal sequence of dermal ridge formation  Blood vessels in the dermis begin as simple, endothelium-lined structures that differentiate from mesenchyme  As the skin grow, new capillaries grow out from the primordial vessels

 Such capillary-like vessels have been observed in the dermis by the end of 5 th week  Some capillaries acquire muscular coats through differentiation of myoblasts developing in the surrounding mesenchyme and become arterioles and arteries  Others become venules and veins  By the end of 1 st trimester, the major vascular organization of the fetal dermis is established

 Most sebaceous glands develop as buds from the sides of developing epithelial root sheaths of hair follicles  The glandular buds grow into the surrounding embryonic connective tissue and branch to form the primordia of several alveoli and their associated ducts  The central cells of alveoli break down forming an oily secretion called sebum

 Sebum is released into the hair follicle and passes to the surface of the skin  In skin it mixes with desquamated peridermal cells to form vernix caseosa  Sebaceous glands independent of hair follicles develop in a similar manner to buds from the epidermis

 Eccrine sweat glands are located in the skin throughout most of the body  They develop as epidermal down growths into the underlying mesenchyme  As bud elongates, its end coils to form the primordium of the secretory part of the gland  The epithelial attachment of the developing gland to the epidermis forms the primordium of the duct

 The central cells of the primordial ducts degenerate, forming the lumen  Peripheral cells of the secretory part of the gland differentiate into myoepithelial and secretory cells  Myoepithelial cells are thought to be specialized smooth muscle cells that assist to expel sweat from the glands  Eccrine sweat glands begin to function shortly after birth

 The distribution of large apocrine sweat glands in humans is mostly confined to axilla, pubic and perineal regions and areola of the nipples  They develop from the stratum germinativum of the epidermis that give rise to hair follicles  The ducts of these glands do not open into the skin surface  They open into the upper part of hair follicles superficial to the opening of sebaceous glands  They secrete during puberty

 They are a modified and highly specialized type of sweat glands  Mammary buds begin to develop during the 6 th week as a solid downgrowths of the epidermis into the underlying mesenchyme  These changes occur due to the inductive influence from the mesenchyme  The mammary buds develop as downgrowths from thickened mammary crest or ridges

 Mammary crest are thickened strips of ectoderm extending from the axillary to the inguinal regions  The mammary crests appear during the 4 th week but normally persist in humans only in pectoral area  Each primary bud soon gives rise to several secondary mammary buds that develop into lactiferous ducts and their branches

 Canalization of these buds is induced by placental sex hormones entering the fetal circulation  By the term, 15 to 20 lactiferous ducts are formed  The fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchyme

 During the late fetal period the epidermis at the site of origin of the mammary gland becomes depressed forming a shallow pit  The nipples are poorly formed and depressed in newborn infants  Soon after birth the nipples usually rise from the mammary pits  The smooth muscle fibers of the nipple and areola differentiate from surrounding mesenchymal cells

 The rudimentary mammary glands of newborn males and females are identical  Some secretion, often called witch’s milk may be produced  This is caused by maternal hormones passing through the placental membrane into the fetal circulation  Only main lactiferous ducts are formed at birth and mammary glands remain underdeveloped until puberty

 In females the breasts enlarge rapidly during puberty due to development of mammary glands and accumulation of fat  Growth of the duct system also occurs because of the raised levels of circulating estrogens  Progestogens, prolactin, corticoids and growth hormone also play a role  If pregnancy occurs, the mammary glands complete their development owing to the raised estrogen and progesterone level

 The intralobular ducts undergo rapid development forming buds that become alveoli  The breasts become hemispherical in shape largely because of fat deposition  Full development occurs at about 20 years

 It refers to excessive development of the male mammary tissue  The rudimentary mammary glands in males normally undergo no postnatal development  It occurs in most newborn males because of stimulation of the glandular tissue by maternal sex hormones  The effect disappears in few weeks

 During midpuberty about two-thirds of boys develop varying degree of hyperplasia of breasts  The subareolar hyperplasia may persist for a few months to 2 years  A decreased ratio of testosterone to estradiol is found in boys with gynecomastia  About 80% of males with Klinefelter syndrome have gynecomastia

 An extra nipple occurs in about 1% of females  It usually develops just inferior to the normal breast  It is also relatively common in males  Often they are mistaken for moles  Less commonly they appear in the axillary or abdominal regions of females  In these positions the nipples develop from extra mammary buds that develop along the mammary crests

 It usually becomes obvious in women when pregnancy occurs  About one-third of affected persons have two extra nipples  Supernumerary mammary tissue very rarely occurs in a location other than along the course of the mammary crests  It probably develops from tissue that was displaced from these crests

 Sometimes nipples fail to elevate above the skin surface  They remain in their newborn location  Inverted nipples may make breast feeding of an infant difficult  A special exercise can be used to prepare the nipple for feeding an infant