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Published byElwin Atkinson Modified over 9 years ago
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By: Dr. Mujahid Khan
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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It covers the palms and soles and lacks Hair follicles Arrector muscles Sebaceous glands But it has sweat glands
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It covers most of the rest of the body and contains Hair follicles Arrector muscles of hairs Sebaceous glands Sweat glands
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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