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==Chapter VI The Skin and its Accessory Structures== | |||
The Skin. — The two parts of the skin develop from different sources, the epidermis from the ectoderm, and the dermis from the mesoderm. At first the epidermis consists of a single layer of cubical cells, but these rapidly proliferate and by the fifth week have arranged themselves in two layers, an outer or epitrichium, and an inner or epidermis proper. The cells of the epitrichium are irregularly dome-shaped with flattened margins and vesicular nuclei. About the sixth month this layer is cast off and its degenerate cells mixed with the secretion of the sebaceous glands form the vernix caseosa, a substance which covers the fcetus until birth and is supposed to protect it from maceration while it floats in the amniotic fluid. The cells of the epidermis proliferate and the more superficial ones become flattened and cornified and give rise to the stratum corneum. The deepest layer of ectodermal cells forms the stratum germinativum. Superficial to this a layer of granular cells, the stratum granulosum, appears, and superficial to this again, there is a layer of clear cells, the stratum lucidum. These two strata vary greatly in their degree of development in different parts of the body. Differentiation of the mesodermal portion of the skin (the corium) begins in the third month of foetal life and up-growths of the compact corium into the epidermal layers form the characteristic dermal papillae. | |||
==The Nails== | |||
The nails begin to develop towards the end of the third foetal month in relation to shallow epidermal folds at the tips of the digits. At the fifth month, the deep surface of the proximal part of each nail fold (the nail bed) gives rise to a plate of compact keratinized cells, the nail plate, which is at first buried in the substance of the stratum corneum. This is pushed forward towards the tip of the digit by continual proliferation of the nail bed which extends distally to the convex border of the lunula. At first, the nail is covered by the outer layers of the stratum corneum but this disappears except at the base of the nail where it forms the eponychium. A similar epidermal thickening is found under the free border of the nail and is known as the hyponychium. During development the nails migrate from the tips of the digits to the dorsum carrying with them the associated palmar or plantar nerves. | |||
==The Hair== | |||
About the third month the first sign of hairs may be seen as thickenings of the stratum germinativum of the epidermis which proliferate and form cylindrical ectodermal down-growths into the corium. The extremities of these become club-shaped and the thickened lower end of each club moulds itself over the summit of a small papilla developed in the mesodermal corium. Proliferation of the central epidermal cells next the papilla results in a rod-like upgrowth, the cells of which become spindle-shaped and keratinized and passing to the surface of the body form the hair shaft. The outer epidermal cells become cuboidal and form the epithelial wall of the hair follicle with which a sebaceous gland is associated [vide iyifvci). The hairs first formed during the foetal period are called “ lanugo †hairs and are cast off at birth. The coarser hair of the adult arises from new follicles. | |||
==Sebaceous Glands== | |||
Solid outgrowth of cells from the epidermis grow from the lining cells of the hair follicles during the fifth foetal month ; they penetrate into the mesoderm of the corium, and their free ends soon become lobed. The central cells undergo fatty degeneration and pass into the hair follicles as sebaceous secretion. The peripheral cells form the lining of the gland, and by multiplication give rise to new generations of central cells, and these in turn degenerate to form the sebum (“ holocrine †secretion). | |||
==Sudoriferous Glands== | |||
The sudoriferous or sweat glands appear about the fifth month as solid columnar ingrowths from the deep layer of the epidermis into the mesoderm. The great majority are separate from the hair follicles. In the sixth and seventh months the ingrowths become convoluted in their deeper parts and the central cells disappear to give a lumen. This does not open on the surface until the surface epidermal cells of the skin are shed. The outer cells of the epidermal ingrowths become transformed into smooth muscle cells which are therefore ectodermal in origin. | |||
==Mammary Glands== | |||
The mammary glands are usually considered to be modified sweat glands. The primordia appear as two thickenings of the ectoderm, one on each side of the trunk during the sixth week of development ; they are known as the milk streaks. Each streak extends from the axillary region above, to the inguinal region below, but very soon in the human the lower two-thirds disappear. Somewhat later the upper portion of the remainder regresses and a small portion in the pectoral region alone persists. This little thickened plaque of ectoderm sinks below the level of the surrounding ectoderm and about the fifth month some fifteen or twenty outgrowths grow from it into the mesoderm. Each is the primordium of a lobe of the breast and becomes canalized to form a lactiferous duct. The blind terminal parts of these ducts branch and the terminal parts become dilated as acini. The epidermal plaque where the lactiferous ducts open to the surface becomes everted to form the nipple shortly before birth. A certain amount of secretion ("witch's milk") composed of fatty degenerating epithelial cells may sometimes be expressed from the nipple for a few days after birth. The mammary glands normally remain in the infantile condition in the male. In the female, the area of the areola around the nipple becomes elevated at puberty and the breast tissue is much increased by repeated branchings of the lactiferous ducts. Deposition of fat between the epithelial structures causes the breast to assume the rounded contour of the adult. | |||
===Anomalies of Development of the Breast=== | |||
# Supernumary glands (hypermastia) or nipples (hyperthelia) are not uncommon in both sexes. They may occur in any part of the line of the original milk streak. In rare instances a supernumerary breast may be functional. | |||
# Gynsecomastia is another developmental anomaly in which a male develops a mammary gland resembling that of the virgin female. | |||
# Absence of the mammary gland (amastia) is a rare condition. | |||
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter VI The Skin and its Accessory Structures
The Skin. — The two parts of the skin develop from different sources, the epidermis from the ectoderm, and the dermis from the mesoderm. At first the epidermis consists of a single layer of cubical cells, but these rapidly proliferate and by the fifth week have arranged themselves in two layers, an outer or epitrichium, and an inner or epidermis proper. The cells of the epitrichium are irregularly dome-shaped with flattened margins and vesicular nuclei. About the sixth month this layer is cast off and its degenerate cells mixed with the secretion of the sebaceous glands form the vernix caseosa, a substance which covers the fcetus until birth and is supposed to protect it from maceration while it floats in the amniotic fluid. The cells of the epidermis proliferate and the more superficial ones become flattened and cornified and give rise to the stratum corneum. The deepest layer of ectodermal cells forms the stratum germinativum. Superficial to this a layer of granular cells, the stratum granulosum, appears, and superficial to this again, there is a layer of clear cells, the stratum lucidum. These two strata vary greatly in their degree of development in different parts of the body. Differentiation of the mesodermal portion of the skin (the corium) begins in the third month of foetal life and up-growths of the compact corium into the epidermal layers form the characteristic dermal papillae.
The Nails
The nails begin to develop towards the end of the third foetal month in relation to shallow epidermal folds at the tips of the digits. At the fifth month, the deep surface of the proximal part of each nail fold (the nail bed) gives rise to a plate of compact keratinized cells, the nail plate, which is at first buried in the substance of the stratum corneum. This is pushed forward towards the tip of the digit by continual proliferation of the nail bed which extends distally to the convex border of the lunula. At first, the nail is covered by the outer layers of the stratum corneum but this disappears except at the base of the nail where it forms the eponychium. A similar epidermal thickening is found under the free border of the nail and is known as the hyponychium. During development the nails migrate from the tips of the digits to the dorsum carrying with them the associated palmar or plantar nerves.
The Hair
About the third month the first sign of hairs may be seen as thickenings of the stratum germinativum of the epidermis which proliferate and form cylindrical ectodermal down-growths into the corium. The extremities of these become club-shaped and the thickened lower end of each club moulds itself over the summit of a small papilla developed in the mesodermal corium. Proliferation of the central epidermal cells next the papilla results in a rod-like upgrowth, the cells of which become spindle-shaped and keratinized and passing to the surface of the body form the hair shaft. The outer epidermal cells become cuboidal and form the epithelial wall of the hair follicle with which a sebaceous gland is associated [vide iyifvci). The hairs first formed during the foetal period are called “ lanugo †hairs and are cast off at birth. The coarser hair of the adult arises from new follicles.
Sebaceous Glands
Solid outgrowth of cells from the epidermis grow from the lining cells of the hair follicles during the fifth foetal month ; they penetrate into the mesoderm of the corium, and their free ends soon become lobed. The central cells undergo fatty degeneration and pass into the hair follicles as sebaceous secretion. The peripheral cells form the lining of the gland, and by multiplication give rise to new generations of central cells, and these in turn degenerate to form the sebum (“ holocrine †secretion).
Sudoriferous Glands
The sudoriferous or sweat glands appear about the fifth month as solid columnar ingrowths from the deep layer of the epidermis into the mesoderm. The great majority are separate from the hair follicles. In the sixth and seventh months the ingrowths become convoluted in their deeper parts and the central cells disappear to give a lumen. This does not open on the surface until the surface epidermal cells of the skin are shed. The outer cells of the epidermal ingrowths become transformed into smooth muscle cells which are therefore ectodermal in origin.
Mammary Glands
The mammary glands are usually considered to be modified sweat glands. The primordia appear as two thickenings of the ectoderm, one on each side of the trunk during the sixth week of development ; they are known as the milk streaks. Each streak extends from the axillary region above, to the inguinal region below, but very soon in the human the lower two-thirds disappear. Somewhat later the upper portion of the remainder regresses and a small portion in the pectoral region alone persists. This little thickened plaque of ectoderm sinks below the level of the surrounding ectoderm and about the fifth month some fifteen or twenty outgrowths grow from it into the mesoderm. Each is the primordium of a lobe of the breast and becomes canalized to form a lactiferous duct. The blind terminal parts of these ducts branch and the terminal parts become dilated as acini. The epidermal plaque where the lactiferous ducts open to the surface becomes everted to form the nipple shortly before birth. A certain amount of secretion ("witch's milk") composed of fatty degenerating epithelial cells may sometimes be expressed from the nipple for a few days after birth. The mammary glands normally remain in the infantile condition in the male. In the female, the area of the areola around the nipple becomes elevated at puberty and the breast tissue is much increased by repeated branchings of the lactiferous ducts. Deposition of fat between the epithelial structures causes the breast to assume the rounded contour of the adult.
Anomalies of Development of the Breast
- Supernumary glands (hypermastia) or nipples (hyperthelia) are not uncommon in both sexes. They may occur in any part of the line of the original milk streak. In rare instances a supernumerary breast may be functional.
- Gynsecomastia is another developmental anomaly in which a male develops a mammary gland resembling that of the virgin female.
- Absence of the mammary gland (amastia) is a rare condition.
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Cite this page: Hill, M.A. (2024, April 18) Embryology Book - Aids to Embryology (1948) 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_6
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
