Book - Text-Book of Embryology 16
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Bailey FR. and Miller AM. Text-Book of Embryology (1921) New York: William Wood and Co.
- Contents: Germ cells | Maturation | Fertilization | Amphioxus | Frog | Chick | Mammalian | External body form | Connective tissues and skeletal | Vascular | Muscular | Alimentary tube and organs | Respiratory | Coelom, Diaphragm and Mesenteries | Urogenital | Integumentary | Nervous System | Special Sense | Foetal Membranes | Teratogenesis | Figures
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The Development of the Integumentary System
The integument consists of the skin and certain accessory structures. The skin is composed of the dermis (or corium) and the epidermis. The accessory structures comprise the hairs, nails, sudoriferous glands, sebaceous glands, and mammary glands. The epidermis (or epithelial layer) and all the accessory structures are derived from the ectoderm; the dermis is mesodermal in its origin. Other appendages of the skin such as scales, feathers, claws, hoofs, and horns which are found only in the lower animals, are ectodermal derivatives and belong in the same class as the accessory structures in man.
The embryonic ectoderm consists primarily of a single layer of cells (Fig. 72). During the latter part of the first month, the single layer gives rise to two layers, of which the outer is composed of irregular flat cells and is known as the epitrichium or periderm, the inner or basal, of larger cuboidal cells which are the progenitors of the epidermal cells and of the accessory structures. The epitrichial cells later become dome-shaped and acquire a vesicular structure, the nuclei becoming less distinct. They persist until the middle of foetal life and are then cast off and mingle with the secretion of the newly formed sebaceous glands as a constituent of the vernix caseosa (see p. 412) . The epidermal cells, constantly increasing in number, soon come to form several layers (4 to 6 in the sixth month). The innermost layer rests upon the basement membrane and is composed of cuboidal or columnar cells rich in cytoplasm ; the outer layers consist of irregular cells with clearer contents and less distinct nuclei.
As development proceeds, the basal layer gives rise to several layers which, together constitute the stratum germinativum. The cells of the innermost layers are constantly proliferating and thus forming new cells which are pushed toward the surface. During the seventh month keratohyalin granules appear in two or three layers which are then known collectively as the stratum granulosum. The clearer cells of the superficial layers undergo a process of degeneration by which their contents are transformed into a horny substance, the nuclei becoming fainter and finally disappearing. These modified or degenerated cells, which are constantly being cast off and replaced by others from the deeper layers, constitute the stratum corneum (Fig. 354). In the thick epidermis, on the palms of the hands and the soles of the feet, for example, a few layers of cells just outside of the stratum granulosum become specially modified (keratinized) to form the stratum lucidum.
In the first month the dermis is represented by closely arranged, spindle-shaped mesenchymal (mesodermal) cells underlying the epidermis, and is separated from the latter by a delicate basement membrane. This mesenchymal tissue gives rise to fibrous connective tissue which, about the third month, becomes differentiated into two layers the dermis proper and the deeper subcutaneous tissue. The papillae develop as little projections of the dermis which grow into the stratum germinativum of the epidermis. In some of these, many blood vessels appear, while in others nerve endings (tactile corpuscles of Meissner) develop, thus giving rise to vascular and nerve papillae. Usually a considerable amount of fat develops in the subcutaneous tissue. Some of the mesencnymal cells of the dermis are transformed into smooth muscle cells which are found in connection with the hairs (arrectores pilorum) , in the scrotum (tunica dartos) , and in the nipples.
Fig. 353. Longitudinal section through the end of the middle finger of a 5 months human foetus. Bonnet.
The dermis has generally been considered as a derivative of the cutis plates (p. 131) which, with the myotomes, constitute the outer walls of the primitive segments, but it is probable that the outer walls of the segments are transformed wholly into muscle tissue (McMurrich).
The pigment in the dermis develops in the form of granules in the connective tissue cells; that in the epidermis appears as granules in the cells of the deeper layers (white races) or of all the layers (dark races). Whether the pigment in the epidermis arises independently or is carried from the dermis by wandering cells is not known.
The nails are derivatives of the epidermal layer of the ectoderm, and correspond morphologically to the claws and hoofs of lower animals. The epidermis on the end of each finger and toe forms a thickening, known as the primitive nail, which is encircled by a faint groove (Zander). This occurs about the ninth week. Later the nail area migrates to the dorsal side of the digit and becomes somewhat sunken below the surface of the surrounding epithelium (Fig. 353). These observations have led to the conclusion that primarily the nails in man occupied positions on the ends of the digits, corresponding to the positions of the claws in lower forms. Furthermore, the fact that the nails (or their anlagen) are at first situated on the ends of the digits and subsequently migrate dorsally would exolain the innervation of the nail region by the palmar (and plantar) nerves.
Fig. 354. Vertical section of the skin of a mouse embryo of 18 mm. Showing early hair germs. Maurer.
After the dorsal migration of the nail area, the epithelium and dermis along, the proximal and lateral edges become still more elevated to form the nail wall, the furrow between the latter and the nail being the nail groove. At the distal edge of the nail area, the epithelium becomes thickened to form the so-called sole plate, which is probably homologous with the more highly developed sole plate in animals with hoofs or claws. The epithelium of the nail area increases in thickness, and, as in the skin, becomes differentiated into three layers (Fig. 353). The outer layers of cells become transformed into the stratum corneum. The cells of the next deeper layers, which acquire keratin granules and constitute the stratum lucidum, degenerate and give rise to the nail substance. Thus the nail is a modified portion of the stratum lucidum. The layers of epithelium beneath the nail form the stratum germinativum, which, with the subjacent dermis, is thrown into longitudinal ridges.
After its first formation, the nail is covered by the stratum corneum and the epitrichium, the two together forming the eponychium. The epitrichium soon disappears; later the stratum corneum also disappears with the exception of a narrow band along the base of the nail.
The formation of nail substance begins during the third or fourth month in the proximal part of the nail area. The nail grows from the root and from the under surface in the region marked by the whitish color (the lunuld). New keratinized cells are added from the subjacent stratum germinativum and become degenerated to form new nail substance which takes the place of the old as the latter grows distally.
The hairs, like the nails, are derivatives of the epidermal layer of the ectoderm. In embryos of about three months, local thickenings of the epidermis appear (beginning in the region of the forehead and eye-brows) and grow obliquely into the underlying dermis in the form of solid buds the hair germs (Fig. 355, I, II). As the buds continue to elongate they become club-shaped and the epithelium at the end of each molds itself over a little portion of the dermis in which the cells have become more numerous and which is known as the hair papilla (Fig. 354).
As the epidermal bud grows deeper, its central cells become spindle-shaped and undergo keratinization to form the beginning of the hair shaft; the peripheral layers constitute the anlage of the root sheath (Fig. 355, III, IV). The hair shaft grows from its basal end, new keratinized cells being added from the epithelium nearest the papilla as the older cells are pushed toward the surface of the skin. The surface cells of the hair shaft become flattened to form the cuticle of the hair (Fig. 355, V). The hairs appear above the surface about the fifth month. Of the cells of the root sheath, those nearest the hair become scale-like to form the cuticle of the root sheath; the next few layers become modified (keratinized) to form Huxley's and Henle's layers. Outside of these is the stratum germinativum, the basal layer of which is composed of columnar cells resting upon a distinct basement membrane. The stratum germinativum is continued over the tip of the papilla, where its cells give rise to new cells for the hair shaft (Fig. 355, V).
The connective tissue around the root sheath becomes differentiated into an inner highly vascular layer, the fibers of which run circularly, and an outer layer, the fibers of which extend along the sheath. The two layers together constitute the connective tissue follicle.
The first formed hairs, which are exceedingly fine and silky, develop in vast numbers over the surface of the embryonic body and are known collectively as the lanugo. This growth is lost (beginning before birth and continuing during the first and second years after), except over the face, and is replaced by coarser hairs. These in turn are constantly being shed during the life of the individual and replaced by new ones. The new hairs probably in most cases develop from the old follicles, the cells over the old papillae proliferating and the newly formed hairs growing up through the old sheaths. In some cases, however, new follicles are formed directly from the epidermis and dermis. In some of the lower Mammals, new hair germs appear as outgrows from the sheaths of old follicles, thus giving rise to tufts of hair. The arrectores pilorum muscles arise from the dermal (mesenchymal) cells and become attached to the follicles below the sebaceous glands.
Fig. 355. Five stages in the development of a human hair. Stohr.
- a, Papilla; b, arrector pili muscle; c, beginning of hair shaft; d, point where hair shaft grows through epidermis; e, anlage of sebaceous gland; l, hair germ; g, hair shaft; h, Henle's layer; i, Huxley's layer; k, cuticle of root sheath; /, inner root sheath; m, outer root sheath in tangential section; n, outer root sheath; o, connective tissue follicle.
The Glands of the Skin
The Sebaceous Glands
These structures usually develop in connection with hairs. From the root sheath a solid bud of cells grows out into the dermis (Fig. 355, IV) and becomes lobed. The central cells of the mass undergo fatty degeneration and the products of degeneration pass to the surface of the skin through the space between the hair and its root sheath. The more peripheral cells proliferate and give rise to new central cells which in turn are transformed into the specific secretion of the gland, the whole process being continuous. On the margins of the lips, on the labia minora'aridon the glans penis and prepuce, glands similar in character to the sebaceous glands arise directly from the epidermis independently of hairs.
The Sudoriferous Glands
The sweat glands : begin to develop during the fifth month as solid cylindrical growths from the deeper layers of the epidermis into the dermis (Fig. 353). Later the deeper ends of the cylinders become coiled and lumina appear. The lumina do not at first open upon the surface but gradually approach it as the deeper epidermal layers replace the more superficial.
The Vernix Caseosa
During foetal life the secretion of the sebaceous glands becomes mingled with the cast-off epitrichial and epidermal cells to form the whitish oleaginous substance (sometimes called the smegma embryonum) that covers the skin of the new-born child. It is collected especially in the axilla, groin and folds of the neck.
The Mammary Glands
In embryos of six to seven mm, or even less, a thickening of the epidermis occurs in a narrow zone along the ventro-lateral surface of the body (Strahl). In embryos of 15 mm this thickening, known as the milk ridge, extends from the upper extremity to the inguinal region (Kallius, Schmidt). Later the caudal end of the ridge disappears, while the cephalic portion becomes more prominent. The further history of the ridge has not been traced, but in embryos considerably older the anlage of each gland is a circular thickening of the epidermis in the thoracic region, projecting into the underlying dermis. It seems most probable that this local thickening represents a portion of the original ridge, the remainder having disappeared. Later the central cells of the epidermal mass become cornified and are cast off, leaving a depression in the skin (Fig. 356). In embryos of 250 mm a number of solid secondary buds have grown out (Fig. 357). These resemble the anlagen of the sweat glands, to which they are generally considered as closely allied (Hertwig, Wiedersheim and others), and represent the excretory ducts. Continued evaginations from the terminal parts of the excretory ducts form the lobular ducts and acini. The acini, however, are scarcely demonstrable in the male, and not even in the female until pregnancy. Lumina appear by a separation and breaking down of the central cells of the ducts and acini, the peripheral cells remaining as their lining.
Fig. 356. Vertical section through the anlage of the mammary gland of a human foetus of 16 cm. Bonnet.
Late in foetal life, or sometimes after birth, the original depressed gland area becomes elevated above the surface to form the nipple. The excretory ducts (15 to 20 in number) which at first opened into the depression, thus come to open on the surface of the nipple. In the area around the nipple the areola numerous sudoriferous and sebaceous glands develop, some of which come to open into the lacteal ducts. Sometimes rudimentary hairs appear. Other glands known as areolar glands (of Montgomery) resembling rudimentary mammary glands also develop from the epidermis of the areola.
After birth the mammary glands continue to grow slowly in both sexes up to the time of puberty. After this they cease to grow in the male, and then atrophy. In the female, growth of the glandular elements goes on, but very slowly, and usually a considerable amount of fat develops in the surrounding tissue, causing the enlargement of the breasts.
The Mammary Glands of Pregnancy
Even in the female, as stated before, acini are scarcely demonstrable until pregnancy. The mamma consists mostly of connective tissue and fat, with scattered groups of duct-like tubules. During pregnancy the tubules give rise to the acini by a process of evagination, the cells increasing in number by mitosis. Toward the end of pregnancy each excretory duct and its smaller ducts and acini form a distinct lobe with a relatively small amount of connective tissue. The epithelium is low or cuboidal, and fat begins to accumulate, in the seventh or eighth month, as droplets in the basal parts of the cells. The droplets increase in number and in size, approaching the inner end of the cell, until finally the cell is practically filled. At the beginning of lactation the fat escapes into the lumen of the acinus, leaving a bit of ragged cytoplasm with a nucleus. This regenerates into a cell capable of further activity; and it is probable that the same cell may become filled with fat and discharge its contents several times during lactation.
Fig. 357. Vertical section of the anlage of the mammary gland of a human foetus of 25 cm. Nagel.
During pregnancy and lactation the acini also contain leucocytes which have wandered through the epithelium from the surrounding tissue. These contain fat droplets and are known as colostrum corpuscles.
At the end of lactation the acini atrophy and disappear, the lobules becoming masses of connective tissue and fat, which contain groups of duct-like tubules and which are so closely joined with one another that they are indistinguishable as lobules.
Anomalies of the Skin
The epidermis may develop to an abnormal degree over the entire surface of the body, forming a horny layer which is broken only where the skin is folded by the movement of the members of the body a condition known as hyperkeratosis. Or the abnormal development may give rise to irregular patches of thick epithelium ichthyosis. In either case, hairs and sebaceous glands are usually absent over the affected areas.
Occasionally pigment develops in excess over larger or smaller areas of the skin, giving rise to the so-called ncevi pigmentosi. In some cases, on the other hand, there is total or almost total lack of pigment in the skin and hair (usually accompanied by defective pigmentation of the iris, chorioid and retina) a condition known as albinism. There are also instances of partial albinism. The influence of heredity in albinism is doubtful, for albinos are usually the children of ordinary parents.
The angiomata (lymphangiomata, haemangiomata) found in the skin are due to dilated lymphatic or blood channels, the color in haemangiomata being due to the haemoglobin in the blood.
Dermoid Cysts. The congenital dermoid cysts not infrequently found in or under the skin are usually situated in or near the line of fusion of embryonic structures, as in the region of the branchial arches, along the ventral body wall and on the back. During the fusion of adjacent structures, portions of the epidermis become constricted from the parent tissue and come to lie in the dermis, where they continue to grow and produce cystic masses and sometimes give rise to hairs and sebaceous glands'. This type of dermoid is to be distinguished from that found for example in the ovary, in which derivatives of all three germ layers are present (see Chap. XX).
Anomalies of the Epidermal Derivatives
Occasionally hair develops in profusion over areas of the skin that naturally possess only a fine, silky growth, such, for example, as a woman's face. Or nearly the entire body may be covered by an unusual amount of hair. Such conditions known as hypertrichosis possibly represent the persistence and continued growth of the lanugo (p. 410) and in this sense are to be regarded as the result of arrested development (Unna, Brandt). Congenital absence of the hair (hypotrichosis, alopecia) is a rare anomaly and is usually accompanied by defective development of the teeth and nails.
Sebaceous cysts, generally regarded as due to accumulation of secretion in the sebaceous glands, sometimes probably represent remnants of displaced pieces of epidermis apart from the hairs (Chiari).
Supernumerary mammary glands (hypermastid) and nipples (hyperthelia) are not infrequently present in both males and females. They are usually situated below the normal mammae (rarely in the axillary region), in a line drawn from the axilla to the groin, and probably represent persistent and abnormally developed portions of the milk ridge (see p. 412). In very rare cases a supernumerary gland develops in some other region (even on the thigh). If the mammary glands are morphologically allied to the sweat glands (p. 413), these misplaced mammae are suggestive of anomalous development of some of the sweat gland anlagen.
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References for Further Study
BROUHA: Recherches sur les di verses phases du developpement et de Pactivite de la mammelle. Arch, de Biol., T. XXI, 1905.
BONNET, R. : Die Mammarorgane im Lichte der Ontogenie und Phylogenie. Ergebnisse d. Anat. u. Entwick., Bd. II, 1892; Bd. VII, 1898.
KALLIUS, E. : Ein Fall von Milchleiste bei einem menschlichen Embryo. Anat. Hefte, Bd. VIII, 1897.
KEIBEL, F., and MALL, F. P.: Manual of Human Embryology, Vol. I, 1910.
KRAUSE, W.: Die Entwickelung der Haut und ihrer Nebenorgane. In Hertwig's Handbuch d. vergleich. u. experiment. Entwick elungslehre der Wirbeltiere, Bd. II, Teil I, 1902.
OKAMURA, T.: Ueber die Entwickelung des Nagels beim Menschen. Arch. f. Dermatol. u. Syphilol., Bd. XXV, 1900.
PIERSOL, G. A. : Teratology. In Wood's Reference Handbook of the Medical Sciences, Vol. VII, 1904.
SCHMIDT, H.: Ueber normale H.yperthelie menschlicher Embryonen und tiber die erste Anlage der menschlichen MilchdrUsen uberhaupt. Morphol. Arbeiten, Bd. XVII, 1897.
SCHULTZE, O.: Ueber die erste Anlage des MilchdrUsen Apparates. Anat. Anz. } Bd. VIII, 1892.
STOHR, P.: Entwiokelungsgeschichte des menschlichen Wollhaares. Anat. Hefte, Bd. XXIII, 1903.
STRAHL, H.: Die erste Entwickelung der Mammarorgane beim Menschen. Verhandl. d. Anat. Gesellsch., Bd. XII, 1898.
ZANDER, R.: Bie friihesten Stadien der Nagelentwickelung und ihre Beziehungen zu den Digitalnerven. Arch. f. Anat. u. PhysioL, Anat. Abth., 1884.
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Text-Book of Embryology: Germ cells | Maturation | Fertilization | Amphioxus | Frog | Chick | Mammalian | External body form | Connective tissues and skeletal | Vascular | Muscular | Alimentary tube and organs | Respiratory | Coelom, Diaphragm and Mesenteries | Urogenital | Integumentary | Nervous System | Special Sense | Foetal Membranes | Teratogenesis | Figures
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