McMurrich1914 Chapter 6

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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.

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McMurrich 1914: General 1 Spermatozoon - Spermatogenesis - Ovum - Fertilization | 2 Ovum Segmentation - Germ Layer Formation | 3 Medullary Groove - Notochord - Somites | 4 Embryo External Form | 5 Yolk-stalk - Belly-stalk - Fetal Membranes Organogeny 6 Integumentary System | 7 Connective Tissues - Skeleton | 8 Muscular System | 9 Circulatory - Lymphatic Systems | 10 Digestive Tract and Glands | 11 Pericardium - Pleuro-peritoneum - Diaphragm | 12 Respiration | 13 Urinogenital System | 14 Suprarenal System | 15 Nervous System | 16 Organs of Special Sense | 17 Post-natal | Figures

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Chapter VI. The Development of the Integumentary System

J. Playfair McMurrich
J. Playfair McMurrich (1859 – 1939)

The Development of the Skin

The skin is composed of two embryologically distinct portions, the outer epidermal layer being developed from the ectoderm, while the dermal layer is mesenchymatous in its origin.


The ectoderm covering the general surface of the body is, in the earliest stages of development, a single layer of cells, but at the end of the first month it is composed of two layers, an outer one, the epitrichium, consisting of slightly flattened cells, and a lower one whose cells are larger and which will give rise to the epidermis (Fig. 81, A). During the second month the differences between the two layers become more pronounced, the epitrichial cells assuming a characteristic domed form and becoming vesicular in structure (Fig. 81, B). These cells persist until about the sixth month of development, but after that they are cast off, and, becoming mixed with the secretion of sebaceous glands which have appeared by this time, form a constituent of the vernix caseosa.


In the meantime changes have been taking place in the epidermal layer which result in its becoming several layers thick (Fig. 81, B), the innermost layer being composed of cells rich in protoplasm, while those of the outer layers are irregular in shape and have clearer contents. As development proceeds the number of layers increases and the superficial ones, undergoing a horny degeneration, give rise to the stratum corneum, while the deeper ones become the stratum Malpighii. At about the fourth month ridges develop on the under surface of the epidermis, projecting downward into the dermis, and later secondary ridges appear in the intervals between the primary ones, while on the palms and soles ridges appear upon the outer surface of the epidermis, corresponding in position to the primary ridges of the under surface.


The mesenchyme which gives rise to the dermis grows in from all sides between the epidermis and the outer layer of the myotomes, which are at first in contact, and forms a continuous layer underlying the epidermis and showing no indications of a segmental arrangement. It becomes converted "principally into fibrous connective tissue, the outer layers of which are relatively compact, while the deeper ones are looser, forming the subcutaneous areolar tissue. Some of the mesenchymal cells, however, become converted into non-striated muscle-fibers, which for the most part are few in number and associated with the hair follicles, though in certain regions, such as the skin of the scrotum, they are very numerous and form a distinct layer known as the dartos. Some cells also arrange themselves in groups and undergo a fatty degeneration, well-defined masses of adipose tissue embedded in the lower layers of the dermis being thus formed at about the sixth month.



Fig. 81. A, Section of Skin from the Dorsum of Finger of an Embryo of 4.5 cm.; B, from the Plantar Surface of the Foot of an Embryo of 10.2 cm et, Epitrichium; ep, epidermis.


Although the dermal mesenchyme is unsegmental in character, yet the nerves which send branches to it are segmental, and it might be expected that indications of this condition would be retained by the cutaneous nerves even in the adult. A study of the cutaneous nerve-supply in the adult realizes to a very considerable extent this expectation, the areas supplied by the various nerves forming more or less distinct zones, and being therefore segmental (Fig. 82). But a considerable commingling of adjacent areas has also occurred. Thus, while the distribution of the cutaneous branches of the fourth thoracic nerve, as determined experimentally in the monkey (Macacus), is distinctly zonal or segmental, the nipple lying practically in the middle line of the zone, the upper half of its area is also supplied or overlapped by fibers of the third nerve and the lower half by fibers of the fifth (Fig. 83), so that any area of skin in the zone is innervated by fibers coming from at least two segmental nerves (Sherrington). And, furthermore, the distribution of each nerve crosses the mid-ventral line of the body, forming a more or less extensive crossed overlap.


And not only is there a confusion of adjacent areas but an area may shift its position relatively to the deeper structures supplied by the same nerve, so that the skin over a certain muscle is not necessarily supplied by fibers from the nerve which supplies the muscle. Thus, in the lower half of the abdomen, the skin at any point will be supplied by fibers from higher nerves than those supplying the underlying muscles (Sherrington), and the skin of the limbs may receive twigs from nerves which are not represented at all in the muscle-supply (second and third thoracic and third sacral).


Fig. 82. Diagram showing the cutaneous Distribution of the Spinal Nerves - (Head.)

The Development of the Nails

The earliest indications of the development of the nails have been described by Zander in embryos of about nine weeks as slight thickenings of the epidermis of the tips of the digits, these thickenings being separated from the neighboring tissue by a faint groove. Later the nail areas migrate to the dorsal surfaces of the terminal phalanges (Fig. 84) and the grooves surrounding the areas deepen, especially at their proximal edges, where they form the nail-folds (nf) , while distally thickenings of the epidermis occur to form what have been termed sole-plates (sp), structures quite rudimentary in man, but largely developed in the lower animals, in which they form a considerable portion of the claws.


Fig. 83. Diagram showing the Overlap of the III, IV, and V Intercostal Nerves of a Monkey. - (Sherrington.)


Fig. 84. Longitudinal Section through the Terminal Joint of the Index finger of an embryo of 4.5 cm. e, Epidermis; ep, epitrichium; nf, nail fold; Ph, terminal phalanx; sp, sole plate.


The actual nail substance does not form, however, until the embryo has reached a length of about 17 cm. By this time the epidermis has become several layers thick and its outer layers, over the nail areas as well as elsewhere, have become transformed into the stratum corneum (Fig. 85, sc), and it is in the deep layers of this (the stratum lucidum) that keratin granules develop in cells which degenerate to give rise to the nail substance (n). At its first formation, accordingly, the nail is covered by the outer layers of the stratum corneum as well as by the epitrichium, the two together forming what has been termed the eponychium (Fig. 85, ep). The epitrichium soon disappears, however, leaving only the outer layers of the stratum corneum as a covering, and this also later disappears with the exception of a narrow band surrounding the base of the nail which persists as the perionyx.


The formation of the nail begins in the more proximal portion of the nail area and its further growth is by the addition of new keratinized cells to its proximal edge and lower surface, these cells being formed only in the proximal part of the nail bed in a region marked by its whitish color and termed the lunula.


The first appearance of the nail-areas at the tips of the digits as described by Zander has not yet been confirmed by later observers, but the migration of the areas to the dorsal surface necessitated by such a location of the primary differentiation affords an explanation of the otherwise anomalous cutaneous nerve-supply of the nail-areas in the adult, this being from the palmar (plantar) nerves.


Fig. 85. Longitudinal section through the nail area in an Embryo of 17 cm. ep, Eponychium; n, nail substance; nb, nail bed; sc, stratum corneum; sp, sole plate. - (Okamura.)

The Development of the Hairs

The hairs begin to develop at about the third month and continue to be formed during the remaining portions of fetal life. They arise as solid cylindrical downgrowths, projecting obliquely into the subjacent dermis from the lower surface of the epidermis. As these downgrowths continue to elongate, they assume a somewhat club-shaped form (Fig. 86, A), and later the extremity of each club moulds itself over the summit of a small papilla which develops from the dermis (Fig. 86, B). Even before the dermal papilla has made its appearance, however, a differentiation of the cells of the downgrowth becomes evident, the central cells becoming at first spindle-shaped and then undergoing a keratinization to form the hair shaft, while the more peripheral ones assume a cuboidal form and constitute the lining of the hair follicle. The further growth of the hair takes place by the addition to its basal portion of new keratinized cells, probably produced by the multiplication of the epidermal cells which envelop the papilla.



Fig. 86. - The Development of a Hair. c, Cylindrical cells of stratum mucosum; hf, wall of hair follicle; m, mesoderm; mu, stratum mucosum of epidermis; p, hair papilla; r, root of hair; s, sebaceous gland. - (Kollmann.)


From the cells which form the lining of each follicle an outgrowth takes place into the surrounding dermis to form a sebaceous gland, which is at first solid and club-shaped, though later it becomes lobed. The central cells of the outgrowth separate from the peripheral and from one another, and, their protoplasm undergoing a fatty degeneration, they finally pass out into the space between the follicle walls and the hair and so reach the surface, the peripheral cells later giving rise by division to new generations of central cells. During fetal life the fatty material thus poured out upon the surface of the body becomes mingled with the cast-off epitrichial cells and constitutes the white oleaginous substance, the vernix caseosa, which covers the surface of the new-born child. The muscles, arrectores pilorum, connected with the hair follicles arise from the mesenchyme cells of the surrounding dermis.


The first growth of hairs forms a dense covering over the entire surface of the fetus, the hairs which compose it being exceedingly fine and silky and constituting what is termed the lanugo. This growth is cast off soon after birth, except over the face, where it is hardly noticeable on account of its extreme fineness and lack of coloration. The coarser hairs which replace it in certain regions of the body probably arise from new follicles, since the formation of follicles takes place throughout the later periods of fetal life and possibly after birth. But even these later formed hairs do not individually persist for any great length of time, but are continually being shed, new or secondary hairs normally developing in their places. The shedding of a hair is preceded by a cessation of the proliferation of the cells covering the dermal papilla and by a shrinkage of the papilla, whereby it becomes detached from the hair, and the replacing hair arises from a papilla which is probably budded off from the older one before its degeneration and carries with it a cap of epidermal cells.


It is uncertain whether the cases of excessive development of hair over the face and upper part of the body which occasionally occur are due to an excessive development of the later hair follicles (hypertrichosis) or to a persistence and continued growth of the lanugo.


The Development of the Sudoriparous Glands

The sudoriparous glands arise during the fifth month as solid cylindrical outgrowths from the primary ridges of the epidermis (Fig. 87), and at first project vertically downward into the subjacent dermis. Later, however, the lower end of each downgrowth is thrown into coils, and at the same time a lumen appears in the center. Since, however, the cylinders are formed from the deeper layers of the epidermis, their lumina do not at first open upon the surface, but gradually approach it as the cells of the deeper layers of the epidermis replace those which are continually being cast off from the surface of the stratum corneum. The final opening to the surface occurs during the seventh month of development.


The Development of the Mammary Glands

In the majority of the lower mammals a number of mammary glands occur, arranged in two longitudinal rows, and it has been observed that in the pig the first indication of their development is seen in a thickening of the epidermis along a line situated at the junction of the abdominal walls with the membrana reuniens (Schulze). This thickening subsequently becomes a pronounced ridge, the milk ridge, from which, at certain points, the mammary glands develop, the ridge disappearing in the intervals. In a human embryo 4 mm. in length an epidermal thickening has been observed which extended from just below the axilla to the inguinal region (Fig. 88) and was apparently equivalent to the milk line of the pig, and in embryos of 14 or 15 mm. the upper end of the line had become a pronounced ridge, while more posteriorly the thickening had disappeared.


Fig. 87. - Lower Surface of a Detached Portion of Epidermis from the Dorsum of the Hand. h, Hair follicle; s, sudoriparous gland. - (Blaschko.)


The further history of the ridge has not, however, been yet traced in human embryos, and the next stage of the development of the glands which has been observed is one in which they are represented by a circular thickening of the epidermis which projects downward into the dermis (Fig. 89, A). Later the thickening becomes lobed (Fig. 89, B), and its superficial and central cells become cornified and are cast off, so that the gland area appears as a depression of the surface of the skin. During the fifth and sixth months the lobes elongate into solid cylindrical columns of cells (Fig. 90) resembling not a little the cylinders which become converted into sudoriparous glands, and each column becomes slightly enlarged at its lower end, from which outgrowths begin to develop to form the acini. A lumen first appears in the lower ends of the columns and is formed by the separation and breaking down of the central cells, the peripheral cells persisting as the lining of the acini and ducts.


The elevation of the gland area above the surface to form the nipple appears to occur at different periods in different embryos and frequently does not take place until after birth. In the region around the nipple sudoriparous and sebaceous glands develop, the latter also occurring within the nipple area and frequently opening into the extremities of the lacteal ducts. In the areola, as the area surrounding the nipple is termed, other glands known as Montgomery' 's glands, also appear, their development resembling that of the mammary gland so closely as to render it probable that they are really rudimentary mammary glands.



Fig. 88. - Milk Ridge (mr) in a Human Embryo. - (Kallius.)


Fig. 89. - Sections through the Epidermal Thickenings which Represent the Mammary Gland in Embryos (A) of 6 cm. and (B) or 10.2 cm.


The further development of the glands, consisting of an increase in the length of the ducts and the development from them of additional acini, continues slowly up to the time of puberty in both sexes, but at that period further growth ceases in the male, while in females it continues for a time and the subjacent dermal tissues, especially the adipose tissue, undergo a rapid development.


The occurrence of a milk ridge has not yet been observed in a sufficient number of embryos to determine whether it is a normal development or is associated with the formation of supernumerary glands (polymastia). This is by no means an infrequent anomaly; it has been observed in 19 per cent, of over 100,000 soldiers of the German army who were examined, and occurs in 47 per cent, of individuals in certain regions of Germany The extent to which the anomaly is developed varies from the occurrence of well-developed accessory glands to that of rudimentary accessory nipples (Jiy perihelia), these latter sometimes occurring in the areolar area of a normal gland and being possibly due in such cases to an hypertrophy of one or more of Montgomery's glands.


Fig. 90. - Section through the Mammary Gland of an Embryo of 25 cm. 1, Stroma of the gland. - (From Nagel, after Basch.)


Although the mammary glands are typically functional only in females in the period immediately succeeding pregnancy, cases are not unknown in which the glands have been well developed and functional in males (gynecomastia). Furthermore, a functional activity of the glands normally occurs immediately after birth, infants of both sexes yielding a few drops of a milky fluid, the so-called witch-milk (Hexenmilch) , when the glands are subjected to pressure.


Literature

J. T. Bowen: "The Epitrichial Layer of the Human Epidermis," Anat. Anzeiger, rv 1889.

Brouha: •' Recherches sur les diverses phases du developpement et de l'activite dela mammelle," Arch, de Biol., xxi, 1905.

G. Burckhard: "Ueber embryonale Hypermastie und Hyperthelie," Anat. Hefte viii, 1897.

H. Head: "On Disturbances of Sensation with Special Reference to the Pain of Visceral Disease," Brain, xvi, 1892; xvn, 1894; and xix, 1896.

E. Kallius: "Ein Fall von Milchleiste bei einem menschlichen Embryo," Anat. Hefte, viii, 1897.

T. Okamura: "Ueber die Entwicklung des Nagels beim Menschen," Archiv fur Dermatol, und Syphilol., xxv, 1900.

H. Schmidt: "Ueber normale Hyperthelie menschlicher Embryonen und uber die erste Anlage der menschlichen Milchdriisen iiberhaupt," Morphol. Arbeiten, xvil, 1897.

C. S. Sherrington: "Experiments in Examination of the Peripheral Distribution of the Fibres of the Posterior Roots of some Spinal Nerves," Philos. Trans. Royal Soc, clxxxiv, 1893, and cxc, 1898.

P. Stohr: " Entwickelungsgeschichte des menschlichen Wollhaares," Anat. Hefte, xxiii, 1903.

H. Strahl: "Die erste Entwicklung der Mammarorgane beim Menschen," Verhandle. Anat. Gesellsch., xii, 1898.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

McMurrich 1914: General 1 Spermatozoon - Spermatogenesis - Ovum - Fertilization | 2 Ovum Segmentation - Germ Layer Formation | 3 Medullary Groove - Notochord - Somites | 4 Embryo External Form | 5 Yolk-stalk - Belly-stalk - Fetal Membranes Organogeny 6 Integumentary System | 7 Connective Tissues - Skeleton | 8 Muscular System | 9 Circulatory - Lymphatic Systems | 10 Digestive Tract and Glands | 11 Pericardium - Pleuro-peritoneum - Diaphragm | 12 Respiration | 13 Urinogenital System | 14 Suprarenal System | 15 Nervous System | 16 Organs of Special Sense | 17 Post-natal | Figures


McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.


Cite this page: Hill, M.A. (2018, December 10) Embryology McMurrich1914 Chapter 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/McMurrich1914_Chapter_6

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