Book - Text-Book of the Embryology of Man and Mammals 16-3
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Development of the Skin and its Accessory Organs
Having now become acquainted with the physiologically more important functions of the outer germ-layer, which consist in the production of the nervous system and the sensory organs, I give a short survey of the changes which take place in the remaining part, which is now designated as primitive epidermis (Hornblatt). This furnishes the whole outer skin of the body or epidermis and the numerous and various organs that are differentiated out of it, such as the nails, the hair, and the sweat-, sebaceous, and milk-glands.
The epidermis of Man is, according to the statements of KOLLIKER, very thin during the first two months of development, and consists of only two single layers of epithelial cells. Of these the superficial layer exhibits flattened, transparent, hexagonal elements ; the deeper one, on the contrary, consists of smaller cells ; so that already there is indicated by this a differentiation into a corneous and a mucous layer. Even now, too, a detachment of epidermal cells begins to manifest itself. For the outer cell-layer is soon found to be in process of decay, with obliterated cell -contours and indistinct nuclei, while a supplementary layer arises beneath it. In many Mammals the dying layer of cells is detached as a continuous sheet and then constitutes for a time a kind of envelope around the whole embryo, to which WELCKEII has given the name epitrichium, because the outgrowing hairs are developed beneath it.
From the middle of embryonic life onward both layers of the epidermis become thicker and the outermost of them contains cornified scales, the nuclei of which have degenerated. From this time onward a more extensive desquaination takes place at the surface, while the loss is made good by cell-divisions in the mucous layer and by the metamorphosis of these products of division into cornified cells. In consequence of this the surface of the embryo becomes up to the time of birth more and more covered with a yellowish-white, greasy mass the siiieynia eiiibryonum or vernix caseosa. This consists of a mixture of detached epidermal scales and of sebaceous secretions, which have been produced by the dermal glands that have arisen meantime. It forms a thick layer, especially on the flexor-side of the joints, on the sole of the foot, the palm of the hand, and on the head. Detached portions of it get into the amniotic fluid and make it turbid. Finally these, as well as some of the detached downy hairs, may be swallowed by the embryo with the amniotic fluid, and thus become a component of the meconium accumulated in the intestine.
The epidermis constitutes only one component of the skin of the adult or of the integument ; the other and more voluminous part the derma or corium is produced by the mesenchyme. The same thing takes place here as in the case of the other membranes and organs of the body. The epithelial layers derived from the primary yennlayers enter into close relationship with the mesenchyme, since they acquire from the latter a connective -tissue foundation that serves for their mechanical support and nutrition. Just as the inner germlayer unites with the intermediate layer to form the mucous membrane of the alimentary canal, as the epithelium of the auditory vesicle with the adjacent connective substance to form the membranous labyrinth, and as the epithelial optic vesicle with the choroid and sclera to form the eyeball, so here also the epidermis unites with the corium to constitute the integument.
During the lirst months the corium forms in Man a layer of closely packed, spindle-shaped cells, and is delimited from the epidermis by a delicate, structureless, smooth-surfaced, bounding membrane (basement membrane), such as exists permanently in the case of the lower Vertebrates. In the third month it is differentiated into the corium proper and the looser subcutaneous tissue, in which there are soon developed clusters of fat cells. From the middle of pregnancy onward the latter so increase in number that the subcutaneous tissue soon becomes a layer of fat covering the whole body. At this time the smooth contour between epidermis and corium is lost, owing to the development on the surface of the latter of small papilla?, which grow into the mucous layer and produce the corpus papilla/re of the skin. The papilla? serve partly for the reception of loops of capillary blood-vessels, and thus effect a better nutrition of the mucous layer ; in part they receive the terminations of tactile nerves (tactile corpuscles), and thus are divided into vascular papilla? and nervous papilla?.
The skin of Vertebrates attains a higher degree of development in consequence of processes similar to those described for the intestinal canal. The epidermis increases its surface outward by the formation of folds, inward by invayinations. Because the evaginated and invaginated parts at the same time alter in many ways their histological peculiarities, there arises a large number of organs of different kinds, which are developed in different ways in the separate classes of Vertebrates and which preeminently determine the external appearance of the animals.
As external processes arise the dermal teeth, and scales, the feathers, hair, and nails. As invaginations of the epidermis are developed the sweat-, sebaceous, and milk-glands. We will begin with the former, and. not to go too far into details, will limit ourselves to the organs of the skin in Mammals.
The most characteristic epidermoidal structures of Mammals and Man are the hairs. One can distinguish two modifications in the method of their development. The ordinary method of development is that which is known in Man. In this case, at the end of the third embryonic month, the mucous layer grows at certain places and forms small solid plugs, the hair-germs, which sink into the underlying corium (fig. 292 B M). By afterwards elongating and becoming thickened at the deep end they assume the shape of a flask. Then there ensues a process similar to that which takes place upon the formation of the teeth. At the bottom of the epithelial plug the adjacent corium grows and forms a richly cellular nodule (yrt), which grows into the epithelial tissue and is the fundament of the connective-tissue hair-papillae, which is early provided with loops of blood-vessels. Around the whole ingrowing germ of the hair the surrounding parts of the corium are afterwards more and more distinctly arranged into special courses of fibres some of which run lengthwise, others in a circular manner and constitute a special, vascular, nutritive envelope, the hair-follicle (fig. 292 G, D, A6).
A somewhat different method of hair-formation has been observed by REISSNER, GOETTE, and FEIERTAG in certain Mammals.
In these the first impulse to the formation of the fundament of a hair is produced by a limited cell-growth of the corium immediately below the epidermis. It produces a small elevation (fig. 292 ^1), \vhich is simply the hair-papilla itself, projecting into the epidermis. Then the papilla is forced farther and farther away from the surface of the skin by the growth of the epidermal cells that cover it, and at last is found far removed from its place of origin and at the deep, somewhat thickened end of a long epithelial plug.
The final result is therefore the same in both cases, only the time of the formation of the first fundament of the papilla and of the epithelial plug is different. In the latter case the papilla arises at the surface of the skin and is forced down by a plug-like epithelial growth ; in the former the epithelial plug first sinks into the underlying tissue and then at its deep end the hair-papilla is formed by a growth of the corium.
The question arises, Which of these two methods of development is to be considered the more primitive? In my opinion it is the formation of the hair^apUla at the surf ace of the skin. For this is unquestionably the simpler and less complete condition, from which the latter is derivable and through which it is explainable. The hairs sink into the underlying tissue for the purpose of better nourishment and attachment. A parallel is furnished by the development of the teeth. In the Selachians the latter arise (so far as they are developed as protective structures in the skin) from papillae which grow from the corium into the epidermis ; in Teleosts and Amphibia, on the contrary, the teeth, which are found distributed over extensive areas in the oral mucous membrane, are established deep down in that membrane, for epithelial growths in the form of plugs first .sink down into the connective tissue, and it is only subsequently that the dental papillre are formed by a process of growth in the connective tissue at the bottom of the epithelial downgrowth.
Let us return after this comparison to the further development of the hair ; this takes place in the same manner in both the cases distinguished above. Tbe epithelial cells which cover the papilla3 multiply and are differentiated into two parts (fig. 292 C) ; first, into cells that are more remote from the papillse, that become spindle-shaped and united into a small cone, and that by cornification produce the first point of the hair (ha), and secondly into cells which immediately invest the papilla, remain protoplasmic, and constitute the matrix the hair- bulb (hz) by means of which the further growth of the hair takes place. The cells of the hair-bulb, which rapidly increase by division, are added below to the first-formed part of the hair, and by cornification contribute to its elongation.
The hair in process of development on the papilla at first lies wholly concealed in the skin and is enveloped on all sides by cells of the epithelial plug, at the bottom of which the first trace of it was formed. From this investment are formed the outer and the inner sheaths of the root (fig. 292 C and D aw and iw). Of these the outer (aw) consists of small protoplasmic cells and is continuous externally with the mucous layer of the epidermis (schl), internally with the hair-bulb (/is). The cells in the inner slieath of the root (iw) assume a flattened form and become cornified.
In consequence of the growth which proceeds from the bulb the hairs are gradually shoved up toward the surface of the epidermis, and at the end of the fifth month in the case of Man begin to break forth to the outside (fig. 292 D ha'}. They protrude more and more above the surface of the skin, even in the embryo, and constitute at many places of the skin, especially on the head, a rather thick covering. On account of their minute size and fineness, and because they fall out soon after birth, they are called the downy hair or lanugo.
Fig. 292. Four diagrams of the development of the hair. A, Development of the hairpapilla on the free surface of the skin, as it occurs, according to GOKTTE, in many Mammals. B, C, 1), Three different stages of the development of the hair in human embryos. ho. Corneous layer of the epidermis ; scltl, mucous layer ; pa, hair-papilla ; hk, germ of hair ; hz, bulb of hair ; ha, yuung hair ; ha', tip of the hair protruding from the hair-follicle ; aw, iw, outer and inner sheath of the root of the hair ; lib, hair-follicle ; td, sebaceous gland.
Each hair is a transitory structure of short duration. Af ter a time it falls out and is replaced by a new one. This process begins even during embryonic life. The hairs that fall off get into the anmiotic fluid, and since with this fluid they are swallowed by the embryo, they form one of the components of the meconmm accumulated in the intestinal canal. A more extensive change takes place in Man soon after birth with the shedding of the downy hair, which is replaced on many parts of the body by a more vigorous growth of hair. In Mammals the shedding of the old and the formation of new hair exhibits a certain periodicity, which is dependent on the warmer and colder periods of the year. Thus they develop a summer and a winter coat. Even in Man the shedding of the hair is influenced, although less noticeably, by the time of year.
The falling off of the hair is initiated by changes in the part resting on the papilla and called the bulb. The cell-multiplication, by means of which the addition of new corneous substance takes place, ceases ; the falling hair becomes detached from its matrix and its deep end looks as though it were split into shreds ; but it is still retained in the hair-follicle by its closely investing sheath, until it is forcibly removed or is crowded out by the supplementary hair that takes its place.
The opinions of investigators still differ concerning the manner in which the supplementary hairs are developed. An especial subject of controversy is the point whether the young hair is formed from an entirely new papilla (STIEDA, FEIERTAG) or from the old one (LANGER, v. EBNER), or whether both methods occur (KOLLIKER, UNNA). It seems to me that the first view is the correct one, and that the shedding of the hairs is due to the atrophy of their papilke. During this slowly occurring process of degeneration, perhaps even before it begins, the substitution is initiated by the occurrence of an active cell-proliferation at a place in the outer sheath of the root which indeed consists of cells rich in protoplasm and by the formation of a new plug, which grows out deeper into the derma from the bottom of the fundament of the old hair. At the blind [deep] end of this secondary hair -form there is then developed from the derma a new papilla, upon which is formed the new hair and its sheaths alongside of and below the old one, in the manner previously described. When it begins to increase in length, it presses against the old hair lying above it, crowds the latter out of its sheaths, until it falls off, and finally itself takes the place of it.
According to this account there would be a certain similarity between the shedding of the hair and that of the teeth, inasmuch as in both cases secondary epithelial processes, from which the new tooth- or hair-papilla begins, arise from the primary fundament, and inasmuch as the new structures by their growth displace the old.
In addition to the development of hairs from old fundaments, a second method of formation, which one might designate as direct or primary, is maintained by many writers (GoETTE, KOLLIKER). It is assumed that even after birth, both in the case of Man and other Mammals, hair-germs are formed directly from the mucous membrane of the epidermis, in the same manner as in the embryo. In how far, at what regions, and up to what age such a direct formation of hair takes place, demands still more detailed and exhaustive investigation.
A second organ resulting from a cornification of the epidermis is the nail, which corresponds in a comparative-anatomical way to the claw- and hoof -like structures of other Mammals. In human embryos only seven weeks old there appear proliferations of the epidermis at the ends of the fingers, which are noticeably short and thick, and likewise at the ends of the toes, which are always less developed than the fingers. In consequence of the proliferations there arise from the loose epidermal cells complicated claw-like appendages, which have been described by HENSEN as predecessors of the nails or primitive nails.
In somewhat older embryos, from the ninth to the twelfth week, ZANDER found the epidermal growth marked off from its surroundings by a ring-like depression. The growth consists of a single layer of cylindrical cells with large nuclei lying on the side toward the derma and corresponding to the rete Malpighii, of two or three layers of polygonal spinous cells, and of a corneous layer.
The territory thus distinguished by a depression and by an altered condition of the cells ZANDER calls the primary basis of the nail (Nagelgrund), and describes it as occupying a greater part of the dorsal, but also a smaller part of the ventral surface of the terminal segment. He infers from this that the nails in Man originally had, like the claws of the lower Vertebrates, a terminal position on the toes and fingers, and that they have secondarily migrated on to the dorsal surface. Thus he explains the fact that the region of the nail is supplied with the ventral nerves of the fingers.
GEGENBAUR subscribes to ZANDER'S view of the terminal position of the fundament of the nail, but, supported by the investigations of BOAS, opposes ZANDER'S assumption of a migration of the fundament of the nail dorsally. He distinguishes in the development of nails and claws two parts (fig. 293), the dorsally located firm nail plate (np} and the plantar horn (Sohlenhorn, sh} connected with it ventrally. Of these the latter arises from the smaller ventral surface of the primary basis of the nail. In unguiculate and ungulate Vertebrates it (fig. 294 sh) is developed to a great extent ; in Man it atrophies, and is recognisable only in an exceedingly reduced condition as nail-iuelt. By this term is meant the welt-like thickening of the epidermis which forms the transition from the bed of the nail to the corrugated skin of the ball of the finger. The nail-plate, on the contrary, is from the beginning exclusively a product of the dorsal surface of the basis of the nail. There is therefore neither in Man nor in other Mammals a dorsal migration of the terminal fundament of the nail, but only a degeneration of its ventral portion, which otherwise furnishes a more complete plantar horn.
Fig. 293. A, Longitudinal section through the toe of a Cercopithecus. B, Longitudinal section through the second finger of Macacus ater. After GEGENBAUR. np, Nail-plate ; sh, plantar horn (Sohlenhorn) ; mo, nail-wall.
Fig. 294. Section through a Dog's toe. After GEGEXBAUR. np, Nail-plate ; sh, plantar horn ; b, ball of toe.
So far as regards the particular events in the development of the nail-plate, the structure is demonstrable in human embryos four months old as a thin flat layer of cornified, closely united cells on the dorsal surface of the primary basis of the nail or the bed of the nail. It is produced by the mucous layer upon which it immediately lies, but continues for a time to be covered by the thin corneous layer that is present at all points of the epidermis. This investment UNNA'S eponychium is not lost until the fifth month. However, notwithstanding their investment, the nails are easily recognisable some weeks before this from their whiteness, in distinction from the reddish or dark red color of the surrounding skin, owing to the addition of now cells from tbo mucous membrane, both from below and from the posterior margin, the nail-plate grows it becomes thickened and increased in surface extent. It is now pushed forward from behind over the bed of tho nail, and at the seventh month its free margin begins to project beyond the latter.
With this the nail has acquired essentially the appearance and condition which it has in the adult. In new-born infants it possesses a margin which projects far over the ball of the finger, and which because it was formed at an early embryonic period is both much thinner and also narrower than the part formed later, which rests on the bed of the nail. This margin is therefore detached soon after birth.
The Glands of the Skin
The glandular structures of the epidermis, which are established by imagination, are of three kinds : sebaceous, sweat-, and urilkglands. They all arise as proliferations of the mucous layer which grow down as solid plugs into the derma, and then undergo further development either according to the tubular or the alveolar type.
The sweat-glands and the ear-wax glands are developed on the tubular plan. They begin in the fifth month to penetrate from the mucous membrane into the cerium ; in the seventh month they acquire a small lumen, take a winding course in consequence of increased growth in length, and become coiled especially at their deep ends, thereby giving rise to the first fundament of the glomerulus.
Sebaceous glands and milk-glands are alveolar structures. The former are either developed directly from the epidermis, as, for example, at the edges of the lips, on the prepuce and on the glans penis, or they are in close connection with the hairs, which is the ordinary condition. In the latter case they are formed as solid thickenings of the outer sheath of the root of the hair near the orifice of the follicle, even before the hairs are completely developed (fig. 292 0, D, td) ; at first they have the form of a flask, then they send out a few lateral buds, which develop club-shaped enlargements at their ends. The glands acquire cavities by the fatty degeneration and disintegration of the interior cells, which are eliminated as a secretion.
The development of the milk-glands, which are more voluminous organs entrusted with an important function and peculiar to the class Mammalia, is of greater interest. Of the numerous works that have appeared concerning them, the comparative-anatomical investigations of GEGENBAUR especially have led to valuable results.
I present at the very beginning of the discussion the following proposition, which is of importance in interpreting the conditions found : each milk-gland in Man is not a simple organ, like an eargland or a submaxillary salivary gland, witli a, simple outlet, but a great glandular complex. Its earliest fundament has been observed in the human embryo at the end of the second month as a considerable thickening of the epidermis (fig. 295) upon the right and left sides of the breast. It has arisen as the result of a special proliferation of the mucous layer, which has sunk into the derma in the form of a hemispherical knob (df). But modifications arise afterwards in the corneous layer also, by its becoming thickened and projecting as a corneous plug into the proliferation of the mucous layer. Ordinarily there is found a small depression (g) at the middle of the whole epithelial fundament.
The proliferation of the epidermis that first appears is not precisely, as assumed by REIN, the first fundament of the glandular parenchyma ; it therefore does not correspond to the epithelial plugs which sink into the derma in the development of the sweat and sebaceous glands, because the further course of development and especially comparativeanatomical studies show, that by the thickening of the epidermis there is only an early delimitation of a tract of the skin, which is subsequently metamorphosed into the nipple-area and papilla, and from the floor of which the separate milk-producing glands at length sprout forth.
The correctness of this view is shown by the following changes : In older embryos the lens-shaped thickening produced by the proliferation of the epidermis has increased at the periphery and has thereby become flattened (fig. 296 df). At the same time it is more sharply defined at the surface, owing to the derma becoming thickened and elevated into a wall (dw) the cutis-wall. Therefore the whole fundament now has the form of a shallow depression (df) of the skin, for which the name gland alar area is very appropriate. For there early grow out from its mucous layer into the derma solid buds (dg), just as at other places the sebaceous glands arise from the epidermis. In the seventh month they are already well developed, and radiate out below and laterally from the pit-like depression. Their number increases up to the time of birth, and the larger ones become covered with solid lateral buds (dfy. Each sprout is the fundament of a milk-producing gland, which opens out on the glandular area (df) by means of a special orifice ; each is morphologically comparable with a sebaceous gland, although its function has become different.
Fig. 295. Section through the fundament of the milk-gland of a female human embryo 10 cm. long, after His. df, Fundament of the glandular area ; g, small depression at its surface.
The name glandular area is also a happily selected one because it presents a point of comparison with the primitive conditions of the Monotremes. For in these animals one does not find, as in the higher Mammals, a sharply differentiated single complex of milk-glands, but instead a somewhat depressed area of the skin, even provided with small hairs, over which are distributed single small glands, the secretion of which is licked up with the . tongue by the young, which are born in a very immature state.
Fig. 296. Section throuh the fundament of the milk-gland of a female human embryo 32 cm. long, after His., Glandular area ; dw, gland-wall ; dg, duct of gland ; db, vesicle of gland.
In the remaining Mammals the glands, in the former case opening separately upon the area, are united into a single organ, which better serves the young in sucking, namely a papilla [nipple] or teat, which encloses all the outlets of the glands and is grasped by the mouth of the suckling. In Man their development begins after birth. The glandular area, which is encircled by the cutis-wall and which before birth was depressed into a pit, now becomes flattened until it lies in the same niveau with the surrounding skin. It is distinguished from the latter by its redder color, which is due to its greater vascularity and the thinner condition of its epidermis. Then during the first years after birth the middle of the glandular area, together with the outlets (ductus lactiferi), which there open out close to one another, is raised up and becomes the nipple, in the derma of which nonstriate muscle-fibres are formed in great numbers ; the remaining part of the area as far as the cutis-wall becomes the areola mammae. The metamorphosis takes place somewhat earlier in the female than in the male.
Soon after birth alterations take place in the still feebly developed glandular tissue. There occurs a transitory swelling of the pectoral glands accompanied with increased blood-pressure, and it becomes possible to press out of the gland a small quantity of a milky fluid, the so-called witches' milk. According to KOLLIKER its formation is due to the originally solid ducts of the glands acquiring at this time a lumen by the fatty degeneration of the central cells, which are dissolved, and, suspended in a fluid, are discharged from the ducts. According to the investigations of BARFURTH, on the contrary, the so-called witches' milk of infants is the product of a genuine transitory secretion, and is like the real milk of the mother both in its morphological and chemical components.
After birth great differences arise between the two sexes in the condition of the milk-glands. Whereas in the male the glandular parenchyma remains stationary in its development, in the female it begins to increase, especially at the time of sexual maturity and still more after the beginning of pregnancy. From the first-formed ducts of the glands there grow out numerous lateral, hollow branches, which become covered with hollow vesicular glands (alveoli) lined with a single layer of cylindrical epithelium. At the same time there are developed in the connective tissue, between the separate lobules of the gland, numerous islands of fat-cells. In consequence the region at which the complex of milk-glands has been formed swells into a more or less prominent elevation, the mamma.
- The development of the hair is inaugurated in human embryos by the growing down of processes of the mucous layer of the epidermis - the hair "germs" - into the underlying derma.
- At the deep end of the hair-germ the vascular hair-papilla is begun by a growth of connective tissue.
- The epithelial hair-germ is differentiated into :
- A young hair, by the cornification of a part of the cells ;
- An actively growing cell-layer situated between the shaft of the hair and the papilla, the bulb, which furnishes the material for the growth of the hair ;
- The outer and the inner sheaths of the root.
- Around the epithelial part of the fundament of the hair there is formed from the surrounding connective tissue the hairfollicle.
- The nails in Man and the claws in other Mammals are developed from a dorsal fundament the nail-plate and a ventral fundament the plantar horn.
- The plantar horn in Man is reduced to the nail-welt.
- The thin nail-plate which is formed at first is for a time covered with a layer of cornified cells, the eponychium, which in Man is shed in the fifth month.
- The milk-gland is a complex of alveolar glands.
- At first there arises a thickening of the mucous layer of the epidermis, which is converted into the glandular area that is afterwards marked off from the surrounding parts by a wall and becomes somewhat depressed.
- From the bottom of the glandular area there grow forth in great numbers the fundaments of alveolar glands.
- After birth the glandular area, embracing the excretory ducts of the glands, is elevated above the surface of the skin, and converted into the nipple and the areola mammse.
- After birth there is a transitory secretion of a small quantity of milk- like fluid the witches' milk.
Development of the Nervous System
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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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