Book - A Text-book of Embryology 14

Chapter XIV. The Development of the Skin and its Appendages

Heisler JC. A text-book of embryology for students of medicine. 3rd Edn. (1907) W.B. Saunders Co. London.

Text-book of Embryology 1907: 1 Male and Female Sexual Elements - Fertilization | 2 Ovum Segmentation - Blastodermic Vesicle | 3 Germ-layers - Primitive Streak | 4 Embryo Differentiation - Neural Canal - Somites | 5 Body-wall - Intestinal Canal - Fetal Membranes | 6 Decidual Ovum Embedding - Placenta - Umbilical Cord | 7 External Body Form | 8 Connective Tissues - Lymphatic System | 9 Face and Mouth | 10 Vascular System | 11 Digestive System | 12 Respiratory System | 13 Genito-urinary System | 14 Skin and Appendages | 15 Nervous System | 16 Sense Organs | 17 Muscular System | 18 Skeleton and Limbs

Early Draft Version of a 1907 Historic Textbook. Currently no figures included and please note this includes many typographical errors generated by the automated text conversion procedure. This notice removed when editing process completed.

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The appendages of the skin include the sebaceous and sweat glands, the mammary glanda, the nails, and the hairs.

THE SKIN.

isiiitiiig of the epidermis or cuticle and of the lorm, or coritim, is derived from two sources, the epithelial epidermis being a product of the ectoderm, and the corinm originating from themesoderm. The nuils and hairs are outgrowths of the epithelial layer, wliile tlio various glands Are lierived from infoldings or in vaginations of the same stratum.

The corinm, the connectivetissue i'"m|Mment n( the skin, is an i>nt}:nhwtli of the cntis plates tif the primitive segments or somites (Fig. l;'.;!). If iirst appeju-s ill (Tilde P)rm in the s^'i'inid niontii lis a layer of spindle-cells liemtfitli the ectoderm. In till' third month, the more siipcrlicial imrt of this layer ac<iiiires more definite and distinctive eharaeter, the nilher loose aggn'gation of cells having diilerentiated into a tissue wich is a niesh-work of handles of white fibrous connective tissue with some intermingled elastic and muscular fibers ; this constitutes the corium proper. The deeper layer of cells becomes a loose, subcutaneous areolar tissue containing a few scattered fat-cells. About a month later the external surface of the primitive corium loses its smooth character and presents numerous little elevations, the villi, which project into the overlying epidermis. The villi, being highly vascular, play an important i)art in the nutrition of the epidermis and being also freely supplied with nerves they sustain an equally important relation to the sensitiveness of the skin.

From the middle of fetal life onward, the fat-cells in the subcutaneous tissue increase in number to such extent that there is formed a continuous and well-marked subcutaneous layer of fat, the panniculus adiposus.

Certain of the cells of the primitive corium differentiate into unstriated muscular tissue, forming thus the muscles of the liair-follicles, the airectores pilonun, as well as the subcutaneous muscular tissue of the dart(»s of the scrotum and penis, and that of the nipple and of the perineum.

The epidermis, consisting of the suj>erficial horny layer and the deeper mucous layer or stratum Malpighii, is entirely an epithelial structure. Its element* are simply the descendants of the early ectodermic cells specially modified to afford the necessary protection to the more sensitive and delicate corium.

The division into the two strata of the epidermis is indicated a? early as the latter part of the first month, at which time the cells of the ectoderm have become arranged into two single layers, a superficial layer of rather large flattened cells and an underlying stratum of smaller elements. The cells of the outer layer, or epitrichiiun, which probably represents the future stratum corneum, successively undergo degeneration and descpiamation, the places of those lost being supplied by the formation of new ones from the deei)er layer. As time goes on, both layers increase in thickness and the hairs and the glands of the skin are gradually formed. With increased proliferation there is increasingly active desquamation of superficial cells, and as the degenerate and cast-off cells become mixed with the products of the sebaceous glands, there is formed a sort of cheesy coating of the skin, the vemix caseosa or smegma embryonmn. This is first easily recognizable in the sixth month, and first covers the entire surface of the body in the eighth month. It serves to protect the epidermis of the fetus from maceration in the amniotic fluid.

The completion of the epidermis, aside from the development of its accessory parts, consists simply in further increase in thickness and in the modification of the superficial cells to produce the characteristic scale-like elements of the corneous layer of the skin, accompanied by the differentiation of the deeper cells into those of the rete mucosum or stratom Malpiglm. The extent to which these modifications are carried varies in different regions of the Ixnly.

The Nails

The nails have their beginning in little clawlike projections, the primitive nails, that appear upon the tips of the still imperfect fingers and toes in the seventh week.* These result from localized proliferation of the cells of the epidermis, Ix'ing entirely epithelial structures. The rudimentary nails project from the tips -of the digits, instead of o<!eupying the dorsal position of the completed structures. The claw-like primitive nail, between the ninth and twelfth "weeks, becomes surrounded l)v a groove, which starves to separate it from the ij:eneral eetodermic surface. Th(»se claw-like rudiments of the human nails are (juite similar to the primitive chiws of many mamY\r.. iw- Longitudinal sec- mals, the ])rinntive nail in ea(^li case

tion throuKh the toe of a <'«'r- •it i i i .1 •!

copitiu<ussoiftir<ieKrni.aur^: inchi(hnir a (lorsa! part, the nailn,>.naiH.iato:^A, plantar horn plate, and a portion which belonjrs

(Sohlenhorn) ; 7IM', nail-wall. * , . ,. .

t<) the ventral surra<*e ol the <ligit, called the plantar horn (Fig. l.'U). The striking difference between the nails of the human a<hilt and the claws and

• Or ninth week, Miiiot. ' A j^eniis of lon^MaikMl African monkeys.

hoofs of many animals is due in great measure to the degree of development to which this ventrally situated plantar horn attains. In the hoofed mammals (Ungulata) and the clawed mammals (Unguiculata), the plantar horn undergoes very great development, whereas in man it retrogrades and leaves no trace except the nail-welt, or the narrow line of thickened epidermis where the distal end of the nailbed merges into the ordinary skin. After the atrophy of the plantar horn, the dorsally situated nail-plate being alone present, the rudimentary nail bears a greater resemblance to the adult condition.

As the nail-plate gradually acquires more distinctive character, the deeper layers of the skin specialize into a structure adapted to its nutrition. This is the nail-bed, a highly vascular and sensitive cushion consisting of the corium and of the stratum Malpighii of the epidermis. It is especially from the proximal part of the nail-bed, representing the matrix of the fully-formed condition, that the nail grows. The rate of growth is such that- the ends of the nails protrude beyond the tips of the digits in the eighth month.

The tissue of the folly-formed nail corresponds to the stratum lucidum of the typical epidermis, developed to an unusual degree. The epitrichium or future stratum comeum, the most superficial layer of the epidermis, does not form a part of the nail, but constitutes a thin covering, the eponychium ; this is lost in the seventh month, with the exception of a small band over the root of the nail, which persists for a short time as the perionyx.

The nails of the toes are always somewhat behind those of the fingers in development.

To repeat, the claw-like rudimentarj- nails appear in the seventh week, the nails are perfectly formed about the twelfth week, and break through their epidermal covering in the seventh month, reaching to or beyond the finger-tips in the eighth month.

The Hair

Each hair consists of the projecting shaft and the embedded root, with its expanded deep extremity, the hair-bulb, the root being embraced by the hair-foUicle. The hair is entirely of ectodermic origlu, being ileriveii from the epidermal Isij-lt of fhi.' Hkin, while the liair-follicle is partly derived from the epidermis and in jjart is a product of the coriiim. The hairs are homologous with the feathers and scales of the lower animals.

The development of the hair is initiated in the third fetal month hy the appearance of small solid masses of epithelium in the stratum Mulpighii of the epidermis. The epithelial plugs or hair-germs grow into the underlying corium and are met by outgrowths or papiUn of the latter, which develop almost simultanoonsly. The papilla; are very vascular and serve for the nutrition of the developing hair.

The root and the shaft of the rudimentary hair result from the specialization of the axial or central cells of the hairgerm. These eells lengthen iu the direction of the long axis of the hair-germ and become hard and corneous, thus constituting the ro'it and the shaft, the cells of the deepest part of the hair-eenn forming the bulb. Tin- irrowtli of the hair in length is due to the proliferation and specialization of the cells of the bulb. The papilla of the niiderlying corinm indents the deep surface of the hair-bulb, this close relation of the two structures enabling the {>apilla the better to fulfil its function of providing nourishment t« the bulb.

b«Jr; te, liuUiof hBtr; Aa, young hair; of Ihelialt: W, balr-n>nide: M.wbBG<

Lir (Hertwlg): ^ and B, Ir-pnpms : U, ireriD of

The hair-follicle, consisting of an outer connective-tissue portion or fibrous layer and an inner epithelial part, the inner and outer root-sheaths, is partly of mesoderniic and partly of ectodermic origin. The inner and outer rootsheaths are produced by the peripheral cells of the hairgerra augmented by cells contributed directly by the stratum Malpighii of the epidermis. The outer fibrous constituent of the follicle results from the mesoderniic cells of the corium that immediately surround the hair-germ.

Gradually increasing in length by the addition of new cells from the hair-bulb, the primitive hair at length protrudes from the follicle as free hair. This first growth of hair is unpigmented and is extremely fine and soft, being known as the lanugo or embryonal down. This appears upon the scalp and some other parts of the body in the fourth month, gradually extending over the entire surface in the succeeding months. In the eighth month the lanugo begins to disappear, but is not lost as a whole until after birth, when the permanent growth of hair takes its i)lace. Upon the face, in fact, the lanugo j)ersists throughout life.

The development of the secondary hair is still a disputed point. It is claimed by some authorities (Stieda, Feiertag) that they develop from entirely new hair-germs. Most authorities hold, however, that the secondary hair develops from the same papilla that produced the hair just lost. According to this view, the empty root-sheath of the cast-oflP hair closes so as to form a cell-cord which represents a hair-germ for the new hair. The cells of this germ in most intimate relation with the underlying papilla produce the new hair in the same manner that hair is produced by the original hair-germs. As the new hair grows toward the surface the old one is gradually crowded out.

The Sebaceous and Sweat-Glands

The sweat-glands, including not only the sweat-glands proper but the ceruminous glands of the external auditory meatus and the glands of Moll of the eyelids, are derived from the ectodermic epithelium. The glands are of the simple tubular type. Each gland develops from a small accumulation of epidermal cells that grows, in the fifth month, from the Malpighian or mucous layer of the epidermis into the underlying corium. The solid epithelial plugs l)ecome tubes in the seventh month by the degeneration and final disappearance of the central cells. The deeper part of the tube becomes coiled and its lining epithelium takes on the (characteristics of secreting cells. Some of the cells of the original epithelial plug undergo specialization into muscular tissue, thus producing the inyolnntary muscles of the sweat-glands.

The sebaceous glands arc dcvelopeci from solid epithelial processes that originate from the deep layer or retc inucosum of the epidermis in a manner similar to that of the development of the sweat-glands. There is the difference, however, that the ej)ithelial plugs acquire lateral branches and thus usually produce glands of the compound saccular or acinous variety. There is the further difference that the epithelial outgrowths generally develop from the ectodcrmic cells of the outer sheath of the root of the hair near the orifice of the follicle (Fig. 136, id), in consecpience of which the ducts of the finished glands usually open into the hair-follicles. In some regions, however — regions devoid of hair, as the prepuce and the glans penis, the labia minora, and the lips — the growth is directly from the stratiini Malpighii, as in the case of the sweat-glands.

The Mammary Gland

The mammary pflnnd represents a number of highly specialized p:lan<ls of the skin, so associated as to constitute the sinjrie adult structure. Its origin, therefore, is to be sought in the cells of the epidermis in common with that of the ordinary glaiuls of the skin.

It is claimed by many authorities, by (ie^enbauer especially, that the mamnue are modified sebaceous glands ; others assert that they are to be elass<'d with the sweat-glands, Ilaidenhain having shown that in the development of the milk-glands there is no fatty metamor])hosis of the central cells as in the sebaceous glands, and Minot enijihasiziiig tin* fact that their mcwle of development closely resembles that of the sweat-glands.

The development of the milk-glands is begun as early as the second month. At this time the deep layer of the epidermis, in the bites of the future gknds, becomes thickened by the multiplication of its cells, the thickened patch encroaching upon the underlying coriuni (Fig. 136, A, b). Thia thickened area enlarges somewhat peripherally and its mai^ins become elevated, owing to wliich latter circumstance the piitch appears relatively depressed [B). The depression is known as the Klaadular area., and it corresponds with the fatnre areola and nipple. In many mammals the development of the milk-glands is initiated by the appearance of a

Fig. IM,— Secllunii representing (hree luceeislvc 8t»Be« of dBTclopmont of tha human DiiimmB (ToiirDeui|; .1, fetiaof SlWmm. (l.S la): fi. of 10.16 eni. (4lii,): C, of '^4.3% GtD. (9.A In.); u. epidermis; A. kggregnlloii of epldcmtKl veils funnlDg ■nUgo of gtand : e, gBlnctophoroiu dui^ts; d, groove limiting glamliitsr area; i, (treat pectoral muscle; /, unslrlalcd muBLnilar tisaue of arvolu; g, subCDtlDeoiu adipose tisaue.

pair of linear thickening;s of the epidermis on the ventrolateral aspect of the bfjdy, called the milk -ridges or milklines, from localized thickenings of which the multiple mammary glands of such animals develop. These railklines have also been observed in the human embryo, but the constancy of their occurrence in man has not as yet been established.

From the bottom of the glandular area, numerous small masses or bad-like processes of cells ^row down into the corinm. Some of the buds acquire lateral branches. By the hollowing out of these cell-buds the latter are transformed into tubes (c), which open upon the glandular area. The branching of the cords begins in the seventh month and is carried on to such a degree that each original cell-cord gives rise to a tubo-racemose gland. The hollowing out of the solid processes begins shortly before birth, but is not completed until after that event. Each cell-cord becomes, in the strict sense, a complete gland, each such individual structure forming a lobe of the mature organ.

This stage of the human mammary gland — that is, a depressed gland-area upon which open individual glands, the nipple being absent^ — is the permanent condition in some of the lowest mammals, as in the echidna, one of the monotremes. In all higher mammals, however, further metamorphoses occur in the tis.sites of the glandular area, and in the human fetus these tissues become the nipple and the surrounding areola.

The nipple is partly formed before birth, but does not become protuberant until post-fetal life. The depressed glandular area rises to the level of the surrounding parts, and its central region, which includes the orifices of the already formed or just forming ducts, swells out into a little prominence, the nipple. This prominence is a protrusion of the epidermis and includes the terminal extremities of the milk-ducts as well as the blood-vessels and connective- tissue elements which surround tli(» ducts. In the dermal constituent of the rudimentary nipple unstriated muscular tissue develops. The region of the glandular area not concerned in the formation of the nipple becomes the areola.

At birth, as above intimated, the manmiary gland is still rudimentary, since many of the ducts have not yet acquired their lumina nor their full degree of c(mij)lexity. Shortly after birth a small quantity of milky secretion, the so-called witches' milk, may be ex])ressed from the glands — in the male and female infant alike. This is true milk according to Rein and Barfruth, but according to Kolliker, it is merely a milky fluid continuing the debris of the degenerated central cells of those rudimentarv ducts that were still solid at birth.

So far, the milk-glands are alike in the two sexes, but while in the male they remain rudimentary structures, they continue to increase both in size and in complexity in the female. The increase aflfeets not only the glandular tissue proper but the connective-tissue stroma as well. At the time of puberty the growth of the glands receives a new impetus, which is very materially augmented upon the occurrence of pregnancy. There may be said, therefore, to be several distinct phases in the development of the milk-glands, first, the embryonic stage ; second, the infantile stage ; third, the stage of maturity beginning at the time of puberty ; and finally, the stage of fhU functional maturity consequent upon preg-> nancy and parturition.