Book - A textbook of histology, including microscopic technic (1910) Special Histology 6
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Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.
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VI. The Skin and its Appendages
A. The Skin (Cutis)
THE skin consists of two intimately connected structures the one, of mesodermic origin, is the true skin, corium or dermis ; the other, of ectodermic origin, is the epidermis or cuticle. The superficial layer of the corium is raised into ridges and papillae which penetrate into the epidermis, the spaces between the papillae being filled with epidermal elements. Thus, the lower surface of the epidermis is alternately indented and raised into a system of furrows and elevations corresponding to the molding of the corium.
In the epidermis two layers of cells may be observed the stratum Malpighii, or stratum germinativum (Flemming), and the horny layer, or stratum corneum. According to the shape and characteristics of its cells, the stratum germinativum may also be divided into three layers first, the deep or basal layer, consisting of columnar cells resting immediately upon the corium ; second, the middle layer, consisting of polygonal cells arranged in several strata, the number of the latter varying according to the region of the body ; and third, the upper layer, or stratum granulosum, which is composed, at most, of two or three strata of gradually flattening cells characterized by their peculiar granular contents.
All these cell layers consist of prickle cells, and for this reason the stratum Malpighii is sometimes known as the stratum spinosum. When these cells are isolated by certain methods, their surfaces are seen to be provided with short, thread-like processes. In section the cells appear to be joined together by their processes. Since it has been proved that the processes of adjacent cells do not lie side by side, but meet and fuse, they must be regarded as belonging alike to both cells. Between the fused processes, which are known as intercellular bridges, there exists a system of channels which is in communication with the lymphatic system of the corium. The prickles just mentioned are variously regarded by different investigators ; some considering them to be exclusively protoplasmic processes of the cells, others regarding them as derived from the membranes of the cells composing the stratum Malpighii. Ranvier and others ascribe a fibrillar structure to the peripheral portion of the cellular protoplasm, and, according to them, these fibrillae, surrounded by a small quantity of indifferent protoplasm, form the processes. Ranvier has also shown that such fibrillae may extend from one cell around several others before reaching their ultimate destination in other cells at some distance. (Fig. 305.) The cells of the stratum granulosum contain peculiar deposits of a substance to which Waldeyer has given the name of keratohyalin. This substance occurs in the form of irregular bodies varying in size and imbedded in the protoplasm. The nuclei of such cells always show degenerative processes, which are possibly due to the formation of the keratohyalin (Mertsching, Tettenhamer). These karyolytic figures and keratohyalin possess in common many apparently identical microchemic peculiarities, and it is very probable that karyolysis and the formation of keratohyalin are processes originally very closely allied i. e., that the keratohyalin is derived from the fragments of the dying nucleus.
Fig. 303. Under surface of the epidermis, separated from the cutis by boiling. The sweat-glands may be traced for a considerable part of their length ; X 4 : a > Sweatgland ; b, longitudinal ridge ; c, depression ; d, cross-ridge.
The stratum corneum forms the outer layer of the epidermis and presents, as a rule, a somewhat differentiated lower stratum. This in concentric lamellae (Kolliker, 89). Here and there between the cornified cells structures may be seen which probably represent the remains of intercellular bridges. The thickness of the epidermis varies greatly according to the locality, and is directly proportionate to the number of its cell layers. As a rule, the stratum Malpighii is thicker than the stratum corneum, but in the palm of the hand and the sole of the foot the latter is considerably the thicker.
Fig- 34- Cross-section of skin of child, with blood-vessels injected ; X 3 latter is more especially noticeable in those regions in which the stratum corneum is highly developed, and is known as the stratum lucidiim. It is quite transparent, this property being due to the presence in its cells of a homogeneous substance, which is in all probability a derivative of the more solid keratohyalin of the stratum granulosum. The cells of the stratum corneum are more or less flattened and cornified, especially at their periphery. This applies more particularly to the superficial cells. In the interior of each cell a more or less degenerated nucleus may be seen, but otherwise its contents are homogeneous, or, at most, arranged
The various layers of the epidermis are in close genetic relationship to one another. The constant loss to which the epidermis is subjected by desquamation is compensated by a continuous upward pushing of its lower elements ; cell-proliferation occurs in the basal cells and adjacent cellular strata of the stratum germinativum (Malpighii), where the elements are often seen in process of mitotic division. The young cells are gradually pushed outward, and during their course assume the general characteristics of the elements
composing the layers through which theypass. For instance, such a cell changes first into a cell of the stratum germinativum ; then, when it commences the formation of keratohyalin, into a cell of the stratum granulosum ; later, into a cell of the stratum lucidum, and finally into an element of the stratum corneum, where it loses its nucleus, cornifies, and at last drops off. The mesodermic portion of the skin, the corium, consists of a loose, subcutaneous connective tissue containing fat, the subcutaneous layer, with the panniculus adiposus, and of the true skin, or corium proper. The amount of adipose tissue in the subcutaneous layer is subject to great variation ; there are, however, a few regions in which there is normally very little or no fat (external ear, eyelids, scrotum, etc.). To the subcutaneous connective tissue is due the mobility of the skin. The corium may be compared to the mucosa of a mucous membrane, and consists of two layers of a deeper and looser pars reticularis, and of a superficial pars papillaris supporting the papillae. The transition from the one to the other is very gradual. Elastic fibers are present in the connective tissue of both layers.
The pars reticularis is made of bundles of connective-tissue fibers arranged in a network, nearly all of the strands of which have a direction parallel with the surface of the skin and are surrounded by a reticulum of rather coarse elastic fibers. In that portion of the pars papillaris bordering upon the epidermis, the interlacing strands of connective tissue, as well as the surrounding reticulum of elastic fibers, are finer, so that the whole tissue is denser. This stratum supports the papillae knob-like or conical elevations of still denser tissue ending in one or more points. We accordingly speak of simple or compound papillae. These structures are especially numerous and well developed in the palm of the hand and sole of the foot, where they are from 1 10 // to 220 fi long. Here they rest upon ridges of the corium, which are nearly always arranged in double rows. According to whether the papillae contain blood-vessels alone, or special nerve-endings also, they are known as vascular or tactile papillae.
Fig. 305. Prickle cells from the stratum Malpighii of man ; X 4^
Fig. 306. Cross-section of human epidermis ; the deeper layers of the stratum Malpighii are not represented ; X 75
The smallest papillae are found in the mammae and scrotum from 30 fj. to 50 ft long. The surface of the pars papillaris is covered by an extremely delicate membrane the basement membrane. According to most authors, the basal cells of the epidermis are simply cemented to this structure. Others believe that the epithelial cells are provided with short basilar processes which penetrate into the basement membrane and meet here with similar structures from the connective-tissue cells of the corium. This would give the basement membrane a fibrillar structure (Schuberg).
The subcutaneous layer contains numerous more or less vertical strands of connective tissue, containing numerous large elastictissue fibers and joining the stratum reticulare of the corium to the superficial fascia of the body or underlying structure, whatever that may be. These strands are the retinaculce cutis, and inclose in their meshes masses of fatty tissue which form the panniculu* adiposus. The latter varies greatly in thickness in different parts of the body. The vertically arranged cords of connective tissue are accompanied by blood-vessels, nerves, and the excretory ducts of glands.
Smooth muscle-fibers are also present in the skin, and around the hair follicles are grouped into bundles. Nearly continuous layers of smooth muscle tissue are found in the subcutaneous layer of the scrotum (forming here the tunica dartos), in the perineum, in the areolae of the mammae, etc. In the face and neck striated muscle-fibers also extend outward into the corium.
Even in the white race certain regions of the epidermis always contain pigment as, for instance, the areolae and mammillae of the mammary glands, the scrotum, labia majora, around the anus, etc. In these regions the epithelial cells and the connective-tissue cells of the pars papillaris corii contain a variable number of small pigment granules. The latter occur chiefly in the basal cells of the epidermis and diminish perceptibly in the cells of the overlying layers, so that in those of the stratum corneum few, if any, are left. In negroes and other colored races the deep pigmentation is due to a similar distribution of the pigment granules in the entire epidermis ; but even here the pigmentation decreases toward the surface, although the uppermost cells of the stratum corneum always contain some pigment. The nuclei of the cells are always free from the coloring-matter. The question as to the origin of the pigment is as yet unsolved. This much is known : that in those regions where pigment is present certain branched and deeply pigmented connective-tissue cells are found immediately beneath the epiderni's, sending out processes which may be traced outward between the cells of the stratum Malpighii (Aeby). This fact has led some authors to believe that the connective tissue is in reality the source of the pigment, and that by some unknown process the latter is taken up and conveyed to the cells of the epidermis. This theory would preclude a direct production of pigment granules in the epidermal cells. But although it can not be denied that the pigment may be derived from the connective tissue, it is hardly logical to assume a priori that epithelial cells are not capable of pigment production, since, in other regions of the body, pigment formation may be observed in cells of undoubted epithelial origin, as, for instance, in ganglion cells and in the pigment epithelium of the retina. An interesting proof that the processes of pigmented connective-tissue cells actually penetrate the epidermis is afforded by the case reported by Karg, of transplantation of a piece of skin from a white man to a negro. After some time the piece of white skin became pigmented. Reinkehas demonstrated that the pigment in certain cells is in combination with certain definite bodies. The latter have been given the botanical name of trophoplasts. If the pigment be removed, colorless trophoplasts are left. They may be tinged with certain stains. In the epidermis of the white race trophoplasts are also constantly present, although they are only slightly or not at all pigmented (Barlow)
Fig. 307. Cross-section of negro's skin, showing the intimate relationship of the pigment cells of the corium to the basilar cells of the epidermis. The latter are more deeply pigmented at their outer ends. The pigment granules may be traced into the outermost layers of the stratum corneum ; X 5 2 5
Fig. 308. A reconstruction showing the arrangement of the blood-vessels in the skin of the sole of the foot (Spalteholz): a, Stratum Malpighii and corium; b, boundary between cutis and subcutis, in the region of the coiled portions of the sweat-glands ; f, subcutis ; J, subpapillary arterial network ; e, cutaneous arterial network ; f, g, //, and t, first, second, third, and fourth venous plexuses.
The following may be said concerning the vascular system of the skin : The arteries which supply the skin with nutriment penetrate the corium and form a characteristic network in its lowest stratum. They also anastomose freely in the fascia and the subcutaneous layer. From this plexus branches pass outward to form a second or subpapillary plexus. From the latter, branches are again given off which, without further anastomoses, pass along beneath the rows of papillae and supply each separate papilla with capillary twigs. These in turn pass over into venous capillaries which unite and form four venous plexuses, one over the other and in general parallel to the surface of the skin. The uppermost venous plexus lies beneath the papillae, each venule corresponding to a single row of papillae and anastomosing with its neighbors. The second plexus is found immediately beneath the first, the third in Papillae. the lower portion of the corium, and the fourth at the junction of the cutis and subcutis. Near the middle of the subcutis the arteries show a circular musculature, but the veins are already thus provided in the
network between the cutis and subcutis, where they also seem to possess valves. As already stated, the subcutaneous fat is divided into lobes by transverse and longitudinal bundles of connective tissue ; a second system of bundles midway between the cutis and fascia separates the panniculus adiposus into an upper and a lower layer. The former is supplied by direct arterial branches ; the latter, by branches passing backward from the cutaneous network. Those regions which are subjected to great external pressure are supplied by a greater number of afferent vessels the caliber of which is increased. In regions where the skin is very mobile the arteries are greatly convoluted. All these vascular peculiarities are present in the newborn (Spalteholz).
The lymph-vessels of the true skin are also distributed in two layers a deep and wide-meshed plexus in the subcutis, and a superficial narrow-meshed plexus immediately beneath the papillae. Into the latter empty the lymph -vessels coming from the papillae. After treating the skin by certain methods, a fine precipitate may be noticed here and there in the papillary region of the corium, a proof that lymph clefts are present. These are regarded as the beginnings of the cutaneous lymphatic system. They may also be traced into the epithelium, where they are in direct communication with the interspinal spaces between the epithelial cells (Unna). Cells are also met with in the interspinal spaces of the epidermis ; these are migratory cells, or cells of Langerhans.
Fig. 309. Nerves of epidermis and papillae from ball of cat's foot ; X 75
The skin owes its great sensitiveness to the numerous nerves and special nerve-endings present, not only in the epithelium, but also in the corium and subcutis. In certain regions of the skin the nerves have been traced into the epithelium. In the finger-tip, for instance, numerous nerves are seen in the epidermis, where they branch and end in telodendria with or without small terminal swellings. There is no direct communication between the terminal nerve filaments and the epithelial cells. (Fig. 309.) In certain peculiarly sensitive regions, as the end of the pig's snout, the nervefibers end in distinct saucer-like discs (tactile menisci) which, as a rule, clasp the lower ends of the basal Malpighian cells.
Fig. 310. Meissner' s corpuscle from man ; Fig. 311. Meissner's corpuscle from man; x 750. x 750.
The special sensory nerve -endings are situated in the corium and subcutis. Of these, we may mention the tactile corpuscles of Meissner, the end-bulbs of Krause, the Pacinian corpuscles, Ruffini's nerve-endings, and the Golgi-Mazzoni corpuscles. All these special sensory nerve-endings with the exception of the two last mentioned have been discussed in a former chapter (p. 169). Meissner's tactile corpuscles are situated in the tactile papillae of the true skin. They are especially numerous in the hand and foot.
In the distal phalanx of the index-finger every fourth papilla is a tactile papilla, containing one or sometimes two corpuscles of Meissner. They are, however, not nearly so numerous in other parts of the hand or in the foot. These corpuscles are further found on the dorsal surface of the hand and volar surface of the forearm, in the nipple and external genitals, in the eyelids (border), and in the lips. In figures 310 and 311 are shown two Meissner's corpuscles, giving the appearance presented by these end-organs when not stained with special reference to nerve terminations. For the latter see figure 137.
The Krause's end-bulbs, both spheric and cylindric, are, as a rule, situated a short distance below the papillary layer, although they are frequently found in the papillae. They occur in man in the conjunctiva, lips, and external genitals, and in the mucous membranes previously mentioned (p. 170). See page 170 and figure 136 for their structure.
In the palm of the hand and sole of the foot, the subcutaneous connective tissue contains numerous Pacinian corpuscles. They occur also along the nerve-fibers of the joints and in the periosteum of the extremities.
Very recently Ruffini demonstrated in the human corium the existence of peculiar nerve end-organs, which consist of a connective-tissue framework supporting a rich arborization of telodendria. They occur side by side with the Pacinian corpuscles and in apparently equal numbers. These nerve terminations resemble in many respects the neurotendinous spindles (see Fig. 145), although they
present certain structural differences. Instead of intrafusal tendon fasciculi, the Ruffini end-organ is composed of white fibrous and elastic tissue. In this end-organ the medullated nerves make long and tortuous turns before becoming nonmedullated, and the terminations of these nerve-fibers occupy the whole of the cross-section.
The Golgi-Mazzoni corpuscle resembles in structure the Pacinian corpuscle, although it possesses fewer lamellae and a relatively larger core, and the nerve - fibers terminating therein are more extensively branched than in the Pacinian corpuscle. Ruffini has found these nerve-endings in the subcutaneous tissue of the fingertips.
The blood-vessels of the skin are richly supplied with the vasornotor nerves, which terminate in the nonstriated muscle of the vessel walls. These vasomotor nerve-fibers are neuraxes of sympathetic neurones.
Fig. 312. Grandry's corpuscles from bill of duck ; X 5
In aquatic birds, and more especially in ducks, the waxy skin of the beak and the cornified portion of the tongue contain the socalled corpuscles of Herbst, which resemble the Pacinian corpuscles in general structure, but have cubical cells in the core. In the same tissues are also found the corpuscles of Grandry, 60 fj. long and 40 IJL broad. They consist of a thin connective-tissue capsule, containing two or three large cells. The nerve-fiber retains its medullary sheath for some distance within the capsule. The axis-cylinder ends in discs situated between the cells inclosed by the capsule.
B. The Hair
The hair and nails are regarded as special differentiations of the skin. Hair is found distributed over almost the entire extent of the skin, varying, however, in quantity and arrangement in different regions. None whatever is present in the palm of the hand and sole of the foot. In the third fetal month small papillary elevations of the skin are seen to develop in those areas in which the hairy growth later appears. Under each of these elevations there occurs a proliferation of the cells of the Malpighian layer downward into the corium. Although the elevations soon disappear, the epithelial ingrowth continues and finally forms the hair germ. This is soon surrounded by a connective-tissue sheath from the corium, in which two layers may be distinguished. At the lower end of the hair germ the corium is pushed upward, forming a papilla which penetrates into the thickened bulb of the germ. This is called the hair papilla. In the mean time the hair germ itself is undergoing marked differentiation. An axial portion, forming later the hair and inner root-sheath, and a peripheral, constituting later the outer rootsheath, are developed. From the latter are derived also the first traces of the sebaceous glands, which in the adult state are in close relationship to the hair and empty their secretion into the space between the hair and its sheath. As soon as the various layers of the hair are complete it grows outward, breaking through the overlying layers of the epidermis.
The visible portion of the hair is called the hair shaft, and that portion below the skin is the hair root. The lower portion of the hair resting upon the papilla is known as the hair bulb, and the sheaths encircling the root and bulb are called the rootsheaths, the entire structure constituting the hair follicle.
The adult hair is covered by a thin cuticle, consisting of overlying plate-like cells, i.i ft thick, most of which possess no nuclei. Beneath the cuticle is the cortical layer, composed of several strata of long, flattened cells from 4. 5 (i to 1 1 ft broad and provided with nuclei. These are also known as the cortical fibers of the hair. Upon treatment with ammonia the fibers separate into delicate fibrils, the hair fibrils (Waldeyer, 82). Scattered between and within the cells of the cortical layer are varying quantities of pigment granules. The axial region of the hair is occupied by the medullary substance, from 1 6 // to 20 // in diameter. This may be lacking ; but if present, consists of from 2 to 4 strata of polygonal, nucleated and pigmented cells. The hair shaft often contains air vesicles.
The inner root-sheath consists of three concentric layers first, of an outer single layer of clear nonnucleated cells, the so-called layer of Henle ; second, of a thicker middle layer, made up of a stratum of nucleated cells containing keratohyalin, the layer of Huxley ; and, third, of an inner cuticle, bordering upon the hair.
Fig- 3 T 3- Transverse section of human scalp; X I2: A P, Musculus arrector pili ; c, corium ; ep, epidermis; fp, hair follicle ; Gap, aponeurosis ; gls, sweat-gland ; glse, sebaceous glands; KH, club-hair; //, papilla of hair; Re, retinacula cutis; Rp, root of hair; Sp, shaft of hair; ts, subcutaneous layer (Sobotta, "Atlas and Epitome of Histology" ).
The outer root-sheath is made up of elements from the stratum germinativum. Here we have to do with prickle cells, surrounded by an outer layer of columnar elements. The connective-tissue portion of the hair follicle is composed of an outer, looser layer of longitudinal fibrous bundles ; of an inner, compacter layer of circular fibers ; and of an innermost well-developed basement membrane the glassy membrane.
At a certain distance above the root bulb all the layers of the epithelial portion of the hair follicle are well developed and distinct from each other. This condition changes toward the hair papilla as well as toward the hair shaft. Below, in the region of the thickened hair bulb, the root-sheaths begin to lessen in thickness, their layers becoming more and more indistinct toward the base of the hair papilla. Finally, all differentiation is lost in the region where they encircle the neck of the papilla. Toward the shaft of the hair, the root-sheath also undergoes changes. In the region into which the sebaceous glands empty, the inner root-sheath disappears, while the outer becomes continuous with the stratum germinativum of the epidermis ; the outer layers of the latter the stratum granulosum, stratum lucidum, and stratum corneum push downward between the outer root-sheath and the hair to the openings of the sebaceous glands.
Fig. 314. Longitudinal section of human hair and its follicle ; X about 300.
Regarding the growth of the hair, two theories are prevalent.
Fig. 315 Cross-section of human hair with its follicle ; X about 300.
The one theory assumes that the elements destined to form the epithelial root-sheaths are derived from the epidermis by a constant process of invagination. The component parts of the hair would thus be continuous with the layers of the root-sheaths, and consequently with those of the epidermis. Thus the basal cells of the external root-sheath would extend over the papilla, and be continuous with the cells of the medulla of the hair (these relations are especially well defined in the rabbit), and the stratum spinosum (middle layer of stratum Malpighii) of the outer root-sheath would be continuous with the cortical substance of the hair. According to this theory also, the layer of Henle would correspond to the stratum lucidum of the epidermis, and at the base of the hair would become its cuticle, while the layer of Huxley would form the cuticle of the inner root-sheath (Mertsching). The other theory assumes that the hair is derived from a matrix, consisting of proliferating cells situated on the surface of the papilla. From these germinal cells would be derived the medullary and cortical substance of the hair, its cuticle, and the inner root-sheath (Unna).
The shedding of hair is common to all mammalia, a phenomenon occurring periodically in the majority of species. In man the process is continuous. Microscopic examination shows that the hair destined to be shed becomes loosened from its papilla by a cornification of the cells of its bulb. At the same time the cortical portion of the hair bulb breaks up into a brush-like mass. Such hairs are called club hairs or bulb hairs, in contradistinction to papillary hairs. In the region of the former papilla there arises, by a proliferation of the external root-sheath, a bud which grows downward, from which a new hair with its sheaths and connective-tissue papilla is developed. The result is that the developing new hair gradually pushes the old hair outward until the latter finally drops out. The exact details of this process have given rise to considerable discussion (yid. Gotte and Stieda, 87).
Adjacent to the hair follicles are bundles of smooth musclefibers, known as the arrectores pilorum. They originate from the papillary layer of the corium and extend to the lower part of the connective-tissue sheath of the hair follicles. In their course they not infrequently encircle the sebaceous glands of the follicle. Since the hair follicles have a direction oblique to the skin surface, forming with it an acute and an obtuse angle, and since the muscle is situated within the obtuse angle, its function may easily be conceived as being that of an erector of the hair. The hair papillae are very vascular.
The nerve-fibers of the hair follicles have recently been studied by a number of investigators, with both the Golgi and the methyleneblue methods. It has been shown that the hair follicles receive their nerve supply from the nerve-fibers which terminate in the immediate skin area. Each follicle receives, as a rule, only one nerve-fiber, which reaches the follicle a short distance below the mouth of the sebaceous gland. The nerve-fiber, on reaching the follicle, loses its medullary sheath and divides into two branches, which surround it in the form of a ring. From this complete or partial ring of nerve-fibers numerous varicose fibers proceed upward parallel to the axis of the follicle for a distance about equal to the cross-diameter of the follicle, to terminate, it would seem, largely outside of the glassy layer (Retzius). In certain mammalia the nerve-fibers end in tactile discs, found in the external root-sheaths of the so-called tactile hairs. The muscles of the hairs receive their innervation through the neuraxes of sympathetic neurones, which reach the periphery from the chain ganglia through the gray rami communicantes. These nerves are known as pilomotor nerves, and when stimulated, excite contraction of the erector muscles of the hairs, causing these to assume an upright position and producing the appearance termed goose skin, or cutis anserina. Langley and Sherrington have made interesting and important observations on the course and distribution of the pilomotor nerves.
Fig. 316 Longitudinal section through hair and hair follicle of cat ; X 160.
C. The Nails
The nails are a peculiar modification of the epidermis. The external arched portion is called the body of the nail ; that area upon which the latter rests, the nail bed, or matrix ; and the two folds of epidermis which overlap the nail, the nail walls. The groove which exists between the nail wall and nail bed is known as the sulcus of the matrix, and the proximal imbedded portion of the nail as the nail root, since all growth of the nail takes place in this region. The nail bed consists of the corium, which is here made up of a dense felt-work of coarse connective-tissue fibers. Immediately beneath the nail the corium is raised into a number of more or less symmetric longitudinal ridges, which again become continuous with the connective-tissue papillae of the skin at the line where the nail projects beyond its bed.
Fig. 317 - Longitudinal section through human nail and its nail groove (sulcus) ; X 34
The depressions between the ridges are occupied by epidermal cells, which also form a thin covering over the ridges themselves.
These cells correspond here to the basilar layer of the stratum Malpighii. The stratum granulosum is not uniformly present, although occurring as isolated areas in the region of the nail root and lunula, the white area of demilunar shape at the proximal portion of the nail. Unna has demonstrated that the pale color of the lunula and root of the nail is due to the presence of keratohyalin. Formerly, this peculiarity was attributed to a difference in the distribution of the vessels in the various portions of the nail bed. The body of the nail, with the exception of the lunula, is transparent a condition which may be explained by the fact that the elements of the nail correspond to those of the stratum lucidum. As a consequence, the vessels of the matrix shine through, except at the lunula, where the keratohyalin granules render the nail opaque.
The nail itself consists of elements homologous to those of the stratum lucidum. They are flat, transparent cells, closely approximated, and all contain nuclei. The cells overlie each other like tiles, and are so arranged that each succeeding lower layer projects
Fig. 318. Transverse section through human r.ail and its sulcus ; X 34 a little further distalward than the preceding. At the period when the nails are formed, about the fourth month of fetal life, sulci are already present. The first trace of the nail is seen as a marked thickening of the stratum lucidum in the region which later becomes the body of the nail ; in this stage the structure is still covered by the remaining layers of the stratum corneum, constituting the eponychium. The embryonal nail then spreads in all directions until it finally reaches the sulcus. Henceforward the growth is uniform. The eleidin normally present in the stratum lucidum of the skin also occurs in the nail, and is derived, as we have already seen, from the keratohyalin. It may readily be conceived that later, when growth is confined to the root of the nail, keratohyalin is also present. As soon as the nail begins to grow forward, in the ninth month, the greater, part of the eponychium is thrown off; but during the entire extrauterine life, a portion of the eponychium is retained at the nail wall, and as hyponychium on the anterior and under surface of the nail.
D. The Glands of the Skin
The glands in the skin are of two kinds sweat-glands and sebaceous glands. In this connection we may also consider the mammary glands, which may be regarded as a modified skin gland.
1. The Sweat-glands
The sweat-glands, or sudoriparous glands, are distributed throughout the entire skin, but are especially numerous in certain regions as, for instance, the axilla, palm of the hand, and sole of the foot. They lie imbedded either in the adipose tissue of the true skin, or still deeper in the subcutaneous connective tissue.
Fig. 319. A, B, Two views of a model of the coiled portion of a sweat-gland from the plantar region of a man, reconstructed by .Bern's wax-plate method ; X Io ( Huber-Adamson).
The sweat-glands are simple tubular in type, and their secreting portion is coiled ; hence the name coil-glands. The coiled portion of these glands measures 0.3 to 0.4 mm. The excretory duct is nearly straight in its course through the corium. From here on its course is spiral, and it should be borne in mind that in its passage through the epidermis it has no other wall than the epidermal cells of the various layers through which it passes, although these cells are arranged concentrically around the lumen of the duct. The duct takes part in the formation of the coiled portion of the gland, forming about onefourth of the length of the tubule which takes part in the formation of the coil. In figure 319 are shown two views of a model of the coiled portion of a sweat-gland from the plantar region of the foot of a man. The length of the tubule in the coiled portion of this gland measures 4.25 mm., of which 1.25 mm. fall to the excretory duct and 3.0 mm. to the secretory tubule.
Fig. 320. Cross - section of tubule of coiled portion of sweat-gland of human axilla. Fixation with sublimate ; X 600.
The blind end of the secreting portion of the tubule and the excretory duct as it enters the coil are usually in close proximity. The secretory portion of the tubule of sweat-glands is lined by a single layer of cubic or columnar cells, with finely granular protoplasm and round or oval nuclei possessing one or two nucleoli. Between this layer of cells and the basement membrane there is found a layer of nonstriated muscle-cells, longitudinally disposed. The portion of the excretory duct found within the coil of the glands is lined by a single layer of short cubic cells, with cuticular border, outside of which there is a delicate basement membrane. The muscular layer is lacking in this and the remaining portion of the duct The excretory portion of the duct passing through the corium is lined by short, somewhat irregularly cubic cells arranged in two layers.
Capillary networks surround the secreting portions of the sweat-glands.
The nerves of the sweatglands have been studied with the aid of the methylene-blue method by Ostroumow, working under Arnstein's direction. These glands receive their innervation through the neuraxes of sympathetic neurones, the terminal branches of which form an intricate network just outside of the basement membrane, known as the epilamellar plexus, From this plexus fine varicose nerve-fibers pass through the basement membrane, and, after coursing a shorter or longer distance with or without further division, end on the gland-cells, often in clusters of small terminal granules united by delicate threads.
The development of the sweat-glands begins in the fifth month of fetal life. At first solid cords grow from the stratum germinativum of the epidermis into the corium. Later, in the seventh month, these become hollow.
Joseph has shown a structural change in the secretory cells 01" the sweat-glands when perspiration was induced by electrical stimulation or by drugs.
With the sweat-glands as here described, and which have, as has been stated, a very wide distribution, we may also class certain skin glands, grouped under the term of "modified sweat-glands," which show certain structural and morphologic peculiarities and are found in special regions of the body. To these belong the axillary glands, the circumanal glands, the ciliary glands or glands of Moll of the eyelid, and the ceruminous glands of the external auditory canal. The axillary glands resemble the sweat-glands in shape and structure, possessing, however, larger and longer tubules. The coiled portions of these glands measure 1.5 to 2 mm., the tubule of the coil attaining a length of 30 mm. In the circumanal region are found several types of sweat-glands, especially in an area having the form of an elliptical ring with a width of about 1.5 cm. and situated about 1.5 cm. from the anus. In this region there are found large sweat-glands, known as the circumanal glands of Gay ; branched sweat-glands of the type of tubulo-alveolar glands ; sweat-glands with relatively straight ducts, ending in a relatively' large saccule or vesicle, from which arise secondary tubules or alveoli ; and, finally, sweat-glands of the type as found in other regions of the body. The ciliary glands or glands of Moll may also be classed as branched glands of the type of tubulo -alveolar glands, with relatively large vesicles. The ceruminous glands are branched tubulo-alveolar glands.
Fig. 321. Tangential section through coiled portion of sweat-gland from human axilla. Sublimate fixation ; X 7
Fig. 322. Model of a sebaceous gland with a portion of the hair follicle, reconstructed by Born's wax-plate method. A, Hair follicle.
2. The Sebaceous Glands
The distribution of the sebaceous glands in the skin is closely connected with that of the hair follicles into which they pour their contents. Exceptions to this rule occur in only a few regions of the body, as, for instance, in the glans penis and foreskin (Tyson's glands), in the labia minora, angle of the mouth, glandule tarsales, and the Meibomian glands of the eyelids, etc. As a rule the sebaceous gland empties by a wide excretory duct into the upper third of the hair follicle. The walls of the duct also produce secretion, and can therefore hardly be differentiated from the rest of the gland. At its base the duct widens and is provided with a number of simple or branched alveoli. The sebaceous glands are therefore of the type of simple branched alveolar glands, varying in length from 0.2 mm. to 0.5 mm. They are surrounded by connective-tissue sheaths, which at the same time cover the hair follicles. Inside of the sheath is the membrana propria, which is a continuation of the glassy membrane of the follicle. The two or three basal strata of glandular cells must be regarded as a direct continuation of the elements of the external root-sheath. In the more centrally placed strata the cells are distinctly changed in character ; their contents consist of fat globules, varying in size and distributed throughout the protoplasm, giving this a reticular appearance, while the nuclei suffer compression from the accumulation of the fat globules and gradually become smaller and more angular. Finally, the cells change directly into secretion, which is then poured into the hair follicle as sebum. It is thus seen that in the secretion of sebum the cells are consumed and must be replaced. This renewal takes place by the constant proliferation of the basilar cells, which push the remains of the secreting cells upward and finally take their places. The final disintegration of the cells occurs either within the gland itself or between the hair follicle and the hair. The secretion contains fatty globules of varying size, which occur either free or attached to cellular detritus.
Fig. 323. Section of alveoli from sebaceous gland of human scalp.
3. The Mammary Glands
The mammary glands are also included among the cutaneous glandular structures. They are developed early, but not until the fifth month is it possible to distinguish a solid central portion, with radially arranged tubules terminating in dilatations. The structures are all derived from the basal layers of the epidermis. From birth to the age of puberry the organs are in a state of constant growth, and are early surrounded by a connective-tissue sheath. The alveoli, which have been developed in the mean time, are still solid and relatively small. Up to the twelfth year the glands remain identical in structure in boys and girls. In the female the mammary glands continue to develop from the age of puberty ; in the male, on the other hand, they undergo a retrograde metamorphosis, ending, finally, in the atrophy of all except the excretory ducts. The mammary glands do not attain their full stage of development in women until the last months of pregnancy, and are functionally active at parturition. The human mammary gland when fully developed has the following structure : It consists of about twenty lobes, separated from each other by connective-tissue septa. These lobes are again divided into a larger number of lobules, and these in turn are composed of numerous irregularly round or oval or even tubular alveoli. The alveoli are provided with small excretory passages, which unite to form the smaller ducts, these in turn uniting to form the larger ducts. Shortly before terminating at the surface of the mammilla, each mammary duct widens into a vesicle, the sinus lactiferus. The number of excretory ducts corresponds to that of the larger lobes. The ducts are lined by simple cubical epithelium, except near their termination in the nipple, where they are lined by stratified pavement epithelium, and surrounded by a fibrous tissue sheath.
Fig. 324. Model of a small portion of a secreting mammary gland ; X 2O (Maziarski, Anatomische Hefte, vol. xvm.J
Fig. 325. From section of mammary gland of nullipara. (From Nagel's "Die weiblichen Geschlechtsorgane," in " Handbuch der Anatomic des Menschen," 1896.)
The epithelium of the alveoli differs according to the state of functional activity. In a state of rest it consists of a single layer of glandular cells of nearly cubical shape which stain deeply, the internal surfaces now and then projecting slightly into the lumen. At the beginning of secretion fat globules make their appearance in the distal ends of the cells. At the same time a corresponding increase in size occurs throughout the entire alveolus. There are as yet current two quite contradictory views as to the manner in which the milk is secreted. According to certain observers, the free ends of the cells, which contain the most fat globules, are constricted off, after which the fat globules are freed in the lumen. The secretory portion of the alveolus is then composed of low epithelial cells, in which the process begins anew. The process of milk secretion therefore consists in throwing off the inner halves of the cells containing the fat globules, and in regeneration of the cells from the nucleated remains of the glandular epithelium. Whether a karyokinetic division of the nuclei occurs in this process is not known, and how often the process of regeneration may be repeated in a single cell is not capable of demonstration. It is -certain, however, that entire cells are destroyed, to be replaced later by new elements. Other observers regard the secretion of milk as occurring without a partial or total destruction of the secretory cells, but after the manner of the secretion of other glands. This latter view seems more in accord with the more recent observations. The membrana propria of the alveoli appears homogeneous. Between it and the glandular cells are so-called basket cells, similar to those in the salivary glands. Benda regards the basket cells as nonstriated muscle elements having a longitudinal direction, making the structure of the alveoli of the mammary gland similar in this respect to that of the secreting portion of the sweat-glands.
The skin of the mammilla is pigmented, and the papillae of its corium are very narrow and long. In the corium are also found large numbers of smooth muscle-fibers, which form circular bundles around the excretory ducts. In the areolse of the mammae are the so-called glands of Montgomery, which very probably represent accessory mammary glands. These are especially noticeable during lactation. The blood-vessels of the mammary gland, the larger branches of which are situated mainly in the subcutaneous tissue, form rich capillary networks about the alveoli.
The mammary glands possess many lymphatics. These are especially numerous in the connective-tissue stroma between the lobules. The lymph-vessels collect to form two or three larger vessels, which empty into the axillary glands. The mammary gland receives its nerve supply from the sympathetic and cerebrospinal nervous systems through the fourth, fifth, and sixth intercostal nerves. The terminations of the nerves in the mammary gland have been studied by means of the methylene-blue method by Dmitrewsky, working in the Arnstein laboratory, who finds that the terminal branches form epilamellar plexuses outside of the basement membrane of the alveoli, from which fine nerve branches pass through the basement membrane and end on the gland cells in clusters of terminal granules united by fine filaments. The nipple has a rich sensory nerve supply. In the connective-tissue papillae are found tactile corpuscles of Meissner.
The milk consists of fat globules of varying size, which, however, do not coalesce an attribute due to the presence of albuminous haptogenic membranes surrounding the globules. Shortly before, and for some days after, parturition the milk contains true nucleated cells in which are fat globules ; these are known as the colostrum corpuscles. They probably represent leucocytes which have migrated into the lumen of the gland and have taken up the fat globules of the milk. This milk is known as colostrum.
Good general views of the skin can be obtained only from sections. Any fixation method may be employed, although alcohol is preferable on account of the better subsequent staining. For detail work Flamming' s solution, corrosive sublimate, or osmic acid is the best. Sectioning of the skin is attended with many difficulties, and large pieces can be cut only in celloidin. Small and medium-sized pieces may be cut in paraffin ; but even in this case the skin must be rapidly imbedded in the paraffin /'. e., it must not remain too long in either alcohol or toluol and the paraffin must have only the consistency necessary to cut well (about 50 C. meltingpoint). In order to obtain good paraffin sections of the skin the following procedure is recommended : Pieces fixed in Flemming's solution or osmic acid are kept in 96% alcohol, then placed for not more than twentyfour hours in absolute alcohol and imbedded in paraffin by means of the chloroform method. In the chloroform, chloroform -paraffin, and pure paraffin they remain for one hour each. The paraffin used should consist of two parts paraffin of 42 C. , and one part paraffin of 50 C. melting-point. The thermostat must be kept at 50 C. (R. Barlow). The sections should not be mounted by the water-albumen method.
In sections of epidermis which have been freshly fixed with osmic acid, the stratum corneum may be clearly differentiated into three layers (probably because of the defective penetration of the reagent) into a blackened superficial, a middle transparent, and a still lower black layer (vid. Fig. 326).
In tissue fixed in alcohol or corrosive sublimate the stratum lucidum stains yellow with picrocarmin, but is very weakly colored by basic anilin stains. In unstained preparations the stratum lucidum is glass-like and transparent. Eleidin is diffusely scattered throughout both the stratum lucidum and stratum corneum. Like keratohyalin, it stains with osmic acid and also with picrocarmin, but not with hematoxylin. Nigrosin stains eleidin, but not keratohyalin.
Keratohyalin is insoluble in boiling water and is not attacked by weak organic acids. It dissolves, however, in boiling acetic acid, but is not changed by the action of pepsin or trypsin. The keratohyalin granules of the stratum granulosum swell in from i c / f to 5 c /o potassiumhydrate solution ; under the influence of heat these granules together with the cells containing them are finally dissolved. They are not attacked by ammonia, and remain unaffected for a long time in strong acetic acid. As ammonia and acetic acid render the remaining portions of the tissue transparent, these reagents may be employed for the rapid identification of keratohyalin. The larger flakes of keratohyalin swell in sodium carbonate solution (i%), but not the smaller granules, and it would seem that the larger granules have less power of resistance than the smaller. Keratohyalin remains unchanged in alcohol, chloroform, and ether, but is digested in trypsin and pepsin (not, however, the keratin). Keratohyalin can be stained with hematoxylin and most of the basic anilin dyes.
The prickles of the cells composing the stratum Malpighii may be seen in very thin sections (not over 3 // in thickness) of skin previously fixed in osmic acid. In this case it is best to employ not Canada balsam, but glycerin, which does not have so strong a clearing action. Isolation of the prickle cells is best accomplished as follows (Schiefferdecker): A fresh piece of epidermis is macerated for a few hours in filtered, cold-saturated, aqueous solution of dry pancreatin ; the whole may then be preserved for any length of time in equal parts of glycerin,
Fig. 326. Transverse section through the human skin. Treated with osmic acid ; X 30 : a, Part of the tortuous duct of a sweat-gland in the epidermis ; b, duct of same sweat-gland in the corium.
water, and alcohol. Small pieces taken from such specimens are readily teased and show both isolated and small groups of attached prickle cells.
The distribution of the pigment in the skin is best studied in unstained sections. With a nearly closed diaphragm and under medium magnification the pigment granules appear darker on raising the tube and lighter upon lowering it.
In sections of skin treated with Flemming's fluid, the structure of the cutis also may be studied. The medullary sheaths of the nervefibers and the fat appear black. In preparations stained with safranin the elastic fibers are colored red and are very distinct (Stohr and O. Schultze). For the orcein method according to Unna, see p. 128.
Hair may be examined in water without further manipulation. The cuticle is then seen to consist of polygonal areas, the border-lines of which correspond to the limits of the flattened cells. By slightly lowering the objective the cortical substance comes into view with its indistinct striation and occasional pigmentation. The medullary substance, if present, may also be seen with its vesicles containing air. Both the cortical and cuticular cells may be isolated, the process consisting in treating the hairs for several days with 33 c / c potassium hydrate solution at room temperature, or in heating the whole for a few minutes. Concentrated or weak sulphuric acid produces the same result. On warming a hair in sulphuric acid until it begins to curl and then examining it in water, we find that the cortical and medullary layers as well as the cuticle are separated into their elements. Treatment of the skin with Miiller's fluid, alcohol, or sublimate is recommended for the examination of hair and hair follicles. The orientation of the specimen should be very precise, in order to obtain exact longitudinal or cross-sections of the hair. There is hardly a structure of the body which is more suitable for staining with the numerous coal-tar colors than the hair and its follicle (Merkel).
The corpuscles of Meissner may be best obtained from the end of the finger. After boiling a piece of fresh skin from the finger-tip for about a quarter of an hour, the epidermis may be easily removed ; the papillae are now seen on the free surface of the cuds. A portion of the latter is cut away with a razor and examined in a 3 % solution of acetic acid. The corpuscles are readily distinguished. Their relations to the nerves should be studied in specimens fixed with osmic acid or gold chlorid. The terminations of the nerves in these end-organs are best seen in preparations stained after the infra vitam methylene-blue method.
The corpuscles of Herbst and Grandry are found in the waxy skin covering the bill, and in the palate of the duck (especially numerous in the tongue of the woodpecker). For the study of the nervous elements the following method is useful : Pieces of the waxy skin are removed with a razor and placed for twenty minutes in 50% formic acid. After washing the specimens for a short time in distilled water they are transferred to a small quantity of i c /c gold chlorid solution (twenty minutes), then again rinsed in distilled water, and placed for from twentyfour to thirty-six hours in the dark in a large quantity (^ liter) of Pichard's solution (amyl alcohol i part, formic acid i part, water 100 parts). After again washing in water the specimens are transferred to alcohols of gradually increasing strengths and finally imbedded in celloidin or celloidin-paraffin.
The Pacinian corpuscles occur in the mesentery of the cat and may be examined in physiologic saline solution.
The nerves of the epidermis are demonstrated by the goldchlorid method (see p. 48). But even here the chrome-silver method and the intra vitam methylene-blue method yield extremely good results, and may be used with great advantage in the study of the nerves in the cutis.
The so-called tactile menisci are very numerous in the snout of the pig and the mole. Bonnet recommends for these structures fixation in -33% chromic acid solution, overstaining with hematoxylin, and differentiation in an alcoholic solution of potassium ferricyanid.
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Reference: Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.
Cite this page: Hill, M.A. (2020, October 22) Embryology Book - A textbook of histology, including microscopic technic (1910) Special Histology 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_A_textbook_of_histology,_including_microscopic_technic_(1910)_Special_Histology_6
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