Book - Human Embryology and Morphology 6
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Chapter VI. The Skin and its Appendages
Considerable assistance in the understanding of the diseases to which the skin is liable and of the nature of the growths which arise from the epidermis, such as corns, bunions and cancer, is to be obtained by studying the manner in which the skin is developed. At first (see Fig. 51) the human embryo is covered by a single layer of epithelium (epiblast or ectoderm) as is the case in the adult amphioxus. By the end of the 1st month there are two layers, the lower representing the germinal layer ; the upper, the corneous layer (Fig. 52). In the 4th month intermediate layers appear, from which the stratum mucosum and the stratum lucidum are differentiated (Fig. 53).
|Fig. 51. The strata of the skin during the first month.||Fig. 52. The strata of the skin during the second month.||Fig. 53. The strata of the skin from the sixth month onwards.|
The epidermis rests at first on undifferentiated mesoblast or mesoderm, consisting of small round cells closely imbedded in a mucoid matrix. This is the normal structure of undifferentiated mesoblast. The superficial mesoblastic cells are subsequently condensed beneath the epidermis to form a corium. They become fibrillated and by the fifth month the mucoid substance has almost disappeared, but even in adult life, when the thyroid is diseased or removed, a mucoid substance may reappear, and a condition resembling the foetal state be thus produced. In the mucous membranes of the lips, anus and vulva the superficial layer of epithelium does not become cornified.
Formation of Dermal Papillae
About the fifth month, the dermal papillae, which are grouped in lines and ridges as is well seen in the palm, are formed in the following manner : Long, linear furrows of epidermis grow down into the dermis (corium) and divide its surface into narrow ridges. These ridges are subsequently subdivided into papillae. The down-growing nature of the epiblastic (epidermal) cells which is here exemplified, is of the greatest clinical importance. The enamel organs, we have seen, arose by a species of downgrowth of the epidermis ; so do hairs, sweat glands and sebaceous follicles. Prolonged pressure and friction welds the corneous cells into a solid plate, such as the callosities seen on the palms of manual labourers. Normal desquamation is arrested ; the cells produced in the deeper layers, unable to grow to the surface, grow inwards and produce corns. In cancer, the epithelial cells of the skin renew their youth and invade the dermis and deeper tissues.
The papillary lines on the palms and fingers give security of grasp (Hepburn). They are arranged in most variable patterns, but the prevailing types in man are those arranged as loops, spirals or whorls, Tig. 54. So variably are the types arranged on the pulps of the digits, that probably no two people show them in the same sequence counting from thumb to little finger in both hands. Hence the impress of the ten finger tips has been successfully used in the identification of criminals.
Fig. 54. The more common patterns formed by the dermal papillae on the tips of the fingers.
Hairs begin to develop in the 5th month. Morphologically a hair may be regarded as a dermal papilla, which has become sunk in the subcutaneous tissue, and capped by a process of epidermis. Hairs appear to have been primarily touch organs and are modifications of the touch bodies found in the skin of Reptilia (Gegenbaur). These touch bodies are composed of epithelial cells, having the same shape and arrangement as those which form the taste buds round the circumvallate papillae of the human tongue. The cells which cap the hair papilla evidently represent the primary sensory cells of the touch bodies ; they are situated in line, and continuous with the basal or germinal layer of the skin (Fig. 55). They produce the cells of the medulla of the hair which bursts through the epidermis (Fig. 55). The primary function of the hairs as touch organs is seen in the vibrissae round the mouths of carnivora, but the hair of man no longer is subservient to the sense of touch.
Fig. 55. Diagram of a Developing Hair.
The first stage in the development of a hair is the ingrowth of epidermis as a solid bud, which pushes in front of it the dermis to form the papilla on which the hair grows (Fig. 55). Only the two deeper of the primary layers of the epidermis are carried inwards to form the hair sheath and hair root.
The hairs produced at the fifth month are fine in texture (lanugo), and by the 7 th month the whole body is covered by it.
The production of hair buds goes on until birth, the later buds and hairs being thicker and stronger. After birth, new hairs are constantly reproduced within the sheaths to replace the old. Probably the manner in which new hairs are produced resembles that of teeth, viz. : from processes of the original bud. Hairs appear first on the head and then on other parts of the body. The comparatively hairless condition of man must be regarded as due to an arrest of development ; the hair distribution of adult man corresponds to a late foetal condition of the anthropoids. Certain sexual hairgrowths appear on the face, pubes and axilla at puberty. Morphologically the pubic region represents the separated axillary regions, and probably the explanation of sexual hairs in the axilla is due to this correspondence, for there is a persistent tendency towards symmetry of development in the upper and lower extremities. The primitive mammary ridges, also sexual structures, end at the axilla and groin.
The nails are made up of the basal, stratum mucosum, and stratum lucidum layers of the skin (Fig. 56), the corneous layer being lost after the 4th month of foetal life. They appear first in the 3rd month as fields of thickened epidermis on the tips of the digits, but are afterwards shifted dorsally, carrying their palmar nerves with them, so that the terminal phalanx is wholly supplied from the palmar digital branches. The nail of the little toe, a digit in a retrograde phase of development, is frequently shaped like a claw, probably a reversion to a primitive form. The nail is produced on the scattered papillae (the matrix) at its root. The area of production is marked by the lunule. On the nail bed, in front of the lunule, the papillae are arranged in longitudinal rows. If the nail be pressed, as by the boot, the lateral papillae, under the nail fold (see Fig. 56) are directed downwards, and their epithelial outgrowths follow the same direction, thus causing ingrowing nail.
Fig. 56. Diagrammatic Section across a Nail.
About the end of the 7th month the matrix of the nail root becomes differentiated, active growth sets in and the terminal margin of the nails become free ; it grows forwards over the corneous layer which covers the terminal row of papillae of the nail bed. The ridge of corneous epithelium under the nail-tip represents the hoof of ungulates.
In the 5 th month solid processes of epidermis grow into the dermis and produce sweat glands. They arise at the same time and in the same manner as, and often in common with, the buds of hair roots and sebaceous glands. They are produced within the papillary ridges, and hence the ducts of sweat glands, as may be seen on the palms and fingers, open along the summits of these. The sweat glands in the axillae are peculiar. In section they resemble the acini of the mammary gland, also believed to be highly modified sweat glands. The axillary glands contain much epithelial debris. They appear to be sexual in nature.
The sebaceous glands are outgrowths from the more superficial part of hair buds (Fig. 55). Their epithelial lining is derived from the germinal layer. In sheaths which have become occluded from the loss of the hair, or when the mouth of the gland is blocked, the secretion is retained, and a sebaceous cyst or wen, so frequently seen in the scalp, is produced. Round the mouth, on the lips and nose, the sebaceous glands, especially in disorders of the sexual organs, are apt to retain their secretions and become inflamed, small pustules being thus produced. The Meibomian glands in the eyelids are modified sebaceous glands. At birth the child is covered by the vernix caseosa, which is composed of desquamated corneous epithelium and the secretion of sebaceous glands.
The mamma is developed in the same manner in both sexes. At puberty the female breast undergoes a great development, while in the male it retains the infantile form.
The Female Breast. — The mamma of the female is of cutaneous origin, and in its earlier stages of development resembles, and probably corresponds to, a collection of sweat glands arising from a small circular depressed area of skin (Fig. 57 C). The manner of its development is the key to its anatomy. The adult female breast is composed of two elements : (a) Glandular tissue derived from the epiblast by a process of inbudding ; (6) An intricate arrangement of connective tissue derived from the mesoblastic subcutaneous tissue over the pectoralis major.
I. Origin of the Glandular Tissue.
(1) The Mammary Line is a slight ridge of epiblast which stretches along the ventral aspect of the body on each side, from the axilla to the groin, and is the first stage of mammary development in mammalian embryoes. In the sow, for instance, mammae are produced along the whole length of the mammary line. Although this stage has not been seen in the human embryo, it probably does occur, for in 5°/ of bodies a more or less distinct trace of a supernumerary mamma or nipple is to be found, and these occur for the greater part in the inguino-axillary line. Such as occasionally occur on the back or thigh are probably of the nature of dermoid tumours. Supernumerary nipples occur much more frequently in men than in women. This one may expect because the more vestigial the condition of an organ, the greater is the tendency to the production of ancestral (atavistic) forms.
Seven stages may be recognised in the developmental history of the glandular mammary tissue.
Four of these take place before birth : 1st (Fig. 57 A). The deeper layer of epiblast thickens over the mammary area ; this thickening represents a part of the mammary ridge or line. This stage is seen in the 2nd month.
2nd (Fig. 57 B). The thickening becomes depressed, thus giving rise to a slight pit on the surface.
3rd (Fig. 57 C). From the depression arises a number of buds, exactly similar to those of sweat gland (5 th month). The stalks of these buds form the epithelial lining of the lactiferous ducts.
4th (Fig. 57 D). The lobular buds, for each bud develops into a lobe, subdivide at their growing extremities. At first solid, they begin to caniculize (7th to 9th months). At or about birth the pit or depression, from which the lobular buds originated, is raised, evaginated and forms the surface of the nipple (Fig. 5 7 D).
Fig. 57. Showing the various stages in tho development of the Mamma.
A. During the 2nd month. B. At the commencement of the 3rd month. C. At the 5th month. D. At birth.
A=Epiblast. b = Subcutaneous tissue (mesoblast). c = Pectoralis major.
Thus the ducts come to open on the apex of the nipple. An ampulla is developed in each duct within the base of the nipple. Stage 3 represents the marsupial — the lowest mammalian form of mamma.
Stages after Birth
Stage 5 occurs at puberty ; the latent infantile lobular buds again undergo a rapid growth and give rise to the minor lobules and acini. Stage 6 occurs towards the end of pregnancy and consists of a renewed production of glandular tissue. Stage 7 sets in with the menopause and is characterized by an atrophy of the glandular tissue formed in the later stages of development.
In the process of subdivision, minor buds of adjacent lobes frequently unite together. Hence it is found difficult, during dissection, to separate the gland into its primary lobes. In any of the three later stages a localized and invading hypertrophy of the cells of the glandular tissue may take place. In this manner cancer is produced. The part played by the lymphatics, which are situated in the mesoblastic tissue of the gland, in the spread of this disease, makes their study important.
II. Origin of the Capsular or Mesoblastic Part of the Gland. — As the glandular buds grow out into the subdermal mesoblastic tissue, which reacts and hypertrophies around the invading processes, they divide it (see Fig. 58) into (a) superficial, and (b) deep layers, these being joined together by (c) interstitial septa. The superficial and deep layers are fused in (d) the circummammary tissue in which the final glandular buds terminate. The processes as they grow outwards also take on (e) perilobular and periductal sheaths. The deep and superficial layers are also connected with the anterior sheath of the pectoral muscles and the skin — for they are all parts of the same subdermal or subcutaneous mesoblastic layer.
Fig. 58. Diagrammatic Section of the Breast to show the arrangement of its Capsule and Lymphatics. The lymphatic vessels are represented by thin wavy lines.
As each part of the capsule carries a network of lymphatic vessels, into which the glandular lymph passes, it will be seen that the arrangement of the parts of the capsule is an important matter in both the physiology and surgery of the gland. The periductal and perilobular lymphatics communicate through the septal or interstitial vessels with the superficial mammary and deep (retro-mammary) lymphatics (Fig. 58). The superficial communicate with the subcutaneous ; the deep with those in the pectoral sheath and thus it will be seen that mammary cancer may spread to the skin or pectoralis major. The deep and superficial join in the circum-mammary lymphatics, and from these pass efferent vessels to the pectoral and central glands of the axilla. The lymph passes from these to the deep axillary and inferior deep cervical glands — all of which are involved in late stages of cancer of the breast. Other efferent vessels pass from the circum-mammary to the anterior intercostal glands of the upper four spaces ; one or two vessels may go to the cephalic gland.
Isolated or semi-isolated small masses of glandular substance are found situated in the circum-mammary tissue, beyond the body of the gland. Some may pierce the sheath of the pectoralis major and become a source of recurrent cancer. The presence of glandular remnants is explained by the fact that, when the primary budding takes place, the subdermal mesoblast is shallow and of small extent ; in the subsequent growth of the thorax, the tissue in which the mamma is developed, is widely spread out.
The position of the Mammary Gland is as a rule wrongly described. Quite a third lies on the serratus magnus and beyond the anterior border of the axilla. The axillary lobe reaches upwards in the axilla to the upper border of the third rib, where it is in contact with the central set of lymphatic glands (Stiles).
Fat begins to be deposited in the subcutaneous tissue during the 5 th month of foetal life. It forms a large element of the mammary gland after puberty. The subcutaneous tissue, out of which the capsule of the gland is formed, normally contains much fat. After lactation, when the glandular tissue atrophies to a considerable extent, a growth of fat replaces it. If no fat is deposited or if it be absorbed, then the breast loses its plump form and hangs on the chest.
The mammary nerves (secretory) come from the 3rd, 4th, and 5th intercostals ; the nipple is supplied from the same nerves. The nipple contains non-striated muscle and is covered with touch papillae and surrounded by modified sweat and sebaceous glands.
To render the glandular mammary tissue clearly recognisable from the surrounding connective tissue, Stiles adopted the method of immersing the mamma in a 5°/ of HN0 3 for two days. The glandular tissue becomes of a dark yellow tint, and thus can be detected even in minute quantities from the surrounding tissue of mesoblastic origin.
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Human Embryology and Morphology (1902): Development or the Face | The Nasal Cavities and Olfactory Structures | Development of the Pharynx and Neck | Development of the Organ of Hearing | Development and Morphology of the Teeth | The Skin and its Appendages | The Development of the Ovum of the Foetus from the Ovum of the Mother | The Manner in which a Connection is Established between the Foetus and Uterus | The Uro-genital System | Formation of the Pubo-femoral Region, Pelvic Floor and Fascia | The Spinal Column and Back | The Segmentation of the Body | The Cranium | Development of the Structures concerned in the Sense of Sight | The Brain and Spinal Cord | Development of the Circulatory System | The Respiratory System | The Organs of Digestion | The Body Wall, Ribs, and Sternum | The Limbs | Figures | Embryology History
Cite this page: Hill, M.A. (2021, April 19) Embryology Book - Human Embryology and Morphology 6. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_and_Morphology_6
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