The skin has greater total mass than any other organ in the body. It consists of two structural units, namely epidermis and dermis, which are strongly attached to each other. The outer epidermis consists of stratified squamous epithelium, superficial layer of which are cornified (keratinized). Epidermis is an avascular structure and derives its nutrition by diffusion of nutrients from the dermis, the second and deeper layer of skin. The dermis is composed of vascularized and irregularly arranged fibroelastic connective tissue. The dermis forms the supporting base for the epidermis, and is separated from it by a well defined basement membrane. The dermis consists of two layers: the superficial papillary layer and the deeper reticular layer. The papillary layer is made up of loose connective tissue. This layer projects as finger like processes known as papillae into the epidermis. Some papillae have nerve endings and are known as tactile papillae and other papillae have capillary plexus and are known as vascular papillae. The distribution of these papillae shows regional differences that are possibly sequelae of mechanical demands. A basement membrane separates the dermis from the epidermis.
The cells of the basal layer of epidermis are attached to the basement membrane and they proliferate throughout life. The progeny cells become displaced further away from their source of nutrients in the dermis. Slowly, these cells die and they are converted into superficial layer of dead keratin, the stratum corneum. Based on the relative thickness of stratum corneum, the skin is classified into two types, namely thick and thin skin. However, it should be clear to the reader, which these terms refer to the thickness of epidermis and not to the thickness of the skin as a whole.
Thin skin covers the entire body except the palms of hands and the soles of feet, which are covered by thick skin. The skin of the back has relatively thick dermis and relatively thin epidermis and keratin. The hypodermis, also known as superficial fascia is composed of loose connective tissue and adipose tissue. Bundles of collagen fibers forming fibrous bands anchor the deep layer of dermis to the underlying fascial sheath of muscles or bone. This arrangement permits a limited amount of free movement of the skin over the underlying tissue.
Functions of Skin
The skin is not a mere envelope wrapped around our bodies but indeed a most versatile organ performing various activities. Being water proof, it prevents the evaporation of water and escape of tissue fluids.
However, epidermis is not entirely impervious to chemicals, i.e. some chemicals can be absorbed through the epidermis and can enter the blood and lymph capillaries in the underlying dermis. Hence, care should be taken to protect the skin from coming in direct contact with harmful chemicals.
Skin does not become desiccated in dry atmosphere, nor does it imbibe water in the bath. Due to a layer of keratin on its outer surface, the epidermis forms an effective barrier against pathogenic organisms. Skin becomes thick (keratinized), where it is subject to rough treatment or friction. Skin is fastened down where it is most likely to be pulled off. Skin has friction ridges that provide a good grip where it is most liable to slip. The pigment melanin present in the skin protects the body from harmful effects of ultraviolet rays in strong sunlight. By sweating, the skin not only lowers the body temperature, but it also acts as an accessory organ of excretion. Skin is a factory for the antirachitic vitamin D (ergosterol), which is produced on exposure of skin to ultraviolet rays present in the sunlight. Even with our ingenious modern machines we cannot create such a tough and durable highly elastic fabric that can withstand heat and cold, wet and dry, acid and alkali, microbial invasion and withstand wear and tear over years and years throughout the entire life span of the person.
The dorsal skin surface of the tip of each finger and toe forms a highly specialized appendage the nail consisting of a dense keratinized plate, the nail plate.
The nail plate has a root, which extends proximally deep to the overhanging nail fold formed by skin, a free end which projects distally and two lateral borders, and two surfaces—a deep surface and a free superficial surface. A white crescent, seen on the free surface distal to the nail fold is known as lunula. The deep surface rests on the nail bed formed by stratified epithelium and white collagen fibers. The nail root and the nail bed extend deeply into the dermis to lie in close apposition to the distal interphalangeal joint, and the dermis beneath the nail plate is firmly attached to the periosteum of the distal phalanx. Growth of nail takes place by proliferation and differentiation of epithelium at the nail root known as nail matrix and at the nail bed as far as the lunula. Beyond this point the nail slides distally on its bed, which does not actively contribute to nail growth. The nail plate adheres to the nail bed till the magin reflecting on its proliferative activity, the nail matrix is thicker than the rest of the nail bed. The highly keratinized free edge of the nail fold is known as eponychium and skin beneath the free end of the nail is known as the hyponychium. Acute illnesses may temporarily arrest the growth of nails and a transverse ridge appears on the each nail when growth is resumed. Presence of transverse ridges on nails is an evidence of past illness. Nails grow approximately three millimeters in a month and the date of past illness can be estimated. All nails do not grow at the same rate, generally the middle finger has the maximum rate of growth and the digiti minimi (fifth finger) has the slowest growth rate. The toenails grow more slowly as compared to the finger nails.
Hairs are present almost over the entire surface of the body except the palms, soles, red of lips, parts of external genitalia, and skin over dorsal surface of distal phalanges of hand and feet. Hairs are also present in the outer one-third part of external acoustic meatus and vestibule of which are lined by skin. Long hairs are present in the scalp, eyebrows, eyelashes, vestibule of nose and external acoustic meatus. At puberty, hairs appear on pubis, external genitalia, axillae and in the male on face.
The hair has a shaft, which projects beyond the surface of the skin, and a deep part known as root. At the deeper end of the root there is a swelling, known as the bulb which fits over a dermal papilla. Old hairs are constantly falling out and new hairs replace them. The life of eye lashes is roughly 3 to 5 months and that of scalp hair is 2 to 4 years. A bundle of smooth muscle fibers known as arrectores pilorum, is attached to the epidermis and to the slanting surface of the hair follicle. When this muscle contracts it makes the hair stand erect. Spasm of arrector pilorum produces “goose skin.” The shaft of hair consists of outer cuticle, cortex and inner medulla. Whiteness of a hair depends upon the pigment melanin in the cortex and the airspaces within it. Degree of differentiation and density of hair follicles varies from region to region (40 to 880/sq. mm).
Sebaceous glands are simple alveolar holocrine glands. They are bottle shaped and they open into the hair follicle. These glands develop mostly from hair follicle. However, they develop in some sites devoid of hair follicles, e.g. eyelids, nipples and labia minora and inner surface of prepuce. These glands are absent in skin of palms and soles of feet. In sections, sebaceous glands look like pale staining flask shaped areas on the slanting side of the hair. Sebaceous glands produce an oily secretion known as sebum, sebum keeps thin skin and its hairs soft, supple, pliable and water proof. Subaceous glands are filled with polyhedral cells which breakdown into secretion and hence the name holocrine, meaning that the whole cell disintegrates into secretion. One or two glands are associated with a hair follicle. The glands in vestibule of nose and external acoustic meatus are largest. Boils and carbuncles start in hair follicles.
Arrector Pili Muscle
In most areas of thin skin, each of the hair follicle has an associated bundle of smooth muscle called arrector pili muscle. The muscle receives this name because, its contraction makes hairs “stand on end.” The arrector pili muscle extends from an attachment near the base of the connective tissue sheath of hair follicle to the papillary layer of dermis and hence it traverses the dermis obliquely. Together with the sebaceous gland, it is situated on the side of the obtuse angle formed by the hair follicle with the papillary layer of dermis. Because of this arrangement, when the arrector pili muscle contracts, it not only causes the hair to stand up a little straighter but also gives the sebaceous gland a gentle squeeze, helping to express more of sebum on to the hair and the skin surface. The arrector pili muscles are innervated by the sympathetic division of the autonomic nervous system, and cold is a well-known stimulus for the reflex that leads to their contraction, causing goose-skin.
Apocrine Sweat Glands
Apocrine sweat glands are microscopically similar to eccrine sweat glands except that they open into the upper part of the hair follicle; however, their distribution is limited to axillae, the areolae of breasts, and the pubic and perineal regions, and they secrete only after puberty. When produced, their cloudy looking secretion does not give any odor. However, as soon as certain of its constituents have been acted upon by bacteria, they liberate distinctly unpleasant odors. Offensive body odors are no longer socially acceptable, and they are kept under control by improving personal hygiene apocrine sweat glands have a relatively large secretory portion, and their duct is narrow. Their secretory portion is surrounded by myoepithelial cells, innervated by sympathetic nerves contraction of these cells can bring about expression of the secretion, under sexual excitement and stress. Their name is misnomer, because it is now well known that they secrete by merocrine mode of secretion.
Sweat glands help in regulation of body temperature because their watery secretion, hypotonic with respect to plasma—sweat or perspiration withdraws heat from the body, through evaporation of its water content. Sweat glands are simple tubular glands of merocrine type. Each gland has a coiled secretory unit lying in the subcutaneous tissue and a long tortuously running duct which passes through the dermis. The duct enters the epidermis between two ridges and proceeds spirally to the skin surface. In the stratum corneum, it is represented merely by a cleft between cells.
Apocrine sweat glands are mainly confined to areolae of breasts, axillae and genital regions where they produce a viscid milky white secretion which becomes malodorous after the action of skin commensal bacteria. Apocrine sweat glands do not become functional until puberty. In women these glands undergo cyclical changes under the influence of hormones of the menstrual cycle. Sweat glands are present in the skin of all parts of body except the red of lips and glans penis. They are most numerous in the axillae, palms and soles. The ceruminous glands in outer part of external acoustic meatus and ciliary glands of eyelids are modified sweat glands. In the axilla, external genitalia and around the anus there are modified sweat glands which produce an odor and they are known as “sexual skin glands.”
The autonomic nerve fibers which control sweating, travel to the sweat glands along with the peripheral cutaneous nerves which provide general sensations any damage to the nerves will result in loss of sweating along with sensory loss.
The salty taste of sweat secretion is due to sodium chloride. This is the reason, why sweat glands are described as accessory organ of excretion (accessory to kidney). Normal sweat secretion keeps the thick horny layers of palms and soles supple and it increases the friction between the skin and the object grasped.
Blood vessels for the supply of skin run in the hypodermis. The dermis receives two arterial plexuses; one is deeply located near the subcutaneous tissue known as the cutaneous plexus, and the other is in the subpapillary layer lying just beneath the dermal papillae known as subpapillary plexus. The cutaneous plexus fatty tissue of the hypodermis and deeper aspect of dermis, and capillary networks which envelope the hair follicles, deep sebaceous glands and sweat glands. The subpapillary plexus supplies superficial part of dermis and the superficial appendages and it sends capillary loops into the papillae. The returning blood passes through several layers of thin walled subpapillary venous plexuses, thence through the cutaneous venous plexus and finally to the superficial veins. Arteriovenous anastomoses connect some of these arterioles to venules. The arteriovenous anastomoses are sometimes open to increase blood flow in order to facilitate heat loss in hot conditions. These anastomoses are sometimes closed in order to decrease blood flow to minimize heat loss in cold conditions whilst nevertheless maintaining adequate nutritional flow. Lymph vessels of the skin form a plexus at the junction of the dermis and the hypodermis. These vessels receive blind finger like vessels from the papillae and they drain into bigger lymph vessels which accompany main arteries and veins of the region.
The cutaneous nerves have two types of nerve fibers namely afferent somatic fibers for general sensations for perception of touch, pain, heat, cold, and pressure, and efferent autonomic fibers for smooth muscles of blood vessels and arrector pilorum, and for sweat glands and sebaceous glands. Free fibers end between the cells of germinative layer and hence intradermal injections may cause pain, if these fibers are injured. Free nerve endings are present around follicles. Free nerve endings are simplest form of sensory receptors, merely consisting of numerous small terminal branches of affenent nerve endings serving a variety of relatively unsophisticated sensory modalities such as temperature, touch and pain. These nerve fibers are of relatively small diameter with slow rates of conduction, although some of these fibers are myelinated, the nerve endings are devoid of myelin. Touch corpuscles are present around the hair follicles and in the tactile papillae. Pacinian corpuscles for perception of sense of pressure lie in the hypodermis and occure abundantly alongly the sides of digits. Pacinian corpuscles can also detect vibrations. Pacinian corpuscles are large encapsulated found in deeper layers of skin. They range from 1 to 4 mm in long hand in section they have appearance of a cut onion. They consist of delicate capsule enclosing many concentric lamellae of flattened cells. Towards the center of the corpuscle, the lamellae become closely packed and the core consists of a single large unbranched nonmyelinated nerve fiber with several club-like terminals, which become myelinated as it leaves the corpuscle. Meissner's corpuscles are mechanoreceptors and respond to skin displacement due to touch. It is significant that Meissner's corpuscles are most abundantly in regions of substantial tactile sensitivity like palmer surface of fingers, plantar surface of feet, lips, eyelids, external genitalia and nipples. These corpuscles lie just below the dermoepidermal junction in the papillary layer of dermis. These are small encapsulated sensory receptors involved in reception of light discriminatory touch, the degree of discrimination depending upon the proximity of the receptor to one another. They consist of delicate collagenous tissue capsule surrounding a mass of plump oval cells arranged transversely and probably representing specialized Schwann cells. Nonmyelinated branches of large myelinated sensory fibers ramify throughout the cell mass in a helical pattern. Ruffini's corpuscles lie deep in the dermis close to hypodermis and are numerous on the plantar surface of feet and they are robust spindle shaped structures, thought to be mechanoreceptors that respond to tension in the collagen fibers. Krause end bulbs are situated in the papillary of dermis and they occur in the conjunctiva, lining of eyelids, oral mucosa, tongue, pharynx and external genitalia. They are delicate receptors, the function of which has not been established but it appears likely that they are mechanoreceptors.
Regional Differences in Skin
The structure of skin differs considerably from one part of the body to another, the principal difference being in epidermal thickness, the size density and state of activity of hair follicles, the nature and density of sweat glands and sensory receptors.
Skin of the face is highly mobile especially around the orifices of the head (mouth, nose, eyes and ears). Lips are reddish partly because the skin is translucent and partly because the vascular papillae are unusually long reaching very close to the surface of epithelium.
Skin of the scalp is robust due to thick densely collagenous dermis. Hair follicles are numerous and closely packed. In fair people, the hair is light in color and the hair follicles are fewer in number and somewhat smaller in size producing finer hair. The follicles of the scalp are long and have more numerous sebaceous glands than those of other areas. Merocrine sweat glands are more numerous though less prominent due to profusion of other appendages. The skin of scalp is firmly anchored to the dense subcutaneous tissue and the epicranius. These three layers are firmly bound together and are separated from periosteum of outer skull by loose areolar. Scalp infections can easily spread to the meninges causing meningitis if not treated on time. Skin over the dorsal surface of the neck and thorax is thick and less sensitive to cutaneous stimuli.
Skin of the abdomen is loosely attached to the hypodermis. The skin of axillae and pubic region shows a moderate density of hair follicles which tend to be orient obliquely to the skin surface and are often curved rather than straight causing the hair to be curled. Apocrine sweat glands are a common feature of this type of skin and they are associated with hair follicles into which they discharge their secretions.
Microanatomy of Skin
In understanding the microanatomy of epidermis, it is important to appreciate that the keratin that constitutes the outer layer of epidermis is not a cellular secretion but the end result of transformation of epithelial cells known as keratinocytes into squames (scales) of keratin. When these scales are worn away and desquamate from the surface, they are replaced by keratinization of the underlying living cells. This means that there must be as much cellular proliferation in the deepest layer of epidermis as there is loss of keratinized cells from its surface. It takes approximately two weeks for keratinocytes to move from the deepest layer (stratum germinativum) to the superficial layer (stratum corneum) after which they remain in the stratum corneum for approximately two weeks more, giving a total transit time of one month. Because the appearance of keratinocytes changes as they shift from deep to superficial layer the epidermis gives the microscopic appearance of being composed of five different layers described below:
This is the deepest layer, so named because it generates new cells, is made up of low columnar cells attached to the underlying basement membrane by hemidesmosomes. The basement membrane is too thin to be resolved by light microscopy. The basal aspect of each germinal cell is highly irregular and bound to the basement by numerous hemidesmosomes. Like cells of adjacent stratum spinosum, small cytoplasmic projections extend across the intercellular spaces to abut upon those of adjacent cells. Desmosomes bind these contact points. Mitotic figures are most frequently observed in this layer but cell division also occurs lesser extent stratum spinosum.
New keratinocytes are displaced into more superficial layer. In electron microscopy (EM) it has been seen that cells of basal layer have a considerable content of free ribosomes and polyribosomes together with substantial numbers of intermediate filaments of prekeratin type known as tonofilaments, which are destined to become part of keratin. By the time the cells reach the next layer bundles of tonofilaments become wide enough to be seen in light microscope (LM). These bundles look like spines or prickles and hence the second layer of epidermis is known as stratum spinosum, also known prickle cell layer.
Cells of this layer are polyhedral and their borders appear to be separated from one another by small spaces that are traversed by fine spine like processes. In EM it can be seen that each of these spine like processes (prickles) represents a site where desmosomed hold neighboring cell membranes together. The tonofilaments distribute tensile forces from one cell to the next and thereby enable epidermal cells to withstand fairly rough treatment.
Prominent nucleoli and cytoplasmic basophilia indicate active protein synthesis in these cells. A fibrillar protein cytokeratin, the predominant synthetic product of these cells, aggregates to form intracellular fibrils known as tonofibrils which converge upon the desmosomes of the cytoplasmic prickles. Tonofibrils become more prominent towards the stratum granulosum. Tonofibrils are also found in small numbers in cells of stratum basale. In upper levels of stratum spinosum lipid containing granules with illuminated internal structure (lamellar granules) are formed. The contents of these granules are extruded into the intercellular spaces as the cells pass to the more superficial layers. This is why the next layer of epidermis is known as stratum granulosum.
The cells in this layer flat, diamond shaped, placed in multiple layers (4–5 layers). They are characterized by numerous dense basophilic granules, which crowd the cytoplasm and tend to obscure the tonofibrils. In EM these granules appear as stellate or angular electron dense masses. Chemical nature of these keratohyalin granules is distinct from that of fibrous proteins of tonofibrils. The process of keratinization is thought to involve the combination of tonofibril and keratohyalin elements to form the mature keratin complex. In the outer most aspect of stratum granulosum, cell death occurs due to rupture of lysosomal membranes, and released lysosomal enzymes may play an important role in the final process of keratinization.
This layer is thin and appears as a clear bright homogenous line, hence the name stratum lucidum. It consists of closely packed dead cells with no nuclei. The nuclei of dying cell become pyknotic, disintegrate and disappear (karyolysis). These cells are now little more than cell membrane containing prekeratin filaments complexed with amorphous protein.
This layer is fifteen to twenty cells thick. The morphology and staining characteristics of this layer are strikingly different from that of underlying layers. The nuclei and cytoplasmic organelles have disappeared and keratohyalin granules are transformed into amorphous matrix embedded in prekeratin fibrils derived from tonofilaments. The dead and dying cells are flattened, devoid of nuclei and other organelles and filled with mature keratin. In the deeper aspect of this layer, the cornified cells retain their desmosomal junctions and the intracellular keratin has an ordered pattern. In this manner each cell is transformed into one of the squames of keratin 30 to 40 microns in diameter. Between the lowermost constituent layers of stratum corneum there are layers of extracellular lipid that are thought to be derived from lamellar granules of keratinocytes. Towards the surface, the desmosomes and internal structure of the cells become completely disintegrated, a process which precedes desquamation.
In addition to keratinocytes, three other cell types (all poorly stained) are found in the epidermis namely melanoytes, Merkel cells and Langerhans’ cells.
Merkel cells are associated with free nerve endings in thick skin and are presumed to serve as sensory receptors; their ultrastructural features suggest that they form part of the diffuse neuroendocrine system. Their ultrastructural features are similar to those of synapses but no neurotransmitter has yet been demonstrated in them.
Langerhans' cells are specialized representatives of the macrophage-monocyte system which act as antigen-presenting cells in the skin. They are found in small numbers throughout the epidermal layers having long dendretic processes extending between surrounding keratinocytes. Their numbers increase in atopic and other immunologically based skin disorders. When stimulated they migrate via dermal lymphatics to the paracortical zones of regional lymph nodes for presentation of antigen to T lymphocytes. In EM, Langerhans cells can be easily be distinguished from keratinocytes by their obvious lack prekerayin filaments bundles. Their nuclei exhibit angular indentations. Their cytoplasm contains a prominent Golgi apparatus without much rER. Secondary lysosomes and a moderate number of mitochondria are present. Their cytoplasm contains unique granules known as Langerhans cell granules present more in number around the Golgi complex. Their function seems to be served as antigen-presenting cells in the initiation of cutaneous contact hypersensitivity reactions, i.e. contact allergic dermatitis (Figs 1.1 and 1.2).
Pigmentation of Skin
The color of human skin depends on three major factors. Firstly, the skin has an inherent yellowish color due in part to the presence of various carotene pigments in the subcutaneous fat. Secondly, the concentration and state of oxygenation of hemoglobin, and the presence of other pigments such as bile pigments in blood are reflected in skin color. Thirdly, skin color is determined by the amount of pigment melanin present in the epidermis. This factor is the most important variable between individuals of same race and between members of different races.
Melanin is synthesized by melanocytes that originate from neural crest cells (neuroectoderm) and migrate during development to the epidermis, where they become scattered in the basal layers in contact with the basement membrane. Melanocytes have long, dendritic processes which ramify between the epithelial cells but they do not establish cell junctions with the epithelial cells. Their cell bodies are usually located between the basal epithelial cells at higher levels.
The ratio of melanocytes to basal epithelial cells varies from about one in five to one in ten in different regions of the body, being highest in skin of face and external genitalia. The number of melanocytes is relatively constant between different individuals irrespective of race. Differences in skin color are therefore due to the amount of melanin produced rather than the number of melanocytes present.
Melanin synthesis involves conversion of the amino acid tyrosine, via intermediates including dihydroxyphenylalanine (DOPA), to melanin. Within melanocytes, melanin accumulates in immature secretory vesicles known as premelanosomes. These mature to form melanosomes which are then disseminated throughout the long cytoplasmic processes from which they are transferred to surrounding keratinocytes mainly of the basal layers. Hence, pigmented cells of the skin are both the melanocytes that synthesize melanin and the epithelial cells which have taken up melanin. Commonly, the epithelial cells contain much more melanin than the melanocytes themselves. The size, shape, and rate of production of melanosomes varies between individuals of one race and between racial groups. In blond and red-haired people there is biochemical differences in the form of melanin produced. Sunlight promotes melanin synthesis and causes darkening of previously synthesized melanin. In addition, melanin synthesis is stimulated by pituitary hormone, melanocyte-stimulating hormone (MSH). In Caucasian there are merely traces of melanin in the basal epithelial cells and relatively inactive melanocytes. In comparison, the basal epithelial of Negroid skin are packed with melanin and melanocytes themselves are difficult to identify in histology slides.
The columnar arrangement of keratinizing cells reflects functional grouping of cells in the basal layer. The pattern of proliferation and differentiation in the epidermis is far from random. From studies in mouse, it is known that epidermal keratinocytes are derived from groups of 10 to 11 basal epidermal cells that constitute the basal compartment of what is known as epidermal proliferative unit or EPU. Each EPU seems to contain one stem cell that occupies a more or less central position in the basal layer of the unit. As progeny cells in the basal layer begin to differentiate, they are laterally displaced to the periphery of the EPU. Continuing proliferation eventually results in the ejection of such cells into the overlying layer, where they continue to mature and gradually flatten so that they eventually cover the area occupied EPU. Studies indicate that the rate at which maturing postmitotic cells move up in the EPU from its basal layer is approximately one cell per day. Most of the cellular proliferation occurs in the stratum germinativum but in human skin mitotic figures are occasionally observed in the suprabasal layers indicating that at least some of the cells that enter these layers are still capable of mitosis. Psoriasis is a manifestation of accelerated and probably misregulated keratinocyte turnover. In normal skin, the transit time for cell displacement from stratum germinativum to the outer surface of skin is approximately 4 weeks, in cases of psoriasis it is approximately one week because proliferation occurs in the bottom three layers of epidermis instead of being largely confined to stratum germinativum.
The relative content of melanin in epidermis accounts for different skin colors in various races of man (black, brown, yellow and white).
The dermis is made up of two layers of connective tissue which merge with each other. The outer papillary layer which is by far the thinner and it is composed of loose connective tissue. It is called papillary layer because it is represented primarily by the connective tissue papillae that project into the epidermis. This layer extends a short distance below the papillae and then merges with the thicker reticular layer, which consists of dense irregularly arranged connective tissue and comprises the remainder of the dermis. It is known as reticular layer because it is composed of thick bundles of collagen fibers that interlace with one another in a net like manner. Most of the dermal collagen is type I collagen, and roughly 15 percent of it is type III collagen.
Fine elastic fibers are arranged as network in the papillary layer whereas coarser elastic fibers are randomly distributed in the reticular layer. Elastin content of dermis is relatively low.
The papillary layer also differs from the reticular layer in the number of capillaries it contains. The relatively abundant capillaries in the papillary layer extend up into the papillae and provide nourishment for the epidermis. In the reticular layer capillaries are not numerous except in association with epidermal appendages that extend down into the reticular layer.
The cells of dermis are mainly fibroblasts responsible for elaboration of the collagen, elastin and ground substance. Various white blood cells, mast cells and tissue macrophages involved in nonspecific defense and immune functions are also present. In addition there are generally a few adipocytes present in dermis.
Skin can distend considerably when abdomen enlarges during pregnancy. However, excessive stretch breaks the collagen fibers of the dermis. Bands of thin wrinkled skin appear on abdomen, buttocks, thighs and breasts. These are known as striae gravidarum. Stretch marks also form in obese individuals and result from loosening of fascia and reduced cohesion between the collagen fibers as the skin stretches. Stretch marks usually fade after delivery and weight loss, but they never disappear completely. Sweat glands are simple tubular glands particularly numerous in thick skin. It has been estimated that there are 3000 sweat glands per square inch in the palmar skin. Each sweat gland consists of a secretory part and an excretory duct. The secretory unit is situated in the subcutaneous tissue immediately below the dermis. The secretory part is coiled on itself and in sections it looks like a cluster of tubes cut in transverse and oblique sections. The secretory cells are of two types. Most are cuboidal or columnar and have pale cytoplasm containing glycogen. These cells are wider at their base than at their luminal surface. Canaliculi between cells of this type convey sweat to the lumen. Cells of second type are narrower at their base than at their luminal surface. Their cytoplasm contains granules that stain darkly and hence they are known as dark a cell, which distinguishes them, from clear cells described earlier. The luminal diameter of the secretory portion of a sweat gland is roughly equal to the thickness of its walls. Spindle shaped myoepithelial cells, which resemble smooth muscle cells, but are ectodermal in origin are wrapped around this part, on the inner aspect of basement membrane. Contraction of these cells assists in expelling sweat. The connective tissue that surrounds the basement membrane of the secretory portion of the gland is condensed to form a supporting sheath. From the secretory portion of the gland the duct ascends in a tortuous manner to the skin surface. Cells lining the duct stain more deeply than those of the secretory portion of the gland. The duct is lined by two layers of which are smaller than those in the secretory portion. Hence, compared to the secretory the duct has more nuclei in its wall which stain more darkly. The lumen of the duct is narrower than that of secretory portion. This is unusual because, in most glands, luminal diameter of the gland exceeds that the secretory units. The duct follows a helical path through the dermis and enters an inter papillary peg. From this point on, the keratinocytes of the epidermis constitute the walls of the duct. After pursuing a markedly helical course through the epidermis, the duct opens at a sweat pore on the crest of an epidermal ridge.
In third month of intrauterine life, hair follicles develop as invaginations of ectoderm into underlying mesoderm. These invaginations become canalized to form the hair follicles and the mesoderm gives rise to dermis and hypodermis. During fifth and sixth month, the fetus becomes covered with very delicate hair called lanugo. Lanugo is shed before birth except in the region of eyebrows, eyelids and scalp where it becomes somewhat stronger. A few months after birth, these hairs are shed and replaced by slightly thicker ones. Over the rest of the body new hairs appear and the body of the infant becomes covered with a dawny coat of hairs known as villus. No hair follicles are formed after birth. Because of the influence of male sex hormones at puberty course hairs develop in the axiliary and pubic regions, face and to a variable extent on other parts of the body. The course hairs of scalp, eyebrows, eyelashes and those that develop at puberty are known as the terminal hairs. Approximately 95 percent of body hair in males is terminal hair and about 65 percent of body hair in females is villus.
There are two different forms of keratin that are present in hair follicles, which have different physical, chemical properties and different microscopic appearance. Chemically, hard keratin is relatively unreactive and contains more cystine and disulfide bond than soft keratin does. Soft is present in skin of the whole body and hard keratin is present only in cortex and cuticle of hairs and in the nail plates of fingers and toes. Formation of hard keratin requires a steady transition of living epidermal cells to keratin without formation of keratohyalin granules. Hard keratin does not desquamate and is more permanent than soft keratin which is steadily desquamates from the free surface of skin.
The deepest part of the hair follicle gives rise to an important group of cells known as hair matrix. The matrix fits over a tiny connective tissue papilla of mesoderm containing capillaries which serve as a source of tissue fluid and nutrition for the hair. The epidermal down growth that connects the matrix with the surface is canalized and is lined by surface epidermis is called external root sheath. Near the surface of the skin, the external T sheath exhibits all the layers seen in epidermis, including the layer of soft keratin which lines the uppermost part of the follicle. Deep down in the follicle the external root sheath does not exhibit some of the more superficial layers of epidermis. At the bottom of the follicle, the external root sheath becomes continuous with the matrix, the external root sheath consists of only stratum germinativum. Investing the external root sheath there is a fibrous sheath formed by the collagen and elastic fibers of the dermis.
Growth of hair in the follicle is due to the proliferation of cells in the hair matrix. As the uppermost cells in the matrix become displaced further away from the papilla, which is the source of nutrition, they become transformed into keratin. Some cells form cortex and cuticle of hair which are made up of of hard keratin without keratohyalin granules. The cellular region where the transition from cells to hard keratin occurs is called the keratogenous zone. Hairs grow because of continuing proliferation of matrix cells and continuing conversion of their progeny into keratin.
The proliferating matrix cells produce another layer called internal root sheath which surrounds the hair only part way up the follicle. The internal root sheath separates the hair from the external root sheath. Internal root sheath is made up of soft keratin and granules a present in cells which are becoming keratinized. These granules are known as trichohyalin granules.
Endogenous pigment of hair is melanin which is synthesized by melanocytes distributed in the hair matrix close to the papilla. The dendritic processes of melanocytes provide melanin for the epithelial cells. These cells form the cortex of the hair and give it color. In older people the melanocytes exhibits an increasing inability to make tyrosine that results in lack of pigment in the hair cortex turning the hair gray. Hair pigments of only three different colors have been identified under the microscopes which are black, brown and yellow. The yellow melanin is known as pheomelanin and the black and brown are known as eumelanin. The production of these pigments is genetically controlled.
The cross-sectional shape and other features of terminal hairs vary in relation to race. Three chief types of hair can be identified—straight, wavy and wooly. Straight hair is found in the Mongol races, Chinese, Eskimos and Indians. This type of hair is course and round in cross section. Wavy hair is present in Europeans. In cross-section wavy hair is oval in shape. Wooly hair is present in black races and it is elliptical or kidney shaped in cross-section. A terminal hair consists of a central medulla made up of soft keratin and a peripheral cuticle and cortex of hard keratin. Some hairs exhibit no medulla and they show only cuticle and cortex of hard keratin.
The cutical consists of thin, flat scale like cells arranged on the surface of the surface of the hair-like tiles on the roof of a house except that the free edges point upward instead of downward. The free edges of these cells interlock with free edges of cells similar cells that line the internal root sheath and whose free edges point downward. This arrangement renders it impossible to pull out a hair without removing at least a part of the internal root sheath with it. The cells of internal root sheath can be subject to genetic mapping. This is of medicolegal importance. Medulla consists of soft keratin. The cells of medulla are cornified and separated from one another. In these spaces air or liquid may be present.
Hair growth is cyclic and hair coming on a comb does not indicate imminent baldness. Each hair follicle alternates between growing and roo resting matrix continues to proliferate and the hair grows in length. In the resting phase, the matrix becomes inactive and atrophies. The root of the hair then becomes detached phase. During the growing phase, cells of the from the matrix and gradually moves up the follicle, gaining for a time secondary attachment to the external root sheath as the lower end of the hair approaches the neck of the follicle. Eventually the hair falls out of the follicle. A new matrix then develops and this leads to growth of hair from the same follicle.
Hairs of the scalp last approximately 2 to 3 years before entering the resting phase of 3 to 4 months. This enables some people to attain hair length of one meter or more. In other parts of body, however, the growing phase is shorter and the resting phase is relatively longer. In hairs of the eyebrows, the growth phase is 1 to 2 months and the resting phase is 3 to 4 months. However, because neighboring hair follicles are commonly in different phases of their cycle at any given time, the renewal of hair usually goes unnoticed. There is no sound evidence to substantiate the notion that hair cuts or too much of shaving accelerate the rate of growth of hair which is estimated to be roughly 0.4 to 0.5 mm per day in scalp hair (Figs 1.3 and 1.4).
Developmentally, skin is a composite structure with contributions from ectoderm, mesoderm and the neural crest. It develops in common with the nervous system, with the superficial common component, epidermis developing from ectoderm and the dermis being a derivative of mesoderm. Ectoderm gives rise to the epidermis and its appendages namely hair, glands and the nails. Development commences with the development of a superficial protective layer periderm, which consists of a single layer of squamous epithelium deep to which is the basal layer. This periderm is continuously keratinized and desquamates till 21st week of development. During this process, the cells of periderm are replaced by the basal layer. After 21st week, periderm disappears gradually, desquamated layers forming a part of vernix caseosa. Vernix caseosa is cheesy material in the liqnor-amnii, which consists of fetal hair, sebum and cells of amniotic fluid, besides the cells which have desquamated from the skin. At birth, the fetus is covered with this material. While the superficial cells form the periderm, the deeper layers form the basal stratum germinativum. The superficial layers form epidermis which shows formation of ridges. These epidermal ridges are the anatomical basis of Dermatoglyphics—the science, which deals with fingerprints.
Deeper layers continue as stratum germinativum, which gives rise to hair buds, deeper parts of which form the hair follicles and the superficial part form the root sheath. Mesoderm forms the dermis. Superficial outgrowth of the dermis forms the dermal papilla, some of which acquire a plexus of capillaries or nerve. Contributors from the neural crest to the skin are manifested as melanoblasts in the dermis and melanocytes in the epidermis. Melanocytes produce melanin. In individuals with darks skin melanin production starts before birth and is continued after birth as well. In individuals with light skin, melanin production is confined to postnatal life only.
Hair buds arise as solid outgrowths of epidermis into dermis, with the deeper part expanding to form the hair bulb, which receives a dermal core to form a papilla. The peripheral cells form the epithelial root sheath and neighboring mesenchymal cells b form the dermal root sheath. Some cells of the hair bulb grow peripherally and get keratinized to form the hair shaft which after further growth pierces the epidermis to reach the exterior. Melanoblasts migrate into hair bulb and differentiate into melanocytes. Melanin produced by these cells is transferred to hair bulbs before birth. Hair becomes visible by 20th week as lanugo hair. During perinatal period, they are replaced by coarser hair called villus hair. These are found all over the body except, scalp, axillae and pubic region where terminal hair develops. In the males, terminal hair appears on the face and chest as well.
Nails begin to develop at the digital tips by the 10th week. Formation of finger nails is followed shortly by formation of toe nails. Initially nails develop as a thickening of epidermis called the nail field, which gets surrounded by a fold of skin called mail fold. Epidermal cells grow from nail folds get keratinized over a circumscribed area, the nailplate in each digit. Growing nails reach the tips of fingers by the 32nd week and of the toes by the 36th week.
Most sebaceous glands appear as buds of developing epidermal root sheath of the hair follicle all over the body. Sebaceous glands develop independent of hair follicles only in glans penis and the labia majora.
Sweat glands arise as outgrowths of the epidermis into the dermis. Peripheral ectodermal cells of this outgrowth differentiate into secretory and myoepithelial cells. Apocrine sweat gland of the axilla, pubic region and areolae of breast develop in common with the hair follicles, from the stratum germinativum.
Mammary gland is a modified sebaceous gland with a well developed duct system. Ectodermal thickenings called milk ridges extend from axilla to the groin. These milk ridges persist only in the pectoral region in human beings. These milk ridges disappear in all other regions.
During the forth week, a down growth of ectoderm in pectoral region is seen. It rapidly branches into buds. The proximal part develops as duct system and distal part into alveoli. Stroma and fat are derived from adjacent mesenchyme. During the late fetal period, the epidermis at the site of origin gets depressed. Mesenchyme under it proliferates, as a result of which nipple and areola differentiae. At birth only the main duct system is well developed. In females, at puberty the duct system divides further with rapid growth of fat and stroma. At the first pregnancy, the glandular system proliferates and the alveoli develop, which begin to secrete during lactation. The gland involutes in old age. In males, the mammary glands remain rudimentary throughout life. Occasionally, under influence of estrogens, the breasts in the male can undergo and increase in size. This condition is known as gynecomastia.
- David H Comack. Ham's Histology, 9th edn. JB Lippincott Company, London: Philadelphia. 1987.
- Susan Standring. Gray's Anatomy, The Anatomical Basis of Clinical Practice 39th edn. Elsevier Churchill Livingstone. New York; 2005.