Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger,
Peer Review Status: Externally Peer Reviewed
The integument is composed of the skin, which covers the entire body, in addition to accessory organs derived from skin. The accessory organs include the nails, hair, and glands of various kinds.
Skin serves many important functions: (1) It is an impervious barrier that excludes harmful substances and prevents desiccation; (2) it plays an important role in the regulation of body temperature; (3) it readily repairs itself; (4) it receives sensory stimuli (touch, pressure, temperature, and pain); (5) sweat glands excrete waste products; (6) lacrimal glands produce an isotonic saline bathing solution for the eyes; (7) sebaceous and ceruminous glands secrete sebum and cerumen ("wax"), respectively; and (8) mammary glands secrete milk.
Skin consists of two layers: (1) the epidermis, which is classified as keratinized stratified squamous epithelium; and (2) the dermis (corium), which is composed of connective tissue (Plate 135). Beneath the dermis is the hypodermis or subcutaneous superficial fascia, which may be composed primarily of fatty connective tissue, a stored energy reserve.
The exposed surface of skin is not smooth but creased by flexion folds around skeletal joints, and it is also pitted by openings of hair follicles and sweat gland ducts. In addition, a characteristic surface pattern exists on the palms of the hands and soles of the feet, which are often used legally for positive individual identification (e.g., fingerprints).
The most abundant cells in the epidermis (epithelium) are termed keratinocytes because they synthesize keratin in increasing amounts as they progress toward the free surface and exfoliation. Keratin constitutes about 85 per cent of the total protein of the uppermost layer (stratum corneum). Only three other cell types are found in the epidermis; they are not abundant but have significant functional activity, which will be described here. The epithelium varies in thickness in different regions of the body but is usually 0.1 mm thick (given ranges are from 0.07 to 0. 12 mm). In the skin of the palms and soles, however, it may be 0.8 to 1.4 mm in thickness. The epidermis of the palm and sole is thick (so-called thick skin) and has five morphologically distinct layers. From the deepest outward, the layers are (1) stratum basale, (2) stratum spinosum, (3) stratum granulosum, (4) stratum lucidum, and (5) stratum corneum.
The strata basale and spinosum are also referred to as the Malpighian layer. The cells of stratum basale constitute a single layer of columnar or cuboidal cells in contact with the basement membrane and connective tissue of the dermis. The stratum basale is often referred to as the stratum germinativum. Melanin pigment granules are richly concentrated in the basal layer but may be found throughout the stratum Malpighii (Plate 11). Above the basal layer is the stratum spinosum, which is composed of polyhedral cells, the so-called prickle cells (Plate 137). The stratum granulosum is composed of a layer of three to five cells that contains keratohyalin granules of irregular shape that stain with basic dyes. The stratum lucidum consists of a tightly packed layer of cells without nuclei containing a refractile substance called eleidin. This layer is strongly eosinophilic. The most superficial layer, the stratum corneum, is composed of many dead cells without nuclei, which are filled with keratin. The surface cells of this layer are continually being desquamated and replaced by cells that arise from mitotic activity in the basal layer. This activity results in the outward displacement of higher cells toward the free surface until they, too, are exfoliated.
Over most of the body, the epidermis is much thinner and simpler in composition. The strata Malpighii and corneum are always present, and the stratum granulosum, consisting of two or three layers of cells, can usually be seen. The stratum lucidum is rarely seen in thinner epidermis. The epidermis is devoid of blood vessels but is nourished by diffusion from capillaries in the underlying dermis (Plates 138 and 157).
Three additional cell types are found in the epidermis: (1) melanocytes, (2) Langerhans cells, and (3) Merkel's cells.
Melanocytes are found in the basal layer of the epidermis and junctional zone of the dermis, but their long slender processes containing melanin extend outward between keratinocytes. Keratinocytes contain melanin granules, but they are produced only in melanocytes and are transferred to keratinocytes by a process called cytocrine secretion. The number of melanocytes is believed to be similar in all races, differing only in the rate of production and transfer of melanosomes to keratinocytes.
Some dendritic cells located in the upper layers of the epidermis were first described by Langerhans in 1868. In routine sections, they have a dark staining nucleus and a pale cytoplasm. Their dendritic processes can only be seen by special methods (Gairn's gold chloride). These cells are believed to play an important role in contact allergic responses and other cell-mediated skin reactions.
Merkel's cells are found only in the basal layer of the epidermis. These pale staining cells are believed to be paraneurons involved in sensory reception.
The dermis or corium underlying the epidermis is 0.3 to 4.0 mm in thickness and may be divided into two layers, papillary and reticular. The papillary layer includes the ridges and papillae, which protrude between the epidermal pegs. The papillae contain tactile corpuscles of Meissner and small blood vessels (Plate 138). The papillary connective tissue is composed primarily of collagenous and elastic fibers. The reticular layer is composed of coarse interlacing collagenous fibers and an elastic network. Hair follicles and smooth muscle (arrector pili), sweat and sebaceous glands, and Pacinian corpuscles are located in the reticular layer (Plates 143, 145, 146 and 147). in the face and neck, striated muscle fibers (muscles of facial expression) terminate in the dermis.
This layer, not part of the skin, is also called superficial fascia and is a loose network of connective tissue bundles and septa, which blend indistinctly with the dermis. This arrangement of the connective tissue of the superficial fascia allows the movement of skin except on the palm of the hand and sole of the foot, where the skin is firmly anchored to deeper structures. In most places, particularly the abdominal wall, lobules of fat may be abundant; the layer may then also be called the panniculus adiposus.
The epidermal derivatives include the nails, hair, and glands.
The fingernails, found only in man and other primates, are convex rectangular specializations of the epidermis called nail plates. Underlying the nail plate is the nail bed, composed of the germinative layer of the epidermis.
Hair is a characteristic of mammals. These elastic, horny filaments may grow to a length of 5 feet or longer and vary in thickness from 0.005 to 0.2 mm. Hair is found on all parts of the skin except the palm and sole, and the oral, anal, and urogenital orifices. Hair consists of a free shaft and a root located in the dermis and superficial fascia. The hair is surrounded by a tubular epithelial follicle. Associated with the follicle are sebaceous glands and the arrector pili smooth muscle fiber bundle (Plates 83, 145, and 147).
Cutaneous glands include the sebaceous, sweat, lacrimal, and mammary glands. The sebaceous glands produce sebum, an oily substance formed by the degeneration of cells rich in lipid droplets and cellular remnants. The glands discharge their contents by the contraction of the arrector pili muscle and by any pressure applied to the gland. Sweat glands are coiled tubular glands, which are widely distributed and vary regionally. There are approximately 100 per cm3 on the palm and sole. The merocrine sweat gland consists of a coiled secretory tubule and duct. At the periphery of the coiled secretory tubule and enclosed within the basement membrane, spindle-shaped myoepithelial cells wind in longitudinal spirals around the tubule. Myoepithelial cells resemble smooth muscle fibers, and it is believed that their contraction empties the contents of the gland, sweat, onto the surface of the skin (Plate 144).
Another variety of sweat gland is the apocrine gland, which is less widely distributed. These glands are large, branched, and less coiled than the ordinary merocrine glands. The lumen of the secretory tubule is wide, the cells are larger and with distinctive projections from their surfaces, and the myoepithelial cells are larger and more numerous than in ordinary sweat glands. The axillary apocrine glands develop their large size at puberty (Plate 140). In women, apocrine sweat glands show periodic changes with the menstrual cycle.
The glands that produce ear "wax" or cerumen are located in the external auditory meatus (Plate 142). They are similar to axillary apocrine sweat glands but are unusual because the ducts may branch and open into hair sacs along with the sebaceous glands.
The differences between eccrine and apocrine sweat glands follow: (1) Eccrine sweat glands are never connected to hair follicles, whereas apocrine sweat glands are. (2) Eccrine sweat glands produce a watery secretion, whereas apocrine secretion is more viscid. (3) Eccrine sweat glands are innervated by cholinergic (parasympathetic) nerves, whereas apocrine glands are innervated by adrenergic (sympathetic) nerves.
The lacrimal gland is a compound tubuloalveolar serous gland and is an outgrowth of the upper lateral margin of the conjunctiva. The secretion is a clear, salty liquid (tears) that moistens, flushes, and protects the conjunctiva and cornea (Plates 300 and 310).
The mammary glands are specialized cutaneous glands that develop rapidly but incompletely at puberty. Additional differentiation begins during pregnancy, and functional activity begins after childbirth. Marked regression of the glandular tissue occurs when nursing ceases. The gland is made up of 15 to 20 lobes, each with its own duct system surrounded by interlobar connective tissue and fat cells. The glandular epithelium resembles that seen in apocrine sweat glands.
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