Chapter 4: The skin, hair and nails
Layers of skin
Common integument refers to skin and subcutaneous tissue, hair, nails, and breast. The last-named is described with the upper limb. The skin (cutis) provides a waterproof and protective covering for the body, contains sensory nerve endings, and aids in the regulation of temperature. The skin is important, not only in general medical diagnosis and surgery, but also as the seat of many diseases of its own. The study of these is called dermatology (Gk derma, skin).
The area of the body surface is about 2 sq m. The temperature of the skin in general is normally about 32 to 36 degrees C. (90 to 96 degrees F.).
The skin (fig. 4-1) varies in thickness from about 0.5 to 3 mm. It is thicker on the dorsal and extensor than on the ventral and flexor aspects of the body. It is thinner in infancy and in old age. The stretching of the abdominal skin during pregnancy may result in red streaks (striae gravidarum) that remain as permanent white lines (lineae albicantes).
The skin consists of two quite different layers: (1) the epidermis, a superficiallayer of stratified epithelium that develops from ectoderm, and (2) the dermis, or corium, an underlying layer of connective tissue that is largely mesodermal in origin. The dermis makes up the bulk of the skin.
Corium (dermis)
The corium, or dermis, contains down growths from the epidermis, such as hair follicles and glands. It presents a superficial papillary layer of loose collagenous and elastic fibers, together with fibroblasts, mast cells, and macrophages. Elevations (papillae) project toward the epidermis. The thicker, deep reticular layer of the dermis consists of dense, coarse bundles of collagenous fibers. Some of the fibers enter the subcutaneous tissue, where they form bundles between lobules of fat. Smooth muscle is found in some regions (areola and nipple, scrotum and penis, and perineum). In some areas, muscle fibers of skeletal type (e.g., platysma) may be inserted into the skin. In tattooing, foreign particles, such as carbon, are introduced into the dermis.
The skin lies on the subcutaneous tissue ("superficial fascia"), a layer of fatty areolar tissue that overlies the more densely fibrous fascia. It should be remembered that fat is liquid, or nearly so, at body temperature. The subcutaneous tissue serves as a depot for fat storage and aids in preventing loss of heat. When a pinch of skin is picked up, subcutaneous tissue is included. A hypodermic injection is one given into the subcutaneous tissue.
Epidermis
The skin is covered by a film of emulsified material produced by glands and by cornification. The epidermis is an avascular layer of stratified squamous epithelium that is thickest on the palms and soles. The epidermis, where it is thick, presents five layers, as listed in table 4-1. In the outer layers, which may conveniently be grouped as the horny zone, the cells become converted into soft-keratin flakes that are worn away from the surface continuously. The stratum corneum is a tough, resilient, semitransparent cellular mem brane that acts as a barrier to water transfer. Under normal conditions, mitotic figures are practically confined to the deepest layer, the stratum basale, which is, therefore, the normal germinative layer of the epidermis. The various layers show the stages through which the basal cells pass before their keratinization and shedding. The cells of the epidermis are replaced approximately once per month. Keratin is a protein that is present throughout the epidermis, perhaps in a modified form. It is readily hydrated-hence the swelling of skin on immersion in water-and dryness of the skin is due chiefly to a lack of water.
Human epidermis displays a rhythmic mitotic cycle. Mitosis is more active at night, and it is stimulated by a loss of the superficial, or horny, zone. A part, or the whole thickness, of the epidermis may be raised up in the form of blisters by plasma when the skin is damaged (e.g., by a second-degree burn), and prolonged pressure and friction result in callosities and corns.
Several pigments, including melanin, melanoid, carotene, reduced hemoglobin, and oxyhemoglobin, are found in the skin. Melanin, which is situated chiefly in the stratum basale of the epidermis, protects the organism from ultraviolet light.
When an area of epidermis, together with the superficial part of the underlying dermis, is destroyed, new epidermis is formed from hair follicles, and also from sudoriferous and sebaceous glands, where these are present. If the injury involves the whole thickness of the dermis (e.g., in a deep burn), however, epithelization can take place only by a growing over of the surrounding edge of the epidermis or alternatively by the use of an autograft. Free skin grafts of the epidermis and a part or all of the thickness of the dermis can be applied, and vascularization takes place through connections between the subcutaneous vessels and those in the graft. A defect of the skin that extends into the dermis is termed an ulcer.
Lines of thickened epidermis known as papillary ridges form a characteristic pattern on the palmar aspect of the hand and the plantar aspect of the foot. They are concerned with tactile sensation. They contain the openings of the sweat glands and overlie grooves in the dermis; these grooves are situated typically between rows of double ridges known as dermal ridges (fig. 4-1). The papillary ridges appear in fetal life in a pattern that remains permanently. They are especially well developed in the pads of the digits, and finger prints in adults and foot prints in infants are used as a means of identification of an individual.
Specialized structures of skin
Sweat (sudoriferous glands)
The sweat, or sudoriferous, glands regulate body temperature, because perspiration withdraws heat from the body by the vaporization of water. The sweat glands develop in the fetus as epidermal downgrowths that become canalized. They are simple tubular glands, each having a coiled secretory unit in the dermis or in the subcutaneous tissue and a long, winding duct that extends through the epidermis and opens by a pore on the surface of the skin (fig. 4-1). Sweat glands are particularly numerous in the palms and the soles, where they open on the summits of the papillary ridges. The chief stimuli to sweating are heat and emotion. Emotional perspiration occurs characteristically on the forehead, axillae, palms, and soles.
Large sweat glands in certain locations, such as the axilla, areola, external acoustic meatus, and eyelid, develop from hair follicles and differ from the more common (eccrine) glands in being apocrine; that is to say, portions of the secreting cells disintegrate in the process of secretion. The perspiration from the apocrine glands is rich in organic material that is susceptible to bacterial action, resulting in an odor.
Water passes through the epidermis also by diffusion. This is termed insensible perspiration because it cannot be seen or felt.
Hairs
Hairs (or pili; pilus in the singular) are characteristic of mammals. The functions of hair include protection, regulation of body temperature, and facilitation of evaporation of perspiration; hairs also act as sense organs. Hairs develop in the fetus as epidermal downgrowths that invade the underlying dermis. Each downgrowth terminates in an expanded end that becomes invaginated by a mesodermal papilla. The central cells of the downgrowth become keratinized to form a hair, which then grows outward to reach the surface. The hairs first developed constitute the lanugo, or down, which is shed shortly before birth. The fine hairs that develop later constitute the vellus. Although hairs on many portions of the human body are inconspicuous, their actual number per unit area is large. In a few places (such as the palms and the soles and the dorsal aspect of the distal phalanges) the skin is glabrous, that is, devoid of hair.
The shaft of a hair consists of a cuticle and a cortex of hard-keratin surrounding, in many hairs, a soft-keratin medulla (fig. 4-2). Pigmented hairs contain melanin in the cortex and medulla, but pigment is absent from the surrounding sheaths. The color of hair depends mainly on the shade and the amount of pigment in the cortex and, to a lesser extent, on air spaces in the hair. In white hairs pig ment is absent from the cortex, and the contained air is responsible for the whiteness; "gray hair" is generally a mixture of white and colored hairs.
The root of a hair is situated in an epidermal tube known as the hair follicle, sunken into either the dermis or the subcutaneous tissue. The follicle is dilated at its base to form the bulb (matrix).
In the obtuse angle between the root of a hair and the surface of the skin, a bundle of smooth muscle fibers, known as an arrector pili muscle, is usually found. It extends from the deep part of the hair follicle to the papillary layer of the dermis. On contraction it makes the hair erect. The arrectores pilorum are innervated by sympathetic fibers and contract in response to emotion or cold. This results in an unevenness of the surface called "goose pimples" or "goose skin."
Sebaceous glands
Sebaceous glands develop from the epidermis in the fetus, usually from the walls of hair follicles. Sebaceous glands are absent from the palms and the soles. They are simple alveolar glands that form lobes in the dermis, generally in the acute angle between an arrector pili and its hair follicle. The basal cells of the gland proliferate, accumulate fat droplets, and are excreted as sebum through a short, wide duct into the lumen of the hair follicle. Contraction of the arrector pili may perhaps aid in expelling the sebum. Sebum keeps the stratum corneum pliable and, in cold weather, conserves body heat by hindering evaporation. Fat-soluble substances may penetrate the skin through hair follicles and sebaceous glands. Hence ointment vehicles are used when penetration is desired. Medicaments should be rubbed into the skin.
Sebaceous glands that are not related to hairs are found in the eyelids as tarsal glands; these are said to be apocrine in type, as are the ceruminous glands of the external acoustic meatus. Seborrhea involves an excessive secretion of sebum; the sebum may collect on the surface as scales known as dandruff. Acne is a chronic inflammatory condition of the sebaceous glands. When the exit from a sebaceous gland becomes plugged, a blackhead (comedo) forms; complete blockage may result in a wen (sebaceous cyst). At birth an infant is covered with vernix caseosa, a mixture of sebum and desquamated epithelial cells.
Nails
The nails (or ungues; unguis in the singular) are hardenings of the horny zone of the epidermis. They overlie the dorsal aspect of the distal phalanges (fig. 4-3). They protect the sensitive tips of the digits and, in the fingers, serve in scratching. Nails develop in the fetus as epidermal thickenings that undercut the skin to form folds from which the horny substance of the nail grows distally.
The horny zone of the nail is composed of hard-keratin and has a distal, exposed part, or body, and a proximal, hidden portion, or root. The root is covered by a distalward prolongation of the stratum corneum of the skin. This narrow fold is composed of soft-keratin and is termed the eponychium. Distal to the eponychium is the "half-moon," or lunula, a part of the horny zone that is opaque to the underlying capillaries.
Deep to the distal or free border of the nail, the horny zone of the fingertip is thickened and is frequently termed the hyponychium. The horny zone of the nail is attached to the underlying nail bed. The matrix, or proximal part of the bed, produces hard-keratin. Further distally, however, the bed may also generate nail substance. Moreover, the most superficiallayer of the nail may be produced by the epithelium immediately dorsal to the root and proximal to the eponychium. The growth of the nail is affected by nutrition, hormones, and disease. Nail growth involves considerable protein synthesis, as a result of which nonspecific changes occur in the nails in response to various local and systemic disturbances. White spots indicate incomplete keratinization.
Blood supply and innervation of
skin
The skin has a profuse blood supply, which is important in temperature regulation. The subcutaneous arteries form a network in the subcutaneous tissue, and from this is derived a subpapillary plexus in the dermis. Capillary loops in the dermal papillae arise from the subpapillary plexus, and from these loops the avascular epidermis is bathed in tissue fluid. A subpapillary plexus of venules gives the skin its pink color: the vessels become dilated when the skin is heated, and thereby make it look red. Most birthmarks consist of dilated capillaries (hemangioma). The dermis contains a lymphatic plexus that drains into the collecting vessels in the subcutaneous tissue. The cutaneous lymphatics can be shown in vivo by injecting vital dyes, and every intradermal injection is an intralymphatic one.
The skin has a rich sensory innervation (fig. 4-4). The cutaneous nerves pierce the fascia and ramify in the subcutaneous tissue to form plexuses both there and in the dermis. Finer axonal ramifications may run between the deeper cells of the epidermis. The cutaneous nerves supply both the skin and the subcutaneous tissue. The area of distribution of a given nerve, however, varies, and considerable overlapping of adjacent nerve territories takes place.
Nerves supplying the skin may form several different types of nerve endings, and these endings have been related in a general way to the basic types of sensations that can be appreciated in the skin and the subcutaneous tissue, namely, pain, touch, temperature changes, and pressure, or deep touch.
Hairy skin contains simple, free endings and plexuses around the hair follicles. Skin without hair, that of the palm, for example, presents the three types of sensory endings that are characteristic of the somatic nervous system: (1) free nerve endings arising from small myelinated fibers, (2) expanded tips, and (3) encapsulated endings. The basic types of sensation, however, can be elicited from both hairy and glabrous skin. Hence correlations between the type of sensation and a specific type of nerve ending are not justified. Lamellated corpuscles are particularly large, encapsulated endings that are found chiefly in the subcutaneous and deeper tissues.
Additional reading
Jarrett, A. (ed.), The Physiology and Pathophysiology of the Skin. Academic Press, New York, 1973, 1974. Volume 1 is on the epidermis, volume 2 on nerves and blood vessels, and volume 3 on the dermis.
Montagna, W., and Parakkal, P. F., The Structure and Function of Skin, 3rd ed. Academic Press, New York, 1974. A good introduction.
Pinkus, H., Die makroskopische Anatomie der Haut, in Normale und pathologische Anatomie der Haut. ed. by O. Gans and G. K. Steigleder, Springer, Berlin, vol. 2, 1964. An excellent account.
Questions
4-1 How do the two layers of the skin differ?
4-2 Into which layer is a hypodermic injection given?
4-3 What is normally the germinative layer of the epidermis?
4-4 What is a second-degree burn?
4-5 How does water leave the skin?
4-6 What is an arrector pili and how does it function?
4-7 What is the function of sebum?
4-8 Can the different types of nerve endings in the skin be related to the basic types of sensation (pain, touch, temperature changes, and pressure, or deep touch)?
Figure legends
Figure 4-1 General view of skin and subcutaneous tissue. A, "Thin" skin from abdomen. B, "Thick" skin from pal m of hand. X, Y, and Z represent the levels of a superficial (Thiersch) graft, a split thickness graft (including one third to one quarter of the dermis), and a full thickness (Wolfe) graft, respectively. The numerals 1 to 6 represent the levels of degrees of burns, according to Dupuytren's classification. Other classifications of burns are also used.
Figure 4-2 Diagram of a hair follicle. The follicle consists of an external root sheath, mainly the basal-cell layer of the epidermis, and an internal root sheath of soft-keratin, which includes a cuticle firmly anchored to that of the shaft of the hair.
Figure 4-3 Diagram of a sagittal section of a fingernail. The arrow indicates the junction between the root and body of the nail.
Figure 4-4 Diagram of the innervation of the skin. The numerals 1, 2, and 3 indicate the epidermis, dermis, and subcutaneous tissue, respectively. The letters A and C stand for adrenergic and cholinergic nerve fibers, respectively.