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The Skin and Its Appendages
Contents
Skin (Cutis) and Subcutaneous Tissue |
Skin Appendages |
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(Tela Subcutanea) |
664 |
Glands of the Skin |
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Layers of the Skin |
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Hair |
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Epidermis 664 |
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Nails |
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Dermis 666 |
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Subcutaneous Tissue 667 |
Summary 671 |
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Sensory Organs of the Skin 667 |
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Tasks of the Skin |
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664 16 The Skin and Its Appendages
Skin (Cutis) and Subcutaneous Tissue (Tela Subcutanea)
Layers of the Skin
The skin is the common integument (integumentum commune) forming the external surface of the body. It is differentiated in various ways in different regions of the body. The term appendages comprises specific derivatives of the skin such as the sensory organs of the skin, the glands of the skin, nails, and hair.
In an adult the integument has a surface area of about 1.7 m2; it is composed of the skin (cutis) and the subcutaneous tissue (tela subcutanea). The skin includes the epidermis, a stratified squamous keratinous epithelium, and the dermis, a dense network of collagen and elastic fibers (Fig. 16.1). The subcutaneous tissue is firmly attached to the dermis and laced with dense white connective tissue fibers.
Epidermis
The epidermis with its epithelium forms the surface layer of the body surface. It is a stratified squamous keratinized (horny) epithelium, and is in most areas of the body 0.1−0.2 mm thick. On the palms of the hands and the soles of the feet, the epidermis is distinctly thickened (0.8−1.5 mm) (Fig. 16.2a, b). It forms a genetically determined pattern of papillary ridges.
The cells divide constantly in the lowest cellular layers (collectively called germinative stratum, stratum germinale), the basal and prickle (or spinous) cell layers (stratum basale and stratum spinosum) (Fig. 16.1). In this process, one daughter cell migrates to the surface, while the other divides again. As the cells migrate toward the surface to become cornified, they form granules (stratum granulosum). In the palms of the hands and the soles of the feet, they then lose their nuclei and cell boundaries to form an extra layer, the clear layer (stratum lucidum). After reaching the surface, they are shed from the horny layer (stratum corneum) as keratinous scales.
Three other types of cell can be found within the epithelial complex:
Merkel cells, melanocytes, and Langerhans cells. Merkel cells are secondary mechanosensory cells mostly found in sensitive areas of the skin (e. g., fingertips). Melanocytes are large cells with long processes (den-
= stratum 
granulosum)
Skin (Cutis) and Subcutaneous Tissue (Tela Subcutanea) 667
The papillary layer is directly adjacent to the epidermis and interdigitates with it by its dermal papillae. The height and number of these papillae depends on the mechanical demands of their location. Elastic fiber networks arranged in various ways in the reticular layer give the skin its resilience. They are responsible for the gaping of a wound.
Subcutaneous Tissue
The subcutaneous tissue (tela subcutanea) consists of loose, adipose connective tissue, which is subdivided by bands of connective tissue. It connects the skin with the superficial fascia covering the body and enables the skin to slide over it. The subcutaneous fatty tissue is quite variable between individuals and different regions of the body. It serves as a store for fat. Structural fat (e. g., on the soles of the feet) is distinguished from depot fat (e. g., abdominal fat pads). Between the subcutaneous tissue and the skin runs a network of arteries and veins, which sends branches as far as the dermal papillae of the dermis (Fig. 16.1).
Sensory Organs of the Skin
The sensory organs of the epidermis, the dermis, and the subcutaneous tissue include the encapsulated nerve endings (mechanoreceptors) and the free nerve endings (mechanical, pressure, pain, and temperature receptors). Their afferent nerve fibers run in skin nerves, together with autonomic efferent axons that supply the blood vessels, glands and hair muscles.
In addition to the Merkel disks (tactile menisci) in the epidermis, touch receptors occur as tactile cells (Meissner’s tactile corpuscles) in the connective tissue papillae of the dermis, and as lamellar corpuscles (pacinian bodies, Vater−Pacini corpuscles) in the subcutaneous tissues (Fig. 16.1). Free nerve endings can be seen especially in the dermis and in the basket networks of nerves around the hair follicles (Fig. 16.4).
Tasks of the Skin
The skin is an organ the individual layers of which have multiple functions:
668 16 The Skin and Its Appendages
Protection: through its keratinized epithelium and the secretions of its glands the skin protects from mechanical, thermal and chemical damage.
Temperature regulation: body temperature is regulated by constriction and dilation of the skin vessels and by secretion of fluid by the glands of the skin.
Water balance: protection against fluid loss and regulation of the secretion of fluid and salts by glands.
Sensory function: sensory organs of the skin in the form of pain, temperature, pressure, and touch receptors.
Immune function: high number of specific immune cells.
Communication: e. g., expressing autonomic responses by blushing or turning pale.
Skin Appendages
The epithelial appendages of the skin include dermal glands, hair, and nails. The surrounding connective tissue takes part in their formation.
Glands of the Skin
The glands of the skin include eccrine and apocrine sweat glands and sebaceous glands. There are a total of about 2 million eccrine sweat glands, and they are clustered especially in the skin of the forehead, the palms of the hands, and the soles of the feet. Their acid secretion forms an acid protective coat on the surface of the skin, inhibiting bacterial growth. Apocrine sweat glands are especially associated with hair (axillary, head, and pubic hair). Their secretion is rather alkaline and is triggered by sex hormones. Sebaceous glands, like apocrine sweat glands, occur mainly in hairy skin (sebaceous glands of hair follicles) (Fig. 16.4). Their secretion, sebum, is rich in fatty acids; it combines with sweat to make the skin pliant and contributes to the gloss of hair.
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Skin Appendages |
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Meissner’s |
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Shaft of hair |
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tactile corpuscle |
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Epidermis |
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Free nerve |
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ending in |
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epidermis |
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and dermis |
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Root of hair |
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Dermis |
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(radix pili) |
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Sebaceous gland |
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(follicular gland) |
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Basket network |
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of nerve fibers |
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around hair root |
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Hair root sheath |
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Arrector |
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muscle of hair |
Subcutaneous tissue |
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(m. arrector pili) |
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(tela subcutanea) |
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Hair papilla |
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Hair bulb |
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Pacinian body |
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Fig. 16.4 Hair and nerve endings in the scalp (hair papilla + hair bulb = hair follicle). (After Leonhardt)
Hair
Hair is seen as lanugo hair in the newborn and terminal hair in the adult. It serves as a cover for warmth and for touch sensation. A terminal hair sits in a root sheath, into which opens a sebaceous gland. The arrector muscle of hair (m. arrector pili) originates under the sebaceous gland on the side toward which the hair inclines and runs to the epidermis (Fig. 16.4). It erects the hair (sympathetic reaction) while pulling the epidermis into a little pit (goose bumps).
The hair includes a keratinous hair shaft with an epithelial root sheath and a root that is attached to a connective tissue hair papilla by an epithelial hair bulb. Hair bulb and hair papilla together form the hair fol-
670 16 The Skin and Its Appendages
licle, which is supplied by blood vessels and from which hair growth originates. While lanugo originates in the dermis, the roots of terminal hairs lie in the superficial part of the subcutaneous tissue (Fig. 16.4). The color of hair depends among other things on its pigment (melanin) content. If melanin production ceases, or after the deposition of small air bubbles, the hair appears gray or white.
Nails
Nails, like hair, are special epidermal formations. They are horny plates (nail plates) anchored in the nail bed. The nail bed is the epithelial tissue from which the nail grows continuously. The proximal edge of the nail plate lies under the nail sinus (sinus unguis), the lateral edge in the lateral nail fold (Fig. 16.5a−c). Distal to the sinus unguis, the nail-forming
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Nail plate |
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Nail wall |
Root of nail |
b |
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Nail plate |
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Nail bed |
Nail sinus (sinus unguis) |
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Lunule |
Nail fold |
Bony phalanx |
Nail wall |
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Fig. 16.5 a−c Finger nail. (After Rauber-Kopsch)
a View from above; b longitudinal section through nail bed; c cross-section through nail bed
Summary 671
epithelial tissue gleams through the nail as a semilunar whitish area (lunule). Capillaries showing through the nail give it its pink color.
The nails protect the terminal phalanges of the fingers and toes and form a backstop for pressure exerted on the pulp. This makes the nails important accessories to the sense of touch.
Summary The Skin and Its Appendages
Structure of the Skin
The skin is the cover of the body and forms its outer surface (ca. 1.7 m2). It includes the actual skin (cutis) and the subcutaneous tissue (tela subcutanea). The cutis is divided into epidermis and dermis:
Cutis
−Epidermis: stratified keratinous squamous epithelium with germinative layer (stratum germinativum = stratum basale and stratum spinosum), granular layer (stratum granulosum), and horny layer (stratum corneum). Its thickness varies: in general skin (e. g., back, abdomen, arms, legs) it is 0.1−0.2 mm thick; in skin of the soles of the feet and palms of the hands it is 0.8− 1.5 mm). It contains three other kinds of cell: Merkel receptors (mechanoreceptors), melanocytes (pigment cells), and Langerhans cells (immune cells).
−Dermis (also called corium): consists of a dense network of collagen and elastic fibers as well as nerves and blood vessels. It makes the skin resistant to tears and makes it pliable. The dermis includes a papillary layer (stratum papillare) next to the epidermis and a reticular layer (stratum reticulare) next to the subcutaneous tissue.
Subcutis (tela subcutanea, subcutaneous tissue): Consists of loose connective tissue rich in fatty tissue (subcutaneous fat) with numerous blood vessels. It allows the skin to slide over the underlying tissue and serves as a store for fat: structural fat (e. g., in soles of feet) and depot fat (e. g., abdominal fat pad).
Sensory organs of the skin occur in all its layers. They include encapsulated nerve endings (mechanoreceptors: Merkel cells in the epidermis, Meissner’s tactile corpuscles in the stratum papillare,
672 16 The Skin and Its Appendages
and pacinian corpuscles in the subcutaneous tissue) and free nerve endings (pain, pressure, and temperature receptors).
Tasks of the Skin
Protection: by cornification and glandular secretions
Temperature regulation: by dilation and constriction of blood vessels and evaporation of fluids
Water balance: by the regulation of fluid and salt secretion
Sensory function: by the presence of various kinds of receptor
Immune function: by the presence of immune cells
Communication: by the expression of autonomic responses.
Skin Appendages
Skin glands: these include eccrine and apocrine sweat glands and sebaceous glands:
−Eccrine sweat glands (especially on forehead, palms of hands, soles of feet) form an acid secretion (acid protective coat) that inhibits bacterial growth.
−Apocrine sweat glands (axillary, head and pubic hair) form an alkaline secretion and are triggered by sex hormones.
−Sebaceous glands (associated with hair) form sebum that is rich in fatty acids and keeps the skin pliant.
Hair: keeps the body warm and serves the sensation of touch (lanugo hair of the newborn and terminal hair of the adult). Terminal hair sits in a root sheath, into which a sebaceous gland opens. A hair muscle erects it. The hair consists of a horny hair shaft with an epithelial hair root sheath and a root (epithelial hair bulb and connective tissue hair papilla = hair follicle).
Nails: 0.5 mm thick horny plates (nail plates) anchored in an epithelial nail bed (growing zone). They protect the terminal phalanx of the fingers and toes and are an important accessory for the sense of touch (backstop for pressure on the pulp of the finger tip).