Objectives
- List the skin’s functions and relate them to its structure.
- Name the skin’s two major layers. For each, name the basic tissue type that predominates and describe the arrangement and distinguishing features of its constituent layers.
- Name the four cell types of the epidermis and describe their structure, function, and location.
- Relate the steps in cell renewal and keratinization to the epidermal layers.
- Compare and contrast thick and thin skin.
- Describe melanin granule synthesis and turnover.
- Identify and describe the components of hair follicles and nail complexes. Briefly describe nail and hair growth.
- Describe the skin’s blood and nerve supply in terms of structure, function, and location.
- Name and compare three types of glands in skin in terms of structure, function, and location.
- Identify skin type, layers, cell types, hair follicles, and glands in a micrograph of a section of skin.
MAX-Yield™ Study Questions
1. Name the two major layers of the skin (I.B) and compare them in terms of thickness (I.B.3; Fig. 18–1), vascularity (I.B.1 and 3), and embryonic germ layer of origin (I.B.1 and 3).
3. List the four cell types in the epidermis and compare them in terms of their number, location, and primary function (II.A, B.2, C, and D).
4. Compare thick and thin skin in terms of epidermal layers, numbers of hair follicles, sweat glands and sebaceous glands, and location (Table 18–1).
5. Beginning at the surface, list the five layers of the epidermis (II.A.1–5). Compare the keratinocytes in each layer in terms of their cell shape, capacity for cell division, staining properties, and the visibility of their nuclei and other organelles.
6. Compare the structures binding the keratinocytes and melanocytes in the stratum basale to the underlying basal lamina (II.A.1 and B.2).
8. Name the enzyme in melanocyte granules that is primarily responsible for melanin production (II.B.3).
11. Describe Langerhans’ cells (II.C) in terms of their shape, location, staining properties, granules, and immune function.
12. Describe Merkel’s cells (II.D) in terms of the skin type (thick or thin) in which they are most abundant, their association with nerve endings, and two possible functions.
13. Name the two layers of the dermis (III.A and B) and compare them in terms of their primary tissue type, thickness, and location in relation to the epidermis and hypodermis.
15. Sketch a hair follicle in longitudinal section (V.B.1–4; Fig. 18–3). Include and label the following:
Hair bulb
Dermal papilla
Germinal matrix
Melanocytes
Hair shaft
Internal root sheath
External root sheath
Glassy membrane
Connective tissue sheath
Arrector pili muscle
16. Sketch a cross-section through a hair follicle above the bulb (V.B.1–4; Fig. 18–3). Include and label the following:
Medulla
Cortex
Cuticle
Internal root sheath
External root sheath
Glassy membrane
Connective tissue sheath
19. Sketch a longitudinal section of a fingertip through the nail (VI.B; Fig. 18–4). Include and label the following:
Class, as determined by adenomere shape (tubular or acinar; VII.A)
Association with hair follicles (VII.A)
Sites in which they occur without hair follicles (VII.A)
Mode of secretion (merocrine, apocrine, or holocrine; VII.B)
Composition of secretion (VII.B)
Class, as determined by adenomere shape (tubular or acinar)
Distribution in skin
Mode of secretion (merocrine, apocrine, or holocrine)
Composition of secretion
Innervation
23. Sketch a vertical section of skin (Fig. 18–1); show the boundaries of the epidermis, papillary dermis, reticular dermis, and hypodermis. Indicate the location of the two arterial plexuses, the papillary capillaries, and the three venous plexuses (IV.A–D).
24. Name the types of sensory receptors found in the epidermis (I.B.1; II.D), dermal papillae (III.A), and reticular dermis (III.B; 24.II.A–G).
25. Name the components of the skin that play a role in protection from dehydration (II.A.3), abrasion (I.B.2; II.A.1; III.A), infection (II.A.3, C; III.A; 5.III.A.1), and ultraviolet radiation (II.B.5 and 6); regulation of blood pressure (IV.D) and body temperature (IV.D; VIII.A.3); sensory reception (II.D; III.A and B; 24.II.A–G); and excretion (VIII.A.3).
Synopsis
The skin is the largest and heaviest organ. It protects against microorganisms, toxic substances, dehydration, ultraviolet radiation, impact, and friction. It acts as a sensory receptor and has roles in excretion, vitamin D metabolism, and in body temperature and blood pressure regulation.
Human skin (the integument) comprises two types. Thick skin, limited to the palms and soles, lacks hair and has abundant sweat glands. Thin skin has hairs and covers the rest of the body (Table 18–1). Thick or thin, the skin consists of two distinct but tightly attached layers—the epidermis and dermis—which are underlain by the hypodermis (Fig. 18–1).
Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular and nourished by vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings (24.II.A). The epidermal layer is further divided into five layers; these include, in order from superficial to deep, the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale (II.A.1–5). The width of these layers differs in thick and thin skin (see Table 18–1).
Dermal–epidermal junction. The stratum basale is underlain by a basement membrane connecting the epidermis and dermis. The junction has the appearance of zigzagging interdigitations between upward projections of the dermis (dermal papillae) and downward projections of the epidermis (epidermal ridges).
Dermis. This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It is further divisible into a superficial papillary dermis and a deeper reticular dermis. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses (IV.D) that help regulate blood pressure and body temperature. The dermis is richly supplied with free nerve endings, a variety of encapsulated sensory receptors (24.II.B–G), and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
Hypodermis. Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue under the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, activity level, body region, and gender. It is also called subcutaneous fascia
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