THE INTEGUMENTARY SYSTEM

Structure of the Skin

 1. The integumentary system consists of the skin, hair, oil and sweat glands, nails, and sensory receptors.

 2. The skin is the largest organ of the body in weight. The principal parts of the skin are the epidermis (superficial) and dermis (deep).

 3. The subcutaneous layer (hypodermis) is deep to the dermis and not part of the skin. It anchors the dermis to underlying tissues and organs, and it contains lamellated corpuscles.

 4. The types of cells in the epidermis are keratinocytes, melanocytes, intraepidermal macrophages, and 

tactile epithelial cells.

5. The epidermal layers, from deep to superficial, are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum (in thick skin only), and stratum corneum . Stem cells in the stratum basale undergo continuous cell division, producing keratinocytes for the other layers.

 6. The dermis is composed of dense irregular connective tissue containing collagen and elastic fibers. It is divided into papillary and reticular regions. The papillary region contains thin collagen and fine elastic

fibers, dermal papillae, and corpuscles of touch. The reticular region contains bundles of thick collagen and some coarse elastic fibers, fibroblasts and macrophages, adipose tissue, hair follicles, nerves, sebaceous (oil) glands, and sudoriferous (sweat) glands.

 7. Epidermal ridges provide the basis for fingerprints and footprints.

 8. The color of skin is due to melanin, carotene, and hemoglobin.

 9. In tattooing, a pigment is deposited with a needle in the dermis. Body piercing is the insertion of jewelry through an artificial opening.

Accessory Structures of the Skin

 1. Accessory structures of the skin—hair, skin glands, and nails—develop from the embryonic epidermis.

 2. A hair consists of a shaft, most of which is superficial to the surface, a root that penetrates the dermis and sometimes the subcutaneous layer, and a hair follicle.

 3. Associated with each hair follicle is a sebaceous (oil) gland, an arrector pili muscle, and a hair root plexus.

 4. New hairs develop from division of hair matrix cells in the bulb; hair replacement and growth occur in a cyclical pattern consisting of growth, regression, and resting stages.

 5. Hairs offer a limited amount of protection—from the sun, heat loss, and entry of foreign particles into the eyes, nose, and ears. They also function in sensing light touch.

 6. Lanugo of the fetus is shed before birth. Most body hair on males is terminal (coarse, pigmented); most body hair on females is vellus (fine).

 7. Sebaceous (oil) glands are usually connected to hair follicles; they are absent from the palms and soles.

Sebaceous glands produce sebum, which moistens hairs and waterproofs the skin. Clogged sebaceous glands may produce acne.

 8. There are two types of sudoriferous (sweat) glands: eccrine and apocrine. Eccrine sweat glands have an extensive distribution; their ducts terminate at pores at the surface of the epidermis. Eccrine sweat glands are involved in thermoregulation and waste removal and are stimulated during emotional stress. Apocrine

sweat glands are limited to the skin of the axillae, groin, and areolae; their ducts open into hair follicles.

Apocrine sweat glands are stimulated during emotional stress and sexual excitement.

 9. Ceruminous glands are modified sudoriferous glands that secrete cerumen. They are found in the external auditory canal (ear canal).

 10. Nails are hard, dead keratinized epidermal cells over the dorsal surfaces of the distal portions of the digits. The principal parts of a nail are the nail body, free edge, nail root, lunula, hyponychium, nail bed,

eponychium, and nail matrix. Cell division of the nail matrix cells produces new nails.

Types of Skin

 1. Thin skin covers all parts of the body except for the palms, palmar surfaces of the digits, and the soles.

 2. Thick skin covers the palms, palmar surfaces of the digits, and soles. (See Table 5.4.)

 Functions of the Skin

 1. Skin functions include body temperature regulation, blood storage, protection, sensation, excretion and absorption, and synthesis of vitamin D.

 2. The skin participates in thermoregulation by liberating sweat at its surface and by adjusting the flow of blood in the dermis.

 3. The skin provides physical, chemical, and biological barriers that help protect the body.

 4. Cutaneous sensations include tactile sensations, thermal sensations, and pain.

Maintaining Homeostasis: Skin Wound Healing

 1. In an epidermal wound, the central portion of the wound usually extends down to the dermis; the wound edges involve only superficial damage to the epidermal cells.

 2. Epidermal wounds are repaired by enlargement and migration of basal cells, contact inhibition, and division of migrating and stationary basal cells.

 3. During the inflammatory phase of deep wound healing, a blood clot unites the wound edges, epithelial cells migrate across the wound, vasodilation and increased permeability of blood vessels enhance delivery of phagocytes, and mesenchymal cells develop into fibroblast.

4. During the migratory phase, fibroblasts migrate along fibrin threads and begin synthesizing collagen fibers and glycoproteins. 

 5. During the proliferative phase, epithelial cells grow extensively.

 6. During the maturation phase, the scab sloughs off, the epidermis is restored to normal thickness, collagen fibers become more organized, fibroblasts begin to disappear, and blood vessels are restored  to normal. 

Development of the Integumentary System

 1. The epidermis develops from the embryonic ectoderm, and the accessory structures of the skin (hair, nails, and skin glands) are epidermal derivatives. 

 2. The dermis is derived from mesodermal cells.

Aging and the Integumentary System

 1. Most effects of aging begin to occur when people reach their late 40s.

 2. Among the effects of aging are wrinkling, loss of subcutaneous adipose tissue, atrophy of sebaceous glands, and decrease in the number of melanocytes and intraepidermal macrophages.