Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
The Skin and Its Derivatives

The Skin is the external covering of The Human Body, with a total surface area of 1.5–2.0 m². It protects the body against external influences, participates in thermoregulation, and performs receptor, respiratory, and excretory Functions. The skin shields the body from the penetration of microbes and toxic substances. Within 24 hours, about 0.6 liters of Water (sweat) containing metabolic products, including lactic acid, is excreted through the skin. Up to 82% of heat dissipation occurs via the skin.

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Fig. 110. Structure OF THE Skin:

1 — stratum corneum; 2 — epidermis; 3 — stratum basale; 4 — Connective Tissue fibers (Collagen, elastic, and reticular) and Cells; 5 — papillary layer; 6 — bundles of myocytes; 7 — Hair bulb; 8 — reticular layer; 9 — hair ROOT; 10 — sweat gland; 11 — sebaceous gland; 12 — adipose tissue lobules

The skin plays a major role as a Blood reservoir: its Blood Vessels contain up to 0.5 liters of blood. The deep layers of the skin contain A large number of sensitive nerve endings of various structures and functions. Skin derivatives include hair, Nails, Sweat Glands, Sebaceous Glands, and Mammary Glands.

Structure of the skin. The skin consists of two layers: the outer layer, the epidermis, and the inner layer, the dermis or true skin (Fig. 110). The thickness of the skin varies across different areas, ranging from 0.5 to 4 mm. Beneath the skin lies adipose tissue, known as the subcutaneous fat layer.

The epidermis is formed by a stratified keratinized epithelium. The thickest layer of the epidermis is found on the soles and palms, while a thin layer covers the eyelids, neck, chest, thighs, and shoulders. The Superficial layer of the epidermis undergoes keratinization and renews within 7–9 days. The Cells of the deep epidermal layer contain the pigment melanin, The amount of which determines skin color.

The dermis, or true skin, is formed by loose Fibrous connective tissue and is divided into two layers: the papillary and reticular layers, with no sharp boundary between them. The papillary layer forms projections—papillae—that extend into the epidermis and create a strictly individual pattern on the skin surface consisting of ridges and furrows. The reticular layer of the dermis contains thick bundles of connective tissue fibers arranged in a network-like fashion. This layer houses the hair roots and the initial sections of the sweat and sebaceous glands. The fibers of the reticular layer transition into the subcutaneous tissue.

The subcutaneous layer contains adipose tissue, which serves as the body's fat depot and plays a significant role in thermoregulation. The presence of the subcutaneous layer makes the skin mobile.

Skin derivatives. Skin derivatives include hair, nails, and Skin glands.

Hair can be bristle hair (eyebrows, eyelashes, scalp hair) and vellus hair, which is located on most areas of the skin. During Puberty, accelerated hair growth begins in the axillae and pubic region, and in males, on the face, limbs, chest, and abdomen. A hair consists of a shaft, which projects above the skin, and a root, located within the dermis. The hair root is enclosed in a hair follicle. A arrector pili Muscle is attached to the hair follicle, and a sebaceous gland opens into it. A hair lives from several months to 2–3 years. Hairs consist of flat cornified scales containing keratin and pigment granules. With age, the amount of pigment in the hair decreases, and air bubbles appear, resulting in graying hair.

Nails are horny plates lying in nail beds. Nail plates are formed by keratin-containing cornified scales that closely adhere to one another. The sides and root of the nail are covered by skin folds (cuticular folds). Skin glands include sweat, sebaceous, and mammary glands. These glands participate in thermoregulation and The excretion of certain metabolic products.

Sweat glands are tubular in shape. The initial sections of sweat glands are coiled into glomeruli (knots) and are located in the deep layers of the skin. Sweat glands are particularly abundant on the palms, soles, axillae, and groin area. The total number of sweat glands reaches 2.5 million. The secretion of these glands (sweat) contains 98% water and 2% organic and inorganic substances (urea, uric acid, and certain salts), which are eliminated from the body through the skin as Metabolic waste products. Evaporation enhances heat dissipation, thereby lowering body Temperature.

Sebaceous glands are located at the boundary between the papillary and reticular layers of the dermis. There are no sebaceous glands on the palms and soles. The ducts of the sebaceous glands open into the hair follicles. Sebum acts as a lubricant for the epidermis and hair, protects them against water and microorganisms, and softens the skin.

Mammary glands are modified sweat glands in terms of their origin. In males, the mammary glands remain underdeveloped. In females, intensive Development of the mammary glands begins with the onset of puberty, driven by the hormonal function of the Ovaries. During the Climacteric period (the decline in reproductive function after 50–55 years of age), a partial involution of the glands occurs, and glandular tissue is replaced by adipose tissue. During Pregnancy, glandular tissue proliferates, the glands increase in size, and the nipples and areolae darken. After breastfeeding ceases, the size of the mammary glands decreases. The mammary glands are located on the anterior chest wall at the level of the II–IV Ribs. Approximately in the middle of the gland is the nipple, on the tip of which there are 10–15 punctate openings where the excretory ducts of the gland open. The nipple is surrounded by a pigmented areola. Each mammary gland consists of 15–20 lobes separated from one another by connective tissue septa. The lobes are compound tubular-alveolar glands whose ducts open at the tip of the nipple. Each lobe consists of glandular lobules that proliferate during pregnancy and Lactation.

Cases may occur involving the underdevelopment of one or both mammary glands, the appearance of accessory glands (polymastia), or accessory nipples alone. In males, the mammary glands occasionally develop along the female pattern (gynecomastia).