Epidermis.

The epidermis, the thin avascular superficial layer of the skin, is made up of an outer dead cornified portion that serves as a protective barrier and a deeper, living portion that folds into the dermis. Together these layers measure 0.05 to 0.1 mm in thickness. The epidermis is nourished by blood vessels in the dermis. The epidermis regenerates with new cells every 28 days. The two major types of epidermal cells are melanocytes (5%) and keratinocytes (90%).

Melanocytes are contained in the deep, basal layer (stratum germinativum) of the epidermis. They contain melanin, a pigment that gives color to the skin and hair and protects the body from damaging ultraviolet (UV) sunlight. Sunlight and hormones stimulate the melanosome (within the melanocyte) to increase the production of melanin.¹ The wide range of skin color is caused by the amount of melanin produced; more melanin results in darker skin color.²

Keratinocytes are synthesized from epidermal cells in the basal layer. Initially these cells are undifferentiated. As they mature (keratinize), they move to the surface, where they flatten and die to form the outer skin layer (stratum corneum). Keratinocytes produce a fibrous protein, keratin, which is vital to the skin’s protective barrier function. The upward movement of keratinocytes from the basement membrane to the stratum corneum takes approximately 4 weeks. If dead cells slough off too rapidly, the skin will appear thin and eroded. If new cells form faster than old cells are shed, the skin becomes scaly and thickened. Changes in this cell cycle are reflected in many skin problems, such as psoriasis.

Dermis.

The dermis is the connective tissue below the epidermis. Dermal thickness varies from 1 to 4 mm. The dermis is very vascular.

The dermis is divided into two layers, an upper thin papillary layer and a deeper, thicker reticular layer. The papillary layer is folded into ridges, or papillae, which extend into the upper epidermal layer. These exposed surface ridges form congenital patterns called fingerprints and footprints. The reticular layer contains collagen and elastic and reticular fibers.

Collagen forms the greatest part of the dermis and is responsible for the skin’s mechanical strength. The primary cell type in the dermis is the fibroblast. Fibroblasts produce collagen and elastin fibers and are important in wound healing. Nerves, lymphatic vessels, hair follicles, and sebaceous glands are also found in the dermis.

Subcutaneous Tissue.

The subcutaneous tissue lies below the dermis and is not part of the skin. The subcutaneous tissue is often discussed with the skin because it attaches the skin to underlying tissues such as muscle and bone. The subcutaneous tissue contains loose connective tissue and fat cells that provide insulation. The anatomic distribution of subcutaneous tissue varies according to gender, heredity, age, and nutritional status. This layer also stores lipids, regulates temperature, and provides shock absorption.

Skin Appendages.

Appendages of the skin include the hair, nails, and glands (sebaceous, apocrine, and eccrine). These structures develop from the epidermal layer and receive nutrients, electrolytes, and fluids from the dermis. Hair and nails form from specialized keratin that becomes hardened.

Hair grows on most of the body except for the lips, the palms of the hands, and the soles of the feet.³ The color of the hair is a result of heredity and is determined by the type and amount of melanin in the hair shaft. Hair grows approximately 1 cm per month. On average 100 hairs are lost each day. The rate of growth is not affected by cutting.² When lost hair is not replaced, baldness results. The absence of hair may be related to disease, treatment, or heredity.

Nails grow from the matrix. The nail matrix is located at the proximal area of the nail plate. The matrix is commonly called the lunula, which is the white crescent-shaped area visible through the nail plate (Fig. 23-2). The nail bed that is under the nail matrix and nail plate is normally pink and contains blood vessels. The nail plate adheres to and is supported by the nail bed. The cuticle is part of the skin that extends a small distance on the nail plate before being shed (like the stratum corneum). The nail root is bordered by the cuticle and hidden by a fold of skin. Fingernails grow at a rate of 0.7 to 0.84 mm per week, with toenail growth 30% to 50% slower. Nails can be injured by direct trauma. A lost fingernail usually regenerates in 3 to 6 months, whereas a lost toenail may require 12 months or longer for regeneration. Nail growth may vary according to the person’s age and health. Nail color ranges from pink to yellow or brown depending on skin color. Pigmented longitudinal bands (melanonychia striata) may commonly occur in the nail bed in approximately 90% or more of all people with dark skin (Fig. 23-3).

Two major types of glands are associated with the skin: sebaceous and sweat (apocrine and eccrine) glands. The sebaceous glands secrete sebum, which is emptied into the hair follicles. Sebum prevents the skin and hair from becoming dry. Sebum is somewhat bacteriostatic and fungistatic and consists mainly of lipids. These glands depend on sex hormones, particularly testosterone, to regulate sebum secretion and production. Sebum secretion varies across the life span according to sex hormone levels. Sebaceous glands are present on all areas of the skin except the palms and the soles. These glands are most abundant on the face, scalp, upper chest, and back.

The apocrine sweat glands are located in the axillae, breast areolae, umbilical and anogenital areas, external auditory canals, and eyelids. They secrete a thick milky substance of unknown composition that becomes odoriferous when altered by skin surface bacteria. These glands enlarge and become active at puberty because of reproductive hormones.

The eccrine sweat glands are widely distributed over the body, except in a few areas, such as the lips. One square inch of skin contains about 3000 of these sweat glands. Sweat is a transparent watery solution composed of salts, ammonia, urea, and other wastes. The main function of these glands is to cool the body by evaporation, excrete waste products through the pores of the skin, and moisturize surface cells.

Functions of Integumentary System

The skin’s primary function is to protect the underlying tissues of the body by serving as a surface barrier to the external environment. The skin also acts as a barrier against invasion by bacteria and viruses, and it prevents excessive water loss. The fat of the subcutaneous layer insulates the body and provides protection from trauma.

The skin with its nerve endings and special receptors provides sensory perception for environmental stimuli. These highly specialized nerve endings supply information to the brain related to pain, heat and cold, touch, pressure, and vibration.

The skin controls heat regulation by responding to changes in internal and external temperature with vasoconstriction or vasodilation. Heat regulation is related to the skin’s function of excretion. Between 600 and 900 mL of water is lost daily through insensible perspiration. This function of the skin helps maintain homeostasis through fluid and electrolyte balance. In addition, sebum and sweat are secreted by the skin and lubricate the skin surface. Endogenous synthesis of vitamin D, which is critical to calcium and phosphorus balance, occurs in the epidermis. Vitamin D is synthesized by the action of UV light on vitamin D precursors in epidermal cells.

The esthetic functions of the skin include the expression of various emotions, such as anger or embarrassment, and the person’s individual appearance. The skin is also used as a system for the delivery of drugs. An increasing number of systemic drugs are effectively delivered via patches or creams applied directly to the skin.