box 14.1. Functional Considerations: Skin Color 409

box 14.2. Functional Considerations: Hair Growth and Hair Characteristics 415

box 14.3. Functional Considerations: The Role of Sebum 415

box 14.4. Clinical Correlations: Sweating and Disease 419

box 14.5. Clinical Correlations: Skin Repair 421

v overview of the integumentary system

The skin (cutis, integument) and its derivatives constitute the integumentary system. The skin forms the external covering of the body and is its largest organ, constituting 15 to 20% of its total mass. The skin consists of two main layers:

• Epidermis, composed of a keratinized stratified squamous epithelium that grows continuously but maintains its normal thickness by the process of desquamation. Epidermis is derived from ectoderm. • Dermis, composed of a dense connective tissue that imparts mechanical support, strength, and thickness to the skin. Dermis is derived from mesoderm.

The bypodermis contains variable amounts of adipose tissue arranged into lobules separated by connective tissue septa. It lies deep to the dermis and is equivalent to the subcutaneous fascia described in gross anatomy. In well-

nourished individuals and in individuals living in cold climates, the adipose tissue can be quite thick.

The epidermal derivatives of the skin (epithelial skin appendages) include the following structures and integumentary products:

• Hair follicles and hair

• Sweat (sudoriferous) glands

Sebaceous glands

• Mammary glands

The integumentary system performs essential functions related to its external surface location

Skin and its derivatives constitute a complex organ composed of many different cell types. The diversity of these cells and their ability to work together provide a number of functions that allow the individual to cope with the external environment. Major functions of the skin include

• It acts as a barrier that protects against physical, chemical, and biologic agents in the external environment (i.e., mechanical barrier, permeability barrier, ultraviolet barrier).

• It provides immunologic information obtained during antigen processing to the appropriate effector cells in the lymphatic tissue.

• It participates in homeostasis by regulating body temperature and water loss.

• It conveys sensory information about the external environment to the nervous system.

• It performs endocrine functions by secreting hormones, cytokines, and growth factors and converting precursor molecules into hormonally active molecules (vitamin D)-.

• It functions in excretion through exocrine secretion of sweat, sebaceous, and apocrine glands.

In addition, certain lipid-soluble substances may be absorbed through the skin. Although not a function of skin, this property is frequently used in delivery of therapeutic agents.

Skin is categorized as thick or thin, a reflection of thickness and location

The thickness of the skin varies over the surface of the body, from less than 1 mm to more than 5 mm. However, the skin is obviously both grossly and histologically different at two locations, the palms of the hands and the soles of the feet; these areas are subject to the most abrasion, are hairless, and have a much thicker epidermal layer than skin in any other location. This hairless skin is referred to as thick skin. Elsewhere, the skin possesses a much thinner epidermis and is called thin skin. It contains hair follicles in all but a few locations.

The terms thick skin and thin skin, as used in histologic description, are misnomers and refer only to the thickness of the epidermal layer. Anatomically, the thickest skin is found on the upper portion of the back where the dermis is exceedingly thick. The epidermis of the upper back, however, is comparable to that of thin skin found elsewhere on the body. In contrast, in certain other sites such as the eyelid, the skin is extremely thin.

s? layers of the skin Epidermis

The epidermis is composed of stratified squamous epithelium in which four distinct layers can be identified. In the case of thick skin, a fifth layer is observed (Figs. 14.1 and 14.2). Beginning with the deepest layer, these are

• Stratum basale, also called the stratum germinativum because of the presence of mitotically active cells, the stem cells of the epidermis

• Stratum spinosum, also called the spinous or prickle cell layer because of the characteristic light microscopic appearance of short processes extending from cell to cell

• Stratum granulosum, which contains numerous intensely staining granules

• Stratum lucidum, limited to thick skin and considered a subdivision of the stratum corneum

• Stratum corneum, composed of keratinized cells

Differentiation of epithelial cells constitutes a specialized form of apoptosis

Terminal differentiation of the epidermal cells, which begins with the cell divisions in the stratum basale, is considered a specialized form of apoptosis. Cells in the stratum granulosum exhibit typical apoptotic nuclear morphology, including fragmentation of their DNA. However, the cellular fragmentation associated with normal apoptosis does not occur; instead, the cells become filled with filaments of the intracellular protein keratin and are later sloughed from the skin surface.

The stratum basale provides for epidermal cell renewal

The stratum basale is represented by a single layer of cells that rests on the basal lamina. It contains the stem cells from which new cells, the keratinocytes, arise by mitotic division. For this reason, the stratum basale is also called the stratum germinativum. The cells are small and

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