Gartner_BRS Cell Biology & Histology, 9e

12

Skin

I. OVERVIEW—SKIN

The skin, considered to be the heaviest organ (~16% of total body weight), is comprised of two layers: the superficial epidermis , which is composed of epithelium, and the underlying dermis , which con sists of connective tissues. A. The skin contains several epidermal derivatives (sweat glands, hair follicles, sebaceous glands, nails as well as the mammary glands, which are discussed in Chapter 17). The skin along with its derivatives is called the integument . B. The skin protects the body against injury , desiccation, and infection; regulates body temperature; absorbs ultraviolet (UV) radiation, which is necessary for the synthesis of vitamin D; and contains receptors for touch, temperature, and pain stimuli from the external environment. In addition, the skin acts as an excretory organ via sebaceous, sweat, and apocrine glands. C. A deeper superficial fascial layer , the hypodermis , lies under the skin. This layer, which is not considered part of the skin, consists of loose connective tissue that binds the skin loosely to the subjacent tissue. A. Overview—Epidermis 1. The epidermis is the superficial layer of the skin, composed of stratified squamous kerati nized epithelium . This tissue is primarily derived from ectoderm and consists predominantly of keratinocytes and three other types of cells: melanocytes , epidermal dendritic cells ( Lang erhans cells ), and tactile epithelial cells ( Merkel cells ). 2. Regeneration of epidermis occurs continuously by the mitosis of the keratinocytes in the deeper layers. These cells normally divide only at night and are responsible for complete re placement of the epidermal tissue within 30 days on average. 3. Deep downward growths of the epidermis, called epidermal ridges , interdigitate with pro jections of the dermis ( dermal ridges, dermal papillae ), resulting in an undulating interface. Each dermal ridge is often further subdivided into two secondary dermal ridges by a narrow downgrowth of the epidermis, called an interpapillary peg . The epidermal surface forms el evations over the dermal ridges that are visible as fingerprints on the fingertips and whose pattern is genetically determined and thus unique to each individual. 4. Keratinocytes can manufacture and release various signaling molecules, such as interleukins (ILs), interferons, tumor necrosis factors, and colony-stimulating factors that stimulate the im mune system. II. EPIDERMIS

Copyright © 2024 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited.

256