Gartner_BRS Cell Biology & Histology, 9e

BRS Cell Biology and Histology

258

1. The stratum basale ( stratum germinativum ) is the deepest layer of the epidermis and is composed mostly of keratinocytes that are cuboidal to columnar in shape. These mitotically active cells are attached directly to the basal lamina of the basement membrane by hemides mosomes and to each other by desmosomes. Pigment-producing melanocytes and fine sen sory receptors, tactile epithelial cells ( Merkel cells ), are also dispersed throughout this layer. 2. The stratum spinosum consists of a few layers of polyhedral keratinocytes anchored to each other and to the cells in the layer above and below them via numerous desmosomes . The kera tin intermediate fibers that attach to and reinforce the desmosomes are collectively called tono filaments . Histologic processing often shrinks the keratinocytes away from each other, except at the sites of desmosomes, leaving an impression of cells with prickles; hence, keratinocytes in this layer cells are also known as prickle cells . Keratinocytes in the deeper aspects of the stratum spinosum are also mitotically active . Epidermal dendritic cells ( Langerhans cells ) derived from mononuclear phagocytic population of immune cells are also present in this layer. a. In the superficial regions of the stratum spinosum: (1) Keratinocytes contain membrane-coating granules ( Odland bodies , lamellar bodies ) filled with lipids, especially glycosphingolipids, ceramides, and phospholipids. Some of these materials are released into the intercellular spaces, creating lipid-rich sheets that are impermeable to water and many foreign substances . (2) They also form nonmembrane-bound keratohyalin granules , containing proteins filaggrin and trichohyalin . The keratohyalin granules envelop the thin bundles of to nofilaments, causing them to become cross-linked, thereby forming thick bundles of tonofibrils . b. The malpighian layer (stratum malpighii) consists of the stratum spinosum and stratum ba sale. Nearly all of the mitotic activity in the epidermis occurs in this region, and only at night. Interleukin-1 (IL-1) and epidermal growth factor are thought to facilitate, whereas trans forming growth factor suppresses, the mitotic activity of these cells. PEARLS UV radiation is the leading cause of three types of skin cancers. Basal cell carcinoma (BCC) is the most common type of skin cancer (80% of nonmelanoma skin cancers), and squamous cell carcinoma (SCC) is the second most common type (20% of nonmelanoma skin cancers) arising from epidermal keratinocytes. Melanoma arises from melanocytes, and although its incidence is much less than that of BCC and SCC, it is the most aggressive cancer and is associated with a high mortality rate, especially if the cancer has spread. Of the two nonmelanoma cancers, SCC is more aggressive and is associated with a higher mortality rate than BCC. The sun’s UV rays reach the earth as a continuous wavelength, although, customarily, for the purposes of dermatologic studies, it is described as if it were composed of three discontinuous waves of differing lengths, in which UVA is the longest (320-400 nm), UVC is the shortest ( < 290 nm), and UVB occupies a band of intermediate length (290-320 nm). The major component of the sun’s rays (~95%) is the UVA band (320-400 nm), and this is the component believed to be most culpable in cancer, because not only does it have the ability to penetrate the deepest layer of the skin, but it is also the most constant component of the sun’s UV radiation, irrespective of the time of the year, and passes undiminished through clouds and glass. Although UVB radiation also penetrates all layers of the skin and causes all three types of skin cancer, it is unable to penetrate glass, and its intensity varies with the time of day (strongest between 10 am and 4 pm and only during late spring, summer, and early autumn). Most of the damage caused by UVB rays is restricted to the more superficial layers of skin. UVC radiation is unable to penetrate the ozone layer surrounding the earth and is not considered to have any biologic effect. In addition to the cancer-­ producing effects of UVA and UVB radiations, they also cause a condition known as “photoaging,” namely, acceleration in the wrinkling and loss of elasticity of skin. Clinical

Copyright © 2024 Wolters Kluwer, Inc. Unauthorized reproduction of the content is prohibited. Sunscreen with a sun protection factor (SPF) rating of 15 or higher may protect against UVB wavelengths, which cause sunburn, but offers no protection against the longer UVA wavelengths. For the best protec tion, using sunscreens approved by the U.S. Food and Drug Administration (FDA) (containing active ingre dients octocrylene, titanium dioxide, and zinc oxide) that shield the skin from damage from both UVA and UVB is recommended.

3. The stratum granulosum is the most superficial layer of the epidermis in which the three to five layers of flattened keratinocytes are still alive. The cells contain bigger keratohyalin granules , tonofibrils , and membrane-coating granules .