Gartner_BRS Cell Biology & Histology, 9e

BRS Cell Biology and Histology

260

a. The keratohyalin-keratin complex lines the plasma membrane of the stratum corneum cell and is further fortified by three proteins: involucrin , small proline-rich protein , and loric rin , thereby establishing a thickened cornified cell envelope . b. The lipid-rich substance released from the membrane-coating granules in the strata spino sum and granulosum persists and forms a lipid coat around each horny cell. c. The cornified envelope and the lipid coat constitute the compound-cornified cell envelope . 6. The squames in the most superficial layer are continuously shed in the process known as des quamation . The rate of shedding matches the rate of cell renewal in the strata basale and the spinosum, thereby maintaining the thickness of the epidermis as well as the structural stability of the compound-cornified cell envelope. PEARLS Psoriasis vulgaris ( plaque psoriasis ) is a chronic inflammatory condition involving hyperproliferation of the epidermal keratinocytes that manifest as reddened, inflamed patches of skin with whitish, flaky crusts that form almost anywhere on the body. In addition to the increased mitotic activity, the keratinocytes have decreased duration of the cell cycle of the cells in the malpighian layer, resulting in the renewal of epider mis in only days rather than in about a month. The fast-growing epidermis promotes increased blood flow, and inflammation ensues (both causing erythema). Psoriasis is a complex, multifactorial condition whose pathogenesis is still poorly understood, but both genetic and immune-mediated influences have been noted. Once the condition is triggered, there is a substantial leukocyte recruitment, especially the acti vated T cells, and subsequent increase in cytokines, such as IL-12, IL-23, interferon- γ , and tumor necrosis factor- α , in both epidermis and dermis of the skin. Clinical C. Nonkeratinocytes in the epidermis: melanocytes, epidermal dendritic cells, and tactile epithelial cells ( Table 12.2 ) 1. Melanocytes (see Fig. 12.1) are neural crest cell derivatives that migrate into and integrate with the stratum basale and differentiate first into melanoblasts , then premelanocytes that form hemidesmosomes with the basal lamina but do not form adhesive junctions with the keratino cytes. Under the influence of stem cell factor, some premelanocytes may permanently differenti ate into melanocytes and extend cellular processes known as dendrites among the keratinocytes of the stratum spinosum. A single melanocyte contacts a certain number of keratinocytes and supplies them with pigments; this group of cells is known as an epidermal-melanin unit . a. Melanocytes possess melanosomes , lysosome-related organelles in which the pigment melanin is synthesized under the influence of the pituitary hormone melanocyte-stimulating hormone (MSH) . MSH binds to transmembrane melanocortin 1 receptors (MC1Rs) , which activate microphthalmia-associated transcription factor that induces melanin syn thesis by the UV-sensitive enzyme, tyrosinase . MC1R also induces melanocytes to enter the cell cycle and activates DNA repair pathways and stimulates antioxidant formation. The two types of melanin produced by melanocytes are eumelanin and pheomelanin. (1) Eumelanin (or just melanin in this textbook) has two major varieties, brown and black, both imparting a dark coloration to the skin. With age, only the black form of melanin is produced. Melanin is photoprotective (ie, it absorbs UV radiation and protects the DNA of epidermal cells from damage).

Table 12.2 Nonkeratinocytes of the Epidermis

Percentage of Epidermal Cells Function

Cell

Location

Derivation

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Melanocyte

Stratum basale Neural crest cells

2-3 2-4

Synthesis of melanin Phagocytosis Antigen presentation

Epidermal dendritic (Langerhans) cell

Stratum spinosum

Bone marrow

Tactile epithelial (Merkel) cell

Stratum basale Neural crest or

1-5

Mechanoreceptors

nonneural ectoderm