Gartner_BRS Cell Biology & Histology, 9e

Chapter 12 Skin

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a. Keratohyalin granules stain intensely with basophilic stains; thus, this layer is readily appar ent in histologic sections. b. Keratinocytes in the superficial layers of the stratum granulosum form tight junctions with one another and with the cells of the epidermal layer above. The transmembrane proteins claudin and occludin that form tight junctions are abundant in this layer. c. The lipid contents of the membrane-coating granules are released into the extracellular space to form a water-impermeable barrier, preventing nutrients from reaching the most superficial layer of cells of the stratum granulosum and those of the strata lucidum and cor neum. The nutrient-starved keratinocytes undergo apoptosis and become keratohyalin-­ tonofibril–filled “hulls.” The impermeable layer prevents aqueous fluid from entering the epidermal layers from the external environment and also minimizes fluid loss from the body. 4. The stratum lucidum is a clear, homogeneous layer just superficial to the stratum granulosum; it is often difficult to distinguish in histologic sections. It is found only in the thick palmar and plantar skin . The dead keratinocytes in this layer have an abundance of tonofibrils embedded in keratohyalin, frequently referred to as eleidin . 5. The stratum corneum is the most superficial layer of the epidermis ( Fig. 12.2 ), composed of as many as 15 to 20 layers of flattened, dead keratinocytes filled with keratohyalin-keratin complex . These nonviable scale-like flat, polygonal structures called squames (or horny cells) are continuously sloughed off from the most superficial layer.

D

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RL

FIGURE 12.2. A photomicrograph of thick skin from a human palm. Note the sweat gland duct (D) in the middle of the image as it courses in a spiral manner through the thick stratum corneum (SC). The thin stratum lucidum (SL) is situated between the SC and the stratum granulosum (SG). The thick stratum spinosum (SS) and the single-layered stratum basale (SB) constitute the remainder of the epidermis. The dermis is composed of two layers, the narrow papillary layer (PL) that interdigitates with the epidermis and the much thicker reticular layer (RL) whose entire ex tent is not shown in this photomicrograph ( × 270).