Gartner_BRS Cell Biology & Histology, 9e

Chapter 12 Skin

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(2) Pheomelanin is reddish to yellow in color, and this form of melanin gives skin a slightly pinkish coloration. Pheomelanin does not offer the same protection against DNA dam age as does melanin. (3) Mature melanin-containing melanosomes are transported into the dendrites along microtubules, and once at the distal end, they are transferred to F-actin pathways for delivery into the extracellular space. (4) Melanosome transfer from melanocytes to keratinocytes is believed to occur via phago cytosis of the melanin released from melanosomes via exocytosis. b. Once inside the keratinocytes ( Fig. 12.3 ), melanin migrates to the supranuclear region and forms a physical barrier between the keratinocyte’s nucleus and the impinging UV rays, thus preventing possible DNA damage from the UV radiation.

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(1) The melanocytes per unit area of skin in people appear to be the same, regardless of skin color, and account for approximately 3% of the entire epidermal cell population. (2) Pigmentation differences are due to the rate of melanin synthesis, melanosome size, content, rate of transfer, and degradation patterns. PEARLS Melanoma is a form of skin cancer originating in melanocytes in the stratum basale. The early signs of melanoma include alterations of an existing mole that may be accompanied by painful, itching, or burn ing sensations. The color of the lesion varies from black to brown to blue to occasionally even pink. Any pigmented lesion that has changed color; is asymmetric with irregular borders; and has increased in size, especially becoming greater than 6 mm in diameter, should raise suspicion. Melanomas can occur almost anywhere on the body but occur most commonly in areas that are exposed to sunlight. Melanoma can also arise from the melanocytes in the uvea of the eye. Approximately 86% of melanomas are believed to be caused by exposure to UV radiation from the sun. Although malignant melanoma accounts for less than 1% of skin cancer cases, it is responsible for the vast majority of skin cancer deaths. FIGURE 12.3. Electron micrograph of keratinocytes in the stratum basale of skin. Melanin pigment granules (MPG) are abundant in the cytoplasm, having been transferred to the cells from melanocyte processes. A few keratin fila ments (K), mitochondria, and portions of nuclei (N) are observed. The base of keratinocytes in this layer attaches to the basal lamina by hemidesmosomes (H), and they attach to neighboring cells by way of desmosomes ( × 9 500). Clinical

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