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Mid-semester essay exam notes - Skin

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Monash University

S2.11 Skin The skin covers the entire external surface of the human body, where it serves as a protective barrier. The skin consists of 2 layers, the epidermis and the dermis, which rest on a fatty subcutaneous layer. The outer layer of the skin begins as a single layer of ectodermal cells. As development continues, the ectoderm becomes multilayered and the differences in structure become more obvious. The first stage in epidermal layering is the formation of the periderm. Cells of the periderm are involved in water, sodium exchange between the amniotic fluid and the epidermis. By the third month, the epidermis becomes a three-layered structure, with a basal layer, which is an intermediate layer of cells. During the sixth month, the epidermis beneath the periderm differentiates into the definitive layers characteristic of the postnatal epidermis. In the multilayered epidermis, unspecialized cells from the stratum basal differentiate as they move through the various layers toward the surface of the epidermis. The cells produce increasing amounts of intracellular keratins and filaggrin; where the latter is involved in the interconnections of the keratinocytes, the final differentiated form of the epidermal cell. The epidermis is derived primarily from surface ectoderm but is colonized by pigment-containing melanocytes of neural crest origin, antigen-processing Langerhans cells of bone marrow origin, and pressure-sensing Merkel cells of neural crest origin. The epidermis contains no blood vessels and is entirely dependent on the dermis for nutrient delivery and waste disposal. Melanocytes’ primarily function is to produce pigment, melanin, which absorbs radiant energy from the sun and protects the skin from the harmful effects of UV radiation. They are found in the basal layer of the epidermis. Everyone has the same amount of melanocytes, but people have different skin tones because of what happens to the melanocytes, not because of
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