The skin consists of two main layers: the superficial epidermis and the underlying dermis. They derive from two different germ line layers. The epidermis comes from the surface ectoderm. At about three weeks of gestation the epidermis consists of a single layer, while in the 4- to 6-week-old fetus two layers can be distinguished: the superficial periderm and the basal proliferative stratum germinativum. The periderm keratinizes and desquamates, giving rise to cells in the amniotic fluid and the vernix caseosa (the white cheese-like substance which covers the fetal skin until near term). By 8 to 11 weeks of gestation, proliferation of the stratum germinativum has produced several intermediate layers of epidermis, and at birth all the layers normally seen in adult epidermis, including a keratinized layer and the pigmentary cells (melanocytes), are present.
Most of the dermis derives from mesoderm. At about 11 weeks of gestation, mesenchymal cells in the dermis begin to produce collagen and elastin fibers. When the epidermal ridges grow down into the dermis at 11 to 12 weeks and begin to form the ridges and grooves that will produce dermatoglyphics, the dermis projects up into the epidermis, resulting in dermal papillae in which capillary loops and sensory nerve endings will form. A network of blood vessels forms in the fetal skin; some of these vessels are transitory and normally disappear.
The completely developed skin can be considered to have three layers: the epidermis, the dermis, and the subcutis, which is loose subcutaneous tissue. The surface of the epidermis is keratinized and covers an area of approximately 1.5 to 2 m2, weighing 0.5 kg in an adult. There is variation in epidermal thickness, depending on the body area. The skin is thinner, for example, around the eyes and lips, and is thicker in the palmar and plantar areas. On average, the skin is 0.1 cm thick. The dermis is separated from the epidermis by a basal membrane. The dermis consists of collagen, elastin, and reticular fibers, as well as a variety of cell types and a matrix with many complex carbohydrates. The subcutis or subcutaneous tissue connects the skin with underlying tissue and structurally consists of a loose organization of fibers and septa. It contains a variable amount of fat, known as the panniculus adiposus. Skin turgor relates to the amount of subcutaneous fat. Thinning of the panniculus adiposus will result in an increased number of skin creases.
The surface of the skin is complex and varies in different body areas. Small lines can be found in the skin surface; they criss-cross the body surface. These lines are thought to be the result of the configuration of the dermal papillae, which reach up to the epidermis, the underlying collagen bundles, and the pull of muscles underneath the skin. Some regions of the skin are oilier than other, reflecting the presence of a variable number of sebaceous glands. Other areas are covered with numerous vellus hairs, like the superior helices of the ears, or the upper lip in females. Skin texture also depends on the number, function, and size of the sweat glands. Skin can be firm, soft, rough, moist, dry, or oily. The texture of skin changes with age, resulting in uneven atrophy and hyperplasia, development of yellowish thickened plaques and subcutaneous nodules, areas of erythema, telangiectasia, and brown macular irregular pigmented lesions. With aging, progressive degenerative changes in collagen can be seen on histology, and collagen is replaced by elastic fibers. Thinning of the overlying epidermis also occurs with aging.
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Rosacea and Eczema are two skin conditions that are fairly commonly found throughout the world. Each of them is characterized by different features, and can be both discomfiting as well as result in undesirable appearance features. In a nutshell, theyre problems that many would want to deal with.