The skin develops from two sources: the superficial layer (epidermis) from the surface ectoderm and the deep layer (dermis) from mesoderm. Early in development, the fetus is covered with a single layer of ec-todermal cells. In the beginning of the second month through the fourth month, the four layers of the epidermis form: a) the basal layer, which is responsible for the production of new cells and is known as the germinative layer; b) a thick, spinous layer, consisting of large polyhedral cells; (c) the granular layer, the cells of which contain small keratohyalin granules; and (d) the horny layer, forming the tough, scale-like surface of the epidermis, and made up of closely packed, dead cells, loaded with keratin. Also during the first three months, cells of neural crest origin invade the epidermis. These cells (melanocytes) synthesize melanin pigment, which can be transferred to other cells of the epidermis through the dendritic processes.
The dermis develops during the third and fourth months. The dermis consists of a layer of connective tissue and fatty tissue and contains a number of structures, including hair. Hair starts as solid epidermal proliferations penetrating the underlying dermis. Nerve endings and blood vessels develop with the hair papillae, and cells from outbuddings of the follicle walls form the sebaceous glands, which degenerate, thereby forming a fat-like substance that is secreted into the hair follicle and then to the skin.
The mammary glands are a specialized skin structure. The first indication is found in the form of a band-like thickening of epidermis, the mammary line or ridge. This extends on each side of the body from the base of the forelimb to the region of the hindlimb by the seventh week of gestation. Most of the mammary line disappears quickly, but a small portion in the thoracic region persists and penetrates the underlying mesenchyme, forming the breast bud. The bud sprouts 16-24 cords, which ultimately form the lactiferous ducts surrounded by the alveoli of the gland. The ducts at first open into a small epithelial pit in the bud, but shortly after birth, this pit matures into the nipple by proliferation of the underlying mesenchyme . At birth, the breast of both the male and female is identical. At puberty the female breast bud enlarges first, then the mammary glands enlarge, and an extensive deposition of fat occurs. The nipple and areola enlarge as well .
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