The earliest stage of recognizable mammary development is the mammary streak, a pair of thickened areas of darkly stained cells on the ventral side of the embryo. The next major developmental stage is the mammary line that appears as a readily discernable area of stratified cells. The mesenchyme underlying the mammary line is notably denser than that seen under the mammary streak. The mammary line defines the orderly arrangement of mammary glands in mature animals. The bovine mammary line does not extend past the umbilicus, whereas porcine and rodent mammary lines extend into the thoracic region. Thus ruminant mammary glands are located only in the inguinal region, but mammae of pigs and rodents are spaced along the ventral surface.
The mammary hillock and bud occur as the next prominent stages of embryonic mammary development. The mammary bud is a compact cluster of cells separated from the underlying mesenchyme by a well-defined basement membrane. The migration of cells that compose the mammary line results in formation of discrete epithelial buds. Cell proliferation and extension of the mammary bud result in formation of the mammary sprout. As it extends, the mammary sprout gradually invades the underlying mesenchyme. Concurrent proliferation by the mesenchyme surrounding mammary buds and sprouts causes distortion of the overlying dermis and formation of teats/nipples. At birth, the mammary gland consists of short, branching ductal elements embedded in a dense stromal matrix underlying the teat.
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