Embryonic mammary gland development. Embryonic mammary gland development. (A,B) Diagram of an E10.5 mouse embryo (A) showing the position of the milk line (dashed line between limbs), and of an E12.5 mouse embryo (B) showing the positions of the five pairs of mammary placodes, which become mammary buds (MB1-5) along the anteroposterior axis (MB1 and MB5 are hidden by the limb buds and only one flank is shown). (C) Overview of mouse embryonic mammary gland development. Placodes, which are visible at E11.5, transform into bulbs of epithelial cells, which sink into the underlying mesenchyme at E13.5 to become the mammary buds. The mesenchymal cells (orange) that surround the buds condense to become the mammary mesenchyme (grey). By E15.5, these buds elongate to form sprouts, which develop a lumen with an opening to the skin, marked by the formation of the nipple sheath. As the end of pregnancy approaches, at E18.5, the sprouts become small arborized glands that invade what has now become the fat pad (buff). Development is essentially arrested at this stage until puberty. Christine J. Watson, and Walid T. Khaled Development 2008;135:995-1003 © 2008.