any structural anomaly of the morphologically distinct bulb of epithelial cells formed once each mammary placode expands and invaginates into the underlying mesenchyme; in mouse embryos at E12.5, each epithelial bud with its contiguous mesenchyme is elevated above the surrounding ectoderm as a knob or dome; at E13.5, the buds sink into the underlying dermal mesenchyme, and by E14.5 they can no longer be detected externally; in female mouse embryos, five pairs of anlagen or buds are formed along the mammary line between E12.5 and E14.5 by proliferation of basal epidermal cells; while further mammary development is temporarily arrested in females, androgen receptor activation in male embryos causes degeneration of the buds between E13.5 and E15.5; in female mice, further development is resumed at around E15.5 when each bud elongates to form a mammary cord (sprout), invading the underlying fat pad precursor
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