The primary structural differences between pre- and postpubertal mammary glands are the extent of lateral outgrowths along the ducts and the extent of parenchymal penetration of the fat pad. In rodents, ductal branching occurs primarily after puberty. With the first few estrous cycles, ducts reach the margins of the mammary fat pad, the number of lateral outgrowths increase, and spacing between adjacent ductal outgrowths decreases. In ruminants, branching of the ducts occurs simultaneously with ductal elongation. Thus, in these species the extent of ductal elongation is the primary distinction between pre-and postpubertal mammary gland. Postpubertally, the ductal network becomes increasingly dense and displaces much of the mammary stroma. Occasionally, areas of secretory tissue develop and undergo limited differentiation during estrus, but these areas quickly de-differentiate and become quiescent as the estrous cycle progresses.
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