Department of Ecology & Evolutionary Biology, Yale University, New Haven, Connecticut, USA.
Department of Evolutionary Biology, University of Vienna, Vienna, Austria.
J Morphol. 2024 Sep;285(9):e21769. doi: 10.1002/jmor.21769.
The unitary mammary gland is a synapomorphy of therian mammals and is thought to have evolved from the pilosebaceous organ in the mammalian stem lineage from which the lactogenic patch of monotremes is also derived. One of the key lines of evidence for the homology of the nipple and the lactogenic patch is that marsupials have retained a transient hair associated with developing mammary glands. However, these structures have not been documented since the early 20th-century drawings of Ernst Bresslau. In this study, we examine the developing mammary organs of Monodelphis domestica and document the presence of mammary hairs in 12-week-old females, as well as their absence after 18 weeks of age. Histochemical staining for cystine confirms the structures as keratinized hairs. Milk ducts of both juvenile and adult nipples show a division between KRT18 luminal epithelium and KRT14 ACTA2 myoepithelium. These patterns match those in eutherians and suggest a conserved ductal morphology and mechanism of milk expulsion. Finally, PTHLH, a peptide hormone which promotes homeotic transformation of hairy skin into hairless nipples in the mouse, was detected in the Monodelphis milk duct during the mammary hair stage, suggesting that the mutual exclusivity of "hairless nipple" and "hair" organ identity is derived in eutherian mammals. These results reveal shared characteristics of the M. domestica nipple with both the eutherian nipple and the pilosebaceous organ, consistent with the evolutionary derivation of the mammary gland from an ancestral hair organ via developmental individualization of pilosebaceous and mammary identities.
单一乳腺是真兽类哺乳动物的同源特征，据推测它是从哺乳动物谱系中的多毛皮脂腺器官进化而来的，而单孔类动物的泌乳区也是从这个器官衍生而来的。乳头和泌乳区同源性的一个关键证据是，有袋类动物保留了与发育中的乳腺相关的短暂毛发。然而，自从 20 世纪初 Ernst Bresslau 的画作以来，这些结构就再也没有被记录过。在这项研究中，我们研究了 Monodelphis domestica 的发育中的乳腺器官，并记录了 12 周龄雌性乳腺毛发的存在，以及 18 周龄后这些毛发的缺失。胱氨酸的组织化学染色证实了这些结构是角蛋白化的毛发。幼年和成年乳头的乳腺导管显示 KRT18 腔上皮和 KRT14 ACTA2 肌上皮之间的分隔。这些模式与有胎盘类动物的模式相匹配，表明导管形态和乳汁排出机制是保守的。最后，在 Monodelphis 的乳腺导管中检测到 PTHLH 激素，该激素在小鼠中促进多毛皮肤向无毛乳头的同源性转化，这表明“无毛乳头”和“毛发”器官特征的互斥性是在有胎盘类哺乳动物中衍生的。这些结果揭示了 M. domestica 乳头与有胎盘类动物的乳头和多毛皮脂腺器官的共同特征，这与乳腺从祖先的毛发器官通过多毛皮脂腺和乳腺特征的发育个体化进化而来的观点一致。
