The Mammary Hair of Monodelphis Domestica and Homology of the Mammary Pilosebaceous Unit

This study investigates the developing mammary organs of Monodelphis domestica, revealing the presence of mammary hairs in 12-week-old females, which disappear by 18 weeks. Histochemical staining confirms these structures as keratinized hairs. The study finds that the milk ducts in both juvenile and adult nipples exhibit a division between KRT18⁺ luminal epithelium and KRT14⁺ ACTA2⁺ myoepithelium, similar to eutherians, indicating a conserved ductal morphology. The presence of PTHLH in the Monodelphis milk duct during the mammary hair stage suggests that the distinction between "hairless nipple" and "hair" organ identity is a derived trait in eutherian mammals. These findings support the evolutionary derivation of the mammary gland from an ancestral hair organ through the developmental separation of pilosebaceous and mammary identities.