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

The study on Monodelphis domestica explores the evolutionary link between mammary glands in marsupials and monotremes, revisiting Ernst Bresslau's early findings. It examines four female opossums, revealing that mammary hair is present in younger females but absent in older ones, suggesting it is a vestigial feature that sheds before the nipple becomes functional. The research marks the first histological detection of mammary hair in this species, identifying mammary pilosebaceous units in 3-month-old opossums. The findings support the sebogenic hypothesis, indicating that mammary hairs may have originally evolved to aid in milk secretion. The study confirms a conserved mechanism of milk letdown through myoepithelial contraction among therians and suggests an evolutionary link between therian mammary glands and hair organs, highlighting the relevance of Bresslau's work in understanding mammary organ evolution.