FIG. 1. Comparison of lactating and parturient mammary tissues. Hematoxylin-and-eosin staining reveals similarly structured, round alveoli (arrowhead) that contain lipid droplets and milk in glands from mouse (A) and bat (B). In the mouse gland (A), the alveoli are surrounded by adipocytes (a). In the bat, secretory portions of the gland contain a higher proportion of epithelial elements (B, D, and F), and multilocular adipocytes are segregated on the periphery (D and G). The Masson trichrome-stained sections show a higher collagen content (stained blue) in bat tissue (D) compared to mouse tissue (C). Prominent collagen bundles (arrow) are adjacent to large ducts, but not the alveolar tissue, in the mouse gland (E). In the bat, each alveolar unit is surrounded by a robust sheath of collagen (arrow; F). Collagen bundles (arrow) are also found in the adipose portion of the bat gland (G). Hematoxylin and eosin-stained sections from an early pregnant bat that had recently lactated have large alveoli that are lined by cuboidal cells lacking lipid droplet inclusions. Secreted material is present in the lumina (arrow; H). Tissue from a bat with an older pup contains areas that resemble the gland from an animal that had just delivered and is actively nursing (arrow) as well as areas that are devoid of lipid droplets (arrowhead; I). Bar = 25 µm (A, B, E, and F) and 75 µm (C, D, H, and I)