Ultrastructural changes in lactating tissue related to the suppression of milk secretion by concanavalin A

The plant lectin, concanavalin A (Con A) suppresses milk secretion when infused into the mammary gland or when incubated with lactating tissue in vitro. Toward defining its mode of action, we infused Con A into rat and goat mammary glands via the teats and observed effects on lactating cells. Lectin dosages were 2 and 25 mg per gland for rats and goat, respectively. Tissue samples were taken 1 and 3 h post infusion for rats and at 24 h for the goat. Control and Con A-treated tissues were observed by light microscopy and by both thin section and freeze fracture electron microscopy. In comparison to controls, Con A-treated tissues of both species exhibited alveoli with enlarged cells and relatively empty lumina; cells were distended with secretory vesicles and fat droplets. Apical plasma membranes of lectin-affected cells of the rat displayed a marked reduction in the number of microvilli, and exhibited an atypical branching and folded structure. Morphometry was employed to quantitate changes in cell and secretory product parameters in both rat and goat tissue. Microtubule numbers and distribution did not appear to be altered by Con A but considerable changes were noted in the arrangement of microfilaments associated with the secretory surface of lectin-treated epithelial cells. Various related ultrastructural changes and the role of Con A in perturbing the microfilament system are discussed.