Pseudopod formation and phagocytosis of milk components by epithelial cells of the bovine mammary gland.

The non-secretory epithelial cells which line the teat and lactiferous sinuses of the bovine mammary gland produce prominent pseudopodia which are able to ingest milk fat globules and casein micelles by phagocytosis. In the lactating gland, pseudopodia are uncommon and the phagocytosis of milk components less active than in the dry glands of cows which have not been milked for 2-3 weeks. Inside phagocytic vacuoles, casein micelles show morphological changes which suggest that digestion is taking place and the presence of acid phosphatase in vacuoles containing milk fat globules indicates that they too are degraded. The differences between lactating and dry glands in their phagocytic activity are reflected by corresponding differences in the number of epithelial cells containing acid phosphatase activity and in the intracellular distribution of the enzyme. It is suggested that the phagocytosis of milk components from the lumen of the dry gland assists in the resorption of milk and that the phagocytic potential of epithelial cells may, under some circumstances, play a role in the protection of the gland from invading bacteria.