Neutrophil migration through test end tissues of bovine mammary quarters experimentally challenged with Staphylococcus aureus.

Diapedesis and infiltration of neutrophils into internal epithelial tissues of the distal teat end and migration into milk were studied in bovine mammary quarters infected with Staphylococcus aureus. Neutrophil extravasation, penetration of the epithelium, and mode of passage into milk were evaluated by light and transmission electron microscopy. Collection of observations from 10 infected quarters of six cows provided morphologic evidence for the following sequence of events as neutrophils passed from blood into milk. In capillaries of the subepithelial stroma, neutrophils adhered to luminal walls, penetrated endothelia and basal lamina, then migrated across the periendothelial cell layer into extravascular connective tissues adjacent to epithelial linings. The leukocytes then penetrated epithelial basal laminae and migrated between basal epithelial cells to gain access to the luminal cell layer. Possible modes of migration across luminal cells into milk included 1) projection through individual, degenerate luminal cells, 2) penetration between intact epithelia, and 3) passage into milk as luminal cells desquamated in areas of epithelial metaplasia. The first method appeared to be the predominant mechanism of migration. These data suggest that elevated numbers of neutrophils in distal teat end epithelium and in cisternal milk may be instrumental in the initial events that prevent establishment of infection in the bovine mammary gland.