Endotoxin induced migration of leukocytes from blood to milk

Guinea pigs were separated from young on days 5 to 7 of lactation. They were anesthetized with ether and were infused intramammary via the teat canal with either sterile saline (.5 ml) or Escherichia coli endotoxin (026:B6:500 microgram/.5 ml). Each animal served as its own control by having sterile saline in one gland and endotoxin in the other. Animals were sacrificed 4, 6, 8, and 12 h later to determine the time of maximum migration of polymorphonuclear leukocytes from blood to milk. A control group of animals having had no intramammary infusion was sacrificed. Tissues were prepared for observation by light and electron microscopy. Five fields per slide, five slides per animal, and three animals per period were examined by light microscopy, and numbers of polymorphonuclear leukocytes per field were categorized for presence a) in the capillary, b) in interstitial space, c) touching the basal lamina, d) between epithelial cells of an alveolus, and e) in the lumen of an alveolus. Polymorphonuclear leukocytes were in small numbers in the lumen by 4 h and peaked at about 8 h after intramammary infusion of endotoxin. By 12 h numbers had decreased from 8 h. Polymorphonuclear leukocytes were observed by electron microscopy in the intercellular spaces of the alveolar epithelium. A time sequence model using the guinea pig to study mechanisms of leukocyte migration into milk based upon tissue studies may be used in research aimed at controlling mastitis.