Apoptosis of polymorphonuclear leukocytes of the juvenile bovine mammary gland during induced influx

The dynamics of apoptosis of polymorphonuclear leukocytes (PMN) during indu ced influx of PMN into the cavity system of the juvenile bovine mammary gla nd in order to investigate the role of apoptosis of PMN in the resolution o f mastitis was studied. The instillation of a synthetic analogue of muramyl dipeptide into teat sinus of the sixteen mammary glands was followed by a massive influx of PMN culminating after 24 h and resolving after 96 h. Ever y 24 h following the influx, apoptotic PMN were microscopically detected, b ased on morphological characteristics. Twenty four hours after the stimulat ion, apoptotic PMN were already observed, and peak counts of apoptotic PMN were reached 48 h after the stimulation. The lowest differential count of a poptotic PMN, corresponding to the pre-stimulation value, was found 96 h af ter the stimulation. The presence of macrophages (MAC) containing phagocyti zed apoptotic PMN was observed by histochemical staining for myeloperoxidas e (MPO) and electron microscopy. The percentage of MPO-positive macrophages increased during the resolution phase to reach peak values 48 h after the stimulation. Apoptosis of PMN and phagocytosis by macrophages may represent a removal mechanism that is important in the resolution of the induced inf lux of PMN in the cavity system of juvenile bovine mammary gland.