Activation of omental milky spots and milky spot macrophages by intraperitoneal administration of a streptococcal preparation, OK-432.

Omental milky spots are omentum-associated lymphoid tissues that cede peritoneal macrophages and participate in the immunity of the peritoneal cavity. We studied the changing surface features of milky spots and milky spot macrophages of Wistar rats, following the i.p. administration of OK-432, a killed streptococcal preparation (1 Klinische Einheit (unit) in 5 ml of phosphate-buffered saline) by the use of scanning electron microscopy. OK-432-activated macrophages demonstrated marked surface membrane activity and migrated through the stomata of the milky spot into the peritoneal cavity. The characteristic features of activated milky spots and milky spot macrophages were noted as early as 3 h following the administration of OK-432, and continued to be observed until 7 days after the injection. By 14 days after the injection, the structural integrity of the milky spot was partially lost. The activation of milky spots and milky spot macrophages by OK-432 provides a convenient in vivo system for the monitoring and study of i.p. cellular events.