To test the hypothesis that myeloid body (MB) formation results from the shedding of retinal photoreceptor outer segments and the consequent degradation of lipids derived from outer segment disk membranes, the effect of massive outer segment shedding and the disruption of such outer segment shedding on MB formation were examined in the leopard frog. Light entrained frogs were first placed in constant light (700 lux) for 48 hours to inhibit shedding, followed by a 1.5 hours dark priming either in vivo or in vitro, and then returned to light for an additional 4 hours which results in massive outer segment shedding. To serve as a control, the effect of shedding disruption on MB formation was assessed in vivo using light manipulation to inhibit shedding, or mechanical removal of the neurosensory retina in vitro. The results indicate that although the phagosome numbers were clearly elevated in the samples taken from either in vivo or in vitro eye-cup preparations where outer segment shedding had been stimulated, there was no significant concomitant increase in Mbs number over controls kept under constant light for 48 hr or constant light 48 hr plus 1.5 hr in dark, where MBs represent approximately 5% of the total RPE cell area. In contrast, when shedding was interrupted either by removal of the neural retina immediately after in vitro eye-cups were returned to light or by maintaining frogs in dark without light stimulation, the RPE cells contained very few phagosome, yet in both conditions RPE cells showed a two-fold increase in MB area over the shedding-stimulated controls (p = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)