In Vivo & In Vitro Effects of Light on Ommatidial Morphology in the Cockroach, Leucophaea Maderae

Clock-driven endogenous circadian rhythms in ommatidial morphology have been reported in Leucophaea maderae. The parameters used to measure changes in ommatidial morphology over time were palisade layer and rhabdom areas and the organization of lightscreening pigment granules about the rhabdom. The fact that the palisade layer area only partially returned to the light-adapted state in vivo during subjective midday was indicative of a passive mechanism, not involving the clock output. Complete return of the palisade layer area to the light-adapted state during the daytime in animals held under LD 12:12 indicated light might play an active role. Temporal changes in rhabdom area were not previously reported. Light-screening pigment granules changed in concentration about the rhabdom on a daily basis; however, this rhythm did not persist on a circadian basis. Therefore, it seemed that control of pigment granule movement resided, at least in part, within the photoreceptor itself. The focus of this investigation was to determine if control of photomechanical movement in response to light did, in fact, reside within the photoreceptor itself. The research was divided into two phases: 1) a preliminary component, Phase One, where the effect of different lengths of light exposure on dark-adapted ommatidial structure was monitored in vivo in order to determine an effective length of light exposure; 2) Phase Two, the effect of light exposure for a length of 10 minutes, a duration found to be effective in Phase One, on ommatidial morphology in dark-adapted eyes was monitored in vitro. All eye samples were surgically removed from cockroaches entrained to a LD 12:12 photoperiodic cycle during the dark phase. The in vitro samples were maintained in physiological cockroach saline for ten minutes whether treated with light or not. All photoreceptive tissue was fixed, stained, dehydrated, embedded and sectioned in preparation for examination using a Zeiss transmission electron microscope. Palisade layer and rhabdom areas were determined from photomicrographs using a Jandel PC3-D computer program. Light-screening pigment granule organization was determined by counting the number of granules within a 13.5 µ diameter circle centered about the rhabdom. The palisade layer area changes in vivo from dark-adapted to light-adapted were consistent with previous reports; however, the in vitro response was the opposite of that expected. The rhabdom response was significantly different between in vivo and in vitro conditions, however, the trends in response to light for both palisade layer and rhabdom areas were similar. The results with respect to rhabdom area are not similar to those previously reported. The aggregation of light-screening pigment granules in response to light in vivo was similar to the in vitro response, but at a depressed level -- suggesting some level of control at the photoreceptor level and the possibility that some control resided in a central nervous system locus other than or inclusive of the optic lobe pacemaker. Support for separate pathways of control of palisade layer area changes and light-screening pigment granule movement was noted