Distribution of mitochondria within Muller cells - II. Post-natal development of the rabbit retinal periphery in vivo and in vitro: dependence on oxygen supply

The occurrence and localization of mitochondria within glial (Muller) cells and neurons of the peripheral (avascular) rabbit retina was studied electr on microscopically and by immunocytochemical demonstration of the mitochond rial enzyme GABA transaminase (GABA-T). Post-natal development in vivo was compared with development of organ cultures from neonatal rabbit retinae, g rown over 2 weeks in vitro. The adult pattern of mitochondrial localization (restriction to the sclerad end of the cells) was observed from the beginn ing of enzyme expression at early post-natal stages. However, when neonatal retinal pieces were grown in vitro with their vitread surface exposed to t he air, their Muller cells contained mitochondria along most of their lengt h. When functionally developed retinae from postnatal day 14 were explanted in vitro, they retained their sclerad mitochondrial distribution for almos t 24 h but thereafter the inner portions of their cytoplasm became occupied by mitochondria within a few hours. This was achieved mainly by mitochondr ial migration rather than by formation of new mitochondria because it was n ot prevented by cycloheximide-induced inhibition of protein synthesis. Thes e data support the following hypotheses: (1) the mitochondrial. distributio n in Muller cells is determined by the local cytoplasmic O-2 pressure (pO(2 )), (2) existing mitochondria move towards cytoplasmic regions of sufficien t pO(2) by rather rapid migration and (3) the start of this migration is de layed by almost 24 h due to the action of as yet unknown control mechanisms . In contrast, the mitochondrial content of retinal ganglion and amacrine c ells in the vitread retinal layers was virtually independent of the source and level of oxygen supply.