Distribution of mitochondria within Muller cells - I. Correlation with retinal vascularization in different mammalian species

The distribution of mitochondria within retinal glial (Muller) cells and ne urons was studied by electron microscopy, by confocal microscopy of a mitoc hondrial dye and by immunocytochemical demonstration of the mitochondrial e nzyme GABA transaminase (GABA-T). We studied sections and enzymatically dis sociated cells from adult vascularized (human, pig and rat) and avascular o r pseudangiotic (guinea-pig and rabbit) mammalian retinae. The following ma in observations were made. (1) Muller cells in adult euangiotic (totally va scularized) retinae contain mitochondria throughout their length. (2) Mulle r cells from the periphery of avascular retinae display mitochondria only w ithin the sclerad-most end of Muller cell processes. (3) Muller cells from the vascularized retinal rim around the optic nerve head in guinea-pigs con tain mitochondria throughout their length. (4) Muller cells from the peripa pillar myelinated region ('medullary rays') of the pseudangiotic rabbit ret ina contain mitochondria up to their soma. In living dissociated Muller cel ls from guinea-pig retina, there was no indication of low intracellular pH where the mitochondria were clustered. These data support the hypothesis th at Muller cells display mitochondria only at locations of their cytoplasm w here the local O-2 pressure (pO(2)) exceeds a certain threshold. In contras t, retinal ganglion cells of guinea-pig and rabbit retinae display many mit ochondria although the local pO(2) in the inner (vitread) retinal layers ha s been reported to be extremely low. It is probable that the alignment of m itochondria and the expression of mitochondrial enzymes are regulated by di fferent mechanisms in various types of retinal neurons and glial cells.