Changes in distribution of mitochondria in the developing chick retina

The distribution of mitochondria in the developing chick retina was examined by enzyme histochemistry, immunohistochemistry, and electron microscopy. Two distinct phenomena were observed: (1) progressive segregation of mitochondria in specific locations in the developing tissue; and (2) progressive loss of mitochondrial activity from the inner retina as it matures. Densitometric scans of stained tissue sections were used to quantitate the relative amounts of mitochondrial activity in the retinal layers. Mitochondria were localized to tissue regions by transmission electron microscopy, enzyme histochemistry for the inner membrane bound mitochondrial enzymes succinic dehydrogenase and cytochrome oxidase, a combined histochemical and ultrastructural method for cytochrome oxidase, and immunolocalization of the mitochondrial matrix enzyme glutamate dehydrogenase. Seven-day embryonic chick retina has a high number of mitochondrial structures and a high level of activity. As development proceeds, the mitochondria organize into layers within the tissue. However, the relative activity of mitochondria in much of the inner retina decreases. In the post-hatch retina, 50% of the mitochondrial activity is found in less than 10% of the tissue area, in the inner segments of the photoreceptor cells.