Cytoplasmic inclusions within developing mouse retinal photoreceptor cells maintained in culture: possible type A retrovirus particles?

Unusual cytoplasmic inclusions were consistently observed in photoreceptor cells of explants of embryonic mouse retina maintained in culture for 21 days or longer. The inclusions were chiefly spherical (some appeared to be short cylinders) and about 76 nm in diameter. They consisted of a granular/amorphous electron-dense periphery and an electron-lucent core and were usually in clusters, often close to the Golgi apparatus or plasma membrane. They were also commonly present in photoreceptor cell synaptic terminals in the outer plexiform layer. They displayed close relationships with cisterns and tubules of the endoplasmic reticulum and many were located within dilated sacs of reticulum. Narrow, smooth-surfaced tubules appeared to run into or through the cores of some of the inclusion bodies linking them to others. Similar inclusions were not seen in any other cell types in the explants and have not been reported in previous in vivo and in vitro studies of developing or adult mouse retina. The possibility that these inclusions represent Type A retrovirus particles is considered.