Cytological organization of the dorsal lateral geniculate nuclei in mutant anophthalmic and postnatally enucleated mice.

The dorsal lateral geniculate nuclei (dLGN) of anophthalmic and early postnatally enucleated mice were studied to determine the role retinal fibres play in the differentiation of postsynaptic target structures. Cell counts indicate that retinal fibres are necessary for the development and maintenance of the normal number of dLGN neurons and glia. This retinal fibre dependence is greater for animals enucleated on postnatal day 3 than for animals with a congenital absence of optic axons. Golgi analysis reveals, however, that the lack of retinal fibres does not preclude the development of the thalamo-cortical and intrinsic types of dLGN neuron. Ultrastructural analysis reveals that in anophthalmic and early postnatally enucleated mice, dLGN synaptic sites normally occupied by optic axon terminals become innervated by large terminals containing round synaptic vesicles and mitochondria with an electron dense matrix. Significantly, the formation of these replacement terminals does not seem to depend upon either the previous existence of retinal fibres or the postnatal stage at which retinal fibres are removed. The possibility that some of these large replacement terminals originate from cortical or recurrent collateral axons is considered.