Development of terminal arbors of retino-geniculate axons in the kitten--I. Light microscopical observations.

The maturation of terminal arbors of retino-geniculate axons was studied in normal kittens from 1 to 8 postnatal weeks. Horseradish peroxidase injected into the optic tract rostral to the lateral geniculate nucleus gave a dense fill of cut axons and their terminals, resembling results obtained by the Golgi methods. At 1 and 2 weeks postnatal, the overall size and extent of axon arbors is not significantly different than in the adult. However, terminal branches of axon arbors at this age give rise to high variable endings. They terminate in finely divided sprays of finger-like extensions and filopodia bearing small spikes rather than the characteristic clusters and strands of crenulated terminals of adult axons. The bases of these sprays are broad and irregular in contour, with foliate growth-cone-like structures occurring at the ends of some branches. At 3 weeks postnatal, terminal swellings become thicker and more crenulated in contour. By 5 to 6 weeks, axon arbors have adult-like terminals with respect to their size, shape and arrangement, although entire branches may still be immature with irregular terminal swellings. Small slightly indented terminals are also seen for the first time. By 8 weeks, axon arbors are generally mature, but occasionally have a bizarre and immature arrangement of fine extensions or growth-cone-like tips. Although these observations do not establish whether reduction of branches or of terminals takes place during postnatal maturation, they demonstrate that kitten retino-geniculate axon terminal arbors are highly immature and undergo considerable changes during the period optimal for induction of sprouting by eye enucleation. The morphogenetic maturation of terminal branches that begins at 3 weeks also marks a decline in their sprouting capacity, even though remodeling of terminals is not complete until after 8 weeks of age.