The development of major projections to the inferior olivary nucleus

Summary

We have employed degeneration techniques to study the ontogeny of major projections to the inferior olivary nucleus in the North American opossum, a species which is born 12 days after conception and which enjoys a protracted development in an external pouch. Subsequent to spinal lesions a small amount of axonal degeneration can be produced within the edge of the olive before subnuclei can be distinguished (7 days after birth, 24 mm, snout-rump length). Degenerating axons are present more deeply within the olive in animals operated 12 days after birth (30 mm, snout-rump length) and by at least day 16 (36 mm, snout-rump length), they are found in all of the regions they occupy in the adult animal. Subsequent to lesions which undercut all descending mesencephalic and diencephalic systems, a small amount of axonal degeneration is found at the dorsolateral edge of the olive by day 7 (23 mm, snout-rump length). Degenerating axons fill more of the olive, particularly caudally, after comparable lesions in older animals and by day 17 (38 mm, snout-rump length), degeneration is present in all of the olivary regions innervated by midbrain and thalamic axons in the adult opossum. There is some evidence that spinal, mesencephalic and diencephalic axons follow a caudal to rostral gradient in their intraolivary growth. Lesions which undercut neurites growing out of the cerebellum produce evidence for cerebello-olivary connections by day 17. Axons from the cerebral cortex reach their olivary targets considerably later than those from either the spinal cord, mesencephalon, diencephalon or cerebellum. It is not until approximately postnatal day 30 (55 mm, snout-rump length) that degenerating axons can be traced into the olive after lesions of the cortical mantle. These data indicate that the inferior olive receives major connections early in development and that there is an orderly sequence to their growth.