Development of signals influencing the growth and termination of thalamocortical axons in organotypic culture

Explants of embryonic or postnatal rat cortex, organotypically cultured in serum-free medium, maintain their structural integrity and their upper laye rs continue to mature. Coculture of portions of embryonic thalamus with cor tical slices taken at different ages reveals a temporal cascade of cortical signals. (1) Slices of occipital cortex taken at E19 or earlier stimulate axonal outgrowth from explants of embryonic lateral geniculate nucleus but do not allow the fibers to invade. (2) In cortical slices taken after E19 b ut before P2, thalamic axons enter the slice, from any direction, and exten d radially across the entire depth of the cortical plate without branching or terminating. (3) In slices taken after P2, fibers slow down, arborize, a nd terminate in the maturing layer 4 of the cortex. If the thalamic explant is placed against the pial surface of the cortical slice, axons still ente r and branch in the same layer. These findings imply that the developing co rtex expresses a diffusible growth-promoting factor and then itself becomes growth permissive, and finally the maturing layer 4 expresses a "stop sign al." In triple cocultures of one thalamic explant with a "choice" of two ne ighboring slices, thalamic axons will not invade slices of cerebellum but b ehave indistinguishably in response to slices from any region of the hemisp here. Thus the initial tangential distribution of the thalamic projection i n vivo (which is achieved by about E16) is unlikely to be controlled by reg ional variation in signals produced by the cortex. When cortical slices wer e precultured alone for 7-14 days before the addition of an explant of embr yonic thalamus for 4 further days of coculture, the pattern of innervation was more appropriate to the chronological age of the slice than the age at which it was first taken. Thus the timing of the cascade of cortical proper ties is at least partly intrinsically determined. This sequence of expressi on of these signals suggests that they play a part in vivo in controlling t he outgrowth of thalamic fibers, their accumulation under the cortical plat e, their invasion of the plate, and their arborization in layer 4. (C) 1999 Academic Press.