Numerical matching between granule and Purkinje cells in lurcher chimeric mice: a hypothesis for the trophic rescue of granule cells from target-related cell death

Previous studies of wild-type mice or mutant-wild-type mouse chimeras using the neurological mutant staggerer have supported a numerical matching hypothesis for target-related cell death. However, analyses of chimeras of a second neurological mutant, lurcher, have suggested that there may be significant flexibility in the relationship between the numbers of pre- and postsynaptic neurons. Whereas in staggerer chimeras there is a strict proportionality between the number of cerebellar granule cells and their postsynaptic target, the Purkinje cells, in lurcher chimeras, Wetts and Herrup (1983) report a relative increase in granule cell survival. We have reexamined the numerical matching between Purkinje and granule cells in an additional 5 lurcher----wild-type and 4 wild-type----wild-type chimerase. Our findings confirm and extend the results of the previous study to show that there is a close correlation between the number of granule and Purkinje cells in +/Lc chimeras, but for any given number of Purkinje cells in the +/Lc chimeras, more granule cells survive than in staggerer chimeras or inbred mouse strains. Whereas the ratio of granule to Purkinje cells in staggerer chimeras or inbred mouse strains is constant across all target sizes, in +/Lc chimeras the ratio of granule cells to Purkinje cells increases as the number of target neurons decreases. It seems likely that the increased granule cell survival is somehow related to the delayed degeneration of the +/Lc fraction of target cells in the +/Lc chimeras. Among the possible explanations for the observed results, we favor the hypothesis that a trophic factor is produced in +/Lc chimeras in response to the deafferentation of Purkinje cells that is capable of rescuing granule cells from target-related cell death. Our preference is based, in part, on observations of the state of the dendritic tree of the wild-type Purkinje cells that survive in +/Lc chimeras (Caddy et al., 1986).