Correspondence between L7-lacZ-expressing Purkinje cells and labeled olivocerebellar fibers during late embryogenesis in the mouse.

It has been suggested that Purkinje cells (PC) play a role in organizing topographic relationships of several cerebellar afferent systems, including olivocerebellar fibers. This hypothesis is based on the observation that PC in the rat express biochemical heterogeneities during the presumptive period of olivocerebellar fiber ingrowth to the cerebellum. Previous studies designed to investigate the organization of murine olivocerebellar fibers during embryogenesis have suggested that interactions with PC may play a role in segregating olivocerebellar fibers after they enter the cerebellum. To determine whether PC heterogeneities are related to olivocerebellar fiber organization, transgenic mice carrying a beta-galactosidase (beta-gal) reporter gene linked to the promoter from the PC-specific gene L7/pcp-2 were used in neuroanatomical tracing experiments. Expression of the transgene mirrors endogenous L7/pcp-2 expression, which is upregulated earliest in parasagittal strips of the vermal cortex. Studies were conducted in vitro by using brainstem-cerebellar explants from embryonic day 17/18 (E17/18) and 18/19 mice. Applications of neuroanatomical tracer (horseradish peroxidase or neurobiotin) were made in either the caudal medial accessory olive (cMAO) or the rostral olive. These studies indicate that groups of olivocerebellar fibers and clusters of L7/lacZ+ and L7/lacZ-Purkinje cells respect common distribution boundaries during late embryogenesis. The strong correspondence between the distribution patterns generated by these two markers suggests that expression of L7/pcp-2 and the topographic organization of olivocerebellar (OC) fibers are not interdependent, but may be regulated by a common event or interaction, of a presently unknown nature, which occurs earlier during cerebellar development.