Embryonic development of the rat cerebellum. III. Regional differences in the time of origin, migration, and settling of Purkinje cells.

The time of origin, site of origin, migratory path and settling pattern of the Purkinje cells of the cerebellar hemispheres, anterior vermis, and posterior vermis were investigated in thymidine radiograms and plastic-embedded materials from rat embryos ranging in age from 15 to 22 days. In the hemispheres there is a rostral-to-caudal cytogenetic gradient: the Purkinje cells of lobulus simplex, crus I, and crus II are produced earlier than the Purkinje cells of the paramedian lobule and paraflocculus, followed by the Purkinje cells of the flocculus. The Purkinje cells of the vermis, in general, are generated later than those of the hemispheres, and with a reverse gradient from caudal to rostral: the Purkinje cells of the posterior vermis (lobules X-VI) being produced ahead of the Purkinje cells of the anterior posteriorly directed wedge of early-produced Purkinje cells through the vermis. Evidence was obtained that the Purkinje cells of the hemispheres derive from the lateral cerebellar primordium capping the lateral recess of the fourth ventricle anteriorly. The Purkinje cells of the anterior vermis originate from the subisthmal cerebellar primordium medially lining the isthmal canal. The Purkinje cells of the posterior vermis originate in the postisthmal cerebellar primordium overlying the tela choroidea caudally. The young Purkinje cells migrate from the neuroepithelium to the surface of the cerebellum in a strictly caudal-to-rostral order, paralleling the spread of the EGL superficially from posteroventral to anterodorsal. This pattern is independent of the time of origin of Purkinje cells. In the posterior vermis the earliest-settling Purkinje cells of the uvula follow a short radial course, and a discrete Purkinje layer is formed 3 days after they are generated. In the anterior vermis the Purkinje cells of lobulus centralis, which follow an anterodorsal migratory course, are still settling on day E22, 7 days after their production, presumably awaiting the fusion of the cerebellar base anteriorly. The fissura prima forms medially at the interface region of Purkinje cells derived from the postisthmal and subisthmal cerebellar primordia. For 1-2 days after their settling, the Purkinje cells of the newly forming lobules can be distinguished by certain cytological criteria from the Purkinje cells in the more caudally-situated, earlier-settled lobules.