Compartmental organization of Purkinje cells in the mature and developing mouse cerebellum as revealed by an olfactory marker protein-lacZ transgene.

In a line of transgenic mice (HpY-1), the pattern of expression of an olfactory marker protein (OMP)-lacZ fusion gene was analyzed in the cerebellum, where, in adult mice, OMP-lacZ was expressed primarily in Purkinje cells (PCs) of the posterior lobe. The transgene-expressing PCs were organized in parasagittal bands, with a boundary of expression roughly corresponding to the primary fissure that separates the cerebellum into anterior and posterior compartments. The regional expression of the lacZ gene was also analyzed during embryonic and postnatal development of the cerebellum. Within the cerebellum-isthmus region, transgene expression first was detected at embryonic day 13.5 (E13.5) in a cluster of postmitotic cells. By E14.5, lacZ was also expressed by a subpopulation of migrating PCs in the postisthmal and lateral cerebellar primordium, and, by E16.5, transgene-positive PCs formed caudally four sagittal bands symmetric to the medial embryonic fissure. The caudal pattern was retained in postnatal cerebella, where, by postnatal day 0 (P0), transgene-positive PCs in vermal lobules VIII and IX appeared to be organized in two prominent parasagittal compartments on either side of a negative midline band. In early postnatal animals, the transgene was expressed transiently in the anterior lobe vermis. Hence, from P5 onward, transgene expression appeared mostly restricted to the posterior lobe, where it followed a caudal-to-rostral gradient. In the paraflocculus, transgene-expressing PCs were confined to the rostrodorsal portion. The results indicate that the anterior and posterior cerebellar lobes are regulated by distinct ontogenetic programs, and PCs of functionally distinct cerebellar regions express the transgene differentially. Furthermore, the data suggest that ectopic expression of OMP-lacZ in the cerebellum is under the control of regulatory elements that provide positional information for the regional specification of PC subsets.