BEN AS A PRESUMPTIVE TARGET RECOGNITION MOLECULE DURING THE DEVELOPMENT OF THE OLIVOCEREBELLAR SYSTEM

It has been shown previously that in the chick embryo the cell adhesio n molecule BEN/SC1/DM-GRASP is expressed by neurons in the inferior ol ive (IO) and by their terminal axonal arbors in the cerebellar cortex, the climbing fibers (Pourquie et al., 1992b), Here, new information o n the expression of BEN during the formation of the olivocerebellar pr ojection adds the important notion that BEN is also expressed by the c erebellar targets of inferior olivary axons, Purkinje cells (PCs) and deep nuclear neurons. This expression is transient, starting at E7-E8 and vanishing shortly after hatching. More importantly, BEN expression is restricted to precise subsets of IO neurons and PCs. In the cerebe llar cortex, BEN-immunoreactive (BEN-IR) structures are not found rand omly but are distributed according to a reproducible pattern of parasa gittal stripes. A maximum of four distinct sagittal stripes is found i n each lobule, along the whole rostrocaudal extent of the cerebellum. Moreover, BEN-expressing stripes belong to two classes; one contains B EN-IR climbing fibers terminating on BEN-IR PCs and the other, more fr equent class is solely composed of BEN-IR climbing fibers. Organotypic cultures of isolated cerebella have shown that the expression of BEN in the IO and in the cerebellum arise independently, probably because of an intrinsic developmental program. Thus, the cell adhesion molecul e BEN meets all criteria for a recognition molecule involved in the fo rmation of the olivocerebellar projection.