Illegal Immigrations

Previews that will form mossy fibers already in the DRL, before Illegal Immigrations onset of their migration, while Wnt expression is absent in the progenitors of neurons that will form climbing fibers. The members of the large Wnt family are secreted signaling molecules associated with the extracellular In most cases, the constituent neurons of most struc-matrix and have been shown to function in various as-tures in the mammalian central nervous system (CNS) pects of neuronal development, including fate determi-are generated within the embryonic proliferative zones nation, polarity, adhesion, migration, and synapse for-of the same brain subdivision in which they will ulti-mation (Hall et al., 2000; Patapoutian and Reichardt, mately reside in the adult organism (Rakic, 1990), and 2000). Although the precise role of Wnt signaling in di-the interdivisional borders seem to be well respected recting neuronal migration across the interdivisional (Rubenstein et al., 1994). There are some notable excep-border is not known, to my knowledge, this is the first tions where newborn neurons do not obey this law and evidence that the brain stem mossy fiber system can trespass to foreign territories. The oldest and best-be distinguished from the climbing fiber system before known example of this behavior is the migration of late-connections with their respected targets are estab-generating neurons from the rhombic lip of the dorsal lished. However, a possibility that other members of the rombencephalon to the complex of gray nuclei of the large Wnt family or other unrelated molecules might pons (PGN) and the inferior olive of the medulla (Essick, also be expressed in and serve as early markers for 1912). More recently, direct evidence for this hypothesis the progenitors of the climbing fiber system cannot be has come from quail chick chimera fate mapping experi-excluded. It also seems possible that the activity of ments (Wingate and Hatten, 1999). These brainstem different members of the Wnt family could be crucial in structures are also known as precerebellar nuclei be-establishing different modes of migration and patterns cause they provide major cerebellar afferent systems— of connectivity for various other cerebellar populations. the mossy and climbing fibers—to the granule and Pur-The approach used in this study provides a useful para-kinje cells, respectively (Figure 1). Another, atypical digm for how the development of other CNS structures example of transdivisional crossing occurs during late could be analyzed with modern methodology. gestation in the human brain where there is massive Finally, Rodriguez and …