The migrating pathways Trunk

Using the quail-chick chimera system and the HNK-l/NC-1 staining method, the pathways by which neural crest cells migrate and the fate map o f the derivatives was constructed (for review see Bronner-Fraser, 1993). Weston (1963), using the tritiated thymidine method, discovered two primary pathways in the trunk o f chick: dorsolaterally under the ectoderm to give rise to melanocytes, and ventrally through the somite, between the sclerotome and dermamyotome, to give rise to cells in the DRG, sympathetic ganglia, adrenomedullary cells, and aortic plexuses (Figure 1-3).

Interestingly neural crest cells which migrate ventrally move in a segmental fashion through the rostral half o f each somite, but do not move through the caudal half (Rickmann et al., 1985; Weston, 1963, Erickson et al, 1992), while those that migrate dorsally under skin do so in an unsegmented manner (Serbedzija et al., 1989, 1990, Erickson et al, 1992). This population o f cells appears to migrate much later than those that take the ventral pathway (see Erickson and Goins, 1995) to give rise to melanocytes (Dusahne, 1935, Dorris, 1939, Rawles, 1947; Mayer, 1973).

Based on knowledge gained by the chick-quail chimera system and HNK-l/NC-1 staining method, apart from migrating through the two main pathways (the ventral and the dorsal lateral pathways), some cells o f the neural crest population were reported to migrate A. intersomitically, B longitudinally along the neural tube, or C. logitudinally along the dorsal aorta. The intersomitic migration pathway o f neural crest cells is reported using the chick-quail system (Le Douarin, 1982; Teillet et al., 1987). Erickson et al (1989) reported that the crest cells in a 28-somite chick stained with the HNK-1 antibody first collect in a wedge and begin to migrate in the intersomitic space at the 23rd somite pair, preceding the ventral migration. This intersomitic population then migrates longitudinally along the dorsal aorta and probably gives rise to the sympathetic ganglia (Loring and Erickson, 1987; Erickson et al., 1989). The longitudinal migration along the dorsal aspect o f the neural tube was reported by Teillet et al (1987). When migration begins, neural crest cells leave the neural primordium in a uniform manner, and also progress longitudinally along the neural tube from anterior to posterior somitic halves, and vice versa (Teillet et al, 1987). Because o f these pathways are topologically more difficult to dissect and to reconstruct, whether the cells utilizing these pathways contribute to the same population and phenotype o f cells as those migrating through the main pathways, is not clear. Furthermore, reports about these migration pathways contradict each other. The timing o f the intersomitic pathway reported by Erickson et al. and Teillet differs, the form er suggests that intersomitic migration precedes the ventral lateral migration while the latter reported that both events occur at the same time. The evidence for migration logitudinally along the tube was indirect. These inconsistencies in these

reports, however, give an idea o f how difficult it is to describe a highly mobile, three dimensional, heterogenic cell population using morphological evidence.

Head

The cranial neural crest cells differ from the trunk crest in several ways. The cranial crest contributes to the formation o f sensory and autonomic ganglia and gives rise to a range o f mesenchymal tissues produced elsewhere by the mesoderm (Le Lievre and LeDouarin, 1975; Noden, 1975). The migrating pathways o f the cranial crest are much more complex. The migrating cranial neural crest in mouse groups into three broad streams from the dorsal aspect o f the neural tube migrating ventrally from the caudal forebrain, midbrain and hindbrain, while in chick and probably also rat only the midbrain and hindbrain give rise to neural crest (Serbedzija et al., 1992). None or very few neural crest cells were found to migrate into the otic vesicle (Le Douarin, 1982; Lumsden et al., 1991; Serbedzija, 1992). Neural crest arising from the level o f forebrain migrates ventrally in a non-segmented, continuous stream through the mesenchyme between the eye and the diencephalon. The neural crest from the midbrain level migrates ventrolaterally through the mesenchyme between the surface o f mesencephalon and the ectoderm (Le Douarin, 1982; Lumsden et al., 1991; Serbedzija, 1992). Cells from mesencephalic and rhomb encephalic levels migrate into their neighboring branchial arches corresponding to the segmented disposition o f the rhomb encephalon. Branchial arches 1,2 and 3 receive crest cells migrating from rhombomeres 2, 4, and 6 respectively, where the cranial nerve entry/exit points

reside, avoiding mesenchymal space lateral to rhombomere 3 and 5 (Lumsden et al., 1991). Enhanced levels o f cell death were found in the dorsal midline o f rhombormere 3 and 5, suggesting that the lack o f migrating crest from these segments is probably due to elimination instead o f lack o f emigrating cells (Lumsden et al.,

1991). The cranial crest cells populate their destination in a ventral to dorsal fashion, as is found in the trunk area (Serbedzija et al., 1992). As the result o f the segmental migration o f the cranial neural crest, the cranial ganglia are arranged along the the hindbrain facing the even numbered rhomb orm eres. The sensory and motor axons cross the border o f the CNS and PNS via the same entry/exit point, again in the even numbered rhombomeres (Lumsden and Keynes, 1989; Golding et al., 1997).