Chicken embryo neural crest cells that migrate into a paraxial mesoderm constructed of multiple rostral half-somites from quail embryos form unsegmented "polyganglia," instead of distinct dorsal root ganglia (DRG). We report here that the environment that is created by grafting rostral somitic (RS) moieties not only is permissive for neural crest cell migration and consequent DRG formation but also is mitogenic for the DRG precursor cells. On embryonic day 3.5 (E3.5), 1 day after surgery, there is a 42% average increase in volume of the polyganglia compared with the corresponding DRG on the unoperated side. The volume increase is accounted for by an increased number of DRG cells-an average of 46% more cells are found in the polyganglia. The increases in volume and cell number are still present a day later at E4.5 (38% and 52%, respectively) and are observed in both limb-forming and nonlimb-forming regions of the embryonic axis. The mechanism for this increase in cell number and volume in the polyganglia is enhanced proliferative activity. On E3.5 the proportion of cells incorporating thymidine of the total DRG cell number is 45% higher in the polyganglia than the control side, when embryos are given a short pulse before sacrifice. This indicates that the rostral sclerotomal environment stimulates the crest cells to proliferate. The difference in volume between the polyganglia and the normal DRG continues to grow until at least E8, when the polyganglia are twice as large as the control DRG. The continued increase in volume can also be accounted for by the mitogenic effect of the RS grafts, since on E4.5 the percentage of thymidine-labeled cells compared with the total cell number in DRG is 28% higher in the polyganglia than in control ganglia. This study demonstrates that the somitic microenvironment regulates the proliferation of neural crest cells in the nascent DRG.Dorsal root ganglia (DRG) arise from a transient embryonic structure, the neural crest (1, 2). Although the initial migration of the neural crest cells from the neural tube is continuous rostro-caudally (3-5), segmented DRG eventually form because of the constraint placed on the further migration of crest cells by the adjacent mesodermal structures, the somites. Neural crest cells and motor axons are able to enter only the rostral portion of the sclerotome arising from each somite (5, 6-11). In contrast, the caudal half-somites inhibit the entry of neural crest cells, exiting the neural tube opposite them. Being unable to migrate laterally, the crest cells migrate either rostrally or caudally where they contribute to two adjacent DRG (5).It has been shown that experimentally modifying the cranio-caudal organization of the somitic mesoderm impairs the segmental migration of crest cells and segmentation of motoneuron axons into individual nerves (12). In the case of the DRG, construction of a continuous rostral somitic (RS) mesoderm led to the development of large, relatively unsegmented DRG termed polyganglia (13). In contrast, mesoderm consi...