Goldstein R S, Avivi C, Geffen R
Department of Life Sciences, Bar Ilan University, Ramat Gan, Israel.
Dev Biol. 1995 Mar;168(1):214-22. doi: 10.1006/dbio.1995.1073.
We have recently shown that there is very early variation in dorsal root ganglia (DRG) size based on their axial position. From the time of their condensation at Stage (St.) 20 (Embryonic Day 3), before the onset of apoptosis, the ganglia in brachial segments 14 and 15 are more than 80% larger on the average than those in cervical 5 and 6. This difference in volume is due to increased numbers of cells in the brachial DRG. In addition, the rostrocaudal length was found to be significantly greater for brachial ganglia, and the greater length of the brachial ganglia was found to be correlated with a greater length of brachial than cervical sclerotomes. It was therefore proposed that the difference in DRG size at the time of gangliogenesis is likely to arise from colonization by a larger initial number of neural crest cells of the longer adjacent rostral sclerotomes in brachial somites. In the present work, we have performed two types of experiments to test this hypothesis. First, we have performed heterotopic grafts of segmental plate mesoderm from cervical to brachial levels and vice versa. In all of these grafts, the sclerotomes developed with a rostrocaudal extent (length) corresponding to their level of origin in the donor embryo. DRG that formed in the grafted mesoderm attained a length appropriate to that of ganglia developing in segments of the axial level of the donor mesoderm, not that of the host. Second, we have estimated proliferation of DRG cells at St. 20 using HNK-1/bromodeoxyuridine double-stain immunocytochemistry. The percentage of cells in S-phase at both brachial and cervical levels of the neuraxis in newly formed ganglia was observed to be the same. These two lines of evidence strongly argue that the initial difference in size between DRG at different axial levels is not intrinsic, but rather is imposed by the mesodermal microenvironment in which they develop, as is the case for DRG segmentation. This is in contrast to what may occur in the hindbrain, where determination of rhombomere identity and neural crest segmentation are thought to be intrisic to the nervous system.
我们最近发现，背根神经节（DRG）的大小基于其轴向位置存在非常早期的差异。从它们在第20阶段（胚胎第3天）凝聚时起，在凋亡开始之前，第14和15臂段的神经节平均比第5和6颈段的神经节大80%以上。这种体积差异是由于臂部DRG中的细胞数量增加。此外，发现臂部神经节的头尾长度明显更长，并且发现臂部神经节的较长长度与臂部比颈部体节更长有关。因此有人提出，神经节形成时DRG大小的差异可能是由于臂部体节中较长的相邻头侧体节的神经嵴细胞初始数量较多而导致的定植。在本研究中，我们进行了两种类型的实验来检验这一假设。首先，我们进行了从颈部到臂部水平以及反之亦然的节段板中胚层的异位移植。在所有这些移植中，体节以与其在供体胚胎中的起源水平相对应的头尾范围（长度）发育。在移植的中胚层中形成的DRG达到了与供体中胚层轴向水平节段中发育的神经节长度相适应的长度，而不是宿主的长度。其次，我们使用HNK-1/溴脱氧尿苷双重染色免疫细胞化学方法估计了第20阶段DRG细胞的增殖。观察到新形成的神经节中神经轴臂部和颈部水平的S期细胞百分比相同。这两条证据有力地表明，不同轴向水平的DRG之间最初的大小差异不是内在的，而是由它们发育所处的中胚层微环境造成的，DRG的节段划分也是如此。这与后脑可能发生的情况形成对比，在后脑中，菱脑节身份的确定和神经嵴的节段划分被认为是神经系统内在的。
