Proliferative and degenerative events in the early development of chick dorsal root ganglia. I. Normal development

Abstract

Development of the chick dorsal root ganglia was examined in 4.5- to 9.5-day embryos. Tritiated thymidine (3H-TdR) and autoradiography was used to analyze proliferative activity and the Feulgen procedure to analyze degenerative activity in ganglia 12-17. Proliferative activity was found to be elevated through 4.5 days of incubation when as many as 14% of the ganglionic cells become labelled following a one-hour exposure to 3H-TdR. By 6.5 to 7.5 days proliferative activity decreases to 2-4% in the lateroventral (LV) regions and to approximately 1% in the mediodorsal (MD) regions of the ganglia. However, there appears to be increased proliferative activity by the end of the experimental period at 9.5 days. Birthdate studies demonstrate that large-scale neuronal production occurs between 4.5 and 6.5 days in the LV regions and between 4.5 and 7.5 days in the MD regions. After those times ganglionic proliferative activity must be largely nonneuronal in nature. This nonneuronal proliferation is greater in LV than in MD regions and in brachial than in nonbrachial ganglia. Degenerative activiy was found to be absent from the ganglia until after 4.5 days of incubation. It then increases rapidly, and by 5.5 days 5% of the LV cells in nonbrachial ganglia are degenerating. Degenerative activity then declines but is still present at 9.5 days. In contrast to results of an earlier study (Hamburger and Levi-Montalcini, '49), degenerative activity was also found in the LV region of brachial ganglia and the MD regions of brachial and nonbrachial ganglia. The pattern of LV degenerative activity in brachial ganglia is similar to that in nonbrachial ganglia, but the level of activity is lower. In the MD regions degenerative activity increases throughout the experimental period, and by 9.5 days as many as 4% of the MD cells are degenerating.

The microenvironment created by grafting rostral half-somites is mitogenic for neural crest cells.

“…This was not responsible for the increase in DRG cell number observed here because <0.5% of the cells in the ganglionic structures on both sides of the E4.5 animals were pycnotic. This low level of cell death is in agreement with other authors, who Developmental Biology: Goldstein et al found that E4.5 is before the period of massive cell death in the DRG (14,17).…”

“…Grafts in embryos sacrificed on E4.5 were made into two different portions of the neuraxis opposite ganglia [13][14][15][16], the ganglia that participate in the sensory innervation of the wing (16), and at cervical levels opposite ganglia [8][9][10][11][12]. On E4.5, both groups had the same average increases in DRG size compared with controls-38 + 5% for the brachial region (n = 3) and 38 ± 10% for the cervical region (n = 4).…”

“…This is interesting in view of the observation that there is natural variation in proliferation rates in DRG and in the motor column in different portions of the neuraxis. In normal animals more DRG cells and presumptive motoneurons are born in limb-forming regions than in trunk regions as a result of different rates of proliferation (14,17,27).…”

Stem cell factor is a neurotrophic factor for neural crest-derived chick sensory neurons

“…Neuronal responsiveness to SCF seems to show two maxima near E5 and E 10, which are also similar to those found for NGF and BDNF (Lindsay et al, 1985). These maxima correspond to the peak times for the birth of large and small neurons, respectively (McMillan and Simpson, 1978).…”

“…Although the peak time for the birth of small sensory neurons is around El0 (McMillan and Simpson, 1978), small, CGRP-containing sensory neurons are present in the DRG as early as E5 (New and Mudge, 1986). It is possible these are the neurons responding to SCF in our cultures.…”