Abstract
Elevated homocysteine (Hcys) causes a wide spectrum of congenital detects, many of which resemble neural crest (NC) cell ablation. However, the etiological mechanism is not known. The purpose of this study was to determine if Hcys altered the number and/or the proliferation of NC cells or their progenitors /?? Uw. Hcys was experimentally elevated in chicken embryos, 16 or 4 hrs before the stage when NC cells in the cardiac region of the neural tube began forming and migrating away from the neural tube. To evaluate the effect of Hcys on cell proliferation, 5-bromo-2'-deoxyuridine (BrdU) was used to label cells in S- phase of the cell cycle. In control embryos, a significantly higher percentage of newly formed NC cells were in S-phase compared to migrating NC cells or their progenitors within the neural tube. These results parallel those of other investigators showing in the trunk axial level, NC progenitor cells must enter the S-phase in order to segregate from the neural tube Elevated Hcys significantly decreased the number of NC cells but also increased neural tube cell number. Yet, the net total number of these cells was not different from control embryos. Hcys also decreased the BrdU-labeling index for newly formed NC cells These observations suggested Hcys inhibits NC progenitors from becoming NC ceils by blocking or delaying their entry into the S-phase. Hcys also decreased the distance NC cells migrated. Although Hcys-treated embryos had significantly fewer NC cells, there was a significant increase in the number of premigratory NC cells compared to controls. Premigratory NC cells in Hcys- treated embryos also had altered BrdU-labeling patterns and more mitotic figures than control embryos. This suggests Hcys also delayed the onset of N C cell migration once they became segregated from the neural tube. These results support the hypothesis that Hcys reduces the number of NC cells in the cardiac neural tube axial level by altering normal proliferation patterns of NC progenitor cells. It also provides the first in vivio evidence that NC morphogenesis is directly altered by elevated Hcys.