Objective: This study was undertaken to determine the extent of necrotic cell death as a measure of neurologic injury in the preterm and near-term ovine fetal brain in response to intermittent umbilical cord occlusion (UCO) with severe, but limited hypoxia and no cumulative acidosis to ensure longer-term survival.
Study design: Fetal sheep (control and experimental groups at 0.75 and 0.90 of gestation) were studied over 4 days with UCOs performed in the experimental group animals by complete inflation of an occluder cuff for 90 seconds every 30 minutes for 3 to 5 hours each day. Animals were then euthanized and the fetal brain perfusion-fixed and prepared for subsequent histology and assessment of necrotic cell injury by using standard staining with hematoxylin and eosin (H&E), and with a novel fluorescent marker, Fluoro-Jade B, that targets degenerating neurons.
Results: In both preterm and near-term animal groups, UCO caused a large decline in arterial Po(2) (to approximately 7 mm Hg), a modest decline in pH (to approximately 7.30), and a modest rise in Pco(2) (to approximately 61 mm Hg) (all P<.01), but with a return to control values after the occluder release and no cumulative acidosis over each day of study. Overall, very low levels of H&E-stained necrotic-appearing cells and Fluoro-Jade B-stained positive cells were observed across all brain regions studied with values not significantly different from zero, excepting that for the gray matter of the preterm control (by H&E staining), preterm and near-term cord occlusion (by H&E and Fluoro-Jade B staining), and the thalamus of the near-term cord occlusion (by H&E staining) animals. Although there were no differences in the levels of H&E-stained necrotic-appearing cells and Fluoro-Jade B-stained positive cells between respective control and cord occlusion group animals for most of the brain regions studied, a significant increase in Fluoro-Jade B-stained positive cells was observed in the gray matter of both the preterm and near-term cord occlusion animals (P<.05).
Conclusion: Intermittent cord occlusion insult with severe but limited fetal hypoxemia and no cumulative acidosis, was generally well tolerated in both the preterm and near-term animal groups as assessed by measures of necrotic cell injury throughout the brain with minimal evidence for such. However, compensatory mechanisms which are protective for the brain may become limited with repetitive hypoxia insult over time as suggested by the low level of Fluoro-Jade B-stained positive cells noted in the gray matter tissues for both occlusion groups.