Chronic intrauterine hypoxia alters neurodevelopment in fetal sheep

Abstract

Objective: We tested the hypothesis that chronic fetal hypoxia, at a severity present in many types of congenital heart disease, would lead to abnormal neurodevelopment. Methods: Eight mid-gestation fetal sheep were cannulated onto a pumpless extracorporeal oxygenator via the umbilical vessels and supported in a fluid-filled environment for 22 ± 2 days under normoxic or hypoxic conditions. Total parenteral nutrition was provided. Control fetuses (n = 7) were harvested at gestational age 133 ± 4 days. At necropsy, brains were fixed for histopathology. Neurons were quantified in white matter tracts, and the thickness of the external granular layer of the cerebellum was measured to assess neuronal migration. Capillary density and myelination were quantified in white matter. Data were analyzed with unpaired Student t tests or 1-way analysis of variance, as appropriate. Results: Oxygen delivery was reduced in hypoxic fetuses (15.6 ± 1.8 mL/kg/min vs 24.3 ± 2.3 mL/kg/min, P <.01), but umbilical blood flow and caloric delivery were not different between the 2 groups. Compared with normoxic and control animals, hypoxic fetuses had reduced neuronal density and increased external granular layer thickness. Compared with normoxic and control animals, hypoxic fetuses had increased capillary density in white matter. Cortical myelin integrity score was lower in the hypoxic group compared with normoxic and control animals. There was a significant negative correlation between myelin integrity and capillary density. Conclusions: Chronic fetal hypoxia leads to white matter hyper-vascularity, decreased neuronal density, and impaired myelination, similar to the neuropathologic findings observed in children with congenital heart disease. These findings support the hypothesis that fetal hypoxia, even in the setting of normal caloric delivery, impairs neurodevelopment.

Prenatal hypoxemia alters microglial morphology in fetal sheep

“…Ewe anesthesia, fetal cannulation technique, and the Extra-uterine Environment for Neonatal Development (EXTEND) support device have been described previously in detail. [14][15][16] In brief, 8 fetal sheep at gestational age (GA) 109 AE 3 days (term $145 days) were exteriorized via laparotomy and hysterotomy from time-dated ewes. Fetuses were then cannulated via their umbilical vessels and attached to a pumpless low resistance oxygenator circuit.…”

“…Specific neuroanatomic landmarks, including the anterior horn of the lateral ventricle, hippocampus, and inferior cerebellar peduncle, were used to select frontal lobe, temporal lobe, and cerebellar sections, respectively. 15 Periventricular white matter boundaries were defined by a horizontal line drawn tangent to the lateral ventricle at the head of the caudate nucleus, connecting to the fundi of adjacent sulci, and the white matter boundaries at those fundi. 15,21 Gray matter was evaluated adjacent to these boundaries.…”

“…15 Periventricular white matter boundaries were defined by a horizontal line drawn tangent to the lateral ventricle at the head of the caudate nucleus, connecting to the fundi of adjacent sulci, and the white matter boundaries at those fundi. 15,21 Gray matter was evaluated adjacent to these boundaries. Quantitative measures described below reflect these regions of interest (ROIs).…”

“…14 Using this artificial womb model, we have previously shown that prenatal hypoxemia, at levels common in CHD, impairs myelination and reduces neurogenesis. 15 Here we utilized this model to determine whether prenatal hypoxemia induced microglial activation or gliosis to better understand the origins of neurobehavioral disorders in CHD.…”

Commentary: Brain injury in congenital heart disease—Who knows what happened first?

“…The design of the ''artificial womb,'' also called the Extrauterine Environment for Neonatal Development (EXTEND), that assist regulated fetal growth should be equally appreciated. 8 Despite obvious limitations in this innovative model of artificially regulated prenatal hypoxia, as discussed in detail by Lawrence and colleagues, 7 this study and the previous data from the same group present exceedingly important steps toward the understanding of a potential ''heart-brain axis,'' which implies structural brain changes at microscopic level in neonates with congenital heart disease, maybe just beyond the impact of hypoxemia. The future may allow us to study the prenatal brain once successful simulation of different congenital cardiac anomalies in animal models becomes feasible.…”