Background:
The process by which cortical folding occurs during brain development is not fully understood. In-utero conditions that impact fetal growth in general have been intensively studied but their effects on cortical development, in particular the development of cortical layers, are unknown. Our aim was to determine if early-onset placental insufficiency impacts lamination and glial cell populations in the fetal sheep cerebral cortex.
Methods:
Single umbilical artery ligation (SUAL) surgery was performed on pregnant sheep at 62 (n=2) and 70 (n=3) days of gestation (dg); sham controls (n=7) were included for comparison. At 90dg (n=12), brain and body weights were recorded, and brains were perfusion-fixed, and then magnetic resonance imaging (MRI) were acquired to determine the brain volume, gyrification index, cortical surface area, and whole-brain fractional anisotropy. Brains were subsequently processed for immunohistochemistry. The grey matter (parietal cortex) was assessed for the density of oligodendrocytes (Olig2-positive) and microglia (Iba1-positive). Cortical layer V/VI width was measured using sections immunostained for Ctip2, a marker of subcortical projection neurons. Data were assessed using unpaired t-test (significance was p<0.05).
Results:
In SUAL vs. control fetuses at 90dg, there was i) no difference in brain or body weight, or in any of the MRI parameters, ii) an overall marked reduction in layer V/VI thickness (p=0.06) and a significant decrease in Olig2-positive oligodendrocytes in the cortex below gyri (p<0.05) but not sulci, and iii) no difference in the density of cortical Iba1-positive microglia.
Conclusions:
In fetal sheep, early-onset placental insufficiency leads to deficits in layer V/VI subcortical projection neurons and cortical oligodendrocytes in the absence of gross microstructural alterations, and prior to the onset of FGR. Further investigations are underway to assess upper cortical layers I-IV at 90dg, and to determine whether alterations persist to 110dg and are accompanied by changes in patterns of cortical folding.