Quantitative and Qualitative Analysis of Transient Fetal Compartments during Prenatal Human Brain Development

Abstract

The cerebral wall of the human fetal brain is composed of transient cellular compartments, which show characteristic spatiotemporal relationships with intensity of major neurogenic events (cell proliferation, migration, axonal growth, dendritic differentiation, synaptogenesis, cell death, and myelination). The aim of the present study was to obtain new quantitative data describing volume, surface area, and thickness of transient compartments in the human fetal cerebrum. Forty-four postmortem fetal brains aged 13-40 postconceptional weeks (PCW) were included in this study. High-resolution T1 weighted MR images were acquired on 19 fetal brain hemispheres. MR images were processed using in-house software (MNI-ACE toolbox). Delineation of fetal compartments was performed semi-automatically by co-registration of MRI with histological sections of the same brains, or with the age-matched brains from Zagreb Neuroembryological Collection. Growth trajectories of transient fetal compartments were reconstructed. The composition of telencephalic wall was quantitatively assessed. Between 13 and 25 PCW, when the intensity of neuronal proliferation decreases drastically, the relative volume of proliferative (ventricular and subventricular) compartments showed pronounced decline. In contrast, synapse- and extracellular matrix-rich subplate compartment continued to grow during the first two trimesters, occupying up to 45% of telencephalon and reaching its maximum volume and thickness around 30 PCW. This developmental maximum coincides with a period of intensive growth of long cortico-cortical fibers, which enter and wait in subplate before approaching the cortical plate. Although we did not find significant age related changes in mean thickness of the cortical plate, the volume, gyrification index, and surface area of the cortical plate continued to exponentially grow during the last phases of prenatal development. This cortical expansion coincides developmentally with the transformation of embryonic cortical columns, dendritic differentiation, and ingrowth of axons. These results provide a quantitative description of transient human fetal brain compartments observable with MRI. Moreover, they will improve understanding of structural-functional relationships during brain development, will enable correlation between in vitro/in vivo imaging and fine structural histological studies, and will serve as a reference for study of perinatal brain injuries.

Keywords: cerebral cortex; cortical plate; human fetal brain; subplate.

Figure 1

A diagram showing four groups of subjects, taken from Zagreb Neuroembryological Collection, that were included in our study. Quantitative MRI measurements were obtained on fetal brains marked with pink rectangle (Group I and II).

Figure 2

Nissl (B,F), AChE (C,G), and CS-56 immunocytochemistry (D,H) stained coronal brain sections of the 16 PCW human fetus. Corresponding T1-weighted MRI coronal sections of the same brain are shown in (A,E). vz, ventricular zone, ^*periventricular fiber rich layer of subventricular zone; iz, intermediate zone; sp, subplate compartment; cp, cortical plate. External capsule and its radiations are marked by a red dotted curved line (B,F,C,G). Double arrows in (A,B,E,F) indicate the upper subplate compartment. Curved dashed red line in (B,C,F,G) illustrates where the border between subplate and intermediate zone was placed during manual segmentation. Scale bar = 10 mm (A,E).

Figure 3

AChE (A), Nissl (B), PAS (C) stained coronal brain sections with a corresponding T1-weighted MRI slice (D), of the human 24 PCW old fetuses. ic, internal capsule; cc, corpus callosum; vz, ventricular zone; ^*subplate compartment; iz, intermediate zone; sp, subplate compartment; cp, cortical plate. External capsule and its radiations are marked by a red dotted curved line (A). Double arrows in (A,B) indicate the upper subplate compartment. Curved red line in (A–D) illustrates where the border between subplate and intermediate zone was placed during manual segmentation. Scale bar = 10 mm (D).

Figure 4

Extracted lateral and medial cortical plate surfaces in 25 (upper row), 30 (middle row), and 40 (bottom row) PCW old fetal brains. Regional and lobar segmentation was performed manually and the surfaces were divided into 6 lobes: frontal lobe (violet), parietal lobe (red), occipital lobe (green), temporal lobe (beige or jungle green), outer ring of the limbic lobe [gyrus fornicatus encompassing: parahippocampal gyrus (celadon green) and cingulate gyrus with subcallosal area (orange)], and insular lobe (yellow).

Figure 5

Best-fit curves (red lines) with 95% prediction bands (between the dashed red lines) for the relations between age in PCW and relative volumes of cortical plate (A), subplate (B), proliferative compartments (C), and intermediate zone (D) within one hemisphere of the telencephalon.

Figure 6

Mean cortical thickness of cortical plate (green) and subplate compartment (blue) in frontal lobe (A), parietal lobe (B), insular lobe (C), temporal lobe (D), in the outer ring of the limbic lobe [gyrus fornicatus encompassing: parahippocampal gyrus (E) and cingulate gyrus with subcallosal area (F)], and occipital lobe (G) measured in millimeters.

Figure 7

Thickness of cortical plate (upper row) and subplate compartment (bottom row) measured in millimeters (color coded bars) at 13, 16, 18, 24, 30, and 40 PCW (left to right).

Figure 8

Line charts showing the increase of cortical plate surface area of frontal lobe (A), parietal lobe (B), insular lobe (C), temporal lobe (D), outer ring of the limbic lobe [gyrus fornicatus encompassing: parahippocampal gyrus (E) and cingulate gyrus with subcallosal area (F)], and occipital lobe (G) during prenatal brain development.

Figure 9

(A) Correlation matrix (Spearman's correlation coefficient - color code on the right) between all seven regional volumes of cortical plate and subplate across eight subjects, each at a different age (21–30 PCW). Significant correlations are marked with ^*. (B) Uncorrected p-values for the correlation coefficients. (C) Significant correlations (FDR-adjusted p-value < 0.05), where a black matrix entry indicates significance. Regional volumes of the cortical plate are marked with numbers 1–7 [parietal lobe (1), occipital lobe (2), frontal lobe (3), insula (4), cingulate gyrus (5), temporal lobe (6), and parahippocampal gyrus (7)]. Regional volumes of the subplate compartment are marked with numbers 8–14 [parietal lobe (8), occipital lobe (9), frontal lobe (10), insula (11), cingulate gyrus (12), temporal lobe (13), and parahippocampal gyrus (14)].