Sexual dimorphism of sulcal morphology of the ferret cerebrum revealed by MRI-based sulcal surface morphometry

Longitudinal MRI of the developing ferret brain reveals regional variations in timing and rate of growth

Normative ferret brain development was characterized using magnetic resonance imaging. Brain growth was longitudinally monitored in 10 ferrets (equal numbers of males and females) from postnatal day 8 (P8) through P38 in 6-d increments. Template T2-weighted images were constructed at each age, and these were manually segmented into 12 to 14 brain regions. A logistic growth model was used to fit data from whole brain volumes and 8 of the individual regions in both males and females. More protracted growth was found in males, which results in larger brains; however, sex differences were not apparent when results were corrected for body weight. Additionally, surface models of the developing cortical plate were registered to one another using the anatomically-constrained Multimodal Surface Matching algorithm. This, in turn, enabled local logistic growth parameters to be mapped across the cortical surface. A close similarity was observed between surface area expansion timing and previous reports of the transverse neurogenic gradient in ferrets. Regional variation in the extent of surface area expansion and the maximum expansion rate was also revealed. This characterization of normative brain growth over the period of cerebral cortex folding may serve as a reference for ferret studies of brain development.

Neonatal valproic acid exposure produces altered gyrification related to increased parvalbumin-immunopositive neuron density with thickened sulcal floors

Valproic acid (VPA) treatment is associated with autism spectrum disorder in humans, and ferrets can be used as a model to test this; so far, it is not known whether ferrets react to developmental VPA exposure with gyrencephalic abnormalities. The current study characterized gyrification abnormalities in ferrets following VPA exposure during neonatal periods, corresponding to the late stage of cortical neurogenesis as well as the early stage of sulcogyrogenesis. Ferret pups received intraperitoneal VPA injections (200 μg/g of body weight) on postnatal days (PD) 6 and 7. BrdU was administered simultaneously at the last VPA injection. Ex vivo MRI-based morphometry demonstrated significantly lower gyrification index (GI) throughout the cortex in VPA-treated ferrets (1.265 ± 0.027) than in control ferrets (1.327 ± 0.018) on PD 20, when primary sulcogyrogenesis is complete. VPA-treated ferrets showed significantly smaller sulcal-GIs in the rostral suprasylvian sulcus and splenial sulcus but a larger lateral sulcus surface area than control ferrets. The floor cortex of the inner stratum of both the rostral suprasylvian and splenial sulci and the outer stratum of the lateral sulcus showed a relatively prominent expansion. Parvalbumin-positive neuron density was significantly greater in the expanded cortical strata of sulcal floors in VPA-treated ferrets, regardless of the BrdU-labeled status. Thus, VPA exposure during the late stage of cortical neurogenesis may alter gyrification, primarily in the frontal and parietotemporal cortical divisions. Altered gyrification may thicken the outer or inner stratum of the cerebral cortex by increasing parvalbumin-positive neuron density.

Follow-up study of subventricular zone progenitors with multiple rounds of cell division during sulcogyrogenesis in the ferret cerebral cortex

The subventricular zone (SVZ) of the developing cerebral cortex appears transiently during cortical neurogenesis and is known as the second proliferative zone that contains intermediate progenitor cells and self-renewable neuronal stem cells-the so-called basal radial glia (bRG). The present study attempted to track the differentiation and migration dynamics of SVZ progenitors undergoing multiple cell divisions at the late stage of neurogenesis in a course of sulcogyrogenesis in the ferret, a gyrencephalic mammal. Ferret pups were given a 5-ethynyl-2'-deoxyuridine (EdU) injection on postnatal day (PD) 5 followed by a 5-bromo-2'-deoxyuridine (BrdU) injection on PD 7. The 48 h interval between EdU and BrdU injections covered the minimum times for the first and second S-phase of self-renewing bRG. Two h after BrdU injection, EdU/BrdU-double labeled cells were found in the inner or outer SVZ (iSVZ and oSVZ), more than 80% of which were Sox2-positive. Furthermore, 95.8% of EdU/BrdU-double labeled Sox2-positive progenitors in the iSVZ and 84.2% in the oSVZ were also Pax6-positive, defining these progenitors as bRG. On PD 20, all EdU/BrdU-double labeled cells were NeuN-immunopositive, and more than 60% of these were parvalbumin-immunopositive. EdU/BrdU-double labeled neurons were distributed densely in the superficial portion of the outer cortical stratum. Cluster analysis divided the gyral and sulcal regions into higher and lower density groups, respectively, based on the diversity of the cortical density of EdU/BrdU-double labeled neurons. The higher density group included the gyral and sulcal regions of the prefrontal, parietooccipital and/or cingulate cortex, corresponding to cortical regions associated with evolutionary expansion. Although a limited population of neurons within a narrow time window of cortical neurogenesis was tracked, the present findings suggest that neurons derived from bRG at the late stage of neurogenesis express parvalbumin during corticohistogenesis. Due to the diversity of sulcogyral distributions, neurons derived from bRG may be implicated in evolutionary cortical expansion.

Characterization of White Matter Tracts by Diffusion MR Tractography in Cat and Ferret that Have Similar Gyral Patterns

The developmental relationships between gyral structures and white matter tracts have long been debated, but it is still difficult to discern whether they influence each other's development or are causally related. To explore this topic, this study used cats and ferrets as models for species that share similar gyral folding patterns and imaged with diffusion magnetic resonance imaging to compare white matter innervations in homologous gyri and other brain regions. Adult cat and ferret brains were analyzed via diffusion spectrum imaging tractography and homologous regions of interest were compared. Although similar genetic lineage and gyral structures would suggest analogous white matter tracts, tractography reveals significantly differing white matter connectivity in both the visual and auditory cortices. Similarities in connectivity were concentrated primarily in the highly conserved cerebellar region. These results correlate well with existing histological and functional studies of both species. Our results indicate that, while the 2 species may share similar gyral structures, they utilize different white matter connectivity; suggesting that while species may share similar gyral structures, they can develop different underlying white matter connectivity.

Biphasic aspect of sexually dimorphic ontogenetic trajectory of gyrification in the ferret cerebral cortex

The present study characterized quantitatively sexual dimorphic development of gyrification by MRI-based morphometry. High spatial-resolution 3D MR images (using RARE sequence with short TR and minimum TE setting) were acquired from fixed brain of male and female ferrets at postnatal days (PDs) 4-90 using 7-tesla preclinical MRI system. The gyrification index was evaluated either throughout the cerebral cortex (global GI) or in representative primary sulci (sulcal GI). The global GI increased linearly from PD 4, and reached a peak at PD 42, marking 1.486±0.018 in males and 1.460±0.010 in females, respectively. Sexual difference was obtained by greater global GI in males than in females on PD 21 and thereafter. Rostrocaudal GI distribution revealed an overall male-over-female sulcal infolding throughout the cortex on PD 21. Then, an adult pattern of sexually dimorphic cortical convolution was achieved so that gyrification in the temporo-parieto-occipital region was more progressive in males than in females on PD 42, and slightly extended posteriorly in males until PD 90. In the sulcal GI, sulcus-specific male-over-female GI was revealed in the rhinal fissure, and presylvian sulcus on PD 42, and additionally in the coronal, splenial, lateral, and caudal suprasylvian sulci on PD 90. The current results suggest that age-related sexual dimorphism of the gyrification was biphasic in the ferret cortex. A male-over-female gyrification was allometric by PD 21, and was thereafter specific to primary sulci located on phylogenetically newer multimodal cortical regions.

Regional difference in sulcal infolding progression correlated with cerebral cortical expansion in cynomolgus monkey fetuses

The present study aimed to specify the cerebral sulci developed by cortical expansion in cynomolgus monkey fetuses. The degree of sulcal infolding was evaluated by the gyrification index (GI), which was quantified using ex vivo magnetic resonance imaging. The correlation of cortical volume with the sulcal GI was most frequent during embryonic days (EDs) 100 to 120. Interestingly, the high correlation was marked during EDs 140 to 150 in restricted primary sulci in prefrontal, parietotemporal and medial temporal regions. The present results suggest that cortical expansion is involved in gyral demarcation by sulcal infolding, followed by the sulcal infolding progression in phylogenetically-newer cortices.

Population based MRI and DTI templates of the adult ferret brain and tools for voxelwise analysis

Non-invasive imaging has the potential to play a crucial role in the characterization and translation of experimental animal models to investigate human brain development and disorders, especially when employed to study animal models that more accurately represent features of human neuroanatomy. The purpose of this study was to build and make available MRI and DTI templates and analysis tools for the ferret brain as the ferret is a well-suited species for pre-clinical MRI studies with folded cortical surface, relatively high white matter volume and body dimensions that allow imaging with pre-clinical MRI scanners. Four ferret brain templates were built in this study – in-vivo MRI and DTI and ex-vivo MRI and DTI – using brain images across many ferrets and region of interest (ROI) masks corresponding to established ferret neuroanatomy were generated by semi-automatic and manual segmentation. The templates and ROI masks were used to create a web-based ferret brain viewing software for browsing the MRI and DTI volumes with annotations based on the ROI masks. A second objective of this study was to provide a careful description of the imaging methods used for acquisition, processing, registration and template building and to demonstrate several voxelwise analysis methods including Jacobian analysis of morphometry differences between the female and male brain and bias-free identification of DTI abnormalities in an injured ferret brain. The templates, tools and methodological optimization presented in this study are intended to advance non-invasive imaging approaches for human-similar animal species that will enable the use of pre-clinical MRI studies for understanding and treating brain disorders.

Regional hypoplasia of somatosensory cortex in growth-retarded mice (grt/grt)

Growth-retarded mouse (grt/grt) is a spontaneous mutant that is known as an animal model for primary congenital hypothyroidism caused by resistance to TSH signaling. The regional pattern of cerebral cortical hypoplasia was characterized in grt/grt mice. Ex vivo computed tomography (CT)-based volumetry was examined in four regions of the cerebral cortex, i.e., prefrontal, frontal, parietal and occipito-temporal regions, which were demarcated by structural landmarks on coronal CT images. A region-specific reduced volume of the parietal cortical region covering most of the somatosensory cortex was noted in grt/grt mice rather than in both heterozygous (grt/+) and wild-type (+/+) mice. We concluded that the cortical hypoplasia in grt/grt was seen in identical cortical regions corresponding to human congenital hypothyroidism.

Differential cortical laminar structure revealed by NeuN immunostaining and myeloarchitecture between sulcal and gyral regions independent of sexual dimorphisms in the ferret cerebrum

The purpose of this study was to quantitatively clarify differences in laminar structure and myeloarchitecture of sulcal and gyral regions of the cerebral cortex of ferrets. Histological sections of cerebrum from male and female ferrets at postnatal day 90 were made at the coronal plane, and were immunostained with anti-NeuN or anti-myelin basic protein (MBP). Thickness was estimated in the entire depth or three strata, that is, layer I, outer (layers II-III) and inner (layers IV-VI) strata of the neocortex in representative five sulcal and seven gyral regions. As with the entire cortical depth, outer and inner strata were significantly thinner in the sulcal bottoms than in the gyral crowns, whereas layer I had about twofold greater thickness in the sulcal bottoms. However, thicknesses of the entire cortical depth and each cortical stratum were not statistically different among five sulcal regions or seven gyral regions examined. By MBP immunostaining, myelin fibers ran tangentially through the superficial regions of layer I in gyral crowns. Those fibers were relatively denser in gyri of frontal and temporal regions, and relatively sparse in gyri of parietal and occipital regions, although their density in any gyri was not different between sexes. These results show a differential laminar structure and myeloarchitecture between the sulcal and gyral regions of the ferret cerebral cortex present in both sexes. Myelination of layer I tangential fibers varied among primary gyri and was weaker in phylogenetically higher-order cortical gyri. Anat Rec, 299:1003-1011, 2016. © 2016 Wiley Periodicals, Inc.