Dynamic Expression Patterns of Progenitor and Neuron Layer Markers in the Developing Human Dentate Gyrus and Fimbria.

The molecular mechanisms that orchestrate the development of the human dentate gyrus are not known. In this study, we characterized the formation of human dentate and fimbrial progenitors and postmitotic neurons from 9 gestational weeks (GW9) to GW25. PAX6+ progenitor cells remained proliferative until GW16 in the dentate ventricular zone. By GW11, the secondary dentate matrix had developed in the intermediate zone, surrounding the dentate anlage and streaming toward the subpial layer. This secondary matrix contained proliferating PAX6+ and/or TBR2+ progenitors. In parallel, SOX2+ and PAX6+ fimbrial cells were detected approaching the dentate anlage, representing a possible source of extra-dentate progenitors. By GW16, when the granule cell layer could be delineated, a hilar matrix containing PAX6+ and some TBR2+ progenitors had become identifiable. By GW25, when the 2 limbs of the granule cell layer had formed, the secondary dentate matrix was reduced to a pool of progenitors at the fimbrio-dentate junction. Although human dentate development recapitulates key steps previously described in rodents, differences seemed to emerge in neuron layer markers expression. Further studies are necessary to better elucidate their role in dentate formation and connectivity.

The hippocampal commissure: a new finding at prenatal 3D ultrasound in fetuses with isolated complete agenesis of the corpus callosum.

OBJECTIVE: The aim of this research was to determine the prevalence and sonographic appearance of the hippocampal commissure in fetuses with isolated complete agenesis of the corpus callosum by three-dimensional neurosonography in the multiplanar mode. METHODS: This was a multicenter observational study. Stored volume datasets of fetuses with isolated complete agenesis of the corpus were retrospectively retrieved for analysis in three tertiary centers. The presence or absence of the hippocampal commissure was independently evaluated in the coronal and midsagittal planes by two operators. Postnatal follow-up was obtained in all cases. RESULTS: From November 2007 to February 2013, 41 cases between 19 and 30 weeks of gestation were retrieved for analysis. The hippocampal commissure was visible in the coronal and sagittal planes in 27/41 (65.8%), absent or not clearly recognizable in the remaining 14 cases. The qualitative analysis of the two operators was concordant in 100% of cases. CONCLUSIONS: In more than half of fetuses with complete callosal agenesis, the hippocampal commissure may be visualized at prenatal ultrasound. This is a residual interhemispheric connection, which in normal cases is hidden by the corpus callosum itself. Further research is needed to establish if this has an impact on postnatal outcome.

Microtubule-associated protein 6 mediates neuronal connectivity through Semaphorin 3E-dependent signalling for axonal growth.

Structural microtubule associated proteins (MAPs) stabilize microtubules, a property that was thought to be essential for development, maintenance and function of neuronal circuits. However, deletion of the structural MAPs in mice does not lead to major neurodevelopment defects. Here we demonstrate a role for MAP6 in brain wiring that is independent of microtubule binding. We find that MAP6 deletion disrupts brain connectivity and is associated with a lack of post-commissural fornix fibres. MAP6 contributes to fornix development by regulating axonal elongation induced by Semaphorin 3E. We show that MAP6 acts downstream of receptor activation through a mechanism that requires a proline-rich domain distinct from its microtubule-stabilizing domains. We also show that MAP6 directly binds to SH3 domain proteins known to be involved in neurite extension and semaphorin function. We conclude that MAP6 is critical to interface guidance molecules with intracellular signalling effectors during the development of cerebral axon tracts.

Choice of diffusion tensor estimation approach affects fiber tractography of the fornix in preterm brain.

BACKGROUND AND PURPOSE: Neonatal DTI enables quantitative assessment of microstructural brain properties. Although its use is increasing, it is not widely known that vast differences in tractography results can occur, depending on the diffusion tensor estimation methodology used. Current clinical work appears to be insufficiently focused on data quality and processing of neonatal DTI. To raise awareness about this important processing step, we investigated tractography reconstructions of the fornix with the use of several estimation techniques. We hypothesized that the method of tensor estimation significantly affects DTI tractography results. MATERIALS AND METHODS: Twenty-eight DTI scans of infants born <29 weeks of gestation, acquired at 30-week postmenstrual age and without intracranial injury observed, were prospectively collected. Four diffusion tensor estimation methods were applied: 1) linear least squares; 2) weighted linear least squares; 3) nonlinear least squares, and 4) robust estimation of tensors by outlier rejection. Quality of DTI data and tractography results were evaluated for each method. RESULTS: With nonlinear least squares and robust estimation of tensors by outlier rejection, significantly lower mean fractional anisotropy values were obtained than with linear least squares and weighted linear least squares. Visualized quality of tract reconstruction was significantly higher by use of robust estimation of tensors by outlier rejection and correlated with quality of DTI data. CONCLUSIONS: Quality assessment and choice of processing methodology have considerable impact on neonatal DTI analysis. Dedicated acquisition, quality assessment, and advanced processing of neonatal DTI data must be ensured before performing clinical analyses, such as associating microstructural brain properties with patient outcome.

Identification of the fetal hippocampus and fornix and role of 3-dimensional sonography.

OBJECTIVES: The purposes of this study were to identify the fetal hippocampus and fornix using 3-dimensional sonography, to measure their curved length during pregnancy, and to describe a systematic method for volume data set analysis of the fetal hippocampus and fornix. METHODS: Three-dimensional volumes of the fetal brain were acquired prospectively in 34 patients between 14 and 37 weeks' gestation. Volumes were acquired with trans-abdominal and transvaginal transducers. All volumes were analyzed offline by 2 examiners separately. The feasibility of identifying the fetal hippocampus and fornix was analyzed. The curved length of the hippocampus-fornix structure was measured on the right and left hemispheres. RESULTS: The fetal hippocampus and fornix were identified bilaterally in 32 of 34 fetuses (94%) at gestational ages of 14 weeks 5 days to 37 weeks 1 day (mean, 23 weeks 3 days). In 1 fetus (3%), only one side was shown, and in another fetus (3%), both sides were obscured by acoustic shadows. A systematic approach for identification of the fetal hippocampus is described. Linear growth of the fetal hippocampus and fornix was shown during pregnancy and was correlated with both the gestational week and the head circumference (R = 0.71 and 0.74, respectively; P = .01). The length of the hippocampus and fornix did not differ between the left and the right hemispheres (P = .598). CONCLUSIONS: The fetal hippocampus and fornix can be identified by a systematic analysis of 3-dimensional data set volumes. The normal hippocampus and fornix show linear growth throughout pregnancy.

Fetal fornix transection and gestation length in sheep.

Experiments in several species indicate that the hippocampus influences hypothalamo-pituitary-adrenal (HPA) axis function. In fetal sheep, simultaneous ACTH and cortisol rises over the last 30 days of gestation peak at term and are necessary for birth. We hypothesized that if the fetal hippocampal formation is functional in late gestation, loss of hippocampal input to the HPA axis following fetal fornix transection would change gestation length in comparison to controls. At 118-121 days of gestation (dG), stereotaxic technique was used in fetal sheep to sham transect (SHAM; n = 8) or transect (FXTX; n = 6) the dorsal fornix at the level of the hippocampal commissure. No differences were found between SHAM and FXTX fetuses in daily hormone profiles over the last week of gestation or in gestation length (148.0 +/- 1.2 vs. 149.0 +/- 0.4 dG, respectively). We conclude that the fetal hippocampus is immature in late gestation and we speculate that an immature hippocampus is necessary for the loss of negative feedback control that gives rise to the long term, simultaneous increases in ACTH and cortisol that are indispensable for labor and delivery at term in sheep.

Coordinated functions of Netrin-1 and Class 3 secreted Semaphorins in the guidance of reciprocal septohippocampal connections.

An essential characteristic of the CNS function is the formation of reciprocal connections between brain areas. Although the mechanisms controlling the establishment of neuronal connections are being determined, very little is known about the development of reciprocal connections, which often course along identical pathways. Here, we show that Netrin-1, expressed along the fimbria, chemoattracts both septohippocampal and hippocamposeptal fibers. Moreover, we show that both Semaphorins 3A and 3F expressed in regions nearby the septum prevent the growth of septal axons into these regions. Blocking experiments with recombinant ecto-Neuropilins indicate that both Semaphorins 3A and 3F act cooperatively in the repulsion of septal axons. Furthermore, netrin-1-deficient mice develop a reduced septohippocampal projection. We conclude that the coordinated actions of Netrin-1 and Semaphorins 3A and 3F cooperate in the development of septohippocampal and hippocamposeptal connections, indicating that the same molecular cues serve the construction of reciprocal connections in both directions of growth.

Temporal and spatial regulation of chondroitin sulfate, radial glial cells, growing commissural axons, and other hippocampal efferents in developing hamsters.

We investigated the time and space relationship between growth of hippocampal efferents, particularly those forming the hippocampal commissure, and expression of extracellular matrix components related to radial glial cells. Developing hamster brains from embryonic day (E) 13 to postnatal day (P) 7 had 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) crystals implanted into the hippocampus or were processed for fluorescent immunohistochemistry against chondroitin sulfate (CS) glycosaminoglycans and glial fibrillary acidic protein (GFAP). The first, pioneer fibers from the hippocampus were seen crossing the midline at E15 and arriving at the contralateral hippocampus 24-48 hours later (P1), followed closely by a thick front of growing fibers. Before E15, CS expression was preceded by septal fusion and was concomitant with formation of the commissural tract. On E15, CS expression formed a U-shaped border below the fimbria. From E15 to P3, CS became expressed between the hippocampal commissure and the third ventricle and at the caudal borders of the fornix columns. As the hippocampal commissure expanded, CS expression became gradually lighter to virtually disappear by P7. On E15 and P1, GFAP-positive radial glial cells were present caudal (but not rostral) to the commissure at the midline, partially overlapping CS expression. Similar cells were present dorsal to the fimbria, extending their processes perpendicularly over the growing axons. The data reveal that CS and radial glial cells form a tunnel surrounding the developing fimbria and a border at the midline caudal to the hippocampal commissure. It is suggested that these cellular and molecular borders play a role in guidance of hippocampal efferents.

A role for cingulate pioneering axons in the development of the corpus callosum.

In many vertebrate and invertebrate systems, pioneering axons play a crucial role in establishing large axon tracts. Previous studies have addressed whether the first axons to cross the midline to from the corpus callosum arise from neurons in either the cingulate cortex (Koester and O'Leary [1994] J. Neurosci. 11:6608-6620) or the rostrolateral neocortex (Ozaki and Wahlsten [1998] J. Comp. Neurol. 400:197-206). However, these studies have not provided a consensus on which populations pioneer the corpus callosum. We have found that neurons within the cingulate cortex project axons that cross the midline and enter the contralateral hemisphere at E15.5. By using different carbocyanine dyes injected into either the cingulate cortex or the neocortex of the same brain, we found that cingulate axons crossed the midline before neocortical axons and projected into the contralateral cortex. Furthermore, the first neocortical axons to reach the midline crossed within the tract formed by these cingulate callosal axons, and appeared to fasciculate with them as they crossed the midline. These data indicate that axons from the cingulate cortex might pioneer a pathway for later arriving neocortical axons that form the corpus callosum. We also found that a small number of cingulate axons project to the septum as well as to the ipsilateral hippocampus via the fornix. In addition, we found that neurons in the cingulate cortex projected laterally to the rostrolateral neocortex at least 1 day before the neocortical axons reach the midline. Because the rostrolateral neocortex is the first neocortical region to develop, it sends the first neocortical axons to the midline to form the corpus callosum. We postulate that, together, both laterally and medially projecting cingulate axons may pioneer a path for the medially directed neocortical axons, thus helping to guide these axons toward and across the midline during the formation of the corpus callosum.

Novel genes expressed in the developing medial cortex.

Two cDNAs, M1 and M2, recently isolated by the differential display method from embryonic rat cerebral hemisphere were characterized and their patterns of spatiotemporal expression analysed in developing rat forebrain by in situ hybridization histochemistry and correlative immunocytochemistry. Neither gene bears any sequence homology to other known genes. Both genes are particularly expressed in medial regions of the cerebral hemisphere and M2 in the roof of the adjacent diencephalon. M1 expression is highly localized and confined to the neuroepithelium of the hippocampal rudiment from embryonic day (E) 12 onward. Its location corresponds to the fimbrial anlage, and immunocytochemical localization of M1 protein indicates its expression in radial glial cells. M2 expression at E12 is more extensive in the medial cerebral wall, extending into the preoptic region and beyond the hippocampus into dorsal hemisphere and into the dorsal diencephalon, with caudal extension along the dorsal midline and in the zona limitans intrathalamica. Later, M2 expression is found in association with the corpus callosum, hippocampal commissure, fimbria, optic nerve, stria medullaris, mamillothalamic tract and habenulopeduncular tract. M1 and M2 expression domains corresponding to the locations of fiber tracts are present prior to the arrival of the earliest axons, as identified by neuron specific markers. These findings suggest M1 and/or M2 genes are involved in early regional specification of the hippocampus and related structures in paramedian regions of the forebrain, and that cell populations expressing these genes in advance of developing axonal pathways may be involved in the early specification of tract location.