Molecular guidance cues in the development of visual pathway

Unraveling the developmental heterogeneity within the human retina to reconstruct the continuity of retinal ganglion cell maturation and stage-specific intrinsic and extrinsic factors

Tissue development is a complex spatiotemporal process with multiple interdependent components. Anatomical, histological, sequencing, and evolutional strategies can be used to profile and explain tissue development from different perspectives. The introduction of scRNAseq methods and the computational tools allows to deconvolute developmental heterogeneity and draw a decomposed uniform map. In this manuscript, we decomposed the development of a human retina with a focus on the retinal ganglion cells (RGC). To increase the temporal resolution of retinal cell classes maturation state we assumed the working hypothesis that that maturation of retinal ganglion cells is a continuous, non-discrete process. We have assembled the scRNAseq atlas of human fetal retina from fetal week 8 to week 27 and applied the computational methods to unravel maturation heterogeneity into a uniform maturation track. We align RGC transcriptomes in pseudotime to map RGC developmental fate trajectories against the broader timeline of retinal development. Through this analysis, we identified the continuous maturation track of RGC and described the cell-intrinsic (DEGs, maturation gene profiles, regulons, transcriptional motifs) and -extrinsic profiles (neurotrophic receptors across maturation, cell-cell interactions) of different RGC maturation states. We described the genes involved in the retina and RGC maturation, including de novo RGC maturation drivers. We demonstrate the application of the human fetal retina atlas as a reference tool, allowing automated annotation and universal embedding of scRNAseq data. Altogether, our findings deepen the current knowledge of the retina and RGC maturation by bringing in the maturation dimension for the cell class vs. state analysis. We show how the pseudotime application contributes to developmental-oriented analyses, allowing to order the cells by their maturation state. This approach not only improves the downstream computational analysis but also provides a true maturation track transcriptomics profile.

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in the adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity," which has been examined during or after the classic "critical period." Because peripheral perturbations can alter the auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters the ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo widefield imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activities in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs, shifting the excitation-inhibition balance toward excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity", which has been examined during or after the classic 'critical period'. Because peripheral perturbations can alter auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the classic critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activity in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs shifting the excitation-inhibition balance towards excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Thalamocortical circuits for the formation of hierarchical pathways in the mammalian visual cortex

External sensory inputs propagate from lower-order to higher-order brain areas, and the hierarchical neural network supporting this information flow is a fundamental structure of the mammalian brain. In the visual system, multiple hierarchical pathways process different features of the visual information in parallel. The brain can form this hierarchical structure during development with few individual differences. A complete understanding of this formation mechanism is one of the major goals of neuroscience. For this purpose, it is necessary to clarify the anatomical formation process of connections between individual brain regions and to elucidate the molecular and activity-dependent mechanisms that instruct these connections in each areal pair. Over the years, researchers have unveiled developmental mechanisms of the lower-order pathway from the retina to the primary visual cortex. The anatomical formation of the entire visual network from the retina to the higher visual cortex has recently been clarified, and higher-order thalamic nuclei are gaining attention as key players in this process. In this review, we summarize the network formation process in the mouse visual system, focusing on projections from the thalamic nuclei to the primary and higher visual cortices, which are formed during the early stages of development. Then, we discuss how spontaneous retinal activity that propagates through thalamocortical pathways is essential for the formation of corticocortical connections. Finally, we discuss the possible role of higher-order thalamocortical projections as template structures in the functional maturation of visual pathways that process different visual features in parallel.

CPMF-Net: Multi-Feature Network Based on Collaborative Patches for Retinal Vessel Segmentation

As an important basis of clinical diagnosis, the morphology of retinal vessels is very useful for the early diagnosis of some eye diseases. In recent years, with the rapid development of deep learning technology, automatic segmentation methods based on it have made considerable progresses in the field of retinal blood vessel segmentation. However, due to the complexity of vessel structure and the poor quality of some images, retinal vessel segmentation, especially the segmentation of Capillaries, is still a challenging task. In this work, we propose a new retinal blood vessel segmentation method, called multi-feature segmentation, based on collaborative patches. First, we design a new collaborative patch training method which effectively compensates for the pixel information loss in the patch extraction through information transmission between collaborative patches. Additionally, the collaborative patch training strategy can simultaneously have the characteristics of low occupancy, easy structure and high accuracy. Then, we design a multi-feature network to gather a variety of information features. The hierarchical network structure, together with the integration of the adaptive coordinate attention module and the gated self-attention module, enables these rich information features to be used for segmentation. Finally, we evaluate the proposed method on two public datasets, namely DRIVE and STARE, and compare the results of our method with those of other nine advanced methods. The results show that our method outperforms other existing methods.

Transient Subplate Sublayer Forms Unique Corridor for Differential Ingrowth of Associative Pulvinar and Primary Visual Projection in the Prospective Visual Cortical Areas of the Human Fetal Occipital Lobe

Cytoarchitectonical parcellation of the visual cortex into the striate and extrastriate cortex requires complex histogenetic events within a precise spatio-temporal frame to attain the specification of areal domains and associated thalamocortical connections during the fetal brain development. We analyzed a deep subplate cellular monolayer (subplate "corridor" cells) present during a restricted period of 13-15 postconceptional weeks, showing the 3D caudo-ventro-medial position in the human fetal occipital lobe, corresponding to the segregation point of pulvinocortical and geniculocortical fibers at the prospective area 17/18 border. Immunofluorescence stainings revealed subplate "corridor" cells as the specific class of the deepest subplate neurons (NeuN+, Tbr1+, Cplx3+) expressing axon guidance molecules (Sema-3A+, EphA6+), presumably for the attraction of pulvinocortical axons and the repulsion of geniculocortical axons growing at that time (SNAP25+, Syn+, FN+). Furthermore, quantitative analysis of the subplate "corridor" region of interest, considering cell number, immunofluorescence signal intensity per cell and per region, revealed significant differences to other regions across the tangential circumference of the developing cerebral wall. Thus, our study sheds new light on the deepest subplate sublayer, strategically aligned along the growing axon systems in the prospective visual system, suggesting the establishment of the area 17/18 border by differential thalamocortical input during the fetal brain development.

Case report: Unilateral optic nerve aplasia and developmental hemi-chiasmal dysplasia with VEP misrouting

Purpose

To describe the trans-occipital asymmetries of pattern and flash visual evoked potentials (VEPs), in an infant with MRI findings of unilateral optic nerve aplasia and hemi-chiasm dysplasia.

Methods

A child with suspected left cystic microphthalmia, left microcornea, left unilateral optic nerve aplasia, and hemi-chiasm underwent a multi-channel VEP assessment with pattern reversal, pattern onset, and flash stimulation at the age of 16 weeks.

Results

There was no VEP evidence of any post-retinal visual pathway activation from left eye with optic nerve aplasia. The VEP trans-occipital distribution from the functional right eye was skewed markedly across the midline, in keeping with significant misrouting of optic nerve fibres at the chiasm. This was supported by the anatomical trajectory of the optic chiasm and tracts seen on MRI.

Conclusion

This infant has chiasmal misrouting in association with unilateral optic nerve aplasia and unilateral microphthalmos. Chiasmal misrouting has not been found in patients with microphthalmos or anophthalmos, but has been reported after early eye loss in animal models. Our findings contribute to our understanding of the discrepancy between the visual pathway physiology of human unilateral microphthalmia and animal models.

Cross-Talk of Low-Level Sensory and High-Level Cognitive Processing: Development, Mechanisms, and Relevance for Cross-Modal Abilities of the Brain

The emergence of cross-modal learning capabilities requires the interaction of neural areas accounting for sensory and cognitive processing. Convergence of multiple sensory inputs is observed in low-level sensory cortices including primary somatosensory (S1), visual (V1), and auditory cortex (A1), as well as in high-level areas such as prefrontal cortex (PFC). Evidence shows that local neural activity and functional connectivity between sensory cortices participate in cross-modal processing. However, little is known about the functional interplay between neural areas underlying sensory and cognitive processing required for cross-modal learning capabilities across life. Here we review our current knowledge on the interdependence of low- and high-level cortices for the emergence of cross-modal processing in rodents. First, we summarize the mechanisms underlying the integration of multiple senses and how cross-modal processing in primary sensory cortices might be modified by top-down modulation of the PFC. Second, we examine the critical factors and developmental mechanisms that account for the interaction between neuronal networks involved in sensory and cognitive processing. Finally, we discuss the applicability and relevance of cross-modal processing for brain-inspired intelligent robotics. An in-depth understanding of the factors and mechanisms controlling cross-modal processing might inspire the refinement of robotic systems by better mimicking neural computations.