Molecular programs of regional specification and neural stem cell fate progression in macaque telencephalon.

During early telencephalic development, intricate processes of regional patterning and neural stem cell (NSC) fate specification take place. However, our understanding of these processes in primates, including both conserved and species-specific features, remains limited. Here, we profiled 761,529 single-cell transcriptomes from multiple regions of the prenatal macaque telencephalon. We deciphered the molecular programs of the early organizing centers and their cross-talk with NSCs, revealing primate-biased galanin-like peptide (GALP) signaling in the anteroventral telencephalon. Regional transcriptomic variations were observed along the frontotemporal axis during early stages of neocortical NSC progression and in neurons and astrocytes. Additionally, we found that genes associated with neuropsychiatric disorders and brain cancer risk might play critical roles in the early telencephalic organizers and during NSC progression.

Wide-ranging migration and destination of early olfactory placode-derived neurons in chick embryos.

Cell migration from the olfactory placode (OP) is a well-known phenomenon wherein various cell types, such as gonadotropin-releasing hormone (GnRH)-producing neurons, migrate toward the telencephalon (TEL) during early embryonic development. However, the spatial relationship between early migratory cells and the forebrain is unclear. We examined the early development of whole-mount chick embryos to observe the three-dimensional spatial relationship among OP-derived migratory neurons, olfactory nerve (ON), and TEL. Migratory neurons that express highly polysialylated neural cell adhesion molecule (PSA-NCAM) emerge from the OP and spread over a relatively wide TEL area at the Hamburger and Hamilton (HH) stage 17. Most migratory neurons form a cellular cord between the olfactory pit and rostral TEL within HH18-20. The earliest axons from the olfactory epithelium (OE) were detected along this neuronal cord using DiI-labeling at HH21. Furthermore, a few PSA-NCAM-positive neurons were dispersed around the cellular cord and over the lateral TEL at HH18. A long cellular cord branch extending to the lateral TEL was transiently observed within HH18-24. These results suggest a novel migratory route of OP-derived neurons during the early developmental stages. Following GFP vector introduction into the OP of HH13-15 embryos, labeled neurons were detected around and within the lateral TEL at HH23 and HH27. At HH36, labeled cells were observed in the rostral-lateral TEL, including the olfactory bulb (OB) region. GFP-labeled and calretinin-positive neurons were detected in the OB, suggesting that early OP-derived neurons enter the forebrain and function as interneurons in the OB.

Mouse and human share conserved transcriptional programs for interneuron development.

Genetic variation confers susceptibility to neurodevelopmental disorders by affecting the development of specific cell types. Changes in cortical and striatal γ-aminobutyric acid­expressing (GABAergic) neurons are common in autism and schizophrenia. In this study, we used single-cell RNA sequencing to characterize the emergence of cell diversity in the human ganglionic eminences, the transitory structures of the human fetal brain where striatal and cortical GABAergic neurons are generated. We identified regional and temporal diversity among progenitor cells underlying the generation of a variety of projection neurons and interneurons. We found that these cells are specified within the human ganglionic eminences by transcriptional programs similar to those previously identified in rodents. Our findings reveal an evolutionarily conserved regulatory logic controlling the specification, migration, and differentiation of GABAergic neurons in the human telencephalon.

DCC regulates astroglial development essential for telencephalic morphogenesis and corpus callosum formation.

The forebrain hemispheres are predominantly separated during embryogenesis by the interhemispheric fissure (IHF). Radial astroglia remodel the IHF to form a continuous substrate between the hemispheres for midline crossing of the corpus callosum (CC) and hippocampal commissure (HC). Deleted in colorectal carcinoma (DCC) and netrin 1 (NTN1) are molecules that have an evolutionarily conserved function in commissural axon guidance. The CC and HC are absent in Dcc and Ntn1 knockout mice, while other commissures are only partially affected, suggesting an additional aetiology in forebrain commissure formation. Here, we find that these molecules play a critical role in regulating astroglial development and IHF remodelling during CC and HC formation. Human subjects with DCC mutations display disrupted IHF remodelling associated with CC and HC malformations. Thus, axon guidance molecules such as DCC and NTN1 first regulate the formation of a midline substrate for dorsal commissures prior to their role in regulating axonal growth and guidance across it.

Diversity in striatal synaptic circuits arises from distinct embryonic progenitor pools in the ventral telencephalon.

Synaptic circuits in the brain are precisely organized, but the processes that govern this precision are poorly understood. Here, we explore how distinct embryonic neural progenitor pools in the lateral ganglionic eminence contribute to neuronal diversity and synaptic circuit connectivity in the mouse striatum. In utero labeling of Tα1-expressing apical intermediate progenitors (aIP), as well as other progenitors (OP), reveals that both progenitors generate direct and indirect pathway spiny projection neurons (SPNs) with similar electrophysiological and anatomical properties and are intermingled in medial striatum. Subsequent optogenetic circuit-mapping experiments demonstrate that progenitor origin significantly impacts long-range excitatory input strength, with medial prefrontal cortex preferentially driving aIP-derived SPNs and visual cortex preferentially driving OP-derived SPNs. In contrast, the strength of local inhibitory inputs among SPNs is controlled by birthdate rather than progenitor origin. Combined, these results demonstrate distinct roles for embryonic progenitor origin in shaping neuronal and circuit properties of the postnatal striatum.

A role for sustained MAPK activity in the mouse ventral telencephalon.

The MAPK pathway is a major growth signal that has been implicated during the development of progenitors, neurons, and glia in the embryonic brain. Here, we show that the MAPK pathway plays an important role in the generation of distinct cell types from progenitors in the ventral telencephalon. Our data reveal that phospho-p44/42 (called p-ERK1/2) and the ETS transcription factor Etv5, both downstream effectors in the MAPK pathway, show a regional bias in expression during ventral telencephalic development, with enriched expression in the dorsal region of the LGE and ventral region of the MGE at E13.5 and E15.5. Interestingly, expression of both factors becomes more uniform in ventricular zone (VZ) progenitors by E18.5. To gain insight into the role of MAPK activity during progenitor cell development, we used a cre inducible constitutively active MEK1 allele (RosaMEK1DD/+) in combination with a ventral telencephalon enriched cre (Gsx2e-cre) or a dorsal telencephalon enriched cre (Emx1cre/+). Sustained MEK/MAPK activity in the ventral telencephalon (Gsx2e-cre; RosaMEK1DD/+) expanded dorsal lateral ganglionic eminence (dLGE) enriched genes (Gsx2 and Sp8) and oligodendrocyte progenitor cell (OPC) markers (Olig2, Pdgfrα, and Sox10), and also reduced markers in the ventral (v) LGE domain (Isl1 and Foxp1). Activation of MEK/MAPK activity in the dorsal telencephalon (Emx1cre/+; RosaMEK1DD/+) did not initially activate the expression of dLGE or OPC genes at E15.5 but ectopic expression of Gsx2 and OPC markers were observed at E18.5. These results support the idea that MAPK activity as readout by p-ERK1/2 and Etv5 expression is enriched in distinct subdomains of ventral telencephalic progenitors during development. In addition, sustained activation of the MEK/MAPK pathway in the ventral or dorsal telencephalon influences dLGE and OPC identity from progenitors.

Developmental Differences Between the Limbic and Neocortical Telencephalic Wall: An Intrasubject Slice-Matched 3 T MRI-Histological Correlative Study in Humans.

The purpose of the study was to investigate the interrelation of the signal intensities and thicknesses of the transient developmental zones in the cingulate and neocortical telencephalic wall, using T2-weighted 3 T-magnetic resonance imaging (MRI) and histological scans from the same brain hemisphere. The study encompassed 24 postmortem fetal brains (15-35 postconceptional weeks, PCW). The measurements were performed using Fiji and NDP.view2. We found that T2w MR signal-intensity curves show a specific regional and developmental stage profile already at 15 PCW. The MRI-histological correlation reveals that the subventricular-intermediate zone (SVZ-IZ) contributes the most to the regional differences in the MRI-profile and zone thicknesses, growing by a factor of 2.01 in the cingulate, and 1.78 in the neocortical wall. The interrelations of zone or wall thicknesses, obtained by both methods, disclose a different rate and extent of shrinkage per region (highest in neocortical subplate and SVZ-IZ) and stage (highest in the early second half of fetal development), distorting the zones' proportion in histological sections. This intrasubject, slice-matched, 3 T correlative MRI-histological study provides important information about regional development of the cortical wall, critical for the design of MRI criteria for prenatal brain monitoring and early detection of cortical or other brain pathologies in human fetuses.

Differential roles of ARID1B in excitatory and inhibitory neural progenitors in the developing cortex.

Genetic evidence indicates that haploinsufficiency of ARID1B causes intellectual disability (ID) and autism spectrum disorder (ASD), but the neural function of ARID1B is largely unknown. Using both conditional and global Arid1b knockout mouse strains, we examined the role of ARID1B in neural progenitors. We detected an overall decrease in the proliferation of cortical and ventral neural progenitors following homozygous deletion of Arid1b, as well as altered cell cycle regulation and increased cell death. Each of these phenotypes was more pronounced in ventral neural progenitors. Furthermore, we observed decreased nuclear localization of ß-catenin in Arid1b-deficient neurons. Conditional homozygous deletion of Arid1b in ventral neural progenitors led to pronounced ID- and ASD-like behaviors in mice, whereas the deletion in cortical neural progenitors resulted in minor cognitive deficits. This study suggests an essential role for ARID1B in forebrain neurogenesis and clarifies its more pronounced role in inhibitory neural progenitors. Our findings also provide insights into the pathogenesis of ID and ASD.

Tetrabromobisphenol A Disturbs Brain Development in Both Thyroid Hormone-Dependent and -Independent Manners in Xenopus laevis.

Although tetrabromobisphenol A (TBBPA) has been well proven to disturb TH signaling in both in vitro and in vivo assays, it is still unclear whether TBBPA can affect brain development due to TH signaling disruption. Here, we employed the T3-induced Xenopus metamorphosis assay (TIXMA) and the spontaneous metamorphosis assay to address this issue. In the TIXMA, 5-500 nmol/L TBBPA affected T3-induced TH-response gene expression and T3-induced brain development (brain morphological changes, cell proliferation, and neurodifferentiation) at premetamorphic stages in a complicated biphasic concentration-response manner. Notably, 500 nmol/L TBBPA treatment alone exerted a stimulatory effect on tadpole growth and brain development at these stages, in parallel with a lack of TH signaling activation, suggesting the involvement of other signaling pathways. As expected, at the metamorphic climax, we observed inhibitory effects of 50-500 nmol/L TBBPA on metamorphic development and brain development, which was in agreement with the antagonistic effects of higher concentrations on T3-induced brain development at premetamorphic stages. Taken together, all results demonstrate that TBBPA can disturb TH signaling and subsequently interfere with TH-dependent brain development in Xenopus; meanwhile, other signaling pathways besides TH signaling could be involved in this process. Our study improves the understanding of the effects of TBBPA on vertebrate brain development.

Conserved Gsx2/Ind homeodomain monomer versus homodimer DNA binding defines regulatory outcomes in flies and mice.

How homeodomain proteins gain sufficient specificity to control different cell fates has been a long-standing problem in developmental biology. The conserved Gsx homeodomain proteins regulate specific aspects of neural development in animals from flies to mammals, and yet they belong to a large transcription factor family that bind nearly identical DNA sequences in vitro. Here, we show that the mouse and fly Gsx factors unexpectedly gain DNA binding specificity by forming cooperative homodimers on precisely spaced and oriented DNA sites. High-resolution genomic binding assays revealed that Gsx2 binds both monomer and homodimer sites in the developing mouse ventral telencephalon. Importantly, reporter assays showed that Gsx2 mediates opposing outcomes in a DNA binding site-dependent manner: Monomer Gsx2 binding represses transcription, whereas homodimer binding stimulates gene expression. In Drosophila, the Gsx homolog, Ind, similarly represses or stimulates transcription in a site-dependent manner via an autoregulatory enhancer containing a combination of monomer and homodimer sites. Integrating these findings, we test a model showing how the homodimer to monomer site ratio and the Gsx protein levels defines gene up-regulation versus down-regulation. Altogether, these data serve as a new paradigm for how cooperative homeodomain transcription factor binding can increase target specificity and alter regulatory outcomes.

Prenatal alcohol exposure is a leading cause of interneuronopathy in humans.

Alcohol affects multiple neurotransmitter systems, notably the GABAergic system and has been recognised for a long time as particularly damaging during critical stages of brain development. Nevertheless, data from the literature are most often derived from animal or in vitro models. In order to study the production, migration and cortical density disturbances of GABAergic interneurons upon prenatal alcohol exposure, we performed immunohistochemical studies by means of the proliferation marker Ki67, GABA and calretinin antibodies in the frontal cortical plate of 17 foetal and infant brains antenatally exposed to alcohol, aged 15 weeks' gestation to 22 postnatal months and in the ganglionic eminences and the subventricular zone of the dorsal telencephalon until their regression, i.e., 34 weeks' gestation. Results were compared with those obtained in 17 control brains aged 14 weeks of gestation to 35 postnatal months. We also focused on interneuron vascular migration along the cortical microvessels by confocal microscopy with double immunolabellings using Glut1, GABA and calretinin. Semi-quantitative and quantitative analyses of GABAergic and calretininergic interneuron density allowed us to identify an insufficient and delayed production of GABAergic interneurons in the ganglionic eminences during the two first trimesters of the pregnancy and a delayed incorporation into the laminar structures of the frontal cortex. Moreover, a mispositioning of GABAergic and calretininergic interneurons persisted throughout the foetal life, these cells being located in the deep layers instead of the superficial layers II and III. Moreover, vascular migration of calretininergic interneurons within the cortical plate was impaired, as reflected by low numbers of interneurons observed close to the cortical perforating vessel walls that may in part explain their abnormal intracortical distribution. Our results are globally concordant with those previously obtained in mouse models, in which alcohol has been shown to induce an interneuronopathy by affecting interneuron density and positioning within the cortical plate, and which could account for the neurological disabilities observed in children with foetal alcohol disorder spectrum.

The Polycomb group protein Ring1 regulates dorsoventral patterning of the mouse telencephalon.

Dorsal-ventral patterning of the mammalian telencephalon is fundamental to the formation of distinct functional regions including the neocortex and ganglionic eminence. While Bone morphogenetic protein (BMP), Wnt, and Sonic hedgehog (Shh) signaling are known to determine regional identity along the dorsoventral axis, how the region-specific expression of these morphogens is established remains unclear. Here we show that the Polycomb group (PcG) protein Ring1 contributes to the ventralization of the mouse telencephalon. Deletion of Ring1b or both Ring1a and Ring1b in neuroepithelial cells induces ectopic expression of dorsal genes, including those for BMP and Wnt ligands, as well as attenuated expression of the gene for Shh, a key morphogen for ventralization, in the ventral telencephalon. We observe PcG protein-mediated trimethylation of histone 3 at lysine-27 and binding of Ring1B at BMP and Wnt ligand genes specifically in the ventral region. Furthermore, forced activation of BMP or Wnt signaling represses Shh expression. Our results thus indicate that PcG proteins suppress BMP and Wnt signaling in a region-specific manner and thereby allow proper Shh expression and development of the ventral telencephalon.

Repression of Irs2 by let-7 miRNAs is essential for homeostasis of the telencephalic neuroepithelium.

Structural integrity and cellular homeostasis of the embryonic stem cell niche are critical for normal tissue development. In the telencephalic neuroepithelium, this is controlled in part by cell adhesion molecules and regulators of progenitor cell lineage, but the specific orchestration of these processes remains unknown. Here, we studied the role of microRNAs in the embryonic telencephalon as key regulators of gene expression. By using the early recombiner Rx-Cre mouse, we identify novel and critical roles of miRNAs in early brain development, demonstrating they are essential to preserve the cellular homeostasis and structural integrity of the telencephalic neuroepithelium. We show that Rx-Cre;DicerF/F mouse embryos have a severe disruption of the telencephalic apical junction belt, followed by invagination of the ventricular surface and formation of hyperproliferative rosettes. Transcriptome analyses and functional experiments in vivo show that these defects result from upregulation of Irs2 upon loss of let-7 miRNAs in an apoptosis-independent manner. Our results reveal an unprecedented relevance of miRNAs in early forebrain development, with potential mechanistic implications in pediatric brain cancer.

A Chromatin Accessibility Atlas of the Developing Human Telencephalon.

To discover regulatory elements driving the specificity of gene expression in different cell types and regions of the developing human brain, we generated an atlas of open chromatin from nine dissected regions of the mid-gestation human telencephalon, as well as microdissected upper and deep layers of the prefrontal cortex. We identified a subset of open chromatin regions (OCRs), termed predicted regulatory elements (pREs), that are likely to function as developmental brain enhancers. pREs showed temporal, regional, and laminar differences in chromatin accessibility and were correlated with gene expression differences across regions and gestational ages. We identified two functional de novo variants in a pRE for autism risk gene SLC6A1, and using CRISPRa, demonstrated that this pRE regulates SCL6A1. Additionally, mouse transgenic experiments validated enhancer activity for pREs proximal to FEZF2 and BCL11A. Thus, this atlas serves as a resource for decoding neurodevelopmental gene regulation in health and disease.

Vascularized human cortical organoids (vOrganoids) model cortical development in vivo.

Modeling the processes of neuronal progenitor proliferation and differentiation to produce mature cortical neuron subtypes is essential for the study of human brain development and the search for potential cell therapies. We demonstrated a novel paradigm for the generation of vascularized organoids (vOrganoids) consisting of typical human cortical cell types and a vascular structure for over 200 days as a vascularized and functional brain organoid model. The observation of spontaneous excitatory postsynaptic currents (sEPSCs), spontaneous inhibitory postsynaptic currents (sIPSCs), and bidirectional electrical transmission indicated the presence of chemical and electrical synapses in vOrganoids. More importantly, single-cell RNA-sequencing analysis illustrated that vOrganoids exhibited robust neurogenesis and that cells of vOrganoids differentially expressed genes (DEGs) related to blood vessel morphogenesis. The transplantation of vOrganoids into the mouse S1 cortex resulted in the construction of functional human-mouse blood vessels in the grafts that promoted cell survival in the grafts. This vOrganoid culture method could not only serve as a model to study human cortical development and explore brain disease pathology but also provide potential prospects for new cell therapies for nervous system disorders and injury.

The fornix acts as a permissive corridor for septal neuron migration beyond the diencephalic-telencephalic boundary.

Neuronal migration is essential for constructing functional neural networks. Two posterior septal (PS) nuclei, the triangular septal nucleus and bed nuclei of the anterior commissure, are involved in fear and anxiety. During development, glutamatergic PS neurons undergo long-distance rostrodorsal migration from the thalamic eminence (TE) of the diencephalon, then settle in the caudalmost telencephalon. However, the developmental behavior of PS neurons and the guidance structures facilitating their migration remain unknown. We previously demonstrated the migration of PS neurons along the fornix, a major efferent pathway from the hippocampal formation. Here, we show that the postcommissural fornix is essential for PS neuron migration which is largely confined to its axonal tract, which grows in the opposite direction as PS neuron migration. Fornical axons reach the TE prior to initiation of PS neuron rostrodorsal migration. Ectopic expression of Semaphorin 3 A in the dorsomedial cortex resulted in defective fornix formation. Furthermore, loss of the postcommissural fornix stalled PS neuron migration resulting in abnormal accumulation near their origin. This suggests that PS neurons utilize the postcommissural fornix as a permissive corridor during migration beyond the diencephalic-telencephalic boundary. This axonal support is essential for the functional organization of the heterogeneous septal nuclear complex.

Physical interactions between Gsx2 and Ascl1 balance progenitor expansion versus neurogenesis in the mouse lateral ganglionic eminence.

The Gsx2 homeodomain transcription factor promotes neural progenitor identity in the lateral ganglionic eminence (LGE), despite upregulating the neurogenic factor Ascl1. How this balance in maturation is maintained is unclear. Here, we show that Gsx2 and Ascl1 are co-expressed in subapical progenitors that have unique transcriptional signatures in LGE ventricular zone (VZ) cells. Moreover, whereas Ascl1 misexpression promotes neurogenesis in dorsal telencephalic progenitors, the co-expression of Gsx2 with Ascl1 inhibits neurogenesis. Using luciferase assays, we found that Gsx2 reduces the ability of Ascl1 to activate gene expression in a dose-dependent and DNA binding-independent manner. Furthermore, Gsx2 physically interacts with the basic helix-loop-helix (bHLH) domain of Ascl1, and DNA-binding assays demonstrated that this interaction interferes with the ability of Ascl1 to bind DNA. Finally, we modified a proximity ligation assay for tissue sections and found that Ascl1-Gsx2 interactions are enriched within LGE VZ progenitors, whereas Ascl1-Tcf3 (E-protein) interactions predominate in the subventricular zone. Thus, Gsx2 contributes to the balance between progenitor maintenance and neurogenesis by physically interacting with Ascl1, interfering with its DNA binding and limiting neurogenesis within LGE progenitors.

Developmental dynamics of the periventricular parietal crossroads of growing cortical pathways in the fetal brain - In vivo fetal MRI with histological correlation.

The periventricular crossroads have been described as transient structures of the fetal brain where major systems of developing fibers intersect. The triangular parietal crossroad constitutes one major crossroad region. By combining in vivo and post-mortem fetal MRI with histological and immunohistochemical methods, we aimed to characterize these structures. Data from 529 in vivo and 66 post-mortem MRI examinations of fetal brains between gestational weeks (GW) 18-39 were retrospectively reviewed. In each fetus, the area adjacent to the trigone of the lateral ventricles at the exit of the posterior limb of the internal capsule (PLIC) was assessed with respect to signal intensity, size, and shape on T2-weighted images. In addition, by using in vivo diffusion tensor imaging (DTI), the main fiber pathways that intersect in these areas were identified. In order to explain the in vivo features of the parietal crossroads (signal intensity and developmental profile), we analyzed 23 post-mortem fetal human brains, between 16 and â€‹40 GW of age, processed by histological and immunohistochemical methods. The parietal crossroads were triangular-shaped areas with the base in the continuity of the PLIC, adjacent to the germinal matrix and the trigone of the lateral ventricles, with the tip pointing toward the subplate. These areas appeared hyperintense to the subplate, and corresponded to a convergence zone of the developing external capsule, the PLIC, and the fronto-occipital association fibers. They were best detected between GW 25-26, and, at term, they became isointense to the adjacent structures. The immunohistochemical results showed a distinct cellular, fibrillar, and extracellular matrix arrangement in the parietal crossroads, depending on the stage of development, which influenced the MRI features. The parietal crossroads are transient, but important structures in white matter maturation and their damage may be indicative of a poor prognosis for a fetus with regard to neurological development. In addition, impairment of this region may explain the complex neurodevelopmental deficits in preterm infants with periventricular hypoxic/ischemic or inflammatory lesions.

march5 Governs the Convergence and Extension Movement for Organization of the Telencephalon and Diencephalon in Zebrafish Embryos.

MARCH5 is a RING finger E3 ligase involved in mitochondrial integrity, cellular protein homeostasis, and the regulation of mitochondrial fusion and fission. To determine the function of MARCH5 during development, we assessed transcript expression in zebrafish embryos. We found that march5 transcripts were of maternal origin and evenly distributed at the 1-cell stage, except for the mid-blastula transition, with expression predominantly in the developing central nervous system at later stages of embryogenesis. Overexpression of march5 impaired convergent extension movement during gastrulation, resulting in reduced patterning along the dorsoventral axis and alterations in the ventral cell types. Overexpression and knockdown of march5 disrupted the organization of the developing telencephalon and diencephalon. Lastly, we found that the transcription of march5 was tightly regulated by the transcriptional regulators CHOP, C/EBPα, Staf, Znf143a, and Znf76. These results demonstrate the essential role of March5 in the development of zebrafish embryos.

Conserved and divergent expression dynamics during early patterning of the telencephalon in mouse and chick embryos.

The mammalian and the avian telencephalon are nearly indistinguishable at early embryonic vesicle stages but differ substantially in form and function at their adult stage. We sequenced and analyzed RNA populations present in mouse and chick during the early stages of embryonic telencephalon to understand conserved and lineage-specific developmental differences. We found approximately 3000 genes that orchestrate telencephalon development. Many chromatin-associated epigenetic and transcription regulators show high expression in both species and some show species-specific expression dynamics. Interestingly, previous studies associated them to autism, intellectual disabilities, and mental retardation supporting a causal link between their impaired functions during telencephalon development and brain dysfunction. Strikingly, the conserved up-regulated genes were differentially enriched in ontologies related to development or functions of the adult brain. Moreover, a differential enrichment of distinct repertoires of transcription factor binding motifs in their upstream promoter regions suggest a species-specific regulation of the various gene groups identified. Overall, our results reveal that the ontogenetic divergences between the mouse and chick telencephalon result from subtle differences in the regulation of common patterning signaling cascades and regulatory networks unique to each species at their very early stages of development.