The chick/quail transplantation model to study central nervous system development

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Quail-chick grafting experiments corroborate that Tbr1-positive eminential prethalamic neurons migrate along three streams into hypothalamus, subpallium and septocommissural areas

The prethalamic eminence (PThE), a diencephalic caudal neighbor of the telencephalon and alar hypothalamus, is frequently described in mammals and birds as a transient embryonic structure, undetectable in the adult brain. Based on descriptive developmental analysis of Tbr1 gene brain expression in chick embryos, we previously reported that three migratory cellular streams exit the PThE rostralward, targeting multiple sites in the hypothalamus, subpallium and septocommissural area, where eminential cells form distinct nuclei or disperse populations. These conclusions needed experimental corroboration. In this work, we used the homotopic quail-chick chimeric grafting procedure at stages HH10/HH11 to demonstrate by fate-mapping the three predicted tangential migration streams. Some chimeric brains were processed for Tbr1 in situ hybridization, for correlation with our previous approach. Evidence supporting all three postulated migration streams is presented. The results suggested a slight heterochrony among the juxtapeduncular (first), the peripeduncular (next), and the eminentio-septal (last) streams, each of which followed differential routes. A possible effect of such heterochrony on the differential selection of medial to lateral habenular hodologic targets by the migrated neurons is discussed.

Role of Shh in the development of molecularly characterized tegmental nuclei in mouse rhombomere 1

Hindbrain rhombomeres in general are differentially specified molecularly by unique combinations of Hox genes with other developmental genes. Rhombomere 1 displays special features, including absence of Hox gene expression. It lies within the hindbrain range of the Engrailed genes (En1, En2), controlled by the isthmic organizer via diffusion of FGF8. It is limited rostrally by the isthmus territory, and caudally by rhombomere 2. It is double the normal size of any other rhombomere. Its dorsal part generates the cerebellar hemispheres and its ventral part gives rise to several populations, such as some raphe nuclei, the interpeduncular nucleus, the rhabdoid nucleus, anterior, dorsal, ventral and posterodorsal tegmental nuclei, the cholinergic pedunculopontine and laterodorsal tegmental nuclei, rostral parts of the hindbrain reticular formation, the locus coeruleus, and part of the lateral lemniscal and paralemniscal nuclei, among other formations. Some of these populations migrate tangentially before reaching their final positions. The morphogen Sonic Hedgehog (Shh) is normally released from the local floor plate and underlying notochord. In the present report we explore, first, whether Shh is required in the specification of these r1 populations, and, second, its possible role in the guidance of tangentially migrating neurons that approach the midline. Our results indicate that when Shh function is altered selectively in a conditional mutant mouse strain, most populations normally generated in the medial basal plate of r1 are completely absent. Moreover, the relocation of some neurons that normally originate in the alar plate and migrate tangentially into the medial basal plate is variously altered. In contrast, neurons that migrate radially (or first tangentially and then radially) into the lateral basal plate were not significantly affected.

Avian-induced pluripotent stem cells derived using human reprogramming factors

Avian species are important model animals for developmental biology and disease research. However, unlike in mice, where clonal lines of pluripotent stem cells have enabled researchers to study mammalian gene function, clonal and highly proliferative pluripotent avian cell lines have been an elusive goal. Here we demonstrate the generation of avian induced pluripotent stem cells (iPSCs), the first nonmammalian iPSCs, which were clonally isolated and propagated, important attributes not attained in embryo-sourced avian cells. This was accomplished using human pluripotency genes rather than avian genes, indicating that the process in which mammalian and nonmammalian cells are reprogrammed is a conserved process. Quail iPSCs (qiPSCs) were capable of forming all 3 germ layers in vitro and were directly differentiated in culture into astrocytes, oligodendrocytes, and neurons. Ultimately, qiPSCs were capable of generating live chimeric birds and incorporated into tissues from all 3 germ layers, extraembryonic tissues, and potentially the germline. These chimera competent qiPSCs and in vitro differentiated cells offer insight into the conserved nature of reprogramming and genetic tools that were only previously available in mammals.

The derivatives of the Wnt3a lineage in the central nervous system

The dorsal midline of the vertebrate neural tube is a source of signals that direct cell fate specification and proliferation. Using genetic fate mapping in the mouse and a previously generated Wnt3aCre line, we report here that genetically labeled cells of the Wnt3a lineage migrate widely from the dorsal midline into the dorsal half of the adult brain and spinal cord, contributing to diverse structures in the diencephalon, midbrain, and brainstem and extensively populating the rostral spinal cord. Conspicuously, many of these structures are linked in specific functional networks. Wnt3a lineage cells populate nuclei of the central auditory system from the medulla to thalamus, and the trigeminal sensory system from the cervical spinal cord to the midbrain. Our findings reveal the rich contributions of the Wnt3a lineage to a variety of brain structures and show that functionally integrated nuclei can share a molecular identity, provided by transient gene expression early in their development.

A model of early molecular regionalization in the chicken embryonic pretectum

The pretectal region of the brain is visualized as a dorsal region of prosomere 1 in the caudal diencephalon, including derivatives from both the roof and alar plates. Its neuronal derivatives in the adult brain are known as pretectal nuclei. The literature is inconsistent about the precise anteroposterior delimitation of this region and on the number of specific histogenetic domains and subdomains that it contains. We performed a cross-correlated gene-expression map of this brain area in chicken embryos, with the aim of identifying differently fated pretectal domains on the basis of combinatorial gene expression patterns. We examined in detail Pax3, Pax6, Pax7, Tcf4, Meis1, Meis2, Nkx2.2, Lim1, Dmbx1, Dbx1, Six3, FoxP2, Zic1, Ebf1, and Shh mRNA expression, as well as PAX3 and PAX7 immunoreaction, between stages HH11 and HH28. The patterns analyzed serve to fix the cephalic and caudal boundaries of the pretectum and to define three molecularly distinct anteroposterior pretectal domains (precommissural, juxtacommissural, and commissural) and several dorsoventral subdomains. These molecular specification patterns are established step by step between stages HH10 and HH18, largely before neurogenesis begins. This set of gene-architectonic data constitutes a useful scaffold for correlations with fate maps and other experimental embryologic results and may serve as well for inquiries on homologies in this part of the brain.

Postulated boundaries and differential fate in the developing rostral hindbrain

The vertebrate brain is progressively regionalized during development in a process whereby a precise spatio-temporal arrangement of gene expression patterns and resulting intercellular and intracellular signals drive patterning, growth, morphogenesis, and final fates, thus producing ordered species-specific differentiation of each territory within a shared morphotype. Before genetic and molecular biology tools started to be used to uncover the underlying mechanisms that control morphogenesis, knowledge on brain development largely depended on descriptive analysis and experimental embryology. The first approach allowed us to know how the brain develops but not why. The second provided insights into inductive and field histogenetic phenomena, requiring causal explanation. In this review, we focused on the regionalization of the rostral hindbrain, defined as isthmus plus rhombomere 1, which is the least understood part of the hindbrain. We addressed what is known about the formation of boundaries in this area and the fate of diverse neuroepithelial portions. We introduced to this end some fate-mapping data recently obtained in our laboratory. Starting from the background of pioneering morphological studies and available fate mapping data, we establish correlation with current knowledge about how morphogens, transcription factors, or other signaling molecules map onto particular territories, from where they may drive morphogenetic interactions that generate final fates step by step.

A distinct preisthmic histogenetic domain is defined by overlap of Otx2 and Pax2 gene expression in the avian caudal midbrain

Correlative in situ hybridization of Otx2, Pax2, Gbx2, and Fgf8 mRNA probes in adjacent serial sections through the chicken midbrain and isthmus at early to intermediate stages of development served to map in detail the area of overlap of Otx2 and Pax2 transcripts in the caudal midbrain. The neuronal populations developing within this preisthmic domain made up a caudal part of the midbrain reticular formation, the interfascicular nucleus, and the magnocellular (pre)isthmic nucleus, plus the corresponding part of the periaqueductal gray. The torus semicircularis-the inferior colliculus homolog-expressed Otx2 in its ventricular lining exclusively, but it never expressed Pax2. The parvicellular isthmic nucleus, although placed inside the midbrain lobe, never expressed Otx2, and its cells rapidly down-regulated an early transient Pax2 signal; this pattern is consistent with its reported isthmic origin and forward tangential translocation. This analysis reveals the existence of four distinct midbrain histogenetic domains along the longitudinal axis, at least for the alar plate. These presumably result from step-like isthmic organizer effects on Otx2-expressing midbrain neuroepithelium at different distances from a caudal FGF8 morphogen source (isthmic Fgf8-positive domain). The final phenotypes of these domains are histologically diverse and make up the griseum tectale (rostrally), the optic tectum, the torus semicircularis, and the presently characterized preisthmic domain (lying closest to the isthmic organizer). Available comparative data for reptiles and mammals suggest the general validity of this scheme.

Imaging brain development and organogenesis in zebrafish using immobilized embryonic explants

Owing to its optical clarity and rapid rate of development, the zebrafish embryo is an ideal model system for studying the cellular mechanics of organogenesis. Unfortunately, extended time-lapse recordings of zebrafish embryos are often disrupted by the extension and straightening of the embryonic axis, as well as movement artifacts associated with developing musculature. In addition, the embryo's massive yolk cell often prevents optical access to tissues of interest. To circumvent these imaging problems, we have developed a procedure to deflate and mechanically remove the yolk cell. A "paralyzing" agent, AMP-PNP (a membrane-impermeant nonhydrolyzable analog of ATP), is first injected into the embryo's contractile yolk cell. The yolk cell is then removed using sharpened tungsten needles. Deyolked embryos, or organ rudiments explanted from them, are then immobilized on a microscope coverslip using a thin plasma clot. This plasma clot immobilization allows novel mountings of the explants so that ventral, lateral, and even cross-sectional fields of views are possible using high numerical aperture objectives. We show that isolated head rudiments undergo normal morphogenesis and gene expression for at least 1 day after being explanted into organotypic culture. These procedures can be used to study the cellular mechanics of organogenesis in "deyolked" embryos, as well as in tissues explanted from green fluorescent protein transgenic animals.

Temporal sequence of gene expression leading caudal prosencephalon to develop a midbrain/hindbrain phenotype

Transplantation of prosomeres 1-2 into the cerebellar plate were used, by using chick/quail chimeras, to analyse the temporal sequence of the genetic cascade leading the graft to develop a midbrain/hindbrain phenotype. Our results show that (1) at Hamburger and Hamilton (HH) stage 13, Pax2 and En2 are already induced within the graft, before all other genes of the cascade, whereas misexpression of Fgf8 is also observed within the contiguous host cerebellar plate; (2) within the graft, Otx2 repression and Gbx2 induction (see Hidalgo-Sánchez et al. [1999] Development 126:3191-3203) are secondary events that affect, from stages HH14-15, the areas in contact with the host Gbx2/Fgf8-expressing cerebellar plate; (3) at these stages, the repressed Otx2 territory extends beyond the areas induced to express Gbx2, with the two territories not abutting before HH17-18; (4) Fgf8 expression becomes progressively induced within the Otx2-repressed/Gbx2-induced territory, starting at HH15-16. Our results support the hypothesis that the host-Gbx2/graft-Otx2 interface could trigger the genetic cascade induced within the graft and that the Gbx2-induced domain could play a key role during the establishment of the induced intragraft midbrain/hindbrain boundary.