The Midbrain Preisthmus: A Poorly Known Effect of the Isthmic Organizer.

This essay reexamines molecular evidence supporting the existence of the 'preisthmus', a caudal midbrain domain present in vertebrates (studied here in the mouse). It is thought to derive from the embryonic m2 mesomere and appears intercalated between the isthmus (caudally) and the inferior colliculus (rostrally). Among a substantial list of gene expression mappings examined from the Allen Developing and Adult Brain Atlases, a number of quite consistent selective positive markers, plus some neatly negative markers, were followed across embryonic stages E11.5, E13.5, E15.5, E18.5, and several postnatal stages up to the adult brain. Both alar and basal subdomains of this transverse territory were explored and illustrated. It is argued that the peculiar molecular and structural profile of the preisthmus is due to its position as rostrally adjacent to the isthmic organizer, where high levels of both FGF8 and WNT1 morphogens must exist at early embryonic stages. Isthmic patterning of the midbrain is discussed in this context. Studies of the effects of the isthmic morphogens usually do not attend to the largely unknown preisthmic complex. The adult alar derivatives of the preisthmus were confirmed to comprise a specific preisthmic sector of the periaqueductal gray, an intermediate stratum represented by the classic cuneiform nucleus, and a superficial stratum containing the subbrachial nucleus. The basal derivatives, occupying a narrow retrorubral domain intercalated between the oculomotor and trochlear motor nuclei, include dopaminergic and serotonergic neurons, as well as a variety of peptidergic neuron types.

Cyp1B1 expression patterns in the developing chick inner ear.

BACKGROUND: Retinoic acid (RA) plays an important role in organogenesis as a paracrine signal through transcriptional regulation of an increasing number of known downstream target genes, regulating cell proliferation, and differentiation. During the development of the inner ear, RA directly governs the morphogenesis and specification processes mainly by means of RA-synthesizing retinaldehyde dehydrogenase (RALDH) enzymes. Interestingly, CYP1B1, a cytochrome P450 enzyme, is able to mediate the oxidative metabolisms also leading to RA generation, its expression patterns being associated with many known sites of RA activity. RESULTS: This study describes for the first time the presence of CYP1B1 in the developing chick inner ear as a RALDH-independent RA-signaling mechanism. In our in situ hybridization analysis, Cyp1B1 expression was first observed in a domain located in the ventromedial wall of the otic anlagen, being included within the rostralmost aspect of an Fgf10-positive pan-sensory domain. As development proceeds, all identified Fgf10-positive areas were Cyp1B1 stained, with all sensory patches being Cyp1B1 positive at stage HH34, except the macula neglecta. CONCLUSIONS: Cyp1B1 expression suggested a possible contribution of CYP1B1 action in the specification of the lateral-to-medial and dorsal-to-ventral axes of the developing chick inner ear.

Fate map of the chicken otic placode.

The inner ear is an intricate three-dimensional sensory organ that arises from a flat, thickened portion of the ectoderm termed the otic placode. There is evidence that the ontogenetic steps involved in the progressive specification of the highly specialized inner ear of vertebrates involve the concerted actions of diverse patterning signals that originate from nearby tissues, providing positional identity and instructive context. The topology of the prospective inner ear portions at placode stages when such patterning begins has remained largely unknown. The chick-quail model was used to perform a comprehensive fate mapping study of the chick otic placode, shedding light on the precise topological position of each presumptive inner ear component relative to the dorsoventral and anteroposterior axes of the otic placode and, implicitly, to the possible sources of inducing signals. The findings reveal the existence of three dorsoventrally arranged anteroposterior domains from which the endolymphatic system, the maculae and basilar papilla, and the cristae develop. This study provides new bases for the interpretation of earlier and future descriptive and experimental studies that aim to understand the molecular genetic mechanisms involved in otic placode patterning.

Origin and plasticity of the subdivisions of the inferior olivary complex.

The precerebellar nuclei (PCN) originate from the rhombic lip, a germinal neuroepithelium adjacent to the roof plate of the fourth ventricle. We first report here that, in chicken, the Brn3a-expressing postmitotic medullary cells that produce the inferior olive (ION, the source of cerebellar climbing fibres) originate from a dorso-ventral domain roughly coinciding with the hindbrain vestibular column. Whereas Foxd3 expression labels the whole mature ION but is only detected in a subpopulation of ION neuroblasts initiating their migration, we report that Brn3a allows the visualization of the whole population of ION neurons from the very beginning of their migration. We show that Brn3a-positive neurons migrate tangentially ventralwards through a characteristic dorso-ventral double submarginal stream. Cath1 expressing progenitors lying just dorsal to the ION origin correlated dorso-ventral topography with the prospective cochlear column (caudal to it) and generate precerebellar nuclei emitting mossy-fiber cerebellar afferents. We used the chick-quail chimaera technique with homotopic grafts at HH10 to determine the precise fate map of ION precursors across the caudal cryptorhombomeric subdivisions of the medullary hindbrain (r8-r11). We demonstrate that each crypto-rhombomere contributes to two lamellae of the ION, while each ION sub-nucleus originates from at least two contiguous crypto-rhombomeres. We then questioned how rhombomere identity is related to the plasticity of cell type specification in the dorsal hindbrain. The potential plasticity of ectopically HH10 grafted ION progenitors to change their original fate in alternative rostrocaudal environments was examined. Heterotopic grafts from the presumptive ION territory to the pontine region (r4-r5) caused a change of fate, since the migrated derivatives adopted a pontine phenotype. The reverse experiment caused pontine progenitors to produce derivatives appropriately integrated into the ION complex. Grafts of ION progenitor domains to myelomeres (my) 2-3 also showed complete fate regulation, reproducing spinal cord-like structures, whereas the reverse experiment revealed the inability of my2-3 to generate ION cell types. This was not the case with more caudal, relatively less specified myelomeres (my5-6). Interestingly, when heterotopically grafted cells are integrated dorsally, they do not change their phenotype. Our results support the hypothesis that positional information present in the hindbrain and spinal cord at early neural tube stages controls the specific fates of ventrally migrating PCN precursors.

Multiple origins, migratory paths and molecular profiles of cells populating the avian interpeduncular nucleus.

The interpeduncular nucleus (IP) is a key limbic structure, highly conserved evolutionarily among vertebrates. The IP receives indirect input from limbic areas of the telencephalon, relayed by the habenula via the fasciculus retroflexus. The function of the habenulo-IP complex is poorly understood, although there is evidence that in rodents it modulates behaviors such as learning and memory, avoidance, reward and affective states. The IP has been an important subject of interest for neuroscientists, and there are multiple studies about the adult structure, chemoarchitecture and its connectivity, with complex results, due to the presence of multiple cell types across a variety of subnuclei. However, the ontogenetic origins of these populations have not been examined, and there is some controversy about its location in the midbrain-anterior hindbrain area. To address these issues, we first investigated the anteroposterior (AP) origin of the IP complex by fate-mapping its neuromeric origin in the chick, discovering that the IP develops strictly within isthmus and rhombomere 1. Next, we studied the dorsoventral (DV) positional identity of subpopulations of the IP complex. Our results indicate that there are at least four IP progenitor domains along the DV axis. These specific domains give rise to distinct subtypes of cell populations that target the IP with variable subnuclear specificity. Interestingly, these populations can be characterized by differential expression of the transcription factors Pax7, Nkx6.1, Otp, and Otx2. Each of these subpopulations follows a specific route of migration from its source, and all reach the IP roughly at the same stage. Remarkably, IP progenitor domains were found both in the alar and basal plates. Some IP populations showed rostrocaudal restriction in their origins (isthmus versus anterior or posterior r1 regions). A tentative developmental model of the structure of the avian IP is proposed. The IP emerges as a plurisegmental and developmentally heterogeneous formation that forms ventromedially within the isthmus and r1. These findings are relevant since they help to understand the highly complex chemoarchitecture, hodology and functions of this important brainstem structure.

Origin of Neuroblasts in the Avian Otic Placode and Their Distributions in the Acoustic and Vestibular Ganglia.

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. This intricate sensory organ originates from the otic placode, which generates the sensory elements of the membranous labyrinth, as well as all the ganglionic neuronal precursors. How auditory and vestibular neurons establish their fate identities remains to be determined. Their topological origin in the incipient otic placode could provide positional information before they migrate, to later segregate in specific portions of the acoustic and vestibular ganglia. To address this question, transplants of small portions of the avian otic placode were performed according to our previous fate map study, using the quail/chick chimeric graft model. All grafts taking small areas of the neurogenic placodal domain contributed neuroblasts to both acoustic and vestibular ganglia. A differential distribution of otic neurons in the anterior and posterior lobes of the vestibular ganglion, as well as in the proximal, intermediate, and distal portions of the acoustic ganglion, was found. Our results clearly show that, in birds, there does not seem to be a strict segregation of acoustic and vestibular neurons in the incipient otic placode.

CRABP-I Expression Patterns in the Developing Chick Inner Ear.

The vertebrate inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions, regarded as an excellent system for analyzing events that occur during development, such as patterning, morphogenesis, and cell specification. Retinoic acid (RA) is involved in all these development processes. Cellular retinoic acid-binding proteins (CRABPs) bind RA with high affinity, buffering cellular free RA concentrations and consequently regulating the activation of precise specification programs mediated by particular regulatory genes. In the otic vesicle, strong CRABP-I expression was detected in the otic wall's dorsomedial aspect, where the endolymphatic apparatus develops, whereas this expression was lower in the ventrolateral aspect, where part of the auditory system forms. Thus, CRABP-I proteins may play a role in the specification of the dorsal-to-ventral and lateral-to-medial axe of the otic anlagen. Regarding the developing sensory patches, a process partly involving the subdivision of a ventromedial pro-sensory domain, the CRABP-I gene displayed different levels of expression in the presumptive territory of each sensory patch, which was maintained throughout development. CRABP-I was also relevant in the acoustic-vestibular ganglion and in the periotic mesenchyme. Therefore, CRABP-I could protect RA-sensitive cells in accordance with its dissimilar concentration in specific areas of the developing chick inner ear.

Distinct and redundant expression and transcriptional diversity of MEIS gene paralogs during chicken development.

Members of the Meis family of TALE homeobox transcription factors are involved in many processes of vertebrate development and morphogenesis, showing extremely complex transcriptional and spatiotemporal expression patterns. In this work, we performed a comprehensive study of chicken Meis genes using multiple approaches. First, we assessed whether the chicken genome contains a Meis3 ortholog or harbors only two Meis genes; we gathered several lines of evidence pointing to a specific loss of the Meis3 ortholog in an early ancestor of birds. Next, we studied the transcriptional diversity generated from chicken Meis genes through alternative splicing during development. Finally, we performed a detailed analysis of chick Meis1/2 expression patterns during early embryogenesis and organogenesis. We show that the expression of both Meis genes begins at the gastrulation stage in the three embryonic layers, presenting highly dynamic patterns with overlapping as well as distinct expression domains throughout 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.

Origin of acoustic-vestibular ganglionic neuroblasts in chick embryos and their sensory connections.

The inner ear is a complex three-dimensional sensory structure with auditory and vestibular functions. It originates from the otic placode, which generates the sensory elements of the membranous labyrinth and all the ganglionic neuronal precursors. Neuroblast specification is the first cell differentiation event. In the chick, it takes place over a long embryonic period from the early otic cup stage to at least stage HH25. The differentiating ganglionic neurons attain a precise innervation pattern with sensory patches, a process presumably governed by a network of dendritic guidance cues which vary with the local micro-environment. To study the otic neurogenesis and topographically-ordered innervation pattern in birds, a quail-chick chimaeric graft technique was used in accordance with a previously determined fate-map of the otic placode. Each type of graft containing the presumptive domain of topologically-arranged placodal sensory areas was shown to generate neuroblasts. The differentiated grafted neuroblasts established dendritic contacts with a variety of sensory patches. These results strongly suggest that, rather than reverse-pathfinding, the relevant role in otic dendritic process guidance is played by long-range diffusing molecules.

Trio controls the mature organization of neuronal clusters in the hindbrain.

During the embryonic development of the hindbrain, movements of neuronal clusters allow the formation of mature "pools", in particular for inferior olivary (ION) and facial motor (fMN) nuclei. The cellular mechanisms of neuron clustering remain uncharacterized. We report that the absence of the Rho-guanine exchange factor Trio, which can activate both RhoG and Rac1 in vivo, prevents the proper formation of ION and fMN subnuclei. Rac1, but not RhoG, appears to be a downstream actor in Trio-induced lamellation. In addition, we report that Cadherin-11 is expressed by a subset of neurons through the overall period of ION and fMN parcellations, and defects observed in trio mutant mice are located specifically in Cadherin-11-expressing regions. Moreover, endogenous Cadherin-11 is found in a complex with Trio when lamellation occurs. Altogether, those results establish a link between Trio activity, the subsequent Rac1 activation, and neuronal clusters organization, as well as a possible recruitment of the Cadherin-11 adhesive receptor to form a complex with Trio.

Quantitative analysis of neural plate thickness and cell density during gastrulation in the chick embryo.

We quantitatively analyzed the developing prospective neural and non-neural ectoderm during chicken gastrulation on semithin transverse sections. At stage PS8 (primitive streak stage 8 of Lopez-Sanchez et al. [C. Lopez-Sanchez, L. Puelles, V. Garcia-Martinez, L. Rodriguez-Gallardo, Morphological and molecular analysis of the early developing chick requires an expanded series of primitive streak stages, J. Morphol. 264 (2005) 105-116.], equivalent to stage HH4), the thickest area of the ectoderm agrees in extent with the fate-mapped neural plate we had reported previously. The thickness of the median ectoderm is constantly higher up to a distance of 250mum from Hensen's node, and thickness decreases along a mediolateral gradient with a further drop at the prospective lateral border of the neural plate. A higher cell density of the developing ectoderm also coincided with the prospective neural plate. We observed that cell death does not play an important role in the spatial definition of the neural plate.

Fgf19 expression patterns in the developing chick inner ear.

The inner ear is a complex sensorial structure with hearing and balance functions. A key aim of developmental biology is to understand the molecular and cellular mechanisms involved in the induction, patterning and innervation of the vertebrate inner ear. These developmental events could be mediated by the expression of regulating genes, such as the members of the family of Fibroblast Growth Factors (Fgfs). This work reports the detailed spatial and temporal patterns of Fgf19 expression in the developing inner ear from otic cup (stage 14) to 8 embryonic days (stage 34). In the earliest stages, Fgf19 and Fgf8 expressions determine two subdomains within the Fgf10-positive proneural-sensory territory. We show that, from the earliest stages, the Fgf19 expression was detected in the acoustic-vestibular ganglion and the macula utriculi. The Fgf19 gene was also strongly, but transiently, expressed in the macula lagena, whereas the macula neglecta never expressed this gene in the period analysed. The Fgf19 expression was also clearly observed in some borders of various sensory elements. These results could be useful from further investigations into the role of FGF19 in otic patterning.

Changes in fiber arrangement in the retinofugal pathway of the turtle Mauremys leprosa: an evolutionarily conserved mechanism.

In spite of the numerous reports on the optic fiber distribution in the optic nerve and tract of vertebrates, there have been few studies of the visual pathway in reptiles. The arrangement of fibers in the optic nerve and tract of the turtle Mauremys leprosa was studied by placing a small granule of carbocyanine dye (DiI or DiA) in one of the four quadrants of the retina. The labeled fibers were traced through transverse sections of the retinofugal pathway with confocal microscopy. Retinal axons displayed a quadrant-specific order along the optic nerve. However, retinal ganglion cell axons were re-organized as they passed through the chiasmatic region of the optic pathway. In the optic tract, the nasal and temporal fibers remained intermingled, but there was segregation of dorsal from ventral fibers. This re-ordering is similar to that described in other vertebrates, suggesting the existence of an evolutionarily conserved mechanism.

Early development of the optic nerve in the turtle Mauremys leprosa.

We show the distribution of the neural and non-neural elements in the early development of the optic nerve in the freshwater turtle, Mauremys leprosa, using light and electron microscopy. The first optic axons invaded the ventral periphery of the optic stalk in close relationship to the radial neuroepithelial processes. Growth cones were thus exclusively located in the ventral margin. As development progressed, growth cones were present in ventral and dorsal regions, including the dorsal periphery, where they intermingled with mature axons. However, growth cones predominated in the ventral part and axonal profiles dorsally, reflecting a dorsal to ventral gradient of maturation. The size and morphology of growth cones depended on the developmental stage and the region of the optic nerve. At early stages, most growth cones were of irregular shape, showing abundant lamellipodia. At the following stages, they tended to be larger and more complex in the ventral third than in intermediate and dorsal portions, suggesting a differential behavior of the growth cones along the ventro-dorsal axis. The arrival of optic axons at the optic stalk involved the progressive transformation of neuroepithelial cells into glial cells. Simultaneously with the fiber invasion, an important number of cells died by apoptosis in the dorsal wall of the optic nerve. These findings are discussed in relation to the results described in the developing optic nerve of other vertebrates.

Fgf3 and Fgf16 expression patterns define spatial and temporal domains in the developing chick inner ear.

The inner ear is a morphologically complex sensory structure with auditory and vestibular functions. The developing otic epithelium gives rise to neurosensory and non-sensory elements of the adult membranous labyrinth. Extrinsic and intrinsic signals manage the patterning and cell specification of the developing otic epithelium by establishing lineage-restricted compartments defined in turn by differential expression of regulatory genes. FGF3 and FGF16 are excellent candidates to govern these developmental events. Using the chick inner ear, we show that Fgf3 expression is present in the borders of all developing cristae. Strong Fgf16 expression was detected in a portion of the developing vertical and horizontal pouches, whereas the cristae show weaker or undetected Fgf16 expression at different developmental stages. Concerning the rest of the vestibular sensory elements, both the utricular and saccular maculae were Fgf3 positive. Interestingly, strong Fgf16 expression delimited these Fgf16-negative sensory patches. The Fgf3-negative macula neglecta and the Fgf3-positive macula lagena were included within weakly Fgf16-expressing areas. Therefore, different FGF-mediated mechanisms might regulate the specification of the anterior (utricular and saccular) and posterior (neglecta and lagena) maculae. In the developing cochlear duct, dynamic Fgf3 and Fgf16 expression suggests their cooperation in the early specification and later cell differentiation in the hearing system. The requirement of Fgf3 and Fgf16 genes in endolymphatic apparatus development and neurogenesis are discussed. Based on these observations, FGF3 and FGF16 seem to be key signaling pathways that control the inner ear plan by defining epithelial identities within the developing otic epithelium.

Expression patterns of Irx genes in the developing chick inner ear.

The vertebrate inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. The molecular patterning of the developing otic epithelium creates various positional identities, consequently leading to the stereotyped specification of each neurosensory and non-sensory element of the membranous labyrinth. The Iroquois (Iro/Irx) genes, clustered in two groups (A: Irx1, Irx2, and Irx4; and B: Irx3, Irx5, and Irx6), encode for transcriptional factors involved directly in numerous patterning processes of embryonic tissues in many phyla. This work presents a detailed study of the expression patterns of these six Irx genes during chick inner ear development, paying particular attention to the axial specification of the otic anlagen. The Irx genes seem to play different roles at different embryonic periods. At the otic vesicle stage (HH18), all the genes of each cluster are expressed identically. Both clusters A and B seem involved in the specification of the lateral and posterior portions of the otic anlagen. Cluster B seems to regulate a larger area than cluster A, including the presumptive territory of the endolymphatic apparatus. Both clusters seem also to be involved in neurogenic events. At stages HH24/25-HH27, combinations of IrxA and IrxB genes participate in the specification of most sensory patches and some non-sensory components of the otic epithelium. At stage HH34, the six Irx genes show divergent patterns of expression, leading to the final specification of the membranous labyrinth, as well as to cell differentiation.

Spatial and temporal patterns of proliferation and differentiation in the developing turtle eye.

Here we show for the first time different aspects of the pattern of neurogenesis in the developing turtle retina by using different morphological and molecular clues. We show the chronotopographical fashion of occurrence of three major aspects of retinal development: (1) morphogenesis of the optic primordia and emergence of the different retinal layers, (2) the temporal progression of neurogenesis by the cessation of proliferative activity, and (3) the apparition and cellular localization of different antigens and neuroactive substances. Retinal cells were generated in a conserved temporal order with ganglion cells born first, followed by amacrine, photoreceptor, horizontal and bipolar/Müller cells. While eventually expressed in many types of retinal neurons, Islet1 was permanently expressed in differentiating and mature ganglion cells. Calbindin-immunoreactive elements were found in the ganglion cell layer and the inner nuclear layer. Interestingly, at later stages the amount of expressing cells in these layers was reduced dramatically. On the contrary, the number of calbindin-immunoreactive photoreceptors increased as development proceeded. In addition, calretinin expressing cells were prominent in the horizontal cell bodies, and their processes extending into the outer plexiform layer were also strongly labeled. Finally, the synthesis of gamma-aminobutyric acid (GABA) was detected in developing and matured horizontal and amacrine cells. All these maturational features began in the dorso-central area, in a region slightly displaced towards the temporal retina.

Developmental changes in the fibre population of the optic nerve follow an avian/mammalian-like pattern in the turtle Mauremys leprosa.

The changes in the axon and growth cone numbers in the optic nerve of the freshwater turtle Mauremys leprosa were studied by electron microscopy from the embryonic day 14 (E14) to E80, when the animals normally hatch, and from the first postnatal day (P0) to adulthood (5 years on). At E16, the first axons appeared in the optic nerve and were added slowly until E21. From E21, the fibre number increased rapidly, peaking at E34 (570,000 fibres). Thereafter, the axon number decreased sharply, and from E47 declined steadily until reaching the mature number (about 330,000). These observations indicated that during development of the retina there was an overproduction and later elimination of retinal ganglion cells. Growth cones were first observed in the optic nerve at as early as E16. Their number increased rapidly until E21 and continued to be high through E23 and E26. After E26, the number declined steeply and by E40 the optic nerve was devoid of growth cones. These results indicated that differentiation of the retinal ganglion cells occurred during the first half of the embryonic life. To examine the correlation between the loss of the fibres from the optic nerve and loss of the parent retinal ganglion cells, retinal sections were processed with the TUNEL technique. Apoptotic nuclei were detected in the ganglion cell layer throughout the period of loss of the optic fibres. Our results showed that the time course of the numbers of the fibres in the developing turtle optic nerve was similar to those found in birds and mammals.