No abstract

The patterns of the Gbx2, Pax2, Wnt1, and Fgf8 gene expression were analyzed in the chick with respect to the caudal limit of the Otx2 anterior domain, taken as a landmark of the midbrain/hindbrain (MH) boundary. The Gbx2 anterior boundary is always concomitant with the Otx2 posterior boundary. The ring of Wnt1 expression is included within the Otx2 domain and Fgf8 transcripts included within the Gbx2 neuroepithelium. Pax2 expression is centred on the MH boundary with a double decreasing gradient. We propose a new nomenclature to differentiate the vesicles and constrictions observed in the avian MH domain at stage HH10 and HH20, based on the localization of the Gbx2/Otx2 common boundary.

During embryonic development, tangentially migrating precerebellar neurons emit a leading process and then translocate their nuclei inside it(nucleokinesis). Netrin 1 (also known as netrin-1) acts as a chemoattractant factor for neurophilic migration of precerebellar neurons (PCN) both in vivo and in vitro. In the present work, we analyzed Rho GTPases that could direct axon outgrowth and/or nuclear migration. We show that the expression pattern of Rho GTPases in developing PCN is consistent with their involvement in the migration of PCN from the rhombic lips. We report that pharmacological inhibition of Rho enhances axon outgrowth of PCN and prevents nuclei migration toward a netrin 1 source, whereas inhibition of Rac and Cdc42 sub-families impair neurite outgrowth of PCN without affecting migration. We show, through pharmacological inhibition, that Rho signaling directs neurophilic migration through Rock activation. Altogether, our results indicate that Rho/Rock acts on signaling pathways favoring nuclear translocation during tangential migration of PCN. Thus, axon extension and nuclear migration of PCN in response to netrin 1 are not strictly dependent processes because: (1)distinct small GTPases are involved; (2) axon extension can occur when migration is blocked; and (3) migration can occur when axon outgrowth is impaired.

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.

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.

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

No abstract

Retinoic acid (RA), an active metabolite of vitamin A, is a diffusible molecule that regulates the expression of several families of genes, playing a key role in specification processes during chordate development. With the aim of defining its possible role in the developing chick inner ear, we obtained in this work a detailed spatiotemporal distribution of the enzymes involved in its synthesis, the retinaldehyde dehydrogenases (RALH1-4). Our results showed that, in contrast to the mouse inner ear, Raldh3 expression was the only Raldh gene detected in the developing chick inner ear, where it appears as early as stage 18. During inner ear morphogenesis, Raldh3 expression was predominantly observed in the endolymphatic system. The Raldh3 expression pattern delimited totally or partially the Bmp4-positive presumptive territories of vestibular sensory epithelia by stage 24 and the basilar papilla at stage 34, suggesting a possible involvement of RA in their specification. In addition, several vestibular sensory areas showed some Raldh3-expressing cells close to the Raldh3-positive domain. These results suggest that the RA signaling pathway may play a role in the initial patterning of the otic epithelium and cell differentiation therein, providing local positional information. Having in mind this Raldh3 expression pattern, we discuss the regulatory interactions among the RA, bone morphogenetic protein, and fibroblast growth factor signaling pathways in the specification of otic sensory elements. Our investigation may underpin further experimental studies aimed at understanding the possible role of signaling pathways in patterning of the developing chick inner ear.