Invagination of the otic placode: normal development and experimental manipulation

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

Pax2 coordinates epithelial morphogenesis and cell fate in the inner ear

Crucial components of the vertebrate eye, ear and nose develop from discrete patches of surface epithelium, called placodes, which fold into spheroids and undergo complex morphogenesis. Little is known about how the changes in cell and tissue shapes are coordinated with the acquisition of cell fates. Here we explore whether these processes are regulated by common transcriptional mechanisms in the developing ear. After specification, inner ear precursors elongate to form the placode, which invaginates and is transformed into the complex structure of the adult ear. We show that the transcription factor Pax2 plays a key role in coordinating otic fate and placode morphogenesis, but appears to regulate each process independently. In the absence of Pax2, otic progenitors not only lose otic marker expression, but also fail to elongate due to the loss of apically localised N-cadherin and N-CAM. In the absence of either N-cadherin or N-CAM otic cells lose apical cell–cell contact and their epithelial shape. While misexpression of Pax2 leads to ectopic activation of both adhesion molecules, it is not sufficient to confer otic identity. These observations suggest that Pax2 controls cell shape independently from cell identity and thus acts as coordinator for these processes.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Apical constriction: a cell shape change that can drive morphogenesis

Biologists have long recognized that dramatic bending of a cell sheet may be driven by even modest shrinking of the apical sides of cells. Cell shape changes and tissue movements like these are at the core of many of the morphogenetic movements that shape animal form during development, driving processes such as gastrulation, tube formation, and neurulation. The mechanisms of such cell shape changes must integrate developmental patterning information in order to spatially and temporally control force production-issues that touch on fundamental aspects of both cell and developmental biology and on birth defects research. How does developmental patterning regulate force-producing mechanisms, and what roles do such mechanisms play in development? Work on apical constriction from multiple systems including Drosophila, Caenorhabditis elegans, sea urchin, Xenopus, chick, and mouse has begun to illuminate these issues. Here, we review this effort to explore the diversity of mechanisms of apical constriction, the diversity of roles that apical constriction plays in development, and the common themes that emerge from comparing systems.

Slits and Robos in the developing chicken inner ear

Mechanosensory hair cells in the chick inner ear synapse onto afferent neurons of the statoacoustic ganglion (SAG). During development, these neurons extend a central process to the brain and a peripheral process into one of eight sensory organs. A combination of cues, including chemoattractants and chemorepellents, direct otic axons to their peripheral targets. As a first step in evaluating the role of known axon guidance molecules, Slits and Robos, we examined expression of their transcripts in the chick inner ear from embryonic day 2-11 (Hamburger and Hamilton stages 14-37). Robo2 and slit2 are in migrating neuroblasts and the SAG, while both slits and robos are in the otic epithelium. We speculate that this family of signaling molecules may be involved in repulsion, first of otic neuroblasts and then of otic axons. Later our expression data revealed a potentially novel role for these molecules in maintaining sensory/nonsensory boundaries.

Early development of the cranial sensory nervous system: from a common field to individual placodes

Sensory placodes are unique columnar epithelia with neurogenic potential that develop in the vertebrate head ectoderm next to the neural tube. They contribute to the paired sensory organs and the cranial sensory ganglia generating a wide variety of cell types ranging from lens fibres to sensory receptor cells and neurons. Although progress has been made in recent years to identify the molecular players that mediate placode specification, induction and patterning, the processes that initiate placode development are not well understood. One hypothesis suggests that all placode precursors arise from a common territory, the pre-placodal region, which is then subdivided to generate placodes of specific character. This model implies that their induction begins through molecular and cellular mechanisms common to all placodes. Embryological and molecular evidence suggests that placode induction is a multi-step process and that the molecular networks establishing the pre-placodal domain as well as the acquisition of placodal identity are surprisingly similar to those used in Drosophila to specify sensory structures.

Determination of the embryonic inner ear

Inner ear induction, like induction of other tissues examined in recent years, is likely to be comprised of several stages. The process begins during gastrulation when the ectoderm is competent to respond to induction. It appears that a signal from the endomesoderm underlying the otic area during gastrulation initiates induction complemented by a signal from presumptive neural tissue. By the neural plate stage, a region of ectoderm outside the neural plate is "biased" toward ear formation; this process may be part of a more general "placodal" bias shared by several sensory tissues. Induction continues during neurulation when a signal from neural tissue (possibly augmented by mesoderm underlying the otic area) results in ectoderm committed to otic vesicle formation at the time of neural tube closure. Studies on several gene families implicate them in the ear determination process. Fibroblast Growth Factor (FGF) family members are clearly involved in induction: FGFs are appropriately expressed for such a role, and have been shown to be essential for inner ear development. FGFs also have inductive activity, although it is not clear if they are sufficient for ear induction. Activation of transcription factors in the otic ectoderm, for example, by Pax gene family members, provides evidence for important changes in the responding ectoderm beginning during gastrulation and continuing through specification at the end of neurulation, although few functional tests have defined the role of these genes in determination. The challenge remains to merge embryologic data with gene function studies to develop a clear model for the molecular basis of inner ear induction.

Perturbation of extracellular matrix prevents association of the otic primordium with the posterior rhombencephalon and inhibits subsequent invagination

In the avian embryo, the otic primordia become visible by Hamburger and Hamilton stage 10 as a pair of thickened regions of head ectoderm. In contrast to other epithelial primordia, invagination occurs by means of formation of a series of folds in distinct areas of the primordium, giving the otic vesicle a box-like appearance. Because previous work has shown that otic invagination is ATP and calcium independent, it is unlikely that cytoskeletal changes are the primary mechanism responsible for invagination as in other epithelial primordia. Interaction of the primordium with surrounding tissues may provide the force for otic invagination. These extracellular forces may be transduced through extracellular matrix macromolecules and their cell surface receptors. This investigation tests the hypothesis that fusion of the otic and hindbrain basal laminae between stages 11 and 13 is necessary for normal invagination. Perturbation of binding of the otic primordium to the neural tube was accomplished by means of microinjection of antibodies to various extracellular matrix components and integrin subunits into the head mesenchyme in the otic region at stage 10. Only antibodies to laminin and integrins caused detachment of the otic primordium from the hindbrain. These experiments suggest that fusion of the otic and hindbrain basal laminae is required for subsequent invagination and, furthermore, that this event is mediated by components of the extracellular matrix.

Hmx2 homeobox gene control of murine vestibular morphogenesis

Development of the vertebrate inner ear is characterized by a series of genetically programmed events involving induction of surface ectoderm, preliminary morphogenesis, specification and commitment of sensory, nonsensory and neuronal cells, as well as outgrowth and restructuring of the otocyst to form a complex labyrinth. Hmx2, a member of the Hmx homeobox gene family, is coexpressed with Hmx3 in the dorsolateral otic epithelium. Targeted disruption of Hmx2 in mice demonstrates the temporal and spatial involvement of Hmx2 in the embryonic transition of the dorsal portion (pars superior) of the otocyst to a fully developed vestibular system. In Hmx2 null embryos, a perturbation in cell fate determination in the lateral aspect of the otic epithelium results in reduced cell proliferation in epithelial cells, which includes the vestibular sensory patches and semicircular duct fusion plates, as well as in the adjacent mesenchyme. Consequently, enlargement and morphogenesis of the pars superior of the otocyst to form a complex labyrinth of cavities and ducts is blocked, as indicated by the lack of any distinguishable semicircular ducts, persistence of the primordial vestibular diverticula, significant loss in the three cristae and the macula utriculus, and a fused utriculosaccular chamber. The developmental regulators Bmp4, Dlx5 and Pax2 all play a critical role in inner ear ontogeny, and the expression of each of these genes is affected in the Hmx2 null otocyst suggesting a complex regulatory role for Hmx2 in this genetic cascade. Both Hmx2 and Hmx3 transcripts are coexpressed in the developing central nervous system including the neural tube and hypothalamus. A lack of defects in the CNS, coupled with the fact that not all of the Hmx2-positive regions in developing inner ear are impaired in the Hmx2 null mice, suggest that Hmx2 and Hmx3 have both unique and overlapping functions during embryogenesis.

Remarks on the inner ear of elasmobranchs and its interpretation from skeletal labyrinth morphology

The structure and function of the craniate inner ear is reviewed, with 33 apomorphic characters of the membranous labyrinth and associated structures identified in craniates, gnathostomes, and elasmobranchs. Elasmobranchs are capable of low-frequency semi-directional phonoreception, even in the absence of any pressure-to-displacement transducer such as ear ossicles. The endolymphatic (parietal) fossa, semicircular canals, and crista (macula) neglecta are all adapted toward phonoreception. Some (but not all) of the morphological features associated with phonoreception can be inferred from the elasmobranch skeletal labyrinth. Endocranial spaces such as the skeletal labyrinth also provide suites of morphological characters that may be incorporated into phylogenetic analyses, irrespective of how closely these spaces reflect underlying soft anatomy. The skeletal labyrinths of Squalus and Notorynchus are compared using silicone endocasts and high-resolution CT-scanning. The latter procedure offers several advantages over other techniques; it is more informative, nondestructive, preserves relationships of surrounding structures, and it can be applied both to modern and fossil material.

Chondroitin sulphate proteoglycan is involved in lens vesicle morphogenesis in chick embryos

Proteoglycans have been implicated in the invagination and formation of various embryonal cavitied primordia. In this paper the expression of chondroitin sulphate proteoglycan (CSPG) is analysed in the lens primordium during lens vesicle formation, and demonstrate that this proteoglycan has a specific distribution pattern with regard to invagination and fusion processes in the transformation of placode into lens vesicle. More specifically, CSPG was detected in: (1) the apical surface of lens epithelial cells, where early CSPG expression was observed in the whole of the lens placode whilst in the vesicle phase it was restricted to the posterior epithelium; (2) intense CSPG expression in the basal lamina, which remained constant for the entire period under study; (3) CSPG expression in the intercellular spaces of the lens primordium epithelium, which increased during the invagination of the primordium and which at the vesicle stage was more evident in the posterior epithelium; and (4) CSPG expression on the edges of the lens placode both prior to and during fusion. Treatment with beta- D -xyloside causes significant CSPG depletion in the lens primordium together with severe alterations in the invagination and fusion of the lens vesicle; this leads to the formation of lens primordia which in some cases remain practically flat or show partial invagination defects or fusion disruption. Similar results were obtained by enzyme digestion with chondroitinase AC but not with type II heparinase, which indicates that alterations induced by beta- D -xyloside were due to interference in CSPG synthesis. The findings demonstrate that CSPG is a common component of the lens primordium at the earliest developmental stages during which it undergoes specific modifications. It also includes experimental evidence to show that 'in vivo' CSPG plays an important role in the invagination and fusion processes of the lens primordium.

A fate map of chick otic cup closure reveals lineage boundaries in the dorsal otocyst

The vertebrate inner ear is structurally complex, consisting of fluid-filled tubules and sensory organs that subserve the functions of hearing and balance. The epithelial parts of the inner ear are derived from the otic placode, which deepens to form a cup before closing to form the otic vesicle. We fate-mapped the rim of the otic cup to monitor the cellular movements associated with otocyst formation and to aid in interpreting the changing gene expression patterns of the early otic field. Twelve sites around the rim, defined as positions of a clock face, were targeted by iontophoretic injection of fluorescent, lipophilic dye. Labeled cells were imaged 24 and 48 h after injection. The data show that the entire dorsal rim of the otic cup becomes the endolymphatic duct (ED), while the posteroventral rim becomes the lateral otocyst wall. Two intersecting boundaries of lineage restriction were identified near the dorsal pole: one bisecting the ED into anterior and posterior halves and the other defining its lateral edge. We hypothesize that signaling across compartment boundaries may play a critical role in duct specification. This model is discussed in the context of mouse mutants that are defective in both hindbrain development and ED outgrowth.

Molecular genetics of pattern formation in the inner ear: do compartment boundaries play a role?

The membranous labyrinth of the inner ear establishes a precise geometrical topology so that it may subserve the functions of hearing and balance. How this geometry arises from a simple ectodermal placode is under active investigation. The placode invaginates to form the otic cup, which deepens before pinching off to form the otic vesicle. By the vesicle stage many genes expressed in the developing ear have assumed broad, asymmetrical expression domains. We have been exploring the possibility that these domains may reflect developmental compartments that are instrumental in specifying the location and identity of different parts of the ear. The boundaries between compartments are proposed to be the site of inductive interactions required for this specification. Our work has shown that sensory organs and the endolymphatic duct each arise near the boundaries of broader gene expression domains, lending support to this idea. A further prediction of the model, that the compartment boundaries will also represent lineage-restriction compartments, is supported in part by fate mapping the otic cup. Our data suggest that two lineage-restriction boundaries intersect at the dorsal pole of the otocyst, a convergence that may be critical for the specification of endolymphatic duct outgrowth. We speculate that the patterning information necessary to establish these two orthogonal boundaries may emanate, in part, from the hindbrain. The compartment boundary model of ear development now needs to be tested through a variety of experimental perturbations, such as the removal of boundaries, the generation of ectopic boundaries, and/or changes in compartment identity.

Cell proliferation and cell death in the developing chick inner ear: spatial and temporal patterns

Morphogenesis of the inner ear is a complex process in which the balance of cell division and death is presumed to play an important role. Surprisingly, there are no reports of a systematic comparison of these two processes in individual ears at different stages of development. This study presents such an analysis for the chicken otocyst at stages 13-29 (embryonic days 2.5-6). To detect proliferating cells, we used exposure to bromodeoxyuridine. To detect apoptotic cells, we used nuclear condensation and fragmentation or terminal dUTP nick-end labeling (TUNEL). The spatial and temporal locations of proliferating and dying cells were mapped across serial sections, revealing regional differences in proliferation within the otocyst epithelium that are more complex than previously reported. In addition, almost all of the previously identified "hot spots" of cell death correspond spatially to regions of reduced cell proliferation. An exception is the ventromedial hot spot of cell death, which is intermingled with proliferating cells when it first appears at stages 19-23 before becoming a cold spot of proliferation. The results further show that the inferior part of the otocyst has a high level of proliferation, whereas the superior part does not. Since the superior part of the otocyst demonstrates outward expansion that is comparable to the inferior part, it appears that regional outgrowth of the otic vesicle is not necessarily coupled to cell proliferation. This study provides a basis for exploring the regulation and function of cell proliferation and cell death during inner ear morphogenesis.

Monoclonal antibody GL1 and its possible involvement in the morphogenesis of the otic vesicle

In a previous study, a monoclonal antibody (MAB) named GL1 was identified that is expressed in a precise pattern during gastrulation and early neurulation stages in chick embryos. In this article we have further investigated the expression pattern of this MAB in the chick embryo. GL1 antigen is present in several organs that seem not to be related developmentally. Among them, GL1 is present during the early steps of the otic placode formation, in the pharyngeal endoderm, in some neural crest cells, in the somites, and in the ventricular surface of the nervous system. The distribution in the nervous system is well patterned with two broad lines of expression in the ventricular side of the metencephalic region, a unique and centered expression in the border between the metencephalon and the myelencephalon and again in two lines running along the myelencephalon and the rostral spinal cord. Additionally, GL1 can be induced by members of the FGF family, and we have used this system to elucidate its role in otic placode formation. The results obtained reveal that GL1 can be a useful marker for the study of developmental processes in the endoderm, the otic anlage, and the apical surface of the nervous system. Biochemical analysis of the antigen recognized by this MAB must be carried out to elucidate the molecular nature of the antigen.

A role for neural cell adhesion molecule in the formation of the avian inner ear

The inner ear forms by a series of folds within an ectodermal placode. Previous work has shown that changes in surrounding tissues play a more prominent role in invagination than changes in the cytoskeleton of the primordium. Interference with the integrity of the extracellular matrix causes abnormalities in the folding process, primarily related to abnormalities in the paraxial mesoderm which lies ventral to the placode. In this study, the role of the neural cell adhesion molecule (N-CAM) was investigated, based on the expression of this component of the plasmalemma at the time the otic placode begins to fold. Microinjection of blocking antibodies to N-CAM into the paraxial mesoderm adjacent to the otic placode resulted in two major classes of defects, detachment of the primordium from the neural tube and interference with formation of the folds. Microinjection of saline, control immunoglobulin, or antibody against cytoplasmic domain had no effect. These defects correlate with the pattern of N-CAM expression at the time of injection, along the neural ectoderm and otic epithelium and the mesenchyme cells ventral to the primordium. It seems likely that N-CAM is playing a role in heterophilic associations rather than through the homophilic binding domain during formation of the otic vesicle.

Bottle cells are required for the initiation of primary invagination in the sea urchin embryo

Invagination of epithelial tissue occurs during gastrulation, neurulation, and organogenesis in many organisms. However, the underlying morphogenetic mechanisms of invagination are not understood. To elucidate these mechanisms, we have analyzed the initial invagination of the vegetal plate in the sea urchin embryo, a process termed primary invagination. At the onset of invagination, a ring of cells with highly constricted apices (bottle cells) encircles a group of two to eight round, central cells. To investigate the morphogenetic role of the bottle cells in the process of primary invagination, we have undertaken a series of laser ablation studies in which different proportions of various cell types were ablated and the effects were recorded using 4-D microscopy. Elimination of a 90 degrees-180 degrees arc of bottle cells markedly retards invagination, but only within the ablated region. Ablation of other cell types does not result in a statistically significant effect on primary invagination. These studies indicate that the number and arrangement of the bottle cells are critical factors for proper initiation of invagination. In addition, we have used the perturbing anti-hyalin antibody mAb183 to show that cell attachment to the hyaline layer is necessary for bottle cell formation and the initiation of primary invagination.

Morphological and quantitative studies in the otic region of the neural tube in chick embryos suggest a neuroectodermal origin for the otic placode

Careful histological observation of the development of the anlage of the inner ear in chicken embryos led us to question the traditional view of otic placode (OP) formation. First, morphological studies in the cephalic region carried out on stages preceding the appearance of the placodal epithelium revealed that the medial placodal cells are continuous temporally and spatially with cells belonging to the neural fold (NF). Second, both the formation of the basal lamina between the dorsal region of the neural tube (NT) and ectoderm and the pattern of formation of the neural crest present distinctive characteristics between otic levels and regions located anteriorly and posteriorly. Third, numerical comparisons of parameters for the NT and the OP between different levels of the rhombencephalon allowed us to assign a differential behaviour in the growth pattern of the otic region. These results indicated that the medial part of the OP is not derived from already independent ectoderm that increases in thickness under the influence of the NT (as previously accepted) but that it develops directly from the NFs. Although we do not exclude other possibilities, we propose that at least a proportion of the OP cells originate directly from cells committed to be neural crest. After this incorporation, basal laminal formation would delimit the NT from the OP without transition of the otic cells to ectoderm. This hypothesis would imply that part of the otic cells originate directly from neuroepithelial cells having a neuroectodermal (rather than the previously established ectodermal) origin.

Involvement of extracellular matrix in the formation of the inner ear

Formation of the inner ear from the optic placode differs from invagination of other cup-shaped organ primordia. Activation of the actin cytoskeleton seems to play a limited role because precocious invagination does not occur upon treatment with activators of a contractile event and cannot be prevented by inhibitors. In this study, the possibility that invagination is mediated by changes in the surrounding mesenchyme was tested by treating embryos with agents which interfere with the integrity of extracellular matrix. Enzymes degrading hyaluronate and/or chondroitin sulfate were microinjected into the otic region prior to folding. Synthesis of chondroitin sulfate proteoglycan was inhibited by microinjection of beta-xyloside. All treatments inhibited otic pit formation by interfering with fold formation within the placode. Immunocytochemical procedures showed depletion of the appropriate extracellular matrix components for a short time period after enzyme treatments and for up to 24 hr after beta-xyloside injection. Invagination of the otic primordium is concluded to be controlled in part by anchorage of the epithelium to adjacent structures and possibly by expansion of the mesenchymal extracellular matrix.

Development of cranial flexure and Rathke's pouch in the chick embryo

Experiments were done to investigate the cause of the cranial (mesencephalic) flexure of the chick brain during stages 10 to 14. Measurements of the length and thickness of the roof and floor of the mesencephalon gave values similar to the values obtained previously by others. The labeling index was determined in the roof and floor of the prosencephalon, mesencephalon, and rhombencephalon as a preliminary measure of cell division. The labeling index was about the same in all regions, and was high enough to suggest that most of the cells were dividing. The labeling indices did not suggest that differential growth was caused by differential rates of cell division in the roof and floor of the mesencephalon. It was found through time lapse photography that the foregut and heart remained stationary along the rostrocaudal axis, whereas the prosencephalon moved rostrally and the mesencephalon underwent flexure. Measurements suggested that the neural tube cranial to the otic primordium grew in volume exponentially at a rate consistent with the labeling index. The rostral tip of the neural tube was observed to be linked to the rostral tip of the foregut by the ectoderm that formed Rathke's pouch at the neural tube and the pharyngeal membrane (prospective stomodeum) at the foregut. As the neural tube grew in length, the link between the neural tube and the foregut did not. We suggest that because of this link, the growing neural tube had to bend around the foregut, forming the cranial flexure, and the ectoderm folded where it attached to the prosencephalon, forming Rathke's pouch.

Immunolocalization of basal lamina components during development of chick otic and optic primordia

Immunolocalization of laminin, fibronectin, and type IV collagen was examined during early morphogenetic shape changes of the avian inner ear and eye. The ear was studied from formation of the otic placode to invagination of the otic pit and the eye from the optic vesicle stage to formation of an optic cup. Distribution and intensity of immunoreactivity were compared in the two organ primordia and in adjacent epithelial layers. Laminin formed a continuous layer at the basal surface of the otic ectoderm and adjacent neural tube at all stages. The basal surfaces of the optic and lens epithelia also were continuously covered with laminin throughout development. The otic placode became attached to the neural ectoderm through a single layer of fibronectin and collagen IV between the layers of laminin. The ring-like attachment between the edges of the optic cup and lens primordium had the same structure. In addition, the central regions of the optic and lens primordia were attached by fibrils containing type IV collagen, whereas finer strands containing fibronectin and laminin also connected the otic epithelium and neural tube. The results are discussed in terms of models of invagination for the two primordia.

Shaping, invagination, and closure of the chick embryo otic vesicle: scanning electron microscopic and quantitative study

Scanning electron microscopy, light microscopy, and morphometric analysis were used to study the morphological changes of the otic placode and vesicle before and during invagination and closure processes. Our results reveal that the otic placode undergoes shaping between stages HH9 and HH12; during this period the rostrocaudal axis is shortened, while the lateromedial axis of the placode lengthens. The presence of long cytokinesis bridges during this period suggests that cellular displacements after mitosis may participate in the shaping of the otic placode. The shaping process appears to facilitate the approach of the otic placode to the neural tube. From stage HH12 on, the otic anlage gradually becomes a U-shaped structure with its medial portion in intimate apposition to the rhombencephalic neural tube. The coincidence in time between the beginning of intimate otic anlage-rhombencephalon contact and active invagination suggests that these two processes are related. Changes occurring at the edges of the otic vesicle until their disappearance in stage HH17 suggest that, in addition to a process of invagination, the edges of the otic anlage become bent. During closure, cells at the edges of the otic vesicle differ in apical morphology according to their topographical location: The cells between the rostral and lateral edges have elongated apices, in contrast with the polygonal shape of the cell apices in other places of the edges. In the opposite side (between the caudal and medial edges) cell death is observed. Closure of the otic vesicle conceptualized as a zipper-like model is discussed. We propose that early development of the otic anlage takes place in four stages: 1) shaping (stages HH9-11); 2) triggering of the invagination (stage HH12); 3) early invagination and lateral bending (stages HH13-15); and 4) late invagination and closure (stages HH16-17).

Patterns of cell movement in early organ primordia of the chick embryo

Purse-string constriction of the cytoskeleton at cell poles is generally accepted as the causal mechanism for invagination during early stages of organ formation. However, it is known that other cell movements, including intercalation, play a role in the organotypic shape changes that occur during gastrulation and neurulation. Such cell movements have not been investigated in pouching and branching epithelial primordia. There is reason to suspect that cells within these organ primordia might exchange their neighbors for others, that is, intercalate or translocate, at sites of sharp folding such as borders with the surrounding epithelial sheet or where a bend occurs within the primordium. The greatest difficulty in identifying these movements has been the need to use intact embryos so that the processes are not distorted. This study explores the possibility of using time-lapse video recording to identify cell movement at these locations. Three organ primordia were tested: otic and thyroid placodes, which had not been tested previously, and neural plate as a control, where movements of this sort have been documented. Embryos or parts containing the primordia were immobilized and cell apices visualized with Hoffman modulation contrast optics. Recordings to an optical memory disc recorder were transferred to a microcomputer for image analysis. The viewing procedure allows reasonably clear visualization of cell apices, and image analysis permits tracking of a number of adjacent cell apices over an extended time period. Several types of movement were found to occur within cell sheets, and the relative abundance of each type depends on the specific primordium. In the neural plate, some cells move many cell diameters from their neighbors. In the other two primordia, most cells show limited shifts in position relative to their neighbors except at regions where folds are formed. In other regions, adjacent cells move as a unit. Knowledge of the movements which occur in any particular primordium is essential to an understanding of the mechanisms controlling its formation.