Two cell autonomous fluorescent labels (DiI and Hoechst) were used as vital markers in a fate map study of the Xenopus neural plate and ridge. Most areas of the brain derive from the neural plate in a fate map that is consistent with the topology of a sheet rolling into a tube, i.e., neighboring areas are maintained as neighbors. This has enabled us not only to plot the fates of larval brain structures, but also to suggest their primordial orientation in the neural plate. Since overlapping areas of the plate gave rise to overlapping regions of the central nervous system (CNS), we have been able to construct a space-filling model of the neural plate, whereby the number of founder cells for each brain region fate-mapped may be estimated roughly. Much of the telencephalon, ventral forebrain, and dorsal brain stem derives from the neural ridge and not the neural plate in the stage 15 Xenopus embryo. The structures of the forebrain were examined in detail because there were indications of substantial cell movements in this region. The anterior pituitary arises from the mid-anterior ridge, while hypothalamic structures arise from the midline regions of the anterior neural plate. Consistent groups of ventral hypothalamic structures were labeled when fluorescent markers were applied to these parts of the neural plate, indicating stereotyped cell movements. Detailed comparisons were made between the fate map of the Ambystoma neural plate (Jacobson, 1959) and that of Xenopus.

Cranial placodes are local thickenings of the vertebrate head ectoderm that contribute to the paired sense organs (olfactory epithelium, lens, inner ear, lateral line), cranial ganglia and the adenohypophysis. Here we use tissue grafting and dye injections to generated fate maps of the dorsal cranial part of the non-neural ectoderm for Xenopus embryos between neural plate and early tailbud stages. We show that all placodes arise from a crescent-shaped area located around the anterior neural plate, the pre-placodal ectoderm. In agreement with proposed roles of Six1 and Pax genes in the specification of a panplacodal primordium and different placodal areas, respectively, we show that Six1 is expressed uniformly throughout most of the pre-placodal ectoderm, while Pax6, Pax3, Pax8 and Pax2 each are confined to specific subregions encompassing the precursors of different subsets of placodes. However, the precursors of the vagal epibranchial and posterior lateral line placodes, which arise from the posteriormost pre-placodal ectoderm, upregulate Six1 and Pax8/Pax2 only at tailbud stages. Whereas our fate map suggests that regions of origin for different placodes overlap extensively with each other and with other ectodermal fates at neural plate stages, analysis of co-labeled placodes reveals that the actual degree of overlap is much smaller. Time lapse imaging of the pre-placodal ectoderm at single cell resolution demonstrates that no directed, large-scale cell rearrangements occur, when the pre-placodal region segregates into distinct placodes at subsequent stages. Our results indicate that individuation of placodes from the pre-placodal ectoderm does not involve large-scale cell sorting in Xenopus.

The fate of the anterior neural ridge was studied by following the relative movements of simultaneous spot applications of DiI and DiO from stage 15 through stage 45. These dye movements were mapped onto the neuroepithelium of the developing brain whose shape was gleaned from whole-mount in situs to neural cell adhesion molecule and dissections of the developing nervous system. The result is a model of the cell movements that drive the morphogenesis of the forebrain. The midanterior ridge moves inside and drops down along the most anterior wall of the neural tube. It then pushes forward a bit, rotates ventrally during forebrain flexing, and gives rise to the chiasmatic ridge and anterior hypothalamus. The midanterior plate drops, forming the floor of the forebrain ventricle, and, keeping its place behind the ridge, it gives rise to the posterior hypothalamus or infundibulum. The midlateral anterior ridge slides into the lateral anterior wall of the neural tube and stretches laterally into the optic stalk and retina, and then rotates into a ventral position. The lateral anterior ridge converges to the most anterior part of the dorsal midline during neural tube closure, then rotates anteriorly, and gives rise to telencephalic structures. Whole-mount bromodeoxyuridine labeling at these stages showed that cell division is widespread and relatively uniform throughout the brain during the late neurula and early tailbud stages, but that during late tailbud stages cell division becomes restricted to specific proliferative zones. We conclude that the early morphogenesis of the brain is carried out largely by choreographed cell movements and that later morphogenesis depends on spatially restricted patterns of cell division.

Characterization of Kif2a in Xenopus embryos identifies new roles for the Kif2a microtubule depolymerase in coordinating cytokinesis and centrosome coalescence. In addition, defects in mitosis can inhibit large-scale developmental movements in vertebrate tissues.

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Laboratory and field studies were initiated to study the life-history patterns of populations of Ambystoma gracile from three permanent lakes located at different altitudes in southwestern British Columbia. Laboratory studies indicated clear differences between low- and high-altitude populations with respect to larval growth, timing of metamorphosis, size at metamorphosis, and the incidence of neoteny. Field studies indicated that low-altitude neotenous females became sexually mature at a length of 7.1 cm (snout to vent length, SVL); but owing to the shorter seasons within higher altitude lakes, neotenous females required more seasons to attain sexual maturity. High- and low-altitude males attained sexual maturity at 7.4-cm SVL, but a few smaller, sexually mature males were discovered within the high-altitude lake. Neotenous females reproduced annually within the low-altitude lake, but high-altitude neotenous females did not reproduce during successive years. Possible genetic adaptations to low and high altitudes are discussed.

This study describes a whole embryo and embryonic field analysis of retinoic acid's (RA) effects upon Xenopus laevis forebrain development and differentiation. By using in situ and immunohistochemical analysis of pax6, Xbf1, and tyrosine hydroxylase (TH), gene expression during eye field, telencephalon field, and retinal development was followed with and without RA treatment. These studies indicated that RA has strong effects upon embryonic eye and telencephalon field development with greater effects upon the ventral development of these organ fields. The specification and determination of separate eye primordia occurred at stage-16 when the prechordal plate reaches its most anterior aspect in Xenopus laevis. Differentiation of the dopaminergic cells within the retina was also affected in a distinct dorsoventral pattern by RA treatment, and cell type differentiation in the absence of distinct retinal laminae was also observed. It was concluded that early RA treatments affected organ field patterning by suppression of the upstream elements required for organ field development, and RA's effects upon cellular differentiation occur downstream to these organ determinants' expression within a distinct dorsoventral pattern.

Functional forebrain development is the result of a complex series of early developmental processes which include cell division, cellular rearrangements, tissue-tissue interactions, cellular determinative and differentiation events, and axonogenesis. In these studies, Xenopus laevis embryos were examined for early forebrain neuronal determination, differentiation and axonogenesis with special emphasis on the hypothalamic area known to be involved in regulating pars intermedia function. Whole brain acetylcholine esterase (AChE) histochemistry was used to follow the early pattern of forebrain neuronal differentiation, and whole brain acetylated-tubulin immunocytochemistry was done to follow early forebrain axonogenesis. AChE histochemistry indicated that the source of the tract of the postoptic commissure (stpoc) was the first forebrain area to begin differentiation (stage 22). Whole brain immunocytochemistry for acetylated-tubulin indicated that the tpoc is also the first forebrain tract to develop (at stage 25/26). The main forebrain tracts have developed and become interconnected by stage 35/36. The forebrain undergoes a pronounced extension, with much cellular mixing and rearrangement during stages 37/38 to 43/44. This results in bending and contortions in the already developed tracts. Whole brain immunocytochemistry for tyrosine hydroxylase and extirpation of the stage 14 presumptive suprachiasmatic (SC) area indicated that the dopaminergic cells of the SC are determined by stage 14 and initially undergo differentiation between stages 37/38 and 40. Tadpoles with stage 14 presumptive SC extirpated lacked TH-positive tracts to the pars intermedia, lacked most midline TH-positive forebrain cells, and also failed to background adapt to white background. Thus, the SC tracts to the pars intermedia that inhibit melanotrope secretion probably form during the extension stages of 37/38 and contact the pars intermedia by stage 40 when animals are first capable of background adaptation.