Congress Abstracts - Society for Developmental Biology

function validated with explant studies, we describe the network of interactions between the transcription factors barhl2, iroquois,
orthodentricle and pax6 that controls establishment of the presumptive MDO (pre-MDO), and we identify the master patterning
homeogenes that specify the MDO identity. We establish that the pre-MDO emerges from early neurulation onwards inside the
prosomere p2 in a domain that expresses barhl2 and otx2, but is devoid of pax6. The pre-MDO expresses iroquois (irx)3, whereas the
prospective thalamus (prothalamus) expresses irx1/2 and early on irx3. The MDO anterior boundary forms at the neurula stage
whereas the MDO caudal boundary develops at tailbud stage through cross-repressive interactions between the iroquois factors and
the shh-dependent recruitment of irx3 expressing cells from the prothalamus into the MDO. In explants, cells expressing a MDO or a
thalamic signature recapitulate in vivo cellular behavior: when in contact with secreted Shh, MDO-cells are competent to express shh
and segregate from cells of unrelated lineages, whereas thalamic-cells do not express shh and develop boundaries with a MDO-like
territory. Barhl2 function in controlling the levels and activity of ß-Catenin participates in MDO patterning. The specification
mechanism we describe provides new insights on caudal forebrain development.
Program/Abstract # 41
Dual placode/neural crest origin of olfactory sensory neurons
Ankur Saxena, Marianne Bronner (Caltech, USA)
The cranial ganglia and sense organs arise from two cell types: neural crest and ectodermal placodes. Both undergo cell migration
and/or dynamic cell rearrangements to reach their final configuration. Most cranial peripheral neurons are derived from the placodes,
with glia cells coming from the neural crest. In the olfactory system, the classical view has therefore been that the olfactory placode
forms all olfactory sensory neurons. In contrast, we show that cranial neural crest cells migrating from the neural tube are the primary
source of microvillous sensory neurons. In studying the olfactory expression of a novel protein termed olfactin, we observed
significant cell migration into the developing olfactory epithelium. Using photoconversion-based fate mapping and live cell tracking
coupled with laser ablation in zebrafish embryos, we determined that neural crest precursors were migrating from the neural tube to
surround the olfactory epithelium where they condensed to form the nasal cavity. Eventually, a subset of these cells, coexpressing
Sox10 protein and a neurogenin1 reporter, ingressed into the epithelium, intercalated amongst placode-derived cells, and differentiated
into microvillous sensory neurons. Timed loss-of-function analysis using a photo-morpholino revealed a critical requirement for
Sox10 in microvillous neurogenesis, and we are now investigating the roles of olfactin and other proteins during this dynamic process.
Taken together, these findings demonstrate for the first time a Sox10-dependent cranial neural crest migratory contribution to
olfactory sensory neurons and provide important insights into the assembly of the nascent olfactory system.
Program/Abstract # 42
The roles of Atoh1 in the developing cerebellum under the influence of multiple organizers
Mary Green, Richard Wingate (King’s College of London, UK)
The domain of the developing cerebellum in dorsal rhombomere 1 is specified by signals from two adjacent organising centres, the
boundary of the roof plate of the fourth ventricle and the midbrain-hindbrain boundary (isthmus). The cerebellar rhombic lip
comprises a dorsal progenitor population, defined by the expression of the transcription factor Atoh1, which sequentially gives rise to
both extra-cerebellar and cerebellar populations of glutamateric neurons. Atoh1 expression in the cerebellar rhombic lip is dependent
on tissue interaction with the adjacent roof plate boundary. We find that, in chick, the isthmus abuts an expanded region of Atoh1
expression from which extra-cerebellar rhombic lip derivatives are born. Through surgical manipulation of the isthmus and roof plate
in cultured explants we demonstrate that this expanded region of Atoh1 is dependent upon the isthmus for its induction and
maintenance. We also demonstrate the role of isthmic Fgf8 in the maintenance of this expanded domain through in ovo electroporation
of Fgf8 and a dominant-negative FGF receptor. Whilst FGF signalling can modulate the expression of Atoh1 in rhombomere 1, we
show that this occurs independently of cell specification at the rhombic lip. Instead we show that loss of isthmic FGF signalling results
in a reduction in the overall size of the cerebellum by mediating changes in growth in rhombomere 1. Analysis of FGF receptor
knockout mutants and FGF hypomorph mice confirmed the results of dominant negative over-expression in chick. Together this work
demonstrates the existence of an FGF-induced Atoh1 population that does not contribute specific rhombic lip derivatives but instead
regulates overall cerebellar growth.
Program/Abstract # 43
Evolutionary origin of the turtle body plan from genomic, anatomical and developmental perspectives
Shigeru Kuratani (RIKEN, Japan); Hiroshi Nagashima (Niigata U, Japan); Naoki Irie (RIKEN, Japan)
The turtle shell represents an example of evolutionary novelty, with its unusual topography of musculoskeletal elements achieved by
the developmental repatterning of mesodermal tissues. During development, growth of the turtle ribs is arrested in the axial part and
allowed to grow laterally towards the turtle-specific carapacial ridge (CR), thereby encapsulating the scapula inside the ribcage. The
CR supports the fan-shaped patterning of the ribs concomitant with marginal growth of the carapace by specific expression of some
regulatory genes. The developmental background of the turtle shell is also consistent with the recently discovered fossil species,
Odontochelys. By generating and analyzing draft genomes of Chinese soft-shelled turtle (Pelodiscus sinensis) and the green sea turtle
(Chelonia mydas), we confirmed a close relationship of turtles to the bird/crocodilian lineage, which split ~267.9-248.3 million years
ago (Upper-Permian to Triassic period). We also found extensive expansion of olfactory receptor genes in these turtles. Embryonic
gene expression analysis revealed an hourglass-like divergence between turtle and chicken embryogenesis, with maximal conservation
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