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1: Dev Genes Evol. 2007 Mar;217(3):177-88. Epub 2007 Jan 11.

Brain induction in ascidian embryos is dependent on juxtaposition of
FGF9/16/20-producing and -receiving cells.

Miyazaki Y, Nishida H, Kumano G.

Department of Biological Sciences, Graduate School of Science, Osaka University, 
1-1 Machikaneyama-cho, Toyonaka, Osaka, 560-0043, Japan.

Coordinated regulation of inductive events, both spatially and temporally, during
animal development ensures that tissues are induced at their specific positions
within the embryo. The ascidian brain is induced in cells at the anterior edge of
the animal hemisphere by fibroblast growth factor (FGF) signals secreted from
vegetal cells. To clarify how this process is spatially regulated, we first
identified the sources of the FGF signal by examining the expression of brain
markers Hr-Otx and Hr-ETR-1 in embryos in which FGF signaling is locally
inhibited by injecting individual blastomeres with morpholino oligonucleotide
against Hr-FGF9/16/20, which encodes an endogenous brain inducer. The blastomeres
identified as the inducing sources are A5.1 and A5.2 at the 16-cell stage and
A6.2 and A6.4 at the 24-cell stage, which are juxtaposed with brain precursors at
the anterior periphery of the embryo at the respective stages. We also showed
that all the cells of the animal hemisphere are capable of expressing Hr-Otx in
response to the FGF signal. These results suggest that the position of inducers, 
rather than competence, plays an important role in determining which animal cells
are induced to become brain tissues during ascidian embryogenesis. This situation
in brain induction contrasts with that in mesoderm induction, where the positions
at which the notochord and mesenchyme are induced are determined mainly by
intrinsic competence factors that are inherited by signal-receiving cells.

PMID: 17216525 [PubMed - indexed for MEDLINE]

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