Tissue interactions in the developing chick diencephalon.
BACKGROUND
The developing vertebrate brain is patterned first by global signalling gradients that define crude anteroposterior and dorsoventral coordinates, and subsequently by local signalling centres (organisers) that refine cell fate assignment within pre-patterned regions. The interface between the prethalamus and the thalamus, the zona limitans intrathalamica (ZLI), is one such local signalling centre that is essential for the establishment of these major diencephalic subdivisions by secreting the signalling factor Sonic hedgehog. Various models for ZLI formation have been proposed, but a thorough understanding of how this important local organiser is established is lacking.


RESULTS
Here, we describe tissue explant experiments in chick embryos aimed at characterising the roles of different forebrain areas in ZLI formation. We found that: the ZLI becomes specified unexpectedly early; flanking regions are required for its characteristic morphogenesis; ZLI induction can occur independently from ventral tissues; interaction between any prechordal and epichordal neuroepithelial tissue anterior to the midbrain-hindbrain boundary is able to generate a ZLI; and signals from the dorsal diencephalon antagonise ZLI formation. We further show that a localised source of retinoic acid in the dorsal diencephalon is a likely candidate to mediate this inhibitory signal.


CONCLUSIONS
Our results are consistent with a model where planar, rather than vertical, signals position the ZLI at early stages of neural development and they implicate retinoic acid as a novel molecular cue that determines its dorsoventral extent.
Guinazu
Maria Flavia
MF
MRC Centre for Developmental Neurobiology, Guy's Hospital Campus, King's College, London SE1 1UL, UK. fguinazu@nimr.mrc.ac.uk
Chambers
David
D
Lumsden
Andrew
A
Kiecker
Clemens
C
D000818:Animals; D019521:Body Patterning / Q000502:physiology*; D016380:Brain Tissue Transplantation / Q000379:methods; D002642:Chick Embryo; D003370:Coturnix; D004027:Diencephalon / Q000166:cytology / Q000196:embryology* / Q000378:metabolism*; D018507:Gene Expression Regulation, Developmental / Q000235:genetics / Q000502:physiology; D006131:Growth Inhibitors / Q000378:metabolism; D053823:Hedgehog Proteins / Q000378:metabolism; D008636:Mesencephalon / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D054259:Neural Tube / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D009924:Organ Culture Techniques; D012249:Rhombencephalon / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D013234:Stem Cells / Q000166:cytology / Q000378:metabolism; D013788:Thalamus / Q000166:cytology / Q000196:embryology / Q000378:metabolism; D018183:Transplantation Chimera; D014212:Tretinoin / Q000378:metabolism*
D006131:Growth Inhibitors; D053823:Hedgehog Proteins; D014212:Tretinoin