Patterning signals acting in the spinal cord override the organizing activity of the isthmus

The regionalization of the neural tube along the anteroposterior axis is established through the action of patterning signals from the endo-mesoderm including the organizer. These signals set up a pre-pattern which is subsequently refined through local patterning events. The midbrain-hindbrain junction, or isthmus, is endowed with such an organizing activity. It is able to induce graded expression of the Engrailed protein in the adjacent mesencephalon and rhombencephalon, and subsequently elicits the development of rectal and cerebellar structures. Ectopically grafted isthmus was also shown to induce Engrailed expression in diencephalon and otic and pre-otic rhombencephalon. Fgf8 is a signalling protein which is produced by the isthmus and which is able to mimic most isthmic properties. We show here that the isthmus, when transposed to the level of either rhombomere 8 or the spinal cord, loses its ability to induce Engrailed and cerebellar development in adjacent tissues. This is accompanied by the down-regulation of fgf8 expression in the grafted isthmus and by the up-regulation of a marker of the recipient site, Hoxb-4. Moreover, these changes in gene activity in the transplant are followed by a transformation of the fate of the grafted cells which adjust to their novel environment. These results show that the Tale of the isthmus is not determined at 10-somite stage and that the molecular loop of isthmic maintenance can be disrupted by exogenous signals. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.

[Genetic control of rhombencephalon development by Hox genes studied in bird embryo by the quail-chick chimera method]

Anne Grapin-Botton

The rhombencephalic neural tube is transiently segmented along the anteroposterior axis into 8 rhombomeres. Each rhombomere, as well as its derived neural crest cells, is characterized by the expression of a specific set of Hox genes which constitute its Hox code. This code is supposed to define the morphogenetic program of these cells according to their position. We took advantage of the quail/chick chimera system to study the regulation of Hox gene expression in neural tube and neural crest cells. We have therefore ectopically transplanted the presumptive territories of the future rhombomeres and studied the evolution of their Hox code. We evidence in the posterior rhombencephalon and the spinal cord a posteriorising signal able to induce Hox gene expression, to repress anterior molecular markers and to control the subsequent development of the neural tube. This signal is conveyed horizontally in the plane of the neuroepithelium and vertically from the mesoderm to the ectoderm. The anteroposterior identity of the neural crest cells seem independent from this inducer after formation of the neural fold.

1997

Generation of segment polarity in the paraxial mesoderm of the zebrafish through a T-box-dependent inductive event

Andrew Charles Oates, Laurel Ann Rohde

The first morphological sign of vertebrate postcranial body segmentation is the sequential production from posterior paraxial mesoderm of blocks of cells termed somites. Each of these embryonic structures is polarized along the anterior/posterior axis, a subdivision first distinguished by marker gene expression restricted to rostral or caudal territories of forming somites. To better understand the generation of segment polarity in vertebrates, we have studied the zebrafish mutant fused somites (fss), because its paraxial mesoderm lacks segment polarity. Previously examined markers of caudal half-segment identity are widely expressed, whereas markers of rostral identity are either missing or dramatically down-regulated, suggesting that the paraxial mesoderm of the fss mutant embryo is profoundly caudalized. These findings gave rise to a model for the formation of segment polarity in the zebrafish in which caudal is the default identity for paraxial mesoderm, upon which is patterned rostral identity in an fss-dependent manner. In contrast to this scheme, the caudal marker gene ephrinA1 was recently shown to be down-regulated in fss embryos. We now show that notch5, another caudal identity marker and a component of the Delta/Notch signaling system, is not expressed in the paraxial mesoderm of early segmentation stage fss embryos. We use cell transplantation to create genetic mosaics between fss and wild-type embryos in order to assay the requirement for fss function in notch5 expression. In contrast to the expression of rostral markers, which have a cell-autonomous requirement for fss, expression of notch5 is induced in fss cells at short range by nearby wild-type cells, indicating a cell-non-autonomous requirement for fss function in this process. These new data suggest that segment polarity is created in a three-step process in which cells that have assumed a rostral identity must subsequently communicate with their partially caudalized neighbors in order to induce the fully caudalized state. © 2005 Elsevier Inc. All rights reserved.

2005

Signals from lateral plate mesoderm instruct endoderm toward a pancreatic fate

Anne Grapin-Botton, Mithlesh Kumar

During embryonic development, organs arise along the gut tube as a series of buds in a stereotyped anterior-posterior (A-P) pattern. Using chick-quail chimeras and in vitro tissue recombination, we studied the interactions governing the. induction and maintenance of endodermal organ identify focusing on the pancreas. Though several permissive signals in pancreatic development have been previously identified, here we provide evidence that lateral plate mesoderm sends instructive signals to the endoderm, signals that induce expression of the pancreatic genes Pdx1, p48, Nkx6.1, glucagon, and insulin. Moreover, this instructive signal directs cells to form ectopic insulin-positive islet-like clusters in endoderm that would otherwise form more rostral organs. Once generated, endocrine cells no longer require interaction with mesoderm, but nonendocrine cells continue to require permissive signals from the mesoderm. Stimulation of activin, BMP, or retinoic acid signaling is sufficient to induce Pdx1 expression in endoderm anterior to the pancreas. Lateral plate mesoderm appears to pattern the endoderm in a posterior-dominant fashion as first noted in the patterning of the neural tube at the same embryonic stage. These findings argue for a central role of the mesoderm in coordinating the A-P pattern of all three primary germ layers. (C) 2003 Elsevier Science (USA). All rights reserved.

2003

Generation of segment polarity in the paraxial mesoderm of the zebrafish through a T-box-dependent inductive event

Andrew Charles Oates, Laurel Ann Rohde

2005