Determination of the identity of the derivatives of the cephalic neural crest: incompatibility between Hox gene expression and lower jaw development

In addition to pigment cells, and neural and endocrine derivatives, the neural crest is characterized by its ability to yield mesenchymal cells. In amniotes, this property is restricted to the cephalic region from the mid-diencephalon to the end of rhombomere 8 (level of somites 4/5), The cephalic neural crest is divided into two domains: an anterior region corresponding to the diencephalon, mesencephalon and metencephalon (r1, r2) in which expression of Hox genes is never observed, and a posterior domain in which neural crest cells exhibit (with a few exceptions) the same Hox code as the rhombomeres from which they originate. By altering the normal distribution of neural crest cells in the branchial arches through appropriate embryonic manipulations, we have investigated the relationships between Hox gene expression and the level of plasticity that neural crest cells display when they are led to migrate to an ectopic environment. We made the following observations. (i) Hox gene expression is not altered in neural crest cells by their transposition to ectopic sites. (ii) Expression of Hox genes by the I)A ectoderm does not depend upon an induction by the neural crest. This second finding further supports the concept of segmentation of the cephalic ectoderm into ectomeres (Couly and Le Douarin, 1990), According to this concept, metameres can be defined in large bands of ectoderm including not only the CNS and the neural crest but also the corresponding superficial ectoderm fated to cover craniofacial primordia, (iii) The construction of a lower jaw requires the environment provided by the ectomesodermal components of BA1 or BA2 associated with the Hox gene non-expressing neural crest cells. Hox gene-expressing neural crest cells are unable to yield the lower jaw apparatus including the entoglossum and basihyal even in the BA1 environment. In contrast, the posterior part of the hyoid bone can be constructed by any region of the neural crest cells whether or not they are under the regulatory control of Hox genes. Such is also the case for the neural and connective tissues (including those comprising the cardiovascular system) of neural crest origin, upon which no segmental restriction is imposed. The latter finding confirms the plasticity observed 24 years ago (Le Douarin and Teillet, 1974) for the precursors of the PNS.

The regeneration of the cephalic neural crest, a problem revisited: the regenerating cells originate from the contralateral or from the anterior and posterior neural fold

Anne Grapin-Botton

The mesencephalic and rhombencephalic levels of origin of the hypobranchial skeleton (lower jaw and hyoid bone) within the neural fold have been determined at the 5-somite stage with a resolution corresponding to each single rhombomere, by means of the quail-chick chimera technique. Expression of certain Hox genes (Hoxa-2, Hoxa-3 and Hoxb-4) was recorded in the branchial arches of chick and quail embryos at embryonic days 3 (E3) and E4. This was a prerequisite for studying the regeneration capacities of the neural crest, after the dorsal neural tube was resected at the mesencephalic and rhombencephalic level. We found first that excisions at the 5-somite stage extending from the midmesencephalon down to r8 are followed by the regeneration of neural crest cells able to compensate for the deficiencies so produced. This confirmed the results of previous authors who made similar excisions at comparable (or older) developmental stages. When a bilateral excision was followed by the unilateral homotopic graft of the dorsal neural tube from a quail embryo, thus mimicking the situation created by a unilateral excision, we found that the migration of the grafted unilateral neural crest (quail-labelled) is bilateral and compensates massively for the missing crest derivatives. The capacity of the intermediate and ventral neural tube to yield neural crest cells was tested by removing the chick rhombencephalic neural tube and replacing it either uni- or bilaterally with a ventral tube coming from a stage-matched quail. No neural crest cells exited from the ventral neural tube but no deficiency in neural crest derivatives was recorded. Crest cells were found to regenerate from the ends of the operated region. This was demonstrated by grafting fragments of quail neural fold at the extremities of the excised territory. Quail neural crest cells were seen migrating longitudinally from both the rostral and caudal ends of the operated region and filling the branchial arches located inbetween. Comparison of the behaviour of neural crest cells in this experimental situation with that showed by their normal fate map revealed that crest cells increase their proliferation rate and change their migratory behaviour without modifying their Hox code.

1996

[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

Defined concentrations of a posteriorizing signal are critical for MafB/Kreisler segmental expression in the hindbrain

Anne Grapin-Botton

It has been shown by using the quail/chick chimera system that Hox gene expression in the hindbrain is influenced by positional signals arising from the environment. In order to decipher the pathway that leads to Hox gene induction, we have investigated whether a Hox gene regulator, the leucine zipper transcription factor is itself transcriptionally regulated by the environmental signals. This gene is normally expressed in rhombomeres (r) 5 and 6 and their associated neural crest, MafB/Kr expression is maintained in r5/6 when grafted into the environment of r3/4. On the contrary, the environment of rhombomeres 7/8 represses MafB/Kr expression. Thus, as previously shown for the expression of Hox genes, MafB/Kr expression is regulated by a posterior-dominant signal, which in this case induces the loss of expression of this gene. We also show that the posterior signal can be transferred to the r5/6 neuroepithelium by posterior somites (somites 7 to 10) grafted laterally to r5/6, At the r4 level, the same somites induce MafB/Kr in r4, leading it to behave like r5/6, The posterior environment regulates MafB/Kr expression in the neural crest as it does in the corresponding hindbrain level, showing that some positional regulatory mechanisms are shared by neural tube and neural crest cells. Retinoic acid beads mimic the effect produced by the somites in repressing MafB/Kr in r5/6 and progressively inducing it more rostrally as its concentration increases. We therefore propose that the MafB/Kr expression domain is defined by a molecule unevenly distributed in the paraxial mesoderm, This molecule would allow the expression of the MafB/Kr gene in a narrow window of concentration by activating its expression at a definite threshold and repressing it at higher levels, accounting for its limited domain of expression in only two rhombomeres, It thus appears that the regulation of MafB/Kr expression in the rhombomeres could be controlled by the same posteriorizing factor(s) as Hox genes.

1998

Defined concentrations of a posteriorizing signal are critical for MafB/Kreisler segmental expression in the hindbrain

Anne Grapin-Botton

1998

The regeneration of the cephalic neural crest, a problem revisited: the regenerating cells originate from the contralateral or from the anterior and posterior neural fold

Anne Grapin-Botton

1996