Roles of Hoxa1 and Hoxa2 in patterning the early hindbrain of the mouse

Early in its development, the vertebrate hindbrain is transiently subdivided into a series of compartments called rhombomeres. Genes have been identified whose expression patterns distinguish these cellular compartments. Two of these genes, Hoxa1 and Hoxa2, have been shown to be required for proper patterning of the early mouse hindbrain and the associated neural crest. To determine the extent to which these two genes function together to pattern the hindbrain, we generated mice simultaneously mutant at both loci. The hindbrain patterning defects were analyzed in embryos individually mutant for Hoxa1 and Hoxa2 in greater detail and extended to embryos mutant for both genes. From these data a model is proposed to describe how Hoxa1, Hoxa2, Hoxb1, Krox20 (Egr2) and kreisler function together to pattern the early mouse hindbrain. Critical to the model is the demonstration that Hoxa1 activity is required to set the anterior limit of Hoxb1 expression at the presumptive r3/4 rhombomere boundary. Failure to express Hoxb1 to this boundary in Hoxa1 mutant embryos initiates a cascade of gene misexpressions that result in misspecification of the hindbrain compartments from r2 through r5. Subsequent to misspecification of the hindbrain compartments, ectopic induction of apoptosis appears to be used to regulate the aberrant size of the misspecified rhombomeres.     

Retinoic acid causes abnormal development and segmental patterning of the anterior hindbrain in Xenopus embryos.

Retinoic acid is a very potent teratogen and has also been implicated as an endogenous developmental signalling molecule in vertebrate embryos. One of the regions of the embryo reliably affected by exogenously applied RA is the hindbrain. In this paper, we describe in detail the hindbrain of Xenopus laevis embryos briefly treated with various levels of RA at gastrula stages. Such treatments lead to development of embryos with loss of anterior structures. In addition, RA has a general effect on rhombomere morphology and specific effects on the development of the anterior rhombomeres. This effect is demonstrated using neurofilament antibodies, HRP staining and in situ hybridisation using a probe for expression of the Xenopus Krox-20 gene. Anatomically it is evident that the development of the hindbrain normally anterior to the otocyst (rhombomeres 1-4) is abnormal following RA treatment. Sensory and motor axons of cranial nerves V and VII form a single root and the peripheral paths of V and VII and IX and X are also abnormal, as is the more anterior location of the otocyst. These anatomical changes are accompanied by changes in the pattern of expression for the gene XKrox-20, which normally expresses in rhombomeres 3 and 5, but is found in a single band in the anterior hindbrain of treated embryos which standardly fail to generate the normal external segmental appearance. The results are discussed in terms of both the teratogenic and possible endogenous roles of RA during normal development of the central nervous system. We conclude that low doses of RA applied during gastrulation have specific effects on the anterior Xenopus hindbrain which appear to be evolutionarily conserved in the light of similar recent findings in zebrafish.     

Hox Genes and Segmental Patterning of the Vertebrate Hindbrain

SYNOPSIS. Pattern formation in the developing hindbrain andcranio-facial region has been studied in a range of vertebrateorganisms. The developing hindbrain is transiently segmentedinto units termed rhombomeres which correspond with domainsof gene expression, lineage restriction and neuronal organizationand serve to coordinate the migration of cranial neural crestinto the adjacent branchial arches. In this paper I review thecellular and molecular events underlying both hindbrain segmentationand the acquisition of segmental identity, consolidating recentresults from different model systems. Data suggesting that thevertebrate Hox genes play an important role in specifying positionalvalue to the rhombomeres and cranial neural crest are also examined.I compare expression patterns of the Hox genes between speciesand consider the mechanisms involved in controlling their appropriatespatial regulation. In addition I describe a recently characterizedzebraflsh hindbrain segmentation mutant, Valentino; morphological,cellular and gene expression data for this mutant are helpingto further our understanding of hindbrain patterning.     

Hindbrain respecification in the retinoid-deficient quail

We report here the development and rescue of the truncated hindbrain of retinoid-deprived quail embryos. The embryo is completely rescued by an injection of retinol into the egg; this confirms retinol, or a related retinoid, as a required molecule in hindbrain development. Staging the retinoid replacement enabled us to determine that the 3-4 somite stage is the period when retinoids are required for normal development. Analysis of the development of the retinoid-deprived hindbrain phenotype through somitogenesis has revealed a pathway of retinoid action in early hindbrain regionalization. The hindbrain of the retinoid-deprived embryo is normal in size, during early somitogenesis, but has a respecified pattern of Krox-20 expression. From the earliest expression of Krox-20, at the 5 somite stage, the rhombomere 3 stripe fills the caudal third of the developing hindbrain to the level of the first somite. Morphologically only 2, instead of the normal 5, rhombomere bulges form. These 2 bulges express genes and, later, develop morphology characteristic of rhombomeres 1 and 2 and rhombomere 3. Posterior hindbrain specific genes, Hoxb-1, Fgf3, MafB, and the rhombomere 5 stripe of Krox-20 are never expressed in the head neuroepithelium of these embryos. From the initial formation of the neural plate, there is no evidence of rhombomere 4-7 specific characteristics. These results indicate the specification of the posterior hindbrain is lost and its cells participate in the formation of an enlarged anterior hindbrain. In our previous study, we reported the absence of the posterior hindbrain in retinoid-deprived quails (Maden, M., Gale, E., Kostetskii, I., Zile, M., 1996. Vitamin A-deficient quail embryos have half a hindbrain and other neural defects. Curr. Biol. 6, 417-426). Here, we show this phenotype to be the result of respecification of the hindbrain cells. This provides evidence for a region specific response to a single stimulus, retinol, which suggests a pre-rhombomeric regionalization of the hindbrain.     

How to build a vertebrate hindlbrain. lessons from genetics

During vertebrate embryogenesis, the hindbrain is the site of a segmentation process which leads to the formation, along the anterior-posterior axis, of 7–8 metameres called rhombomeres. This phenomenon plays an essential role in early hindbrain regionalisation and in the specification of the pattern of developing structures in this region of the brain. Data accumulated during the last 10 years have also shown that rhombomeres are units of gene expression and of cell lineage. Hence, a number of regulatory genes are expressed according to segment-specific patterns in the hindbrain and have been implicated in the pattern formation process. In this review, we focus on the analysis of the function and regulation of these genes along the different steps of hindbrain segmentation, from segment delimitation to acquisition of positional identity. On this basis, we propose a model for the control of early hindbrain development.