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Recent studies in the chick have indicated that rhombomeres (r) are segments that underlie the patterning of hindbrain nerves. These segments may also be important for the specification of branchial arch structures since alternating rhombomeres, r2, r4 and r6, each contribute crest to a specific arch. Krox-20 has been implicated in the segmental patterning of the hindbrain in the mouse by its expression prior to segment formation in alternating domains, which later correspond to r3 and r5. Here, we describe the sequence and developmental expression of the Xenopus Krox-20 gene, XKrox-20. Alternating domains of XKrox-20 expression appear in the early neurula, later correspond to r3 and r5, and persist until late tadpole stages. In contrast to this conserved spatial expression in rhombomeres, we find a pattern in the neural crest of Xenopus that appears different from that found in the mouse: expression occurs in crest that migrates from r5 into the third visceral arch. We speculate that this may reflect a distinct route of neural crest migration due to anatomical differences between these systems, rather than a difference in the site of origin of Krox-20-expressing crest.

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 Vll form a single root and the peripheral paths of V and VH 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.