Expression of Krox-20 and hox genes in hindbrain segments and neural crest

The vertebrate hindbrain is morphologically segmented by transient formation of rhombomeres. A transcription factor gene, Krox-20, which is expressed in a segmental manner in the vertebrate hindbrain is thought to play a role in hindbrain segmentation. Vertebrate homologues of Drosophila homeotic genes of the Antennapedia class, the Hox genes, are known to specify antero-posterior positional identity in embryos. It was suggested that Krox-20 may be setting up the anterior limit of the Hox-2 genes in the hindbrain. In this study I have shown that murine 3' Hox-2 genes, Hox-2.H and -2.9, are at their anterior limits of expression in the hindbrain before the onset of Krox-20 expression, hence Krox-20 is not setting up this limit. From the nature of sharpening of expression domains, it seems different mechanisms are involved in the regulation of Krox- 20 and Hox-2 genes. The later modulation of Hox-2 gene expression correlates with the Krox-20 pattern, suggesting involvement of Krox-20 in quantitative regulation of Hox expression in the rhombomeres. Krox-20 is expressed in the neural crest also. This study in mouse and quail reveals a lack of relationship between early expression in migratory crest and later expression in crest denved boundary caps of cranial (branchiomotor and motor) and spinal nerves. The timing of boundary cap expression is unrelated to defining entry/exit points of nerves in the brain wall. Premigratory and migrating crest from the rhombomere 5/6 junction expresses Krox-20, the precise rhombomeric origin not being definable by this study. Stage and tissue specific expression is also observed in mesectodermal derivatives, the tooth germs and whisker fibrils. Expression of Krox-20, Hox-2.9 and -2.6 in cultures of explanted neural crest reveal Krox-20 expression is lacking under in vitro conditions, but that peripheral cells of both midbrain and hindbrain cultures respond to explantation by upregulating Hox-2 expression. Hox genes have been implicated in prespecification of arch crest, according to a Hox code for the branchial region. This study reports Hox-2.8 is expressed in arch one endoderm, contrary to previous suggestions. Also, studying the expression of its paralogue, Hox-1.11 at later stages suggests that Hox genes may not be directly involved in the fine patterning of structures within arches