PROGRESSIVE SPATIAL RESTRICTION OF SEK-1 AND KROX-20 GENE-EXPRESSION DURING HINDBRAIN SEGMENTATION

After segmentation of the vertebrate hindbrain, expression of the zinc -finger gene Krox-20 and the receptor tyrosine kinase gene Sek-1 is pr ecisely restricted to rhombomeres (r)3 and 5. This precise segmental e xpression is likely to reflect a critical requirement for these rhombo meres to acquire a distinct and homogeneous identity and raises the qu estion as to how this relates to the intermingling and restriction of cell movement during segmentation. We have analysed Krox-20 and Sek-1 expression in the mouse and chick hindbrain at single-cell resolution using whole-mount in situ hybridisation and immunocytochemistry. We fi nd that, in the mouse, the presumptive r3 and r5 expression domains ea ch arise as narrow stripes that then broaden, suggestive of a recruitm ent of cells to an r3/r5 identity and/or a segmental regulation of cel l proliferation. In addition, we find that expression of these genes i nitially occurs in fuzzy domains, and that these are progressively res tricted to segmental domains, although occasional ''violating'' cells are observed even after segmentation. We propose that the establishmen t and maintenance of these segmental domains may involve both a dynami c regulation of r3/r5 identity and the restriction of cell movement ac ross rhombomere boundaries. (C) 1996 Academic Press, Inc.