EARLY EXPRESSION OF OSTEOPONTIN IN THE CHICK IS RESTRICTED TO RHOMBOMERE-5 AND RHOMBOMERE-6 AND TO A SUBPOPULATION OF NEURAL CREST CELLS THAT ARISE FROM THESE SEGMENTS

Background: Osteopontin previously has been shown to be expressed in t he early notochord and rostral hindbrain during formation of the rat n euraxis, Both of these sites are known to have important organizing ca pacities associated with the expression of a number of transcription f actors and secreted signaling molecules. Thus multiple types of molecu les are involved in neuraxis patterning, Osteopontin is an extracellul ar matrix molecule, whose expression within the early hindbrain and no tochord suggests that it too could play a role in patterning of the ne uraxis, Methods: By wholemount in situ hybridization, we analyzed oste opontin expression in the chick embryo at Hamburger and Hamilton (HH) stages 5-15, and patterns of expression were compared to those of othe r hindbrain and neural crest cell markers, Early developmental express ion was further examined in the rat by wholemount in situ hybridizatio n and immunohistochemistry. Results: Our results have demonstrated tha t whereas notochord expression of osteopontin is not conserved between rat and chick, expression was clearly present in the chick caudal hin dbrain between HH stages 9 and 11. In the chick, osteopontin expressio n was restricted to rhombomeres 5 and 6 and to a subpopulation of neur al crest cells arising from these rhombomeres, up to HH stage 13, Oste opontin expression was present in a cranial-to-caudal gradient, and in the dorsoventral plane, expression was absent from both the floor pla te and roof plate, with the exception of occasional cells within the r oof plate, Conclusions: Localized expression in the 5th and 6th rhombo meres suggests that osteopontin may have a role in patterning of posto tic neural crest cells that arise from these segments of the hindbrain , In addition, comparison of our current data with those obtained earl ier in the rat suggests that there are significant differences either between the developmental role(s) of osteopontin in the chick and in t he rat, or between osteopontin gene products in the two species. (C) 1 998 Wiley-liss, Inc.