Background: Osteopontin previously has been shown to be expressed in the early notochord and rostral hindbrain during formation of the rat neuraxis. Both of these sites are known to have important organizing capacities associated with the expression of a number of transcription factors and secreted signaling molecules. Thus multiple types of molecules are involved in neuraxis patterning. Osteopontin is an extracellular matrix molecule, whose expression within the early hindbrain and notochord suggests that it too could play a role in patterning of the neuraxis.
Methods: By wholemount in situ hybridization, we analyzed osteopontin expression in the chick embryo at Hamburger and Hamilton (HH) stages 5-15, and patterns of expression were compared to those of other hindbrain and neural crest cell markers. Early developmental expression was further examined in the rat by wholemount in situ hybridization and immunohistochemistry.
Results: Our results have demonstrated that whereas notochord expression of osteopontin is not conserved between rat and chick, expression was clearly present in the chick caudal hindbrain between HH stages 9 and 11. In the chick, osteopontin expression was restricted to rhombomeres 5 and 6 and to a subpopulation of neural crest cells arising from these rhombomeres, up to HH stage 13. Osteopontin expression was present in a cranial-to-caudal gradient, and in the dorsoventral plane, expression was absent from both the floor plate and roof plate, with the exception of occasional cells within the roof plate.
Conclusions: Localized expression in the 5th and 6th rhombomeres suggests that osteopontin may have a role in patterning of postotic neural crest cells that arise from these segments of the hindbrain. In addition, comparison of our current data with those obtained earlier in the rat suggests that there are significant differences either between the developmental role(s) of osteopontin in the chick and in the rat, or between osteopontin gene products in the two species.