BMP-7 INFLUENCES PATTERN AND GROWTH OF THE DEVELOPING HINDBRAIN OF MOUSE EMBRYOS

The expression pattern of bone morphogenetic protein-7 (BMP-7) in the hindbrain region of the headfold and early somite stage developing mou se embryo suggests a role for BMP-7 in the patterning of this part of the cranial CNS, In chick embryos it is thought that BMP-7 is one of t he secreted molecules which mediates the dorsalizing influence of surf ace ectoderm on the neural tube, and mouse surface ectoderm has been s hown to have a similar dorsalizing effect, While we confirm that BMP-7 is expressed in the surface ectoderm of mouse embryos at the appropri ate time to dorsalize the neural tube, we also show that at early stag es of hindbrain development BMP-7 transcripts are present in paraxial and ventral tissues, within and surrounding the hindbrain neurectoderm , and only later does expression become restricted to a dorsal domain. To determine more directly the effect that BMP-7 may have on the deve loping hindbrain we have grafted COS cells expressing BMP-7 into the v entrolateral mesoderm abutting the neurectoderm in order to prolong BM P-7 expression in the vicinity of ventral hindbrain, Three distinct ac tivities of BMP-7 are apparent, Firstly, as expected from previous wor k in chick, BMP-7 can promote dorsal characteristics in the neural tub e, Secondly, we show that it can also attenuate the expression of soni c hedgehog (Shh) in the floorplate without affecting Shh expression in the notochord. Finally, we find that ectopic BMP-7 appears to promote growth of the neurectoderm. These findings are discussed with respect to possible timing mechanisms necessary for the coordination of hindb rain dorsoventral patterning.