GALIS, F.*; VAN DOOREN, T.J.M.; METZ , J.A.J.; WITKAM , A.; WIJNAENDTS, L.C.D.; Leiden University, The Netherlands; idem; idem; idem; Free University Medical Centre, Amsterdam: The coordinated patterning of the embryonic axes and the conservation of the seven cervical vertebrae

The number of cervical vertebrae is extremely conserved in mammals. Yet, studies on fetal deaths in humans show that new mutants with an effect on this number are common. This indicates that evolutionary conservation is caused by strong stabilizing selection. The cause for the selection appears to be that mutations with an effect on the number of cervical vertebrae almost invariably lead to deleterious pleiotropic effects in other parts of the body. Changes in the number of cervical vertebrae usually involve homeotic shifts of several cervical and thoracic vertebrae. These shifts are due to changes in the early antero-posterior (A-P) patterning of the paraxial mesoderm. Normal metazoan development requires the coordinated patterning of the embryonic axes in all germlayers. Our working hypothesis is that the high interactivity during the early patterning of the axes lies at the core of the conservation of the number of cervical vertebrae. We will discuss to which extent data on human fetuses support our working hypothesis.