PAX-1 IN THE DEVELOPMENT OF THE CERVICO-OCCIPITAL TRANSITIONAL ZONE

The Pax-1 gene has been found to play an important role in the develop ment of the vertebral column. The cervico-occipital transitional zone is a specialized region of the vertebral column, and malformations of this region have frequently been described in humans. The exact embryo nic border between head and trunk is a matter of controversy. In order to determine a possible role of Pax-1 in the development of the cervi co-occipital transitional zone we studied the expression of this gene in a series of quail embryos and murine fetuses with in situ hybridiza tion and immunohistochemistry. Pax-1 is expressed in all somites of th e embryo, including the first five occipital ones. During embryonic da ys 3-5 the gene is down-regulated in the caudal direction within the f irst five somites, whereas more caudally Pax-1 is strongly expressed i n the cells of the perinotochordal tube. In 5-day-old quail embryos, t he cartilaginous anlage of the basioccipital bone has developed and th ere is no more expression of Pax-1 in this region. The fusion of the d ens axis with the body of the axis also coincides with switching off o f the Pax-1 gene. More caudally, the gene is continuously expressed in the intervertebral discs of murine embryos and therefore seems to be important for the process of resegmentation. Quail embryos do not poss ess permanent intervertebral discs. ''Hyper-'' or ''hyposegmentation'' defects may be explained by an over- or under-expression of Pax-1 dur ing development. We also reinvestigated the border between the head an d trunk in chick embryos by performing homotopical grafting experiment s of the 5th somite between chick and quail embryos. Grafted quail cel ls formed mainly the caudal end of the basioccipital bone. They were a lso located in the cranial half of the ventral atlantic arch, and only a few cells were found in the tip of the dens axis.