ROSTRO-CAUDAL POLARITY IN THE AVIAN SOMITE RELATED TO PARAXIAL SEGMENTATION - A STUDY ON HNK-1, TENASCIN AND NEUROFILAMENT EXPRESSION

Segmental organization of the vertebrate body is one of the major patt erns arising during embryonic development. Somites that play an import ant role in this process show intrinsic patterns of gene expression an d differentiation. The somites become polarized in all three dimension s, rostrocaudal, mediolateral and dorsoventral, the quadrants giving r ise to several tissue components. The timing of polarization was studi ed by means of antibodies against HNK-1, tenascin and neurofilament. W hole mounts and serial sections of quail and chick embryos show that s omites are already polarized at the moment of their segregation from t he segmental plate. The rostral hemisomite carries the HNK-1 epitope p referentially, while the caudal hemisomite stains more strongly for te nascin. HNK-1-stained areas in the segmental plate strongly relate to the notochordal sheath, suggesting that axial structures determine the fate of paraxial structures. Neural crest cells were only seen to col onize the rostral part of a somite after they had differentiated into HNK-1 positive cells. Their colonization pattern seems to be guided by the segmental organization of the somite. Moreover, this somite organ ization probably dictates the organization of both sensory and motor f ibres converging towards the segmental dorsal root ganglia, justifying a shift in the connections between neural tube and somites. This segm ental shift takes place over one quarter of a somite length in a rostr al direction.