Anteroposterior patterning is required within segments for somite boundaryformation in developing zebrafish

Somite formation involves the establishment of a segmental prepattern in th e presomitic mesoderm, anteroposterior patterning of each segmental primord ium and formation of boundaries between adjacent segments. How these events are co-ordinated remains uncertain. In this study, analysis of expression of zebrafish mesp-a reveals that each segment acquires anteroposterior regi onalisation when located in the anterior presomitic mesoderm, Thus anteropo sterior patterning is occurring after the establishment of a segmental prep attern in the paraxial mesoderm and prior to somite boundary formation. Zebrafish fss(-), bea(-), des(-) and aei(-) embryos all fail to form somite s, yet we demonstrate that a segmental prepattern is established in the pre somitic mesoderm of all these mutants and hox gene expression shows that ov erall anteroposterior patterning of the mesoderm is also normal. However, a nalysis of various molecular markers reveals that anteroposterior regionali sation within each segment is disturbed in the mutants. In fss(-), there is a loss of anterior segment markers, such that all segments appear posterio rized, whereas in bea(-), des(-) and aei(-), anterior and posterior markers are expressed throughout each segment. Since somite formation is disrupted in these mutants, correct anteroposterior patterning within segments may b e a prerequisite for somite boundary formation. In support of this hypothes is, we show that it is possible to rescue boundary formation in fss(-) thro ugh the ectopic expression of EphA4, an anterior segment marker, in the par axial mesoderm, These observations indicate that a key consequence of the a nteroposterior regionalisation of segments may be the induction of Eph and ephrin expression at segment interfaces and that Eph/ephrin signalling subs equently contributes to the formation of somite boundaries.