Uncoupling segmentation and somitogenesis in the chick presomitic mesoderm.

Little is known about the tissue interactions and the molecular signals implicated in the sequence of events leading to the subdivision of the somite into its rostral and caudal compartments. It has been demonstrated that rostrocaudal identity of the sclerotome is acquired at the presomitic (PSM) level. However, it is not known whether this compartment specification is fully determined in the PSM or whether it is dependent upon maintenance cues from the surrounding environment, as is the case for somite epithelialization. In this report, we address this issue by examining the expression profiles of C-Delta-1 and C-Notch-1, the avian homologues of mouse Delta-like1 (Delta1) and Notch1 which have been implicated in the specification of the somite rostrocaudal polarity in mouse. In chick, these genes are expressed in distinct but partially overlapping domains in the PSM and subsequently in the caudal regions of the somites. We have used an in vitro assay that consists of culturina PSM explants to examine the regulation of these genes in this tissue. We find that PSM explants cultured without overlying ectoderm continue to lay down stripes of C-Delta-1 expression, although epithelialization is blocked. These results suggest that somite rostrocaudal patterning is an autonomous property of the PSM. In addition, they demonstrate that segmentation is not necessarily coupled with the formation of somites.