Acellularity of starfish embryonic mesenchyme cells as shown in vitro

Embryonic mesenchyme cells of the starfish are shown to be unexpectedly fusogenic in vitro. When archenteron complexes (archenterons and varying portions of the extracellular matrix {ecm} surrounding them) are isolated from starfish embryos and inoculated in sea water containing 4% newborn bovine serum, the mesenchyme cells form large syncytia on the substratum underneath each archenteron. These syncytia break into smaller fragments interconnected by fine cell processes within 24h. These networks have been studied morphologically, dynamically and ultrastructurally and found to lack cell borders between the constituent fragments. These fragments contain various numbers of nuclei ranging from 0 to 6 or more and move about constantly over the substratum, sometimes breaking into two and sometimes fusing with neighbouring fragments, so that the overall pattern of the network changes constantly. Our results also indicate that the network is three-dimensional i.e. it has crossing sites, the frequency of which seems to depend on the amount of the ECM excreted on the substratum. A similar network pattern is found among mesenchyme cells in vivo, which suggests that the features found in vitro reflect those in vivo.