Coelomic pouch formation in reconstructing embryos of the starfish Asterina pectinifera

The morphogenetic processes of coelomic pouch (CP) formation in starfish embryos that were experimentally dissociated and induced to undergo reconstruction were studied. An analysis of these embryos randomly chosen from several cultures showed that CP always form on either side of the esophagus, even though the CP formation can differ in timing of initiation and duration, and can vary in number and size from embryo to embryo. Successive observations of CP formation in living embryos revealed two distinct sequences of CP development that were accompanied by different appearances of the blastocoele. These processes were named 'enterocoelic- like' and 'schizocoelic-like' CP formation. The former resembled normal development and occurred in embryos with a transparent blastocoele. The latter was characterized by the aggregation and epithelialization of mesenchyme-like cells on either side of the esophagus and was observed in embryos possessing a cloudy blastocoele. In a few embryos, both types of CP formation were seen in the same individual ('mosaic type' CP formation). Thick sections of embryos possessing a cloudy blastocoele revealed that aggregates of mesenchyme-like cells undergoing CP formation directly contact the developing esophagus. Together, these data demonstrate flexibility in the morphogenetic processes that regulate CP formation, and suggest that positional cues in the esophagus regulate the placement of CP.