Development of the mesendoderm in the dendrobranchiate shrimp Sicyonia ingentis.

Dendrobranchiate shrimp embryos form a 4-cell stage that resembles spiralians in its cell contacts, but cleavage proceeds radially without any further evidence of spiralian character. The fates of Sicyonia ingentis mesendoblasts were followed by nuclear staining and confocal microscopy. The dorsal mesendoblast produced yolk-endoderm, which proliferated from the anterior of the embryo to cover the dorsal interior. The ventral mesendoblast divided into the primordial endoblast and a cell that further divided into the primordial mesoteloblast and the primordial germ cell. The primordial endoblast divided into left and right endoblasts, which then underwent two teloblastic divisions, leaving behind two pairs of smaller descendants and the larger endoblasts at the dorsal posterior. The endoblasts then paused in cell division while extensive morphogenesis occurred in the ectoderm to form the naupliar segments. Ectoteloblasts formed at the posterior. The primordial mesoteloblast underwent two asymmetric divisions, synchronously with the endoblasts, to form two small descendants and M2 at the ventral posterior. From 15 to 18h, M2 divided laterally then dorsal-ventrally to form four descendants. The results extend the cell lineage of S. ingentis from the egg to the nauplius larva, and demonstrate that endoderm forms dorsal to teloblastic mesoderm from an early stage.