Ultrastructure of mesoderm formation and development in Membranipora membranacea (Bryozoa: Gymnolaemata)
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Mesoderm origin in Bryozoa is largely unknown. In this study, embryonic and early larval stages of Membranipora membranacea, a bryozoan exhibiting a planktotrophic cyphonautes larva, are investigated using mainly ultrastructural techniques. Shortly after the onset of gastrulation, an ectodermal cell, which is situated centrally at the prospective anterior pole of the larva, can be recognized by its constricted apical surface and enlarged basal part. It is also distinct from other ectodermal cells by the composition of its cytoplasm. In later stages, it has left the epidermis, lost its epithelial character, and is situated subepithelially, between the basal sides of the ectodermal and endodermal sheets. A blastocoelic cavity is not present at this stage. This cell divides and gives rise to a group of cells forming a muscular and neuronal strand at the anterior side of the larva. The majority of the larval musculature originates from this ingression. Despite this evidence for an ectodermal origin, additional sources of mesoderm can so far not be excluded. The literature on mesoderm origin in Bryozoa is reviewed and the results are compared to known data from other metazoan taxa.