A simplified placenta-like system for the transport of extraembryonic nutrients during embryogenesis of Bugula neritina (bryozoa)

Authors

  • Robert M. Woollacott,

    1. Museum of Comparative Zoology Laboratories, Harvard University, Cambridge, Massachusetts 02138
    2. Santa Catalina Marine Biological Laboratory, University of Southern California, Avalon, California 90704
    Search for more papers by this author
  • Russel L. Zimmer

    1. Museum of Comparative Zoology Laboratories, Harvard University, Cambridge, Massachusetts 02138
    2. Santa Catalina Marine Biological Laboratory, University of Southern California, Avalon, California 90704
    Search for more papers by this author

  • Contribution No. 17 from the Santa Catalina Marine Biological Laboratory.

Abstract

Embryos of the marine cheilo-ctenostome bryozoan Bugula neritina undergo a marked increase in volume (about 500-fold) during embryogenesis while being retained in a brood chamber. Previous morphological studies indicate that shortly after transfer of the zygote to the brood chamber, the epithelium of the maternally-derived portion of the brood chamber, the ooecial vesicle, differentiates in regions adjacent to the embryonary space from a squamous to a columnar form suggesting that the parent is involved as a source of extraembryonic nutrients required for the extensive growth of the embryo.

Results of the present ultrastructural study indicate that hypertrophy of the epithelial cells occurs only in that region of the ooecial vesicle which opposes the embryo, that differentiation (and subsequent regression) of the lining are predictable events correlated with the onset (and termination) of embryonic growth, and that hypertrophied cells are well equipped for the synthesis and transport of macromolecular materials across the vesicle wall to the developing embryo. Further, that portion of the embryo's ectoderm (the presumptive metasomal sac) in contact with this hypertrophied epithelium is morphologically specialized for the uptake of nutrients. Finally, shortly before release of the larva, this intimate association of the metasomal sac tissue and the hypertrophied ooecial vesicle lining epithelium is terminated by invagination of the sac and atrophy of the lining.

Ancillary