Early Stages of the Placental Analogue Formation in Crisiid Bryozoans ()

The placental analogue of cyclostome bryozoans is an example of the transformation of a mechanical structure (membranous sac), which initially performs a hydrostatic function, into a trophic organ. This is an example of functional substitution, or exaptation, a change in the function of an organ that originated in connection with another function. The formation of the placenta in cyclostome bryozoans is carried out due to transdifferentiation and fusion of peritoneal cells of the membranous sac into a trophic syncytium. It is also possible that another source of cells are follicular cells of the ovary. The process of formation of the trophic syncytium, accompanied by its rapid growth and multiple division of nuclei, generally resembles the formation of benign tumors. Such “hyperplasia” allows formation of a large placental structure in a very short period of time, providing nutrition, respiration and removal of metabolic products of numerous embryos. Syncytia formed by embryonic cells are characteristic of mammalian placentas, while syncytia formed by maternal cells are involved in the formation of placentas in bandicoots, skinks, salps, and some onychophorans, serving as examples of convergent evolution. In this paper, we studied the structure of the placental analogue in the cyclostomed bryozoans Crisia eburnea and Crisiella producta, which is another example of a placental syncytium formed by maternal cells.