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

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, namely the placenta. This represents a functional substitution, or exaptation, a change in the function of an organ that originated in connection with another function. The development of the placenta in cyclostome bryozoans involves transdifferentiation and fusion of peritoneal cells of the membranous sac into a trophic syncytium. In addition to peritoneal cells, follicular cells of the ovary can also be potentially involved. The process of formation of the trophic syncytium is accompanied by its rapid growth and multiple division of nuclei, and generally resembles the formation of benign tumors. Such “hyperplasia” enables the formation of a large placental structure in a very short time, providing nutrition, respiration, and removal of metabolic wastes 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, being examples of convergent evolution. In this paper, we studied the structure of the placental analogue in the cyclostome bryozoans Crisia eburnea and Crisiella producta, which is another example of a placental syncytium formed by maternal cells.