The cell lineages of two types of embryo and a hermaphroditic gonad in dicyemid mesozoans.

The present review summarizes the patterns of cell division and the cell lineages of the two types of embryo, namely, the vermiform embryo and infusoriform embryo, of the dicyemid mesozoan Dicyema japonicum. The infusoriform embryo develops from a fertilized egg. The early cleavages are holoblastic and spiral. At around the 20- to 24-cell stage, cleavages become asynchronous and the cleavage pattern changes from spiral to bilateral. The fully formed infusoriform embryo consists of 37 cells and exhibits bilateral symmetry. These 37 cells are produced after only four to eight rounds of cell division. The vermiform embryo develops asexually from an agamete. Cell division proceeds spirally in the early stages, becoming bilateral from the fifth cell division onward. The fully formed vermiform embryo consists of 23 cells and exhibits bilateral symmetry. The 23 cells are produced after only four to six rounds of cell division. During the development of both types of embryo, a few cells consistently undergo extremely unequal divisions; the much smaller daughter cells eventually degenerate and ultimately disappear during embryogenesis. This disappearance may be an example of programmed cell death. The cell lineages appear to be invariant among embryos, apart from the derivation of the lateral cells of infusoriform embryos. In addition to a description of embryogenesis, the present review includes details of development of the functionally hermaphroditic gonad, the infusorigen, and gametogenesis. The infusorigen is formed from an agamete. The first spermatogonium is produced by the second division and the first oogonium is produced by the third division. Oogonia occupy the outer surface of the axial cell of the infusorigen, while spermatogonia are incorporated into the cytoplasm of the axial cell of the infusorigen. The infusorigen consists finally of approximately 20 cells.