Cleavage patterns, cell lineages, and development of a cytoplasmic bridge system in Volvox embryos

We report an extensive scanning electron microscope (SEM) study of cleavage planes, cell shape changes, and cell lineages during cleavage of the asexual embryo of Volvox carteri f. nagariensis. Although our data generally confirm the basic description of cleavage developed by others using light microscopy, there is one important exception. We observed that the fourth cleavage plane is much more oblique than had previously been recognized. We show that, as a result, the four tiers of cells in the 16-cell embryo overlap extensively, and the new generation of asexual reproductive cells, or gonidia, are derived from three of these tiers (rather than two, as previously believed). Our study focused on the development of the highly organized system of cytoplasmic bridges that appears during cleavage. Hundreds of cytoplasmic bridges are formed in each division cycle as a result of incomplete cytokinesis. Existing bridges are conserved and divided between daughter cells while new bridges are formed at each division. Hence, the number of bridges per embryo increases regularly even though the number per cell declines from the fourth cleavage on. The bridges are organized into bands that girdle the cells at a predictable level and exhibit a regular 500-nm interbridge spacing; bridge bands of adjacent cells are in register and form a structural continuum throughout the embryo which we term "The cytoplasmic bridge system." The only place where bridges are not present is along a pair of intersecting slits, called the phialopore. We describe in detail the development of this bridge-free region.