Just six hours after fertilization, a sand dollar (Dendraster eccentricus) egg has become a busily dividing embryo. The embryo is in the early blastula stage consisting of nearly 128 cells, most of them in the metaphase of their eighth mitotic division. However, a small cluster of cells, located in the upper right corner of the image, are on a different schedule. Among them, the four smallest cells at the center of the cluster are in the prophase of the last division they will make before the embryo becomes a larva. They will eventually give rise to the sand dollar germ cells. The cells that immediately surround them have just completed their sixth mitosis and will play an important role in various developmental processes, such as gastrulation and differentiation, and will ultimately be involved in the development of the calcite skeleton.
This image shows approximately one-third of an early blastula of the sand dollar (Dendraster eccentricus), stained for microtubules and projected from serial optical sections obtained via confocal microscopy.
George von Dassow PhD, Oregon Institute for Marine Biology, Charleston, OR