Sea Urchin Embryonic Development (time lapse) Video
This series of video clips shows selected important events in sea urchin embryonic development. 1) The unfertilized egg is about 100 micrometers (µm) in diameter, similar to that of humans, and is surrounded by an extracellular layer called the vitelline layer. Upon fertilization by the first sperm, the vitelline layer becomes raised off the surface of the egg and hardens, forming the protective structure known as the fertilization envelope. All cleavages up to the blastula stage occur within this envelope. 2) During first cleavage, the nuclear envelope breaks down, and the duplicated chromosomes separate into two complete sets, followed by cytokinesis. In the two new cells, or blastomeres, you can clearly see the two new nuclei. 3) Second cleavage, progressing from 2 to 4 cells, is seen here. Cleavages will proceed synchronously, approximately every 30 minutes, passing through the morula stage (16-64 cells) when the cells are loosely attached to each other, up to the blastula stage (more than 128 cells). 4) The blastula stage is seen at the end of this clip. This stage is made up of a hollow ball of 1000 or so cells, arranged in a single-layered epithelium. The cells are tightly packed together, maintaining a space in the center called the blastocoel cavity. 5) At the beginning of gastrulation, a number of cells in the flattened "vegetal pole," shown here at the bottom of the embryo, move as individual cells into the blastocoel cavity. In this cavity the cells migrate around, fuse with each other in a ring, and begin secreting elements of the calcium carbonate skeleton of the embryo. Because these cells are the first to move as individual cells in the embryo, they are called the primary mesenchyme cells (PMCs). The remaining cells in the vegetal pole fill in the gaps, restoring a complete epithelial sheet. 6) While the PMCs are migrating around, archenteron formation, or formation of the embryonic digestive tract, begins. The first stage involves the pushing in of the vegetal pole to form a short, wide, blind-ended tube. 7) This tube then narrows and elongates by a process that includes extensive cell rearrangement. Following this elongation, a subset of cells (secondary mesenchyme cells) at the tip of the archenteron will extend processes that contact a specific site on the inside of the ectodermal wall and tow the archenteron toward that spot. The wall of the ectoderm will bend inward and fuse with the tip of the archenteron to form the mouth. The digestive tract will differentiate into an esophagus, a stomach, and an intestine, and the embryo will begin to feed. Four to 8 or 12 arms will extend, supported by internal skeletal elements. This feeding larva will float around in the plankton, eating algal cells, for 5 or 6 weeks, then will metamorphose into the adult form of the sea urchin.
Credit: Rachel Fink, editor, "A Dozen Eggs," Society for Developmental Biology