Echinoderms - Cleavage

Although cleavage is radial and holoblastic in sea urchin embryos, not all divisions are symmmetric. Although sea urchin embryos undergo radial cleavage, the cleavages are decidely unequal at the fourth cleavage, when the vegetal tier of blastomeres undergoes equatorial cleavages to produce four micromeres at the extreme vegetal pole, and four large macromeres. The animal blastomeres divide symmetically to form 8 mesomeres. Micromere derivatives form skeletogenic, or primary, mesenchyme later in development.




16-cell L. variegatus zygote, viewed from the side (left) and from the vegetal pole (right). Micromeres form at the vegetal pole, macromeres form immediately above them, and eight equal-sized mesomeres form at the animal pole.

Image courtesy of George von Dassow, Center for Cell Dynamics, University of Washington. In order to divide asymmetrically, the vegetal blastomeres at the 8-cell stage must position their mitotic spindles near their vegetal ends, so that the division results in a macromere and a micromere. The image at left, which shows the microtubules of the mitotic spindle at the 4th cleavage, shows how asymmetric this division is.

Once the blastomere at the 8-cell stage are formed, the vegetal cells are "hard-wired" to divide asymmetrically. The cells on the right were formed from cells isolated at the 8-cell cell stage. Can you guess which came from a vegetal blastomere, and which came from an animal blastomere? L. variegatus blastomeres isolated at the 8-cell stage. The pair of daughter cells on the left are a macromere and a micromere; those on the right are two mesomeres (micrograph by Jeff Hardin, Univ. of Wisconsin)

Click on the thumbnails below to see three movies. The first two (left, center) are movies of micromere formation, courtesy of George von Dassow, Center for Cell Dynamics, Univ. of Washington. On the left, a time lapse movie of micromere formation in S. purpuratus (the vegetal pole is towards the left in this movie); center, a rotational view of a late 8-cell embryo stained to visualize tubulin. Note how the spindles are skewed towards the extreme vegetal pole in the vegetal quartet of blastomeres.

The third movie, courtesy of Charles Shuster, New Mexico State Univ., is a vegetal view of micromere formation at the fourth and fifth cleavages in Lytechnius pictus.