Wednesday, May 18, 2011

Spiral cleavage in Calliostoma

These pictures show three views of a 16-celled embryo of the marine gastropod Calliostoma ligatum which displays equal spiral cleavage. Spiral cleavage is a stereotypical cleavage pattern present in many phyla, including Nemertea, Annelida, and Mollusca. Collectively, these phyla are referred to as spiralians. Because of how conserved their early development is scientists can identify homologous body parts of these animals! Spiral cleavage got its name from the way the blastomeres are arranged after the four-cell stage. The four cells divide at oblique angles with respect to the animal-vegetal axis of the egg.

The division from the 4 to 8 cells is typically dextral (clockwise), the next is sinistral (counter-clockwise). Dextral and sinistral divisions alternate creating the “spiral” pattern. Because the position of cells is well conserved, cells receive names in spiralian embryos. The 8 cells in focus on the top image (animal view), are the daughters of the first-quartet micromeres.

The second image (vegetal view) shows the four macromeres in focus. The cells are not necessarily named for their size (although the mircomeres are smaller in this particular embryo) but rather location; the micromeres (and their descendants) are at the animal pole, while the macromeres are always at the vegetal pole.

The bottom image (lateral view) shows the stereotypical “twist” of the blastomeres. You can follow a lineage of cells by zig-zagging up from the vegetal pole. The most in-focus (and largest) cell is the macromere. The cell at 1 o’ clock is its sister, the second-quartet micromere. The two cells above and to the left (at 11 and 12 o’ clock) are the daughters of the first-quartet micromere (generated in the previous division from 4 to 8 cells) in the same quadrant. See veliger larva of Calliostoma ligatum.

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