Early cleavage in echinoderms

Early development in echinoderm embryos follows a regular and predictable pattern. The first and second cleavages are equal and meridional (cut along the animal-vegetal axis of the egg). The third cleavage is equal and equatorial, i.e. perpendicular to the plane of the first two divisions, resulting in two tiers of 4 equal-sized cells. The nature of the fourth cleavage (from 8 to 16 cells) differs between echinoids and asteroids.

The first picture shows a side view of a 16-cell stage of the sea star Pisaster ochraceus (animal pole, marked by the two polar bodies, is at about two o’clock). As you can see here, all cells (12 of 16 are visible in this focal plane) are equal in size.

The second picture shows a side view of a 16-celled embryo of the sand dollar Dendraster excentricus (animal pole up). As in most other echinoids, the four cells of the animal pole tier divided equally and meridionally, producing a tier of eight equal-sized cells (called mesomeres). Four mesomeres are clearly visible in this focal plane. The blastomeres of the vegetal tier divided equatorially and unequally producing four smaller cells at the vegetal pole, called micromeres (two are visible), and four larger cells called macromeres (two are in focus). The micromeres of echinoids give rise to the larval skeletogenic cells. In comparison, asteroids, have no micromeres and no larval skeletal spicules (see a post by Nick Hayman).

Other differences in early development of the two classes are also apparent here. In the asteroids, developing oocytes in the adult ovary are arrested at prophase I. Meiosis is normally completed after fertilization (which traps the polar bodies inside the fertilization envelope). This is why you can see the polar bodies in the top picture. On the other hand, in echinoids oocytes complete meiosis in the ovary (prior to fertilization), thus no polar bodies can be seen inside the fertilization envelope.