Stages of Cell Development in a Sea Urchin

Radial Holoblastic Cleavage

• After fertilization the primary and secondary cleavages happen perpendicularly on the cell meridian creating four polarized sections. Tertiary cleavages divides the polarized sections into a two-tiered structure. The upper tier is the animal half and the lower tier is the vegetal half. The animal tier then divides equally into eight mesomeres (mid-sized cells) and the vegetal tier divides unequally into four macromeres (large-sized cells) and four micromeres (small-sized cells). Animal cells divide equally again leaving two tiers (an1, an2), the macromeres divide meridonally creating a single tire beneath an2 and the micromeres divide into a cluster beneath the macromeres. The sixth cleavages are all equatorial and the seventh division is meridional (divides on the meridian line), leaving a blastula (ball of embryonic cells) of 128 cells.

Blastula Formation

• At this point the cells form a cavity called a blastocoel in the center of the 128-cell cluster. Along with the blastocoel, surrounding cells tighten formation creating an epithelial sheet connecting them to the liquid in the blastocoel. Cell division continues to occur normally until the 9th or 10th division at which point new gene expressions can be seen. Nondividing cells often make cilia, the cells at the vegetal pole thicken making a plate and the cells at the animal pole create an enzyme that frees the embryo from confinement.

Cell Specialization

• By the 60-cell level the direction or fate of most cells has been established. It has been found that at this level, though most cells are already specialized, if moved via external manipulation these cells often will re-specialize according to their new location in the organism. Animal cells generally form larva skin and neurons, veg tier1 produces organ cells, and veg tier2 produces endoderm, body wall, pigment, immunocyte and muscle cells. Micromeres produce skeletal and endoderm cells.

Correlations in Science

• By reviewing the fertilized sea urchin egg and observing its development, the understanding of human embryos is increased significantly. Human gametes (reproductive cells) are similar to sea urchin gametes in size and cellular development into the gastrula (early embryonic) stage. Both deuterostomes, organisms which develop the mouth opening secondary to the anal opening, the urchin and human embryo mouth develops on the opposite side of gastrulation (cell specialization and mapping). The relationship between sea urchins and humans can be viewed on a classification chart.