Fertilization in a sea urchin and a starfish

During the first two weeks of class we focused on the early development of sea urchins (e.g. Strongylocentrotus purpuratus) and sea stars (e.g. Pisaster ochraceous), both in the phylum Echinodermata. By either physically shaking or injecting the adults with 0.53 M KCl, we encouraged the release of gametes and fertilized them to observe development. One of the first changes that can be seen following fertilization of echinoderm eggs is the formation of the fertilization envelope, a visible membrane surrounding a fertilized egg that acts as a physical barrier to prevent polyspermy (fertilization by multiple sperm). It forms from the vitelline layer as it lifts off the egg plasma membrane and is hardened by the enzymes released by cortical granules. This photo shows two eggs of a purple sea urchin, Strongylocentrotus purpuratus - one fertilized (and surrounded by the fertilization envelope), and one unfertilized (without the envelope).

One of the largest noticeable differences between sea stars and sea urchins in early development is the formation of polar bodies. A polar body is a tiny sister-cell of the primary oocyte, produced during meiosis. It contains discarded DNA, and very little of anything else. Polar bodies are not usually observed in sea urchin, because meiosis is completed within the ovary, and spawned eggs have already parted with their polar bodes.
However, we were able observe polar bodies in sea stars. This is because in sea stars, sperm entry occurs before the oocytes have completed meiosis (cell division, reducing the number of chromosomes). Polar bodies form after fertilization and are trapped within the fertilization envelope. The photos here show an immature unfertilized oocyte (with a large nucleus and a nucleolus inside) and a fertilized secondary oocyte with homogenenous cytoplasm, a tight fertilization envelope around it, and one polar body (at about 5 o'clock), in the ochre sea star Pisaster ochraceous. The fertilization envelope in starfish is much closer to the surface of the egg than in sea urchins.