The purpose of this lab was to observe and analyze the processes of fertilization and development in sea urchins. The procedure allowed the viewing of the physical processes of fertilization and cleavage. It also made it possible to later view blastulization, gastrulization, and further development and specialization. The fertilization process witnessed in this lab is typical of fertilization in all sexually reproducing animals. The sea urchin life cycle is almost entirely diploid, which means that all the nuclei in the somatic cells contain two copies of each chromosome. The process that is essentially the opposite of fertilization is meiosis. By this process, cells of a diploid somatic cell replicate their DNA, and each proceeds to divide by two separate processes into four haploid sperm (in males) or one haploid ova (in females).
The process of fertilization in sea urchins is physiologically complex, but conceptually simple. It begins as the sperm approaches the egg. At this point, the sperm has used almost all it’s energy, but has enough remaining to carry through the procedure of fertilization. As the head of the sperm comes in contact with the jelly layer of the egg, the acrosome in its head releases its collection of enzymes. The enzymes digest a cavity in the jelly layer of the ovum. The sperm head continues into the cavity and when it comes in contact with the vitelline layer, specific proteins on its head bind to receptor molecules on the vitelline layer. In species, like the sea urchin, with external fertilization, this step is key; the proteins are species-specific and hence prevent the fertilization of an egg of one species by the sperm of another.
After this initial phase, the sperm proceed through the vitelline layer. The plasma membrane of the sperm then fuses with that of the egg, combining the two cytoplasm. Less than one second after this process occurs, the plasma membrane of the egg cell becomes impermeable to other sperm, to prevent polyspermy. After this, the vitelline layer hardens and separates from the plasma membrane of the egg. The space between the vitelline layer and plasma membrane fills with water, and the vitelline layer becomes a fertilization membrane. Finally, the haploid nuclei of the sperm and egg fuse into a single, diploid zygote nucleus.
The ovum after fertilization has created the fertilization envelope.
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