SEA URCHIN REPORT (Week 2)
William Yin
Steve Droho
Thursday Lab
INTRODUCTION
Sea urchins normally produce millions of gametes for external fertilization and are
commonly used in the laboratory setting for their ease in inducing the spawning of
gametes for fertilization experiments. Characteristics of early fertilization include the
blistering of the fertilization membrane and the subsequent complete formation of the
fertilization envelope. First cleavage occurs typically within an hour and the cells in the
embryo rapidly divide into motile blastulae within a day; by the end of the second day,
the embryo has undergone gastrulation to become a pluteus larva. Calcium ions play a
role in activating the egg and setting in the progress of events that will lead to cleavage,
starting first with the fusion of cortical granules to the plasma membrane. The purpose of
this lab was to conduct and observe the fertilization of sea urchin eggs and explore the
role of calcium in fertilization, as well as observe early urchin embryos in various stages.
RESULTS
Fertilization Reaction
The fertilization reaction begins with the sperm surrounding the urchin egg and making
contact. Shortly after the addition of sperm, each within 3 minutes on three trials, the
sperm entrance point developed the expected blister from the vitelline membrane. The
vitelline membrane became an ellipsis around the circular egg, with the thickest area (the
blister) at the presumed location of the sperm entry point. The completion of the
fertilization membrane took an additional 5 to 8 minutes, and was continuous, with the
thinner regions of the membrane gradually expanding to the thickness of the blister.
Role of Calcium
Figure 1. Drawings of the sea urchin gametes and the extra-embryonic material surrounding the egg. A:
sperm; B: unfertilized eggs; C: jelly coat.
Figure 2. Process of the full fertilization reaction. A: egg before sperm addition; B: early stages; C:
completed fertilization reaction; D: 2-cell and 4-cell stage embryos.
Figure 3. 2 day old urchins. A: young blastula; B: Pluteus larva
DISCUSSION
Roles of Calcium
The five of the events other than the cortical reaction that occur during egg activation
could be: changes in egg structure (formation of the blister and FE), several-fold increase
in protein synthesis, explosive release of Ca ions within the egg, producing a wave of
calcium that travels across it which triggered the egg activation, the egg which has been
blocked at a stage in meiosis, now complete meiosis in egg activation, whereupon the egg
and sperm nuclei fuse to form the diploid zygotic genome, and the fertilized egg enters
mitosis.
I do not think the spindles form because our observations didn't indicate any cleavage in
the cell; besides, the centrioles are sperm contributed and they are used to anchor the
mitotic spindle, if we don't have sperm entering the egg, the centrioles are then absent
and will not be able to carry out normal cleavage by anchoring the mitotic spindles.