The Questions of Developmental Biology

6. In many species, eggs secrete diffusible molecules that attract and activate the sperm.
7. In sea urchins, the acrosome reaction is initiated by compounds in the egg jelly. The acrosomal
vesicle undergoes exocytosis to release its enzymes. Globular actin polymerizes to extend the
acrosomal process. Bindin on the acrosomal process is recognized by a protein complex on the
sea urchin egg surface.
8. In mammals, sperm must be capacitated in the female reproductive tract before they are
capable of fertilizing the egg.
9. Mammalian sperm bind to the zona pellucida before undergoing the acrosome reaction. In the
mouse, this binding is mediated by ZP3 (zona protein 3) and one or many sperm proteins that
recognize it. The mammalian acrosome reaction is initiated on the zona pellucida, and the
acrosomal enzymes are concentrated there.
10. Fusion between sperm and egg is mediated by protein molecules whose hydrophobic groups
can merge the sperm and egg plasma membranes. In sea urchins, bindin may mediate gamete
fusion. In mammals, fertilin proteins in the sperm bind to integrins in the egg and allow the
membranes to fuse.
11. Polyspermy results when two sperm fertilize the egg. It is usually lethal, since it results in
three sets of chromosomes divided among four cells.
12. There are often two blocks to polyspermy. The fast block is electrical and is mediated by
sodium ions: the egg membrane resting potential rises, and sperm can no longer fuse with the egg.
The slow block is physical and is mediated by calcium ions. A wave of calcium ions propagates
from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane.
The released contents of the granules cause the vitelline membrane to rise and to harden into the
fertilization envelope.
13. In mammals, blocks to polyspermy include the modification of the zona proteins by the
contents of the cortical granules. Sperm can no longer bind to the zona.
14. Inositol 1,4,5-triphosphate (IP 3 ) is believed to be responsible for releasing calcium ions from
storage in the endoplasmic reticulum. DAG (diacylglycerol) is thought to initiate the rise in egg
pH. The free calcium ions, supported by the alkalization of the egg, activate egg metabolism,
protein synthesis, and DNA synthesis.
15. The male pronucleus and the female pronucleus migrate toward each other, replicating DNA
as they move.
16. In sea urchins, the two pronuclei merge and a diploid zygote nucleus is formed. In mammals,
the pronuclei disintegrate as they approach each other, and their chromosomes gather around a
common metaphase plate.
17. Some genes are transmitted differently depending on whether they are from the egg or the
sperm. Methylation differences determine if these genes are to be expressed in the early embryo.
18. Microtubular changes cause cytoplasmic movements. These rearrangements of cytoplasm can
be critical in specifying which portions of the egg are going to develop into which organs.