Biology Forums - Study Force

Describe the events that must occur in order for sperm to penetrate an egg. What prevents more than one sperm from entering? In the case of twins, how many sperm entered the egg? Why must a man have millions of sperm in a semen sample to be considered fertile? If only one sperm were present in the semen and that sperm managed to find its way to the egg, could it fertilize the egg? Explain.

The sperm must penetrate the corona radiata and the zona pellucida, which surround the egg, in order to get to its membrane. The acrosome of the sperm contains enzymes that disrupt these barriers. The first sperm to reach the membrane binds to a receptor, the sperm and egg membranes fuse, and the sperm nucleus enters the egg. The cortical reaction ensues, in which granules in the egg spill their contents out of the membrane, which alters it in a way preventing additional sperm from binding. Twins result from normal fertilization, but either two eggs are present or the early embryo splits. Only about 100 sperm manage to find the egg, with the other millions swimming off in the wrong direction. One sperm probably does not contain enough enzymes to allow penetration of the egg barriers.

as what bio man said...a sperm first reaches the egg and must "squeeze" through the cells covering the egg left over from the follice. As it reaches the jelly-like layer, the acrosome (package of enzymes at sperm's head) releases breakdown enzymes that "eat away" or dissolve the jelly. the sperm thus reaches the vitelline layer of the egg where the sperm and egg membrane joins causing the egg membrane to depolarize and reject all other incoming sperm. once the membranes fuse, the sperm releases its nucleus which then fuses with the egg's nucleus to form a zygote nucleus

Thank a lot guys!

In regards to twins:

Dizygotic (fraternal) twins are produced from ther release of two oocytes (female sex cells) and each is fertilized by different sperm. Twins may be of different sex, or the same sex. Genetically they are different.

Monozygotic (identical) twins is a result of one single ooctye which separates  the developing cells into two embryos. Separation usually occurs prior to 8 days after fertilization. They are always of the same sex and also contain the same genetic material.

Co-joined twins are identical twins where the cells have separated later than 8 days resulting in them sharing some body structures.

With sperm it is based on quantity, quality, and motility (the power of spontaneous movement) for classification of fertility.

If you have more than 39 million per/milliliter sperm in semen upon ejaculation you are considered fertile. A low sperm count is regarded as less than 20million per milliliter in semen upon ejaculation.

Quality is based on if 4% of your sperm is normal shape and structure, then you are considered fertile.

Motility is based on if 40% of the sperm is moving than you are regarded as fertile.

Strange, my fiancee was asking me about this a while a go. Thanks.