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  • Fertilization is the epic story of a single sperm facing incredible odds

  • to unite with an egg and form a new human life.

  • It is the story of all of us.

  • During sexual intercourse, about 300 million sperm enter the vagina.

  • Soon afterward, millions of them will either flow out of the vagina,

  • or die in its acidic environment.

  • However, many survive because of the protective elements

  • provided in the fluid surrounding them.

  • Next, the sperm must pass through the cervix, an opening into the uterus.

  • Usually it remains tightly closed, but here the cervix is open for a few days

  • while the woman ovulates.

  • The sperm swim through the cervical mucus,

  • which has thinned to a more watery consistency for easier passage.

  • Once inside the cervix, the sperm continue swimming toward the uterus,

  • though millions will die trying to make it through the mucus.

  • Some sperm remain behind, caught in the folds of the cervix,

  • but they may later continue the journey as a backup to the first group.

  • Inside the uterus, muscular uterine contractions

  • assist the sperm on their journey toward the egg.

  • However, resident cells from the woman's immune system,

  • mistaking the sperm for foreign invaders,

  • destroy thousands more.

  • Next, half the sperm head for the empty fallopian tube,

  • while the other half swim toward the tube containing the unfertilized egg.

  • Now, only a few thousand remain.

  • Inside the fallopian tube, tiny cilia push the egg toward the uterus.

  • To continue, the sperm must surge against this motion to reach the egg.

  • Some sperm get trapped in the cilia and die.

  • During this part of the journey, chemicals in the reproductive tract

  • cause the membranes covering the heads of the sperm to change.

  • As a result, the sperm become hyperactive, swimming harder and faster toward their destination.

  • At long last, the sperm reach the egg.

  • Only a few dozen of the original 300 million sperm remain.

  • The egg is covered with a layer of cells called the corona radiata.

  • The sperm must push through this layer to reach the outer layer of the egg,

  • the zona pellucida.

  • When sperm reach the zona pellucida, they attach to specialized sperm receptors on the surface,

  • which triggers their acrosomes to release digestive enzymes,

  • enabling the sperm to burrow into the layer.

  • Inside the zona pellucida is a narrow, fluid-filled space just outside the egg cell membrane.

  • The first sperm to make contact will fertilize the egg.

  • After a perilous journey and against incredible odds, a single sperm attaches to the egg cell

  • membrane.

  • Within a few minutes, their outer membranes fuse, and the egg pulls the sperm inside.

  • This event causes changes in the egg membrane that prevent other sperm from attaching to

  • it.

  • Next, the egg releases chemicals that push other sperm away from the egg and create an

  • impenetrable fertilization membrane.

  • As the reaction spreads outward, the zona pellucida hardens, trapping any sperm unlucky

  • enough to be caught inside.

  • Outside the egg, sperm are no longer able to attach to the zona pellucida.

  • Meanwhile, inside the egg, the tightly packed male genetic material spreads out.

  • A new membrane forms around the genetic material, creating the male pronucleus.

  • Inside, the genetic material reforms into 23 chromosomes.

  • The female genetic material, awakened by the fusion of the sperm with the egg, finishes

  • dividing, resulting in the female pronucleus, which also contains twenty-three chromosomes.

  • As the male and female pronuclei form,

  • spiderweb-like threads, called microtubules,

  • pull them toward each other.

  • The two sets of chromosomes join together, completing the process of fertilization.

  • At this moment, a unique genetic code arises, instantly determining gender, hair color,

  • eye color and hundreds of other characteristics.

  • This new single cell, the zygote, is the beginning of a new human being.

  • And now the cilia in the fallopian tube gently sweep the zygote toward the uterus

  • where he or she will implant in the rich uterine lining, growing and maturing for the next nine months

  • until ready for birth.

Fertilization is the epic story of a single sperm facing incredible odds

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Fertilization (Conception)

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    keep seeing posted on 2014/04/18
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