Fertilization (Conception)

Fertilization is the epic story of a single *** facing incredible odds to unite with an egg and form a new human life. It is the story of all of us. During *** intercourse, about 300 million *** enter the ***. Soon afterward, millions of them will either flow out of the ***, or die in its acidic environment. However, many survive because of the protective elements provided in the fluid surrounding them. Next, the *** must pass through the ***, an opening into the uterus. Usually it remains tightly closed, but here the *** is open for a few days while the woman ovulates. The *** swim through the cervical mucus, which has thinned to a more watery consistency for easier passage. Once inside the ***, the *** continue swimming toward the uterus, though millions will die trying to make it through the mucus. Some *** remain behind, caught in the folds of the ***, but they may later continue the journey as a backup to the first group. Inside the uterus, muscular uterine contractions assist the *** on their journey toward the egg. However, resident cells from the woman's immune system, mistaking the *** for foreign invaders, destroy thousands more. Next, half the *** 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 *** must surge against this motion to reach the egg. Some *** 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 *** to change. As a result, the *** become hyperactive, swimming harder and faster toward their destination. At long last, the *** reach the egg. Only a few dozen of the original 300 million *** remain. The egg is covered with a layer of cells called the corona radiata. The *** must push through this layer to reach the outer layer of the egg, the zona pellucida. When *** reach the zona pellucida, they attach to specialized *** receptors on the surface, which triggers their acrosomes to release digestive enzymes, enabling the *** to burrow into the layer. Inside the zona pellucida is a narrow, fluid-filled space just outside the egg cell membrane. The first *** to make contact will fertilize the egg. After a perilous journey and against incredible odds, a single *** attaches to the egg cell membrane. Within a few minutes, their outer membranes fuse, and the egg pulls the *** inside. This event causes changes in the egg membrane that prevent other *** from attaching to it. Next, the egg releases chemicals that push other *** away from the egg and create an impenetrable fertilization membrane. As the reaction spreads outward, the zona pellucida hardens, trapping any *** unlucky enough to be caught inside. Outside the egg, *** 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 *** 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.