字幕列表 影片播放
-
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 and 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 is 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 women'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 pushed 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 reached 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 pushed through this layer to reach the outer layer of the egg the Zona Pellucida
-
When sperm reached 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 23 chromosomes
-
as the male and female pronuclei form
-
spider web-like threads called Microtubules pull them toward each other
-
the two sets of Chromosomes joined 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 and the Fallopian Tube gently sweep the Zygote toward the uterus
-
Where he or she will implant in the richer uterine lining
-
Growing and maturing for the next nine months until ready for birth