C2 高級 美國腔 811 分類 收藏
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
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


受孕 (Fertilization)

811 分類 收藏
hkraywai 發佈於 2020 年 2 月 19 日
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