arcanegirl
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How Do the Egg and the Sperm Reach the Fallopian Tube?
The journey of the egg, or ovum, through the fallopian tube and finally into the uterus after fertilization is extraordinarily hazardous. The woman's tube is not simply a passive channel through which the egg is transferred; many events must work in precise synchrony for successful pregnancy to occur.
There are, on the surface of the fimbria, microscopic hairs called cilia, which constantly beat in the direction of the uterus at a fantastically rapid speed and create a kind of conveyor-belt effect for moving the egg into the tube and toward the uterus. The cilia work this magic by digging into the sticky gel, called the cumulus oophorus, that surrounds the egg, and they transport this whole sticky, gooey mass. The egg itself is invisible to the naked eye, but the gel that envelops it is easily visible. If this sticky substance were not present, and the egg were placed bare upon the surface of the fimbria, the beating of the cilia would never move the egg along. The cilia are only able to dig in and transport the egg with this sticky, gooey material encasing it.
The process of grasping the egg and moving it into the interior of the tube requires only about fifteen to twenty seconds. Once the egg is safely within the tube, it is transported quickly toward the narrower region of the tube, the ampullary-isthmic junction, located two-thirds of the way toward the uterus. Here, the egg must wait for a successful sperm coming from the opposite direction to fight its way into the egg's tough outer membrane, the zona pellucida, score a direct hit, and thereby establish pregnancy. While the egg is held in this location by the tight resistance of the narrow region of the tube, the much tinier sperm nonetheless must struggle through this area of resistance to arrive from the opposite direction. Once it is fertilized, the egg must be nourished for several days in the ampulla of the tube before it can be allowed to pass into the uterus. If it is transferred into the uterus too soon, it will not be ready to implant, and it will die. If the transfer of the egg into the uterus is delayed too long, a tubal, or ectopic, pregnancy will occur (the fertilized egg will implant in the tube rather than the womb). Once the egg has been allowed to develop in the tube for three or more days, the isthmus suddenly opens up and the early embryo passes quickly into the uterus. Because the journey of the egg from the ovary to the site of fertilization, its nourishment in the tube, and the precise synchrony of the continuation of its journey into the womb are so intricate, problems with this egg and embryo transport process are frequently responsible for female infertility.
If the egg is not penetrated by sperm soon after ovulation, it becomes overripe and dies. After the egg is released from the ovary, it is only capable of fertilization for about twelve, or possibly at most twenty-four, hours. The likelihood of intercourse taking place during such a specific interval in any month is rather slight. So nature must provide some mechanism for providing a continuous flow of healthy sperm to the site of fertilization. That way, if intercourse is perhaps one or two days off schedule, some sperm can still arrive at the site of fertilization at the right time. For this reason, complicated barriers to sperm transport are necessary.
The success of IVF demonstrates that if eggs can be recovered at precisely the right time, they can be fertilized in the laboratory with only a small number of sperm. Then the complicated barrier mechanisms provided by nature to allow a slow, continuing flow of a small number of sperm at any moment is not necessary and the large numbers of sperm normally required for fertilization through intercourse are not needed.
The journey of the egg, or ovum, through the fallopian tube and finally into the uterus after fertilization is extraordinarily hazardous. The woman's tube is not simply a passive channel through which the egg is transferred; many events must work in precise synchrony for successful pregnancy to occur.
There are, on the surface of the fimbria, microscopic hairs called cilia, which constantly beat in the direction of the uterus at a fantastically rapid speed and create a kind of conveyor-belt effect for moving the egg into the tube and toward the uterus. The cilia work this magic by digging into the sticky gel, called the cumulus oophorus, that surrounds the egg, and they transport this whole sticky, gooey mass. The egg itself is invisible to the naked eye, but the gel that envelops it is easily visible. If this sticky substance were not present, and the egg were placed bare upon the surface of the fimbria, the beating of the cilia would never move the egg along. The cilia are only able to dig in and transport the egg with this sticky, gooey material encasing it.
The process of grasping the egg and moving it into the interior of the tube requires only about fifteen to twenty seconds. Once the egg is safely within the tube, it is transported quickly toward the narrower region of the tube, the ampullary-isthmic junction, located two-thirds of the way toward the uterus. Here, the egg must wait for a successful sperm coming from the opposite direction to fight its way into the egg's tough outer membrane, the zona pellucida, score a direct hit, and thereby establish pregnancy. While the egg is held in this location by the tight resistance of the narrow region of the tube, the much tinier sperm nonetheless must struggle through this area of resistance to arrive from the opposite direction. Once it is fertilized, the egg must be nourished for several days in the ampulla of the tube before it can be allowed to pass into the uterus. If it is transferred into the uterus too soon, it will not be ready to implant, and it will die. If the transfer of the egg into the uterus is delayed too long, a tubal, or ectopic, pregnancy will occur (the fertilized egg will implant in the tube rather than the womb). Once the egg has been allowed to develop in the tube for three or more days, the isthmus suddenly opens up and the early embryo passes quickly into the uterus. Because the journey of the egg from the ovary to the site of fertilization, its nourishment in the tube, and the precise synchrony of the continuation of its journey into the womb are so intricate, problems with this egg and embryo transport process are frequently responsible for female infertility.
If the egg is not penetrated by sperm soon after ovulation, it becomes overripe and dies. After the egg is released from the ovary, it is only capable of fertilization for about twelve, or possibly at most twenty-four, hours. The likelihood of intercourse taking place during such a specific interval in any month is rather slight. So nature must provide some mechanism for providing a continuous flow of healthy sperm to the site of fertilization. That way, if intercourse is perhaps one or two days off schedule, some sperm can still arrive at the site of fertilization at the right time. For this reason, complicated barriers to sperm transport are necessary.
The success of IVF demonstrates that if eggs can be recovered at precisely the right time, they can be fertilized in the laboratory with only a small number of sperm. Then the complicated barrier mechanisms provided by nature to allow a slow, continuing flow of a small number of sperm at any moment is not necessary and the large numbers of sperm normally required for fertilization through intercourse are not needed.
