The Upward Journey of the Spermatozoa
Posted on October 31, 2007
Filed Under Pregnancy |
The midportion of the fallopian tube is the rendezvous point for egg and sperm. Explanations of how spermatozoa ascend from the vagina into the uterus, and from the uterus to the meeting place in the tube, have shifted as knowledge of the subject has increased and clarified. A hundred years ago a spermatozoon was believed to be endowed with instinctive, bloodhound-like qualities which directed it along the proper path to insure fertilization. Today it is known that the fate of the several hundred million spermatozoa depends in part on the phase of the recipient’s menstrual cycle.During the 3 or 4 days before ovulation and the day of ovulation itself, the canal of the cervix, the opening into the uterus from the vagina, is filled with a profuse, transparent, watery, stretchy mucus through which the sperm cells swim with ease. The mucus at this time can be stretched between the fingers for several inches, like the white of an egg or a strand of wool as it is stretched out on the spinning wheel. This characteristic is called spinnbarkeit. The appearance of this profuse mucus explains why some women notice a colorless vaginal discharge each month for 3 to 5 days in midcycle.
Some women occasionally spot or even bleed lightly for up to 48 hours in midmonth, at the time of ovulation. Many experience pain in occasional cycles for 4 or 5 hours on one side or the other of the lower abdomen, depending on whether the egg that particular month was ovulated from the left or right ovary. This is called mittelschmerz (German: mittel - mid, schmerz - pain). At times of the month other than these several days in midcycle, the cervical canal contains a scant, sticky, opaque mucus, onto which sperm are entrapped, quite like flies on flypaper.
During intercourse, the spermatozoa are catapulted into the upper vagina, near the cervix. The sperm cells swim haphazardly in all directions, some into the upper recesses of the vagina, some toward the outside, others away from the middle of the vagina far to one side or the other. Most spermatozoa never reach the protective confines of the cervical canal, but remain in the vagina, exposed to the acidic environment of vaginal secretions. Sperm cells are sensitive to an acid medium, and those remaining in the vagina become motionless and dead within a few hours. A relative few, by sheer spatial accident, immediately gain the sanctuary of the alkaline cervical mucus. This was demonstrated by studies in the 1970s in which cooperating couples notified research physicians as soon as male orgasm had been accomplished. The physicians then took samples of mucus from high up in the cervical canal. Much to the surprise of the scientific community, the cervical mucus was already swarming with sperm cells. Some sperm reach the site of fertilization in the fallopian tube within 5 minutes of ejaculation, the majority, within 4 to 6 hours.
Some of the sperm swim straight up the one-inch canal with almost purposeful success, while others bog down on the way, getting hopelessly stranded in tissue bays and coves. A small proportion of the total number ejaculated eventually reach the cavity of the uterus and begin their upward two-inch excursion through its length. This progress is aided by muscular contractions of the uterus. The undaunted sperm reach the openings of the two fallopian tubes-one on each side of the uterus. From there, they continue their upward journey into one of the tubes.
If the egg has reached the midportion of a tube, a spermatozoon swimming up the opposite tube has no chance of meeting it. Only a few thousand of the four hundred million cells ejaculated ever reach the trysting site, the midsegment of the fallopian tube containing the egg, two or three inches above where the uterus meets the tube. Only a few hundred sperm ever come near the egg cell.
The one sperm that achieves its destiny has won against gigantic odds, several hundred million to one. The baby it engenders has a far greater mathematical chance of becoming pregnant than the sperm had of fathering a baby. No one knows just what selective forces are responsible for the victory. Perhaps the winner had the strongest constitution, perhaps it was the swiftest swimmer of all the contestants entered in the race. Perhaps it was merely the luckiest in finding a fluid current leading straight to the ovum.
The method by which the tiny sperm cells locate the egg is not clearly understood. Because of the few spermatozoa, just several thousand, in the relatively long tube at the time of fertilization, many investigators feel some process other than random encounter is involved. One suggested explanation is that the egg exerts a chemical trapping effect, increasing sperm concentration by making them swim more rapidly when they are headed toward it and less rapidly when going away from it. When sperm egg collision occurs, the sperm immediately becomes bound to the egg’s surface.
If ovulation occurred within several minutes to 24 hours before the sperms journey ends, the ovum will be in the tube, awaiting fertilization. If ovulation took place more than 24 hours before the sperm’s arrival, the egg cell will have begun to deteriorate and fragment by the time the spermatozoon reaches it. If ovulation has not yet occurred, but takes place within 2 or 3 days after intercourse, living spermatozoa will be cruising at the tubal site waiting for the egg .
We have followed the sperm and egg to their meeting place, and we can now observe what happens when they meet-that is, the actual process of fertilization.
Tags:cervix, egg, fallopian tube, lower abdomen, mucus, Pregnancy, sperm cells, spermatozoa, spermatozoon, time of ovulation, uterus, vagina vaginal discharge
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