Sperm transport is a complex process that involves the production of billions of sperm cells in the testicles, storage in the epididymis, and ejaculation through the vas deferens and ejaculatory ducts. Seminal fluid provides nutrients and protection, and only a small number of sperm cells will make it to the egg for fertilization. Capacitation occurs when sperm are released from the seminal fluid and mix with cervical mucus, and fertilization typically occurs in the fallopian tubes where one sperm cell enters the egg and forms a zygote.
The journey of sperm, in humans and animals, is the short but complex history of the beginnings of life. In addition to the basics of mating, creation and conception, scientists have closely studied the exact means of transporting sperm to best facilitate artificial insemination, infertility treatment and contraception. This includes not only understanding what happens to sperm before it leaves the man, but also after it enters the woman.
Common knowledge is that sperm transport begins when sperm are produced in the male testicles, but this is the extent of most people’s knowledge. In each of the testicles, small coils called siminiferous tubules produce approximately 12,000,000,000 sperm cells each month in the average mature male. Prior to maturity, these cells are stored just above the tubules in the epididymis, where they remain until maturity is reached.
When the penis is stimulated, sperm transport moves from the epididymis, through the vas deferens tube, to the ejaculatory ducts. At this point, the cells unite with the seminal fluid produced in the nearby seminal vesicles. This fluid contains food for the cells in the form of glucose and protection from the acidic climate of the vagina in the form of alkaline substances. After ejaculation, semen is pushed through the prostate gland, which adds a thick milky prostate fluid for faster swimming speed through the urethra for propulsion into the vagina.
Sperm transport can take up to 48 hours. This is the time for the sperm to find and fertilize the egg inside the uterus before they die. According to the University of Pennsylvania Medical School, only about 200 of the approximately 300,000,000 sperm cells in each ejaculation will make it to the egg. Only one – and occasionally a few more – will be allowed in to start a new life.
Once deposited at the cervical inlet, seminal fluid will begin to release sperm from its socket. This initiates the process of capacitation, i.e. final maturation and hyperactivation. The cells mix with the acidic cervical mucus, which sloughs off the weak cells and allows the strong cells to enter the cervix.
When the sperm transport arrives at the fallopian tubes, the uterus actually stores thousands of sperm cells in suspended fertility until the egg reaches the central section of the fallopian tubes, called the ampullary-isthmic junction. This is where most human fertilization takes place, as the sperm – coaxed by hormonal and thermal impulses – can get to the immature egg or oocyte. Here, the outer membrane of the egg, called the zona pellucida, allows one sperm cell to enter and then blocks all others. A unicellular zygote is formed between the egg and sperm, which will divide several times over the next nine months to produce offspring.
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