The Journey of Spermatozoa

Through the Male Ducts

From each testicle the spermatozoa slowly pass upward through the epidiymis.The trip through the ducts requires 2 to 4 weeks, the spermatozoa maturing as the journey progresses.

Spermatozoa do not make their own way up the male ducts, since they are motionless at this stage, but are propelled upward by imperceptible contractions of the muscular tissue forming the walls of the epididymis and vas. It is only after the mass of sperm cells is diluted during orgasm by fluid from the prostate and other male glands that they are thrown into vigorous movement. Spermatozoa remain actively motile for as long as 72 hours in the upper reaches of the female reproductive tract.

The Journey of the Egg Down the Tube

After ovulation, the egg, having passed from an ovary into the fallopian tube, travels down the five-inch tube. The muscular walls of the tube encircle a canal that is wide at the ovarian end and narrow at the uterine end. The diameter of the tube at the uterine end is as small as a single broomstraw. Although it would make sense that an egg released from the left ovary would be picked up by the left tube, women who have had one tube and the opposite ovary removed have been known to achieve pregnancy, attesting to the remarkable action of the tubes .

The mechanism that propels the egg downward through the tube toward the uterus seems to be a combination of fluid currents and rhythmic muscular contractions. Many of the cells lining each fallopian tube are ciliated they possess hairlike projections from the surface that beat vigorously. Under the microscope, they look like a field of wheat being blown by the wind. The beating of the cilia causes a fluid current that mostly flows down the tube from the ovarian end toward the uterine end. When the ovum is ovulated from a ripe ovarian follicle, it is surrounded by a thick, loosely adherent covering of some three thousand small cells, the cumulus cells that envelop the egg completely during its residence in the follicle. Some of the cells are brushed loose by the egg’s contact with the sides of the tube, especially with the ciliated cells of the tubal wall.

Sperm Motility and Fertility

If the male does not ejaculate for some time, the spermatozoa that complete the journey from the testes are stored in the ducts and epididymis and may suffer effects of aging, The first ejaculation after a long interval, therefore, may produce cells of impaired motility, For this reason, physicians and midwives recommend frequent inter course ­optimally, every 24 hours during the days immediately preceding ovulation-for most couples desirous of pregnancy who are having difficulty in conceiving,

Fusion of the Two Nuclei

Once the sperm has entered the cytoplasm of the ovum, its head en­larges to form what is called a pronucleus-the nucleus of the mature sperm cell. The ovum also completes its final maturation after entry of the sperm, also forming a pronucleus. Both the male and female pronuclei enlarge and migrate toward the center of the cell. The membranes of each pronucleus disintegrate, and the two nuclei fuse into one nucleus. When this has been accomplished, fertilization is completed. The fertilized ovum now has forty six chromosomes twenty three from the father and twenty-three from the mother. It has become a zygote.

Cleavage-or rapid cell division-ensues within 30 hours of fertilization.The fertilized ovum divides into two cells, the two cells divide into four, the four into eight, and so on, creating-after an average of 266 days from the moment of fertilization or 280 days from the beginning of the last menstrual period-a fully developed baby, weighing from 5½ to over 10 pounds.