The Early Hours of The Fertilized Egg

Sep 26 2007

In the mouse the two-cell egg appears 24 hours after fertilization, and the four-cell stage after 38 to 50 hours. In the human a two-cell stage has been observed within 30 hours after fertilization and sixty-four-cell development in 72 hours. By the fifth day the human embryo is made up of about five hundred cells, the cells having doubled their number about every 12 hours since the time of fertilization. In all mammals, the developing egg in its earliest phase, while still a traveler down the fallopian tube, is a solid mass of cells, aptly termed a morula. It is still round and has increased little if at all in diameter, its contents merely having divided into smaller units.

The egg passes from the tube into the uterus on day 4 or 5, when it is no longer a solid mass of cells, the cells having arranged themselves about the outer surface of the sphere, the center now occupied by fluid. It is termed a blastocyst. By the fifth day the multicellular conceptus begins to increase in size. It floats about the slit like uterine cavity for about 3 more days and then adheres to its inner cell lining, which has been prepared by the special hormone progesterone, fed by the ovary into the bloodstream from which it reaches the uterus. Progesterone has made the lining succulent and full with a network of new and enlarged blood channels coursing through it. The developing egg, possessing an enzyme that digests away the surface cells to which it has adhered, then sinks down into the rich depths of the uterine lining. The process is something like hoeing the firm, dry earth above to plant corn in the soft, moist, nourishing earth beneath. Implantation The Early Hours of The Fertilized Egg occurs around the eighth day. By the tewelth day the human egg is already firmly implanted, but the damaged, superficial uterine lining cells through which it passed have only partially healed to roof over the tissue nest in which the egg now rests. About 6 out of 10 pregnancies implant on the posterior, or rear, wall of the uterine cavity, and 4 in 10 on the anterior, or front, wall. Either site is normal.

When studied under a microscope, the 12-day egg already shows a specialized accumulation of cells which later will form the embryo. The remaining cells become the placenta and membranes. At about the time of implantation, the fertilized egg secretes the pregnancy hormone hCG. This keeps the corpus luteum from regressing. The progesterone secreted by the corpus luteum is necessary for the maintenance of pregnancy. Again, this reminds us of the wondrous interactive rhythms that are required to create new life.

The process of becoming pregnant is now completed, yet the mother to be has not even had time to miss her first menstrual period, and will not experience other symptoms suggestive of pregnancy for several days.