Talking about embryogenesis, the emergence of an embryo, requires that we first answer the question when does pregnancy begin? This can be confusing, because gestational age (the number of weeks and days pregnant that you are) is calculated based on the mother’s last menstrual period (LMP). Due date for delivery is 40 weeks from the LMP, the first day of menstrual bleeding counting as day one of gestation. Of course, you aren’t actually pregnant during the last menstruation prior to pregnancy. Pregnancy begins later, generally during the third gestational week 3, although the exact timing varies, because some women ovulate (release an egg, also called an ovum, from one of the ovaries) a little sooner than two weeks after the first day of menstruation, or a little later, or a lot later than two weeks. For pregnancy to begin, an ovum must be fertilized. This happens in the days following sexual intercourse. Of hundreds of millions of sperm cells enter the vagina, only about one in ten thousand —a couple of hundred in total— are able to swim all the way to the fallopian tube, and there, come into contact with an ovum that is en route through the tube, bound for the uterus. There is a fertile window spanning about four days surrounding ovulation, when sperm cells have the best opportunity to meet an egg that is ripe for fertilization.
Normally, there is only one egg (there can be multiple ova in a woman who is taking fertility hormones), but, even though only one sperm cell can end up combining its genome (its collection of genetic material, its chromosomes) with the genome of the egg, one single sperm cannot fertilize an egg by itself. The two hundred or so sperm that make it to the egg are needed. This is because the ovum is surrounded by the zona pellucida, a protective barrier. Enzymes released by the sperm cells weaken the zona pellucida, enabling one sperm –and only one—to get its nucleus inside the ovum.
That nucleus that gets inside contains the father’s genome in one set of 23 chromosomes. This is called a haploid genome. The egg also has a haploid genome, meaning 23 chromosomes. In contrast, body cells are diploid, meaning that they have a double set of chromosomes, and thus 46 chromosomes in total. But by the end of the fertilization process, the two haploid genomes merge, creating a unique cell, with a distinct, diploid genome. Once fertilization has occurred, the resulting entity is called a zygote, which is still surrounded by the zona pellucida, because the enzymes from all of the sperm cells only weakened that barrier, without destroying it. The zygote doesn’t remain a single-celled zygote for very long. In a process called cleavage, it divides into two identical cells without getting bigger. and then those cells also undergo cleavage, still within the bounds of the zona pellucida. Over a few days, continuing cleavage brings the fertilized entity to a ball sixteen cells, and so on. In the realm of 16-60 cells, the entity is called a morula. Then, two different groups of cells start looking different from one another. An inner mass of cells, called embryoblasts —which will become the embryo that will later become the fetus— form with in an outer later of cells (trophoblasts) that will nourish the growing embryo and later will form most of the placenta. This happens as the morula continues its journey through the fallopian tube.
5-6 days after fertilization, the dividing cells have arranged themselves into a blastocyst, in which the clump of embryoblasts concentrated on one side of the sphere of outer cells, leaving a cavity of fluid. The blastocyst is the entity that implants into the endometrium, the lining of the uterine wall. This is the point where pregnancy actually begins, meaning that you’re not pregnant until the third week of pregnancy.
Often, blastocysts are ejected in what looks and feels like a normal, or slightly heavy, period. But, if healthy, an implanted blastocyst releases increasing levels of a hormone called beta-hCG. Beta-hCG is the hormone the pregnancy tests detect. Home pregnancy tests check for the presence of beta-hCG in the urine. Pregnancy tests at your obstetrician’s office check for it in the urine but also can check for it in the blood and measure the amount of it. Beta-hCG stimulates the ovary to release another hormone, progesterone, which supports the endometrium, thereby maintaining the pregnancy. Beta-hCG also may play a role in preventing the mother’s immune system from rejecting the blastocyst as a foreign entity.
The next milestone in embryogenesis (the emergence of an embryo) is called gastrulation. In gastrulation, the embryoblast, reorganizes into three layers of cells, flattens out, and folds upon itself. The embryo is no longer called a blastocyst. It’s now called a gastrula. The triple layer of cells within the gastrula is less than half a millimeter long, but, if it it survives, it will form your future child. While the embryoblast undergoes gastrulation, the trophoblast (that outer layer of cells) is developing into tissues that will support the embryo as it grows. The placenta that will arise from the trophoblast will deliver oxygen and nourishment from the maternal blood and carry carbon dioxide and waste from the embryo/fetus to the maternal blood.
As a result of folding, the young embryo has the beginnings of a body orientation. One end is on track to become the head and mouth, while the other end will become the anus. Between these two points, the cells of the three layers are on track to form specialized tissues with very different roles. Continued development of these layers will give rise to all of the major organ systems in the weeks to come.