Conception - A New Human Being
One lucky sperm is the first to reach the egg; it emits enzymes that help it first bond to the surface and then penetrates the zona pellucida
, the tough membrane surrounding the oocyte. The egg emits enzymes of its own, preventing any other sperm from penetrating.
Technically, it is only at this point after fertilization, that the egg matures from an oocyte to an ovum. The ovum casts off a polar body, a small, useless cell, in which the egg discards its extra replicated DNA, and results in the proper 23 chromosomes with a single strand of DNA each. This is the final phase of oogenesis (egg formation), and it's triggered only if and when a sperm fertilizes the egg; so most oocytes never become an ovum at all.
I always think of fertilization happening almost instantly; the sperm bores into the egg, their chromosomes combine, and voila, a zygote. In fact, fertilization can take 24 hours to complete, with nearly half that time just for the ovum to shed the polar body. During this time, the sperm loses its tail and its midpiece, until nothing is left but its package of chromosomes.
Day 1: A Zygote, 1-Celled Embryo
At last, the nuclei of the egg and sperm fuse; the egg and sperm cease to exist individually as they unite to form a zygote
. The 23 single chromosomes from the egg combine with the 23 from the sperm, forming a complete set of 23 pairs, 46 in total; and at this moment, the life of a new and unique human being begins. This singular genetic blueprint, defining every trait of this new person, has been uniquely defined among a billion possibilities, and can never be changed; nor will it ever be repeated.
Recall that part of that genetic blueprint is contributed by the last pair of chromosomes, pair #23. The egg has contributed an X chromosome to this pair; and the sperm that fertilized the egg carries either an X or a Y as its 23rd chromosome. If the sperm contributed an X, the embryo will become a female; if it contributed a Y, the embryo will become a male.
The fallopian tube is lined with microscopic beating hairs called cilia, which gently propel the single-celled embryo toward the waiting uterus, a trip that will last a week or so.
Days 2-3: First Cell Divisions
In about 40 hours, the cell will divide, replicating its DNA for the first time. This event will be repeated trillions of times to create all of the cells needed to form a complete person, but each time, the DNA copy will remain the same, inherited from the original zygote created when the egg and sperm unite.
The new cells are called blastomeres, and the following day, they divide again for a total of 4 cells.
Day 4: Morula
The embryo has become a ball of about 32 cells, and is called a morula
(from Latin for mulberry).
With each division, the cells grow smaller. We can now see why the egg cell needed to be so large to start with; the mass of cytoplasm must support all these early cell divisions before the developing embryo reaches the uterus.
Day 5: Blastocyst
The following day, the morula becomes a blastocyst
as a fluid-filled core forms, pushing the cluster of cells to one side within an outer shell of cells.
For the first time, specialized cells have been formed: the outer layer of cells are destined to become the placenta, umbilical cord, and amniotic sac.
The inner cell mass will become the fetus, and will form all of the tissues, organs, and bones of the body. These are the stem cells, cell "blanks" capable of generating the over 200 types of specialized cells in the body, as well as reproducing more stem cells.
(During embryonic stem cell research, this inner cell mass is extracted, destroying the embryo.)
Days 6-7: HCG Production
On days 6 and 7, the developing embryo begins to produce hCG (human chorionic gonatotropin). We would probably not be very interested in this fact, except that hCG is detectable in the mother's urine by pregnancy tests. Sensitive home pregnancy tests can detect even the very small amounts of hCG that will be present by a few days from now, reducing the dreaded 2WW -- the 2-week-wait to find out if you're pregnant.
Choreography by DNA
By day 7, the embryo arrives in the uterus, and may float there for a day or two before implanting in the lining, the endometrium
. The endometrium has been thickening since early in the cycle, in anticipation of nurturing the growing embryo. A placenta begins to form.
By day 22, amazingly, a primitive beating heart exists. The embryo has just survived the most hazardous three weeks of its life. Sadly, as many as half of fertilized zygotes never reach this point, due to malfunctioning chromosomes, improper cell divisions, or failure to implant.
As you have seen, the embryo's development is a complex choreography of the creation of specialized cells in a certain order. All of this activity is directed by instructions in the DNA.
By the second month, sex organs begin to form. If the fetus has an XY chromosome, the instructions in the Y chromosome dictate forming male sex organs; if it has an XX chromosome, the instructions in the X chromosome are carried out to form female sex organs.