The reproductive journey begins at the vagina, where semen is deposited. From there, the spermatozoa enter the uterus through the cervix and then continue along the fallopian tubes, where fertilization takes place.
Out of the nearly 200 sperms that make it to the end of the trip, only one will fully enter the ovum. When this happens, chemical substances are activated to counter the entrance of the rest of the male gametes.
Then, the spermatozoa that entered the ovum losses its tail and moves on towards its encounter with the nucleus. Fertilization takes place when both nuclei (the sperm’s and the ovum’s) fuse together.
This way, a total of 46 chromosomes are put together (each cell contributes with 23), which makes for a complete set of chromosomes for a new individual.
Although half of the genetic information comes from each of the parents, the new being is singular and unique, and from that moment on, it will be known as a zygote.
Division and implantation
The zygote slowly begins its journey through the fallopian (uterine) tubes towards its final destination, the uterus. Since it can’t move on its own, it is pushed along by the cilia that cover the tubaric mucosa.
This journey lasts around eight or nine days, and during that time the zygote continues to change. After 36 hours, thanks to a process called mitosis, the zygote splits into two, forming two new identical cells, each of which contains an exact copy of all the original cell’s genetic information. After that, a consecutive cell division begins. After 72 hours, the zygote is made up of 16 cells and is now called a morula.
It reaches the uterus in this state, where it becomes a more solid mass and continues to split until it is made up of 64 cells, a state known as blastocyst or blastula.
It is a cell mass (embryoblast) covered by an outer layer (trophoblast) and it has a cavity filled with liquid (blastocoele).
This is considered to be the stage prior to the development of the embryo, which will now implant itself onto the walls of the uterus.
After these nine days have passed, the blastocyst will become permanently attached. The trophoblast will aid the growth of an internal exchange structure known as placenta.