The Process of Conception

Emergency contraception is sometimes referred to as the “morning after pill” but it can be used for up to 120 hours after unprotected sex. They prevent fertilization and ovulation but do not affect implantation. Most of them contain high doses of female hormones to prevent pregnancy. There are few side effects of these methods, which decrease the pregnancy chances by 85 percent. IUDs are also used as emergency protection and these are more effective than hormones alone.

THE PROCESS OF CONCEPTION

The process of conception is referred to as human fertilization in humans. It happens in the ampulla of the fallopian tube and represents the union of the sperm and egg to created a zygote cell. It begins with vaginal sex and ejaculation before or during female ovulation. The major exception to this process is in vitro fertilization or artificial insemination, which do not involve sex but still lead to human fertilization.

As you remember from chapter one, the sperm must bind to and fuse with the egg cell or oocyte. The corona radiata surrounds the secondary oocyte and is a layer of follicular cells. There is a drawing out or conical elevation of the yolk or cytoplasm of the egg when the sperm cell is about to pierce the egg. This is also referred to as the reception cone or the cone of attraction. The outside of the yolk changes into a membrane called the perivitelline membrane after the sperm cell has entered. The purpose of this membrane is to prevent more sperm from entering the oocyte. Figure 21 shows the structure of the egg and sperm cell:

Figure 21.

Sperm cells that are freshly ejaculated do not fertilize eggs well. They must undergo capacitation in the female to increase its motility. It needs to be prepared for the acrosome reaction, which is the enzymatic process that allows the sperm to penetrate the zona pellucida, which surrounds the egg cell just inside the corona radiata.

The zona pellucida is made from glycoproteins as part of an extracellular matrix. It binds to sperm cells and triggers the acrosome to burst and release lysozyme, an enzyme used to get through the zona pellucida. There is a process called kin selection, which allows some sperm cells to facilitate the pathway for other sperm cells.

The cortical reaction happens when the zona pellucida has been breached. There are cortical granules that fuse with the cell’s plasma membrane, causing exocytosis of the enzymes in the granules. The zona pellucida glycoproteins then crosslink with one another so no further sperm cells can enter. In that sense, it is both the cortical and acrosome reaction that prevent more than one sperm entering the cell. The tail disintegrates and the second part of meiosis occurs in the egg cell, releasing a polar body and allowing for fusion of the sperm with the egg. Figure 22 shows the zygote initial growth:

Figure 22.

Fertilization marks the inital point of prenatal development. Times measured after this are referred to as the fertilization age, embryonic age, conceptional age, or fetal age. These are all different from the gestational age, which is marked from the first day of the last menstrual period.