Module 4 Lesson 1 - 3

Ovulation to Implantation

---

At ovulation, the egg leaves the ovary. A single sperm nucleus enters the egg, and fertilization occurs in the Fallopian tube. As the zygote moves along towards the uterus, it undergoes several mitotic divisions called cleavage.

Morula

---

Each cell continues to divide into two cells until it reaches a 16-cell stage called morula after about 3 days following the fertilization. The morula enters the uterus and continues to divide into about 100 cells called blastocyst.

Blastocyst

---

The outer layer of the blastocyst is called trophoblast, and a group of cells form on the inside into an inner cell mass. The most amazing thing is that the cells in the inner cell mass are pluripotent. Although pluripotency only lasts for a couple of days, it means each of these cells is capable of becoming almost any cell in the human body.

Implantation

---

After a blastocyst forms, it fuses itself in the lining of the uterus, beginning the process of implantation. With successful implantation, the trophoblast begins to produce human chorionic gonadotropin (hCG) hormone. hCG signals the corpus luteum in the ovary to enlarge and to continue producing progesterone and estrogen. The progesterone continues to maintain the endometrium to support the embryo and the estrogen suppresses ovulation.

Although complications can arise at any point during prenatal development, one of the first problems that can occur is that implantation in the uterine lining is unsuccessful. Implantation can occur in the Fallopian tube or anywhere outside the uterus. Such an event is termed an ectopic pregnancy.

The following three images outline the events that occur during the first week of embryonic development.

Twins

---

Twins can be formed in two ways. In a normal sequence of events, a woman releases one egg every menstrual cycle. However, in about 1% of the ovulation cycles, two eggs mature and are released at about the same time in the same ovulation period. If this happens, each egg is fertilized by a sperm, which results in fraternal (dizygotic) twins. Because fraternal twins are developed from two eggs fertilized by two sperm, they are not identical, and they may or may not be of the same sex. Fraternal twins are as similar as two siblings born at different times. During pregnancy and development, each forms his or her own supporting structures including a placenta. On rare occasions, they may share a placenta.

The other type of twins is less common than fraternal twins and are referred to as identical (monozygotic) twins. They are formed from one egg that is fertilized by one sperm. For identical twins to form, the quickly dividing fertilized cell mass separates into two separate masses. Because sex is a genetic determinant from the combining of the parental chromosomes, identical twins are of the same sex. Monozygotic twins may develop their own separate supporting structures if the embryos separate in the earlier stage, or they may share them if the embryos separate later at the blastocyst stage.

Did You Know?

As the trophoblast increases the production of hCG after implantation, hCG accumulates in the blood and is excreted through urine. A home pregnancy test can be used a few days after implantation to detect the hCG hormone level in urine.