Lesson 1 — Fertilization and Implantation


Self-Check

  1. How many chromosomes are in a fertilized human egg?

  2. Why must the egg be fertilized within a day of its release?

  3. Why do so few sperm arrive in the Fallopian tube?

  4. Why is the first sperm that reaches the egg unlikely to be the sperm that enters and fertilizes it?

  5. Explain the process of cleavage.

  6. How does a morula differ from a blastocyst?

  7. Which groups of cells (trophoblast or inner cell mass) become the embryo?

  8. What is implantation?

  9. What is hCG and how long is it secreted?



Self-Check Answers


1. There are 46 chromosomes that make up the zygote (fertilized egg): 23 from the sperm and 23 from the egg.

 

2. The egg must be fertilized within 12 to 24 hours of release because it loses its capacity to develop further after 24 hours.

 

3. The sperm must survive the acidic environment of the female reproductive tract. In addition, only one Fallopian tube contains an egg each month; many of the sperm may enter the "wrong" Fallopian tube.

 

4. To break through the protective jelly-like layer of the egg requires the actions of several sperm and their acrosome enzymes. Therefore, a sperm that arrives after the first few have breached the barrier is able to utilize its own acrosome enzymes to enter successfully.

 

5. Cleavage is cell division without growth. This is the initial stage of mitotic divisions after the formation of the zygote in the Fallopian tube. The cleavage divisions do not increase the size of the zygote. They increase only the number of cells within it.

 

6. The morula reaches the uterus within 3 to 5 days of fertilization, and upon arrival, it begins to fill with fluid that diffuses from the uterus. Within the fluid-filled space inside the morula, two different groups of cells characterize the blastocyst: those that nourish the embryo and those that become the embyro.

 

7. The inner cell mass develops into the embryo.

 

8. Implantation occurs when the blastocyst nestles in the endometrium (lining of the uterus). This happens between about the 10th and 14th day after fertilization. The blastocyst attaches to the endometrium by secreting enzymes from the trophoblast cells within to digest some of the tissues and blood vessels.

 

9. hCG is human chorionic gonadotropin hormone, which is secreted to maintain the corpus luteum after implantation has occurred. It is secreted at a high level for the first two months, declining to a low level by the end of four months when there is a fully functional placenta to take over the production of estrogen and progesterone from the corpus luteum. At this point, the hormone production of the corpus luteum is no longer important although it continues throughout the pregnancy.



Going Beyond

The development of the embryo is very complex and consists of much more detail than can be presented in this lesson. Remember that all parts of the fetus have developed originally from one fertilized cell.  As cells divide and the organism grows, the cells differentiate and congregate to form all the specialized structures of the body. If you are interested in this process, research how specific structures such as the eyes, ears, muscles, or bones are formed, just to name a few. 

Many other terms define specific processes that occur during fetal development. Research some of the following terms: organogenesis, neurogenesis, angiogenesis, chondrogenesis, myogenesis, osteogenesis, zygocity, and 'fourth germ layer'. (This is not a requirement of this course.)

Another item you may wish to investigate is the Carnegie stages of development. You could find how each stage is defined through the embryo's external and internal development. Include in your research how embryonic cells communicate with each other.