8.2: Germinal Stage

Learning Objectives

1. Describe the process of ovulation, fertilization, and the outcomes if fertilization does or does not occur.
2. Explain the events that occur during the germinal period, from conception to implantation.
3. Define key terms such as zygote, blastocyst, trophoblast, and embryonic disk.
4. Identify the hormonal changes that occur immediately following conception and how pregnancy is detected.
5. Compare monozygotic (identical) and dizygotic (fraternal) twins in terms of conception and genetic similarity.

Once ovulation has occurred, and the egg is released, it waits in the fallopian tube until one of two things happens:

1. A sperm comes along and the two are united, resulting in fertilization of the egg (conception).
2. If the egg is not fertilized, it moves to the uterus and is absorbed by the body. All pregnancy-sustaining hormones decrease, triggering the system to shed the thickened uterine lining, which results in menstruation 14 days later.

The Germinal Period

The germinal period (about 14 days in length) lasts from conception to implantation of the fertilized egg in the lining of the uterus. At ejaculation, millions of sperm are released into the vagina, but only a few reach the egg and typically only one fertilizes the egg. Once a single sperm has entered the egg, the wall becomes hardened and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off, and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. It is typically fertilized in the top section of the fallopian tube and continues its journey to the uterus. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a zygote.

Figure \(\PageIndex{1}\): Sperm and ovum at conception. Image is in the public domain.

During this time, the organism begins cell division through mitosis. After five days of mitosis, there are 150 cells, which is now called a blastocyst. The blastocyst consists of both an inner cell mass and an outer cell mass. The inner group of cells, known as the embryonic disk, will develop into the embryo, while the outer group of cells, referred to as the trophoblast, will form the support system that nourishes the developing organism. This stage ends when the blastocyst fully implants into the uterine wall (U.S. National Library of Medicine, 2015), around 6-12 days after fertilization.

Figure \(\PageIndex{2}\): The cycle of fertilization. Image by Ttrue12 is licensed under CC BY-SA 3.0

Mitosis is a fragile process, and less than 50% of all zygotes survive beyond the first two weeks (Hall, 2004). Some of the reasons for this include:

• the egg and sperm do not join properly, thus their genetic material does not combine;
• there is too little or damaged genetic material;
• the zygote does not replicate;
• or the blastocyst does not implant into the uterine wall.

The failure rate is higher for in-vitro conceptions. Figure \(\PageIndex{2}\) illustrates the journey of the ova from its release to its fertilization, cell duplication, and implantation into the uterine lining. ²²

Immediately after conception, as the zygote begins its journey, the female body triggers an increase in the production of hormones essential for sustaining pregnancy, including progesterone and human chorionic gonadotropin (HCG). HCG is present in the blood right after conception, but is not immediately detectable in urine. Over-the-counter pregnancy tests look for the presence of HCG in urine samples, which is why a positive result may not occur until at least 9-10, or more, days after ovulation.

Twins

Monozygotic or identical twins occur when a fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate, but genetically identical offspring. That is, they possess the same genotype and often the same phenotype. Approximately one-third of twins are monozygotic.

Sometimes, however, two eggs or ova are released and fertilized by two separate sperm. The result is dizygotic or fraternal twins. These two individuals share the same amount of genetic material as would any two children from the same mother and father. In other words, they possess different genotypes and phenotypes.

Older mothers are more likely to have dizygotic twins than are younger mothers, and couples who use fertility drugs are also more likely to give birth to dizygotic twins. ²⁸

Figure \(\PageIndex{3}\): Monozygotic Twins. Image is in the public domain.

Figure \(\PageIndex{4}\): Dizygotic Twins. Image by Jennifer Paris used with permission.

References, Contributors and Attributions

22. Lifespan Development: A Psychological Perspective (pages 42-43) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

28. Lifespan Development: A Psychological Perspective (page 36) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0