Module 4 Fertilization to Birth

2.4.1 page 4

Reflect and Connect

Now that you have read and viewed the many changes that occur throughout embryonic development, complete the following items as your lesson assessment. Once all of the items are complete, submit your work to your instructor for evaluation.

1. Complete the ‘Launch Lab’ on page 507 of the textbook.
2. Complete questions 18, 20, 21, 23 to 26 from ‘Applying Concepts’ and ‘Making Connections’, questions on pages 536 and 537.
3. Complete Lab 1: ‘Comparing Embryonic Structures’.

Reflect on the Big Picture

No other human growth period shows as much change as that of embryonic development.  Understanding what is happening during pregnancy makes most expecting mothers and fathers just a little pre-occupied with their little ‘bundle’. So it was with Jane and John and their immersion into the understanding of how their twins were progressing. It took a bit more understanding to figure out how their twins could be born one white and one black.

The explanation of the twin’s skin colours comes from their parental background. John, is of mixed-race, Jamaican-English heritage, while Jane is of white Scandinavian background. John, then, has sperm that carry genes for both light and dark skin colour. They are trying for another child and are not worried as to what colour their third offspring might be.

Going Beyond

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

There are many other terms that define specific processes that are occurring during fetal development. See if you can find out what some of the following terms mean: organogenesis, neurogenesis, angiogenesis, chondrogenesis, myogenesis, osteogenesis, zygocity and ‘fourth germ layer’.

One last item that you may wish to look into is the Carnegie Stages of development. Find out how each stage is defined through the embryo’s external and internal development. Include in your research how embryonic cells communicate to each other.

Lesson Summary

Prenatal development is divided into two main periods of development. The first period takes place from fertilization to the end of the eighth week of development. This period is termed embryonic development. Other names are given to the embryo to describe the different specific stages of development of the offspring. The period that takes place from the beginning of the ninth week to birth is termed the fetal development period, and the offspring is referred to as the fetus during this time.

Once the egg and sperm have been correctly formed and been released, both must meet inside the female’s reproductive system for ‘natural’ fertilization to occur. The process of the sperm entering the egg, and the combining of 23 chromosomes from the father’s sperm and the 23 chromosomes from the mother’s egg is known as fertilization. Fertilization results in a single cell with 46 chromosomes, now called a zygote. While still in the oviduct, the zygote begins to divide through the process of cleavage, and at the 16 cell stage it is classified as a morula. The morula continues to divide and also begins to fill with fluid. The resulting structure is then called a blastocyst.

The blastocyst forms two distinct layers of cells, an outer layer called the trophoblast, which will develop into the supportive membrane structure called the chorion. The mass of inner cells of the blastocyst will form the embryo. As the blastocyst reaches the uterus, the trophoblast produces and secretes an enzyme which digests some of the endometrial tissue, enabling the blastocyst to implant itself in the uterine lining.

As implantation is being completed, the amniotic cavity develops within the amnion, a type of sac.  From the amniotic cavity, the embryonic disc forms. This disc will undergo the process of gastrulation: the formation of germ tissue layers (endoderm, mesoderm and ectoderm). The event of gastrulation gives rise to a new term for the developing offspring; it is now called a gastrula. During gastrulation a series of changes occurs where cells begin differentiating.

Tremendous growth and organ development happen in the first trimester, with 90% of all organs being formed by the end of the embryonic period. At the same time as the embryonic growth is happening, supporting structures are also developing. The allantois, amnion, chorion and yolk sac are formed to give support to the developing embryo and fetus.  Some of these structures then give rise to the placenta and umbilical cord. All of the supporting structures help to nourish, protect and remove wastes the developing offspring during prenatal development. Once the baby is born all of the supporting structures are expelled from the female’s body along with the fetus.