Module 4 Fertilization to Birth: Lesson 2.4.1

Lesson 2.4.1

Lesson 1—Fertilization and Embryonic Development

Image shows sperm trying to fertilize egg, egg still has cell of follicle around it in places, corona radiata cells

Get Focused

All the structures and functions of the mammalian reproductive systems, including the human reproductive system, are designed to get to the end result of fertilization, implantation and development of an embryo and fetus. This results in the continuation of the species through the birth of a new offspring.

How does an offspring go from a single cell, totally dependent on its internal life in utero, to an offspring made up of billions of cells, who is able to survive independent from the mother? The processes that take place from fertilization through to the maturation of the fetus are astounding, and it all starts with two cells.

Image of developing embryo, head and hand are visible

From previous lessons in this unit, you have gone through the structures and functions of the male and female reproductive systems and the hormones involved which cause the onset and development of these systems. In this lesson, you will go through the events that occur upon fertilization, and those that occur after fertilization, up to, and including, the development of the embryo, which lasts until the end of the eighth week.

Image shows twin embryos developing in separate placentas.

Jane and John, from the Fertility Case Study, were so thrilled when they heard the news that they were going to be parents for the first time. They were still thrilled, but a bit taken aback when they discovered they were going to have twins. Imagine how much more surprised they were when the twins were born and they had two different skin colours! The probability of this is about a million to 1.

Whether Jane and John had twins or a single child, black or white, their babies developed the same way in the uterus. An offspring’s embryonic development occurs as huge array of changes as an egg and sperm unite in fertilization to form a new individual. As the newly formed embryo goes through the different stages of its development, different names are given to its specific state of growth and maturation. Also, these various stages of prenatal development contribute to the hormonal control which sustains the pregnancy.

The development of a prenatal offspring is divided into two development periods. The first takes place from fertilization to the end of the eighth week of pregnancy. During this period the developing offspring is termed an embryo. The second development period takes place from the ninth week to birth. During this period, the developing offspring is called a fetus.

In addition to the development of the embryo, there are supporting structures which must be formed at the same time. The supporting structures are collectively termed the extra-embryonic membranes and provide the embryo, and later the fetus, with means of nutrition, respiration, excretion and protection.

At the end of embryonic development almost all of the organ systems are formed and the embryo appears human like. All of this in just two short months; no wonder pregnant woman are very tired in the first few weeks of their pregnancy. Remember that all the requirements for prenatal growth come from the mother’s body.

Male identical twins. Outside with sport shirts. They're looking at camera. Close-up of smiling faces.

How did Jane and John’s pregnancy come to the formation of twins? There are two ways in which twins can be formed. In a normal sequence of events, a woman releases one egg every menstrual cycle. However, it is not uncommon for two eggs to mature and be released at about the same time in the same ovulation period. If this happens, then each egg is fertilized by two different sperm, which results in fraternal (dizygotic) twins. Fraternal twins are not identical, and may or may not be of the same sex. During pregnancy and development they each form their own supporting structures including a placenta. Jane and John had fraternal twins.

The other type of twins is referred to as identical (monozygotic) twins, as 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 cells. Since sex is a genetic determinate from the combining of the parental chromosomes, identical twins are of the same gender. Monozygotic twins may also develop their own separate supporting structures, or they may share them.

Complications can arise at any point along the way during prenatal development. One of the first problems that can occur is that implantation in the uterine lining does not successfully take place. Implantation can happen in the oviducts or in other areas of the uterus. Such an event is termed an ectopic pregnancy.

For Jane and John, they were thrilled to have their first pregnancy and once they discovered they were having twins, they paid special attention to the prenatal development that would be happening to their soon to be children. During this prenatal time, they read much and discovered more about the amazing development that was taking place with their twins.

In this lesson, to understand fertilization and embryonic development, you will examine the following focusing questions:

What are the sequences of events and hormonal controls that take place throughout fertilization and embryonic development?
What major tissues and organs arise from the ecto-, meso-, and endo-derm?
How are twins formed?

This lesson will take approximately 80 minutes.

Module 4: Lesson 1—Assessment

The assessment for this lesson will consist of a number of questions and a lab, which you will NOT be required to submit to your teacher. You should however watch the tutorial video for this lesson.

Bio30 tut#2.4.1 Fertilization and development