Module 4 Lesson 1 - 2
Lesson 1 — Fertilization and Implantation
Read pages 506 - 519
All the structures and functions of the mammalian reproductive systems, including the human reproductive system, are designed to achieve the end result of fertilization, which involves implantation and development of an embryo and fetus. This results is the birth of new offspring.
An offspring's embryonic development involves a huge array of changes beginning with an egg and sperm uniting in fertilization to form a new individual. The newly formed embryo goes through the various stages of development; each specific state of growth and maturation is given a separate name. These various stages of prenatal development contribute to the hormonal control that sustains the pregnancy.
The development of a prenatal offspring has two development periods: embryonic development and fetal development. The first embryonic development occurs from fertilization to the end of the eighth week of pregnancy. During this period, the developing
offspring is termed an embryo. The second fetal development period occurs from the ninth week to birth, during which the developing offspring is called a fetus. This lesson focuses on embryonic development.
| Number of Weeks in Development
|
Stages in Development
|
| 1 - 2 weeks | pre-embryonic development |
| 3 – 8 weeks | embryo |
| 9 – 38 weeks | fetus |
To understand how two gamete cells, each containing half the number of chromosomes of each parent, can combine and transform into an organism of millions of cells within the first two months of gestation, you need to understand fertilization and embryonic development. To begin to understand how prenatal development occurs, read pages 506 to 519 of the textbook.
Fertilization
Fertilization occurs when an egg is released from an ovary, travels along the Fallopian tube, and combines with a sperm. Each of these gametes contains half the genetic material necessary to form a new being. As their nuclei fuse, they form a single cell called a zygote.
After ejaculation within the vagina, hundreds of millions of sperm
must travel through the acidic environment of the vagina and survive
the hostile environment of the uterus. They must make their way into the
Fallopian tubes, not knowing which ovary released
the egg.
As multiple sperm cells reach the jelly-like coating surrounding
the egg, they burrow through the outer layers. The cap of the sperm,
called the acrosome, releases enzymes that digest a path through the
coating. This path allows the sperm cells to penetrate
the coating farther and move toward the plasma membrane of the
egg. Only one sperm will cross the plasma membrane to fertilize the egg.
As soon as the first sperm fuses with the egg, the plasma membrane
of the egg changes radically to prevent any further binding of sperm.
This prevents multiple sperms from fertilizing one egg. If two sperm
cells were to fertilize an egg, it would have
three copies of the genetic materials, which would not yield a
viable zygote. After one sperm cell enters the egg, the sperm nucleus
and the egg nucleus fuse, completing fertilization.
If fertilization does not occur immediately, sperm can survive in the Fallopian tubes for 3 to 5 days. Unlike sperm, an egg can survive for only 24 hours after ovulation.
Watch and Listen
Now that you have been introduced to the concepts and terminology of embryonic development, watch the following videos to gain further understanding of this important prenatal period.
©Alberta Education. Embryonic Development and Differentiation: From Single Cell to Complex Organism (0.00-28:50); Series 19. LearnAlberta.ca