Module 4 Lesson 1 - 5

Lesson Summary

Prenatal development is  two main periods of development. Embryonic development is from fertilization to the end of the eighth week of development. Other names are given to the embryo to describe the various specific stages of development of the offspring. The ninth week to birth is termed the fetal development period, and the offspring is referred to as the fetus during this time.

After the egg and sperm have been formed correctly 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 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 Fallopian tube, the zygote begins to divide through the process of cleavage. 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 called a blastocyst.

The blastocyst forms two distinct layers of cells. The outer layer is called the trophoblast, which will develop into the supportive membrane structure called the chorion. The trophoblast produces human chorionic gonadotropin (hCG) hormone, which maintains the corpus luteum. The mass of inner cells of the blastocyst form the embryo. As the blastocyst reaches the uterus, the trophoblast produces and secretes an enzyme that digests some of the endometrial tissue, enabling the blastocyst to implant itself in the uterine lining.