Human embryogenesis is a complex process that occurs during the first eight weeks after fertilization. The stages of embryogenesis are described in more detail below.
Image Credit: Andrii Vodolazhskyi / Shutterstock.com
Stage 1: The process of embryogenesis begins with an egg or ovum being fertilized by a sperm cell to form a zygote. The zygote is surrounded by a strong membrane made up of glycoproteins, which the sperm has managed to penetrate. The zygote is formed 24 hours after the egg and sperm nuclei fuse. The zygote is a single diploid cell.
Stage 2: Over the next three days, the zygote undergoes a number of cell divisions, a process referred to as cleavage. Once the embryo has reached the 8-cell stage, it undergoes another process called compactation, which involves the tight binding of the cells to create a compact sphere. After compactation, the embryo is made up of 16 cells, which is referred to as the morula.
Stage 3: After one more day, a cavity referred to as the blastocele develops in the morula. Cells inside the blastocele compact and flatten to form a structure referred to as the blastocyst.
Stage 4: Around 24 hours later, the blastocyst moves towards the womb, where it implants itself in the lining.
Stage 5: Over the next week, the mass of cells rapidly divides, giving rise to a disc-shaped structure that has two layers. One of the layers goes on to become the embryo and the amniotic cavity, while the other layer develops into the yolk sac.
Stage 6: A process called gastrulation also occurs, where the blood system starts to appear in the placenta and blood cells are produced by the yolk sac. A streak of cells becomes apparent on the embryonic disc.