>Embryogenesis

Mouse Embryology

Figure 1: Overview of mouse embryogenesis

The development of a mouse embryo begins when an oocyte is fertilized by a sperm, creating a one-cell zygote. There is no yolk enrichment in mammalian embryos, but there is still a yolk sac. After the formation of the zygote, cleavages occur slowly, about every 12-24 hours. The first cleavage divides the zygote meridonally into 2 cells. The second division is equatorial, creating a 4-cell embryo. At the beginning stages, all of the cells are equivalent. After cleavage, compaction of the embryo causes the cells to be different from one another and leads to formation of the morula. The blastocyst forms next and is made up of an inner cell mass, which will become the mouse embryo, a fluid-filled cavity called the blastocoel, and a trophectoderm, which will become the placenta. The blastocyst forms around embryonic day 3 in mice and about 5 days after fertilization in humans. The next step in mouse embryogenesis is gastrulation, in which the ectoderm, mesoderm, and endoderm are derived from the two cell layers of the egg cylinder. The ordering of these three germ layers is exactly opposite that of most other mammals, including humans. The mouse embryo must turn at around E8.0-8.5 so the dorsal side faces outward and the ventral side faces in. After gastrulation, the neural tube is made and closed during neurulation. The final step in mouse embryogenesis is organogenesis, in which cells migrate and differentiate into organ systems. The embryo will mature inside of the mother until it is ready to be born. The average gestation period of a mouse is 20 days, which is significantly less than humans (280 days).