The transgenic boom is built on application of knowledge learned in traditional studies of mouse embryology and developmental biology. Developmental biology examines the programmed series of events that follows fertilization of the egg. After fertilization the mouse cell divides, or cleaves, into two cells called blastomeres, which cleave into four cells, which cleave into an eight-celled embryo, called a morula.
Cleavage continues as the embryo develops.
The morula rapidly develops into the blastocyst stage about 3.5 days following fertilization. As the embryo grows, cells differentiate into specialized tissues and organs. At birth, the mouse contains the same differentiated cell types and tissues as a human. Remarkably, the mouse pup is fully developed and born only 19 days following fertilization of the egg, compared to approximately 266 days for the human. For the initial 100 hours or so of post-fertilization development, the mouse and human embryos are virtually indistinguishable visually from one another.