Embryo Project Encyclopedia Articles

As mice embryos develop, they undergo a stage of development called gastrulation. The hallmark of vertebrate gastrulation is the reorganization of the inner cell mass (ICM) into the three germ layers: ectoderm, mesoderm, and endoderm. Mammalian embryogenesis occurs within organisms; therefore, gastrulation was originally described in species with easily observable embryos. For example, the African clawed frog (Xenopus laevis) is the most widely used organism to study gastrulation because the large embryos develop inside a translucent membrane. Domestic chicken (Gallus gallus) gastrulation was also an early model organism because researchers could open the egg during development to look inside. Despite the challenges associated with studying mammalian gastrulation, the common house mouse (Mus musculus) has helped to shed light on the unique adaptations associated with mammalian development, and on the subtle differences in structure that give rise to significant divergence in late embryogenesis.

Implantation is a process in which a developing embryo, moving as a blastocyst through a uterus, makes contact with the uterine wall and remains attached to it until birth. The lining of the uterus (endometrium) prepares for the developing blastocyst to attach to it via many internal changes. Without these changes implantation will not occur, and the embryo sloughs off during menstruation. Such implantation is unique to mammals, but not all mammals exhibit it. Furthermore, of those mammals that exhibit implantation, the process differs in many respects between those mammals in which the females have estrous cycles, and those mammals in which the femals have menstrual cycles. Females in the different species of primates, including humans, have menstrual cycles, and thus similar processes of implantation.