Embryo Project Encyclopedia Articles

Fetus in fetu is a rare variety of parasitic twins , where the developmentally abnormal parasitic twin is completely encapsulated within the torso of the otherwise normally developed host twin. In the late eighteenth century, German anatomist Johann Friedrich Meckel was the first to described fetus in fetu, which translates to “fetus within fetus.” Fetus in fetu is thought to result from the unequal division of the totipotent inner cell mass , the mass of cells that is the ancestral precursor to all cells in the body. The unequal division is thought to occur during the formation of the blastocyst, which can also result in parasitic and conjoined twins . Fetus in fetu represents a developmental anomaly that has prompted developmental biologists to further examine the mechanisms for how twins arise.

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.