Diabetes and preimplantation events of embryogenesis.

From animal and human studies is it clear that mammalian embryos are vulnerable to injury during the earliest preimplantation stages of development. Exposure to some agents during this brief period has resulted not only in fetal loss but also in malformations. Thus, the potential for maternal induced embryotoxicity exists earlier than previously expected. This period is marked by a drastic change in metabolic needs at the late morula stage. Glucose becomes the predominant exogenous energy substrate and enters the blastocyst via one of three facilitative glucose transporters, GLUT1, GLUT2, and GLUT3. It has been shown that maternal diabetes adversely affects the preimplantation embryo. Recent work has revealed that hyperglycemia leads to a downregulation of the GLUTs at the blastocyst stage in the mouse. This downregulation results in decreased glucose transport into the blastocyst of diabetic mice and thus lower intraembryonic free glucose levels. The embryos are starving themselves of the key substrate necessary for survival. Maternal diabetes also causes a decrease in the number of cells in rat blastocyst and recently hyperglycemia has been shown to induce apoptosis in the mouse blastocyst via cell death effector pathways involving BAX and caspases. Significant loss of key progenitor cells from the blastocyst may predispose these diabetic embryos to later developmental deficiencies manifesting as dysmorphogenesis, fetal loss or early growth delay. The hypothesis that the hyperglycemia-induced decrease in glucose transport triggers apoptosis is presented and discussed as a novel mechanism to explain preimplantation diabetic embryopathy.