Glucose transport and utilization play a crucial role during preimplantation development of the embryo, especially during the transition into the blastocyst stage. In the earlier developmental stages embryos do not use glucose as the main fuel but prefer more oxidized sources of energy such as pyruvate and lactate. The increased glucose transport indicated in mouse, bovine, porcine, rat, rabbit and human blastocysts is probably caused by the elevated energetic demands resulting from the morphological changes occurring in this stage of development. Moreover, the decreased glucose transport causes apoptotis in blastocyst cells resulting in further malformations of the fetus and even miscarriages. Glucose is transported into the embryo mainly by the membrane glucose transporters glycoproteins (GLUTs) GLUT 1, 2, 3, 4, 8, 9 that are specifically expressed during preimplantation development. GLUT 1, 2, 3, 9 are expressed in the cell membranes while GLUT 4 and 8 - insulin/IGF-I dependent transporters - are localized intracellularly and translocated into the membrane in response to insulin action. Contrary to the above mentioned transporters amongst which GLUT 1 is expressed as early as in oocytes, the expression of GLUT 4 and 8 begins from the blastocyst stage. The increased expression of glucose transporters on the blastocyst stage and especially the specific localization in blastocyst cells indicate their important role in preimplantation development.
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