Preimplantation development of giant triploid zygotes in the mouse

Giant oocytes or two-cell embryos have been reported in various mammalian s pecies. They may arise during multiplication of oogonia, after fusion of tw o oogonia or, more probably, when nuclear division is not accompanied by cy toplasmic division. The ultimate fate of these giant embryos is not well kn own. In our laboratory, giant two-cell mouse embryos have been occasionally observed. Recently, we observed two giant one-tell zygotes in the same spe cies, Both showed two female pronuclei and one male pronucleus, as well as two second polar bodies localized at opposite poles of the embryo. These tw o giant zygotes showed normal viability and developmental capacity, Their t riploid nature was confirmed by cytogenetic analysis, In order to study thi s interesting phenomenon in more detail, we produced giant oocytes containi ng two germinal vesicles by cell fusion and cultured them in vitro. About o ne third of them extruded two first polar bodies; in the second group only one polar body was observed, whilst the last group was without polar bodies . When parthenogenetically activated, the consistent answer analogical to t hat observed in "in vivo" oocytes was only observed when oocytes with two p olar bodies were activated, The implication for IVF technologies is discuss ed.