Maturation of the mouse oocyte in vitro

In the course of examining about 5000 mouse oocytes maturing in vitro, three types of errors of first polar body formation, presumably spontaneous in origin, were observed: (1) twenty (0.4%) oocytes with a very large polar body possessing an organized metaphase II spindle instead of the usual scattered and degenerate polar body chromatin; (2) four (0.08%) oocytes in which two normal-sized polar bodies shared equally all the egg chromatin; and (3) one (0.02%) oocyte with all the chromatin in one group, probably representing failure of polar body formation. Large polar body formation provides a cytologic basis for those cases in man where fertilization of two egg nuclei by two spermatozoa has been postulated as the origin of certain mosaic individuals. The other two types of anomalies would presumably lead to death of the embryo.