DNA strand breaks and phosphatidylserine redistribution in newly ovulated and cultured mouse and human oocytes: occurrence and relationship to apoptosis

Abstract

This study determined the occurrence of two molecular markers of apoptosis, chromosomal DNA strand breaks and oolemma phosphatidylserine redistribution, in >200 uninseminated and unfertilized human oocytes, and >800 newly ovulated and cultured mouse oocytes. DNA breaks were analysed by terminal deoxynucleotidyl transferase-mediated dUDP nick-end labelling (TUNEL) and phosphatidylserine by annexin V staining, with imaging by conventional epifluorescence and scanning laser confocal fluorescence microscopy. More than 300 intact and 500 fragmented mouse oocytes were examined at 24 h intervals during 6 days of culture in three different types of medium. For the human, 205 oocytes were examined at retrieval or at 24 h intervals during 7.5 days of culture in two types of medium. The perifollicular vascularity and the dissolved oxygen content of follicular fluid were determined for most of the follicles from which human oocytes were derived. The results demonstrate that TUNEL fluorescence of metaphase II (MII) chromosomes and annexin V staining of the oolemma in newly ovulated and cultured mouse and human oocytes are rare, and, when detected, are not spatially or temporally related. This finding also applied to mouse oocytes that fragmented during culture and exhibited morphological features that grossly resembled apoptotic body formation. In contrast, TUNEL but not annexin V staining occurred in the first polar body of a relatively high proportion of newly ovulated mouse oocytes, but was rarely detected in newly aspirated human oocytes. For the human, the occurrence of MII chromosomal TUNEL fluorescence was patient-specific and unrelated to perifollicular vascularity or dissolved oxygen content of the corresponding follicular fluid. The pattern of chromosomal TUNEL fluorescence observed in the first polar body and in the MII chromosomes of a very small number of mouse and human oocytes, especially after many days of culture, suggests that DNA strand breaks may not arise by apoptosis-associated endonuclease digestion. The results with these two markers suggest that it is premature to conclude that apoptosis occurs in ovulated oocytes or that such a mechanism is involved in the elimination or prevention of fertilization of oocytes with cytoplasmic or chromosomal defects.