Differences between mature ovarian and oviductal oocytes: a study using the golden hamster

To investigate whether mature ovarian oocytes are physiologically identical with recently ovulated oocytes, hamster oocytes collected from ovaries approximately 1 h before ovulation were compared with the oocytes which had been in the oviduct for 3-4 h. Although these two groups of oocytes were at the metaphase of the second meiosis, there were quantitative differences between the two with respect to (i) sensitivity of cumulus matrix to hyaluronidase and spermatozoa (oviductal greater than ovarian), (ii) size of the perivitelline space (oviductal greater than ovarian), (iii) viscosity of the ooplasm (ovarian greater than oviductal), (iv) responsiveness to Ca2+-ionophore (oviductal greater than ovarian), and (v) time needed in completing meiosis (ovarian greater than oviductal). The most prominent difference was found in the zona pellucida. The zona of the oviductal oocyte was 'heterogeneous' in its optical density and had a stronger acrosome-reaction-inducing ability than that of the ovarian oocyte. When cultured in artificial media (e.g. F-10 medium with serum) for 4 h, the ooplasm of ovarian oocytes became like that of oviductal oocytes. However, their zonae remained unchanged. Zonae of ovarian oocytes became like those of oviductal oocytes only when they were exposed to ampullary and/or isthmic fluids. The zona-altering factors in the oviductal fluid (oviductal glycoproteins), which are apparently integrated into the native zona, may act to enhance the various functions of the zona.