Delayed implantation in the northern fur seal (Callorhinus ursinus) and other pinnipeds.

Delayed implantation is an obligate part of the reproductive cycle of many (possibly all) pinniped species, the duration of delay ranging from 2 to 4 months depending on the length of gestation. Blastocyst reactivation and implantation in the northern fur seal occur during decreasing photoperiods, and are preceded by increase in the circulating concentrations of oestradiol-17 beta and progesterone. However, neither experimental shortening of photoperiod, nor injections of oestradiol-17 beta or progesterone (or both) induced reactivation. Furthermore, these treatments had no effect on the composition or quantity of proteins in uterine flushings, examined by gel filtration (though changes are associated with spontaneous implantation). All treatments involving capture and caging of seals caused an increase in blastocyst mitotic activity (though not expansion), and it is suggested that this represents a response to stress. Experiments in which fur seal or rabbit blastocysts were cultured in vitro failed to identify a growth promoting substance whose absence from uterine fluid may account for the dormancy of blastocysts during diapause; however, these experiments did demonstrate the presence of an inhibitor or rabbit blastocyst growth in fur seal serum. It is suggested that an unidentified compound regulated by the ovary is involved in stimulating the increased uterine proliferation and secretion which are necessary for blastocyst reactivation.