CIRCULATING LEVELS OF OESTRADIOL-17β DURING EARLY PREGNANCY IN THE ALASKAN FUR SEAL SHOWING AN OESTROGEN SURGE PRECEDING IMPLANTATION

From morphological, physiological and biochemical observations, Shelesnyak, Kraicer & Zeilmaker (1963) demonstrated the existence of an `oestrogen surge' in the rat occurring on Day 3 of either pregnancy or pseudopregnancy (spermatozoa in the vaginal smear on Day 0). This observation was confirmed by Shaikh & Abraham (1969) with radioimmunoassay for oestradiol concentration in ovarian venous blood plasma. The oestrogen surge coincides with the period of blastocyst development immediately preceding implantation. Furthermore, an artificial surge produced by exogenous oestradiol-17β in rodents ovariectomized before Day 3 terminates a state of facultative delayed implantation by reactivation of blastocysts in diapause (Cochrane & Meyer, 1957). McLaren (1972) has suggested that ".....the necessity for an oestrogenic intervention at the time of implantation,....., may be characteristic of those species where the embryo undergoes delayed implantation". We were interested in determining whether a representative of a group having obligatory delayed implantation might also have an oestrogen surge