Abstract: 260

LUTEAL REGULATION AND THE TIMING OF EMBRYO IMPLANTATION IN THE CORN MOUSE, Calomys musculinus (MURIDAE, SIGMODONTINAE).

O Buzzio¹ , A Koninckx¹ , N Carreño¹ , A Castro-Vazquez^{1 *}
Laboratorio de Reproducción y Lactancia (LARLAC - CONICET) and Cátedra de Fisiología Normal, Universidad Nacional de Cuyo, Mendoza, Argentina ¹

The corn mouse is a representative of the diverse group (about 250 species) of Neotropical sigmodontine rodents. Previous evidence (Biol Reprod 58:620-625, 1998) has indicated that this mouse displays a peculiar mode of luteal regulation, that includes an ultrashort luteal phase (2-3 days duration) that cannot be prolonged as a consequence of coital stimulation. Therefore, an implantation date earlier than that of other studied murid species was expected. In the present work, direct observations of the timing of implantation were made in females that had copulated during postpartum estrus and that were either lactating (L+) or not (L-). Observations were made on days 3 to 5 of pregnancy (day 1=day after coitus), both at 3-5 AM (morning) and at 5-7 PM (evening). Embryos were first detected in uterine flushings of L- females on the morning of day 3 (1/8 cases) and were present in flushings of all L+ (N=10) and L- females (N=10) by that evening. On the morning of day 4, blastocysts could still be recovered by uterine flushing, both in L+ (6/10) and L- (6/10) females; no implantation swellings could be clearly seen at that time, but the IV injection of Evans blue was followed by endometrial blue spots in all cases (5 L- and 6 L+ females). No blastocysts were recovered by uterine flushing on day 4 evening in all 10 L- females, while they could still be recovered in 3 of 10 L+ females. Blastocysts were also recovered from a single L+ female (out of 10) on the morning of day 5. It is concluded that implantation in the corn mouse occurs about 48 h earlier than in the rat, and that a short lactational delay of implantation may occur in a small proportion of animals.

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