Establishment of pregnancy in the pig: I. Interrelationships between preimplantation development of the pig blastocyst and uterine endometrial secretions.

Pregnancy recognition in pigs occurs between Days 11 and 12 when blastocysts undergo transformation from the spherical to filainentous form. This study evaluated the relationship between blastocyst development and total calcium (Ca), estrone (E, ), estradiol (E2), estriol (E3), E1 sulfate (E1S), E2S, E3S, prostaglandin F (PGF), PGE2, protein (Pr) and acid phosphatase (AP) activity in uterine flushings obtained from pregnant gilts between Days 10 and 14. Data from pregnant gilts were compared to those obtained from uterine flushings collected between Days 10 and 14 of the estrous cycle. Surface and ultrastructural changes in the endometrium associated with blastocyst development were evaluated by scanning (SEM) and transmission (TEM) electron microscopy. Pregnant uterine flushings were analyzed relative to average size of blastocysts recovered: 5 mm spherical, 5-8 mm spherical, 9-50 mm tubular, and >50 mm filamentous Day 12 and Day 14. Nonpregnant gilt uterine flushings were analyzed by Day of the estrous cycle (10.5, 11, 11.5, 12 and 14). Total E2 content increased almost 4-fold in pregnant uterine flushings containing tubular blastocysts (2.3 ng) as compared with flushings with spherical blastocysts (0.6 ng) and continued to increase as the blastocysts became filamentous (4.4 ng) but had declined by Day 14 (0.4 ng). The pattern of change for total recoverable E, and E3 was similar to that for E2 and the highest values were obtained in uterine flushings with filamentous blastocysts. A concomitant increase in E, S and E2 S content also was detected in flushings with tubular and Day 12 filamentous blastocysts. Total Ca, Pr, AP, PGF, and PGE2 content increased in association with increased E2 in flushings. The increase in Pr, AP, PGF and PGE2 content in pregnant flushings continued to Day 14. However, Ca content had declined by Day 14 (0.1 mg) after a transient increase to 1.5 mg in flushings that contained tubular blastocysts (Days 11 and 12). Comparable changes in estrogens, proteins, Ca, PGF and PGE2 content in nonpregnant uterine flushings collected between Days 10.5 and 14 were not detected. Electrophoresis of protein in pregnant uterine flushings indicated that the appearance of three basic uterine proteins (Mr 32K to 60K) were associated with the increase in estrogen. These proteins were detected in flushings with tubular and filamentous blastocysts (Day 12), but were not found until Day 14 in nonpregnant gilts. A synchronized release of secretory vesicles from the glandular epithelium was observed by TEM which indicated a close association between formation of tubular blastocysts, onset of blastocyst estrogen production and increased protein in uterine flushings. Although secretion was detected in nonpregnant glandular epithelium, no synchronized release was observed. Results suggest that estrogen production by tubular and early filamentous blastocysts (Day 11.5-12.0) stimulates secretion of protein from the endometrium which may be mediated through an effect of free Ca on the uterine glandular epithelium. Increases in PGF and PGE2 were associated closely with estrogen production and blastocyst elongation.