and Departments of Physiology4 and Anatomys

The present study established the magnitude of and temporal changes in the pressure within in situ rabbit conceptuses during early pregnancy. At the 7th, 8th, 9th, or 10th day post coitum (dpc), animals were anesthe-tized, and the number and dimensions of implantation sites were recorded. Conceptus pressure was measured by the servo-nulling method employing a glass micro pipette. The mean number of sites was 6/cornu; spacing appeared normal. Implantation dome volume increased from 7 through 10 dpc 0.2 ± 0.03; 0.5 ± 0.1; 1.6 ± 0.2, and 2.7 ± 0.9 cm3, respectively (Day 7 vs. 8, 9 and 10, p<O.o1). Intraconceptus pressure declined between 7 and 10 dpc: 6.3 ± 0.4; 5.7 ± 0.4; 4.5 ± 0.9, and 3.9 ± 0.7mm Hg, respectively (Day 7 vs. 10, p<O.05). Pressure fluctuated; the frequency of change in pressure varied between 7 and 8 dpc (4.2 ± 3.0 to 2.3 ± 3.5 peaks/mm). The amplitude of fluctuation did not vary significantly between 7 and 10 dpc (2.6 ± 0.3; 0.79 ± 0.8 mm Hg, respectively). Simultaneous measurement of pressure within the conceptus and within the adjacent uterine lumen indicated that alterations in luminal pressure only occasionally influenced pressure within the con ceptus. The decline in conceptus pressure suggests that the uterine wall becomes progressively more compliant as blastocyst cavity/yolk sac fluid accumulates within the conceptus. Conceptus expansion resulting from internal pressure may enhance conceptus-uterine metabolic exchange by facilitating apposition of conceptus-uterine surfaces and by increasing the ratio of con ceptus surface to cytoplasmic mass