Early Growth of Rabbit Trophoblast

Several indices of growth are recorded for early trophoblast tissue, as calculated from measurements made on rabbit blastocysts throughout the first ten days of development. Doubling time of trophoblast surface cells, up to six days post coitum, is about eight hours, extending thereafter at the average rate of five hours per day until it is greater than 24 hours by nine to ten days post coitum. The mitotic index stays relatively constant throughout this period, averaging 4.42 per cent. The duration of mitosis in early trophoblast tissue is shown to be about 30 minutes up to six days post coitum. It may lengthen beyond that time to partially account for the change noted in doubling time. Production of syncytiotrophoblast and cell death are also considered to contribute to this change. Calculations are made for the rate of blastocyst fluid accumulation. An acceleration rate constant is observed equal to .222 x 10-6 mm3/trophoblast cell/hour/hour. The existence of some type of transtrophoblast pump is inferred to account for this rapid rate. Correlations between trophoblast cell size, onset of fluid accumulation, and blastulation are noted.