EVIDENCE FOR A PREIMPLANTATION RISE IN OESTRADIOL-17β LEVELS ON DAY 4 OF PREGNANCY IN THE MOUSE

Abstract

Summary. Measured by radioimmunoassay, the peripheral plasma oestradiol concentration reached a peak (21·2±1 pg/ml) between 10.00 to 12.00 hours on Day 4 of pregnancy in mice. These values were significantly different from all others measured between 07.00 and 18.00 hours. To determine the physiological significance of this peak, ovariectomies were performed at selected intervals. Ovariectomy at 08.00 hours or earlier usually prevented implantation. The period from 10.00 to 12.00 hours on Day 4 was found to represent a critical time period before which insufficient oestrogen is secreted, in most instances, to initiate implantation.