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Audrey S. Bingel

Summary. The times at which post-partum ovulation occurred relative to the times of parturition, were similar for CD*-1 mice exposed either to 18L:6D or 10L: 14D and comparable to the times reported previously for mice of the same strain kept under 14L: 10D. When parturition took place close to 'lights off', ovulation tended to occur 13–14 h after littering (i.e. during the last part of the same dark period and the early part of the next light period). Conversely, when parturition took place closer to 'lights on', ovulation tended to be delayed by the equivalent number of hours so that it occurred during the last part of the next dark period and early part of the subsequent light period. This confirmation and extension of earlier work suggests that mice of this strain would be useful for investigating hormonal events associated with the timing of post-partum ovulation in the mouse.

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AUDREY S. BINGEL

Summary.

The time at which ovulation occurred on the morning of oestrus in unmated cyclic mice was investigated; the majority housed with males exhibited 4-day cycles and the majority housed in the absence of males exhibited 5-day cycles. Regardless of housing condition or cycle length, most animals ovulated between 03.00 and 08.00 hours (lights on from 05.00 to 19.00 hours) on the morning of oestrus. The majority of animals injected with barbital at 17.00 hours on the day of pro-oestrus did not ovulate, nor did their ovaries contain stimulated follicles on the morning of 'oestrus.' These results suggest that the critical period for the ovulatory release of LH is similar in 4-day cyclic mice housed in the presence of males and 5-day cyclic mice housed in the absence of males.

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AUDREY S. BINGEL and NEENA B. SCHWARTZ

Summary.

Body, uterine, ovarian and pituitary weights, and pituitary lh content were determined on each day of the oestrous cycle of individually housed, spontaneously cyclic mice. Uterine and pituitary weights varied significantly; uterine weight was maximum at pro-oestrus and pituitary weight was maximum at oestrus. The cyclic changes in the vaginal smear cells were correlated with the cyclic ovarian changes; the morning of di-oestrus I (2 mornings after newly ovulated ova are found in the oviduct) was the most useful marker for cycle stage, since it could always be recognized as the 1st morning on which only leucocytes and nucleated epithelial cells (no cornified cells) were present in the vaginal smear. In additional studies, females, which had been previously grouped (ten/cage), were paired individually with males and killed on days of 'induced' pro-oestrus (pre-ovulatory) or oestrus (post-ovulatory). The organ weights and pituitary lh content of these mice did not differ significantly from those obtained for spontaneously cyclic mice on the corresponding 2 days of the cycle.

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AUDREY S. BINGEL and NEENA B. SCHWARTZ

Summary.

The time at which ovulation occurred in post partum mice under L14:D10 (lights on from 05.00 to 19.00 hours) was investigated. Ovulation tended to occur during the late part of the dark period and early part of the light period, but the exact time depended on the time of delivery. Mice that gave birth between 21.00 and 01.00 hours ovulated approximately 26 hr after delivery. The interval between delivery and ovulation decreased the later in the day that delivery occurred, until it became 12 to 13 hr for mice that gave birth between 17.00 and 21.00 hours.

Barbital was administered at various intervals after delivery in an effort to determine the time of release of the lh responsible for ovulation. Although the data indicate that lh release tended to occur in most mice during the afternoon or early evening, there was no one time of day at which barbital was totally effective in blocking lh release for mice giving birth during different times of the entire 24-hr light and dark period.

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AUDREY S. BINGEL and NEENA B. SCHWARTZ

Summary.

The time at which ovulation occurred on the morning of oestrus in unmated cyclic mice was investigated. Ovulation was found to occur during the late part of the dark period and early part of the light period (lights on from 05.00 to 19.00 hours). Barbital was administered at different times before the expected time of ovulation in an effort to determine the time of the ovulatory release of lh. Barbital injection at 17.00 hours, but not at 21.00 hours, on the day of pro-oestrus prevented the appearance of ovarian evidence of lh release as indicated by the absence both of ovulation and of stimulated follicles.