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R. K. W. Smith and M. H. Johnson

Summary. The third (4-cell) and fourth (8-cell) cell cycles of early mouse development have been analysed in populations of blastomeres synchronized to the preceding cleavage division. DNA content was measured microdensitometrically. The entry of blastomeres into these cell cycles showed considerable heterogeneity both within and between individual embryos. This heterogeneity was greater in the fourth than in the third cell cycle. The component phases of the third cell cycle were estimated as G, = 1 h, S = 7 h, and G2 + M = 2–5 h, and those of the fourth cell cycle as G1 = 2 h, S = 7 h, and G2 + M = 1–3 h.

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S. K. Dey and D. C. Johnson

Summary. The effect of adrenal steroids upon implantation was evaluated by examining the efficacy of oestradiol-17β on the initiation of implantation in ovariectomized, ovariectomized plus adrenalectomized or hypophysectomized pregnant rats treated with progesterone. More oestrogen (× 5) was required in ovariectomized animals to obtain results equivalent to those obtained with the other animal models.

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S. K. Dey and D. C. Johnson

Summary. Mouse embryos recovered on the 4th day of pregnancy produced histamine, as evidenced by the14CO2 produced from carboxy labelled l-histidine, at the rate of 1·5 ± 0·3 (s.e.m.) pmol/embryo per hour. Most (83·2 ± 4·6%) of the embryos flushed from the oviducts on Day 3 of pregnancy (4–8-cell stage) developed into blastocysts within 48 h after being placed in culture. Inclusion of l-histidine hydrochloride (4·7 × 10−4 m) in the culture medium did not alter this development but dl-α-methylhistidine (3·8 × 10−4 m), an inhibitor of histidine decarboxylase, reduced the number of embryos developing into blastocysts to only 10·8 ± 6·8%. A combination of l-histidine and dl-α-methylhistidine in the medium prevented the growth-retarding effect of the latter compound. The results indicate that mouse embryos can produce histamine and suggest that this is necessary for normal development.

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S. Long, S. K. Dey and D. C. Johnson

Summary. An intravenous injection of 2-fluoro-oestradiol simultaneously with an implantation-inducing dose of oestradiol reduced the number of implantation sites in delayed implanting hypophysectomized rats maintained with progesterone. Administration of 2-fluoro-oestradiol 1 h before or after oestradiol had no effect. Furthermore, injection of as much as 500 ng 2-fluoro-oestradiol 48 h before administration of oestradiol failed to have any effect upon implantation, i.e. failure to block implantation was correlated with failure to induce the uterine refractory state. These results suggest that conversion of primary oestrogens to catechol oestrogens could be important for implantation as well as for the induction of the oestrogen refractory state in the uterus.

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RAMA A. VAIDYA, R. H. GLASS, PRAMILA DANDEKAR and K. JOHNSON

Since the introduction of the concept of capacitation, numerous investigators have attempted to define the changes that spermatozoa undergo in the female reproductive tract. Studies utilizing light and electron microscopy have failed to show alterations in sperm structure following incubation in the rabbit uterus for periods up to 15 hr, a length of time sufficient to accomplish capacitation (Bedford, 1970). The loss of tetracycline fluorescence from spermatozoa in the oestrous uterus has been suggested as an indicator of capacitation (Ericsson, 1967), but it has since been shown conclusively that this theory can no longer be considered tenable (Vaidya, Bedford, Glass & Morris, 1969).

Changes in the net negative surface charge of rabbit spermatozoa during passage through the epididymis have been demonstrated by micro-electrophoresis (Bedford, 1963). This communication describes changes

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R. C. Hoversland, S. K. Dey and D. C. Johnson

Summary. Rabbit blastocysts were homogenized by sonication, and centrifuged at 105 000 g for 60 min. The pellet was resuspended and incubated in phosphate buffer containing [1β-3H]testosterone and a NADPH generating system. The amount of 3H2O produced was determined by liquid scintillation counting. Enzyme activity was calculated, after subtracting blank values obtained with boiled embryos, and expressed as pg testosterone aromatized per embryo per hour. Aromatase activity was undetectable to low on Day 5 and increased on Day 6 of pregnancy. There was a 10-fold increase in activity in Day-6 embryos cultured for 24 h, with a further 6-fold increase in activity in Day-6 embryos cultured for 48 h. The enzyme had an apparent K m of 0·77 μm and was completely inhibited by an aromatase inhibitor. The results clearly indicate that the rabbit blastocyst has an increasing capacity for aromatization of testosterone at about the time of implantation.

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K Wellings, J Wadsworth, A Johnson, J Field and W Macdowall

Teenage mothers and their children face poorer prospects in life than do women who delay motherhood until later in life. Moreover, patterns of early childbearing tend to be repeated in subsequent generations. Therefore, an understanding of the factors associated with early fertility is important for the prevention of adverse consequences. This paper uses data from the National Survey of Sexual Attitudes and Lifestyles to explore these associations. Early sexual intercourse is an important predictor of early fertility, as is poor educational attainment, although it is not clear to what extent pregnancy acts to thwart academic ambitions, or to what extent poor educational performance leads to a need to seek personal fulfilment in other than academic goals. Thus, interventions designed to influence age at first intercourse and to improve educational performance both have potential in terms of impacting on teenage pregnancy rates. Family background also exerts a powerful influence on teenage fertility. Young people for whom one or both parents are absent are more likely to become parents early in life. However, the most important factor of family life determining the chances of teenage motherhood appear to be the quality of communication about sexual matters with the home. In terms of outcomes, teenage mothers are more likely to live in social housing, are less likely to be in paid employment and have larger than average sized families. Certain areas of the country, notably the older, run-down industrial areas, have higher rates of teenage motherhood than the newer, more prosperous areas. Because most of these effects are independent of one another, there is potential merit in intervening to prevent unintended conception at several points in a young woman's life. Primary preventive efforts are needed to reduce the rates at which teenage pregnancy occurs in this country. Yet, if the cycle of deprivation that means the children of young mothers themselves enter parenthood early is to be broken, then efforts must also be made to mitigate the effects of teenage fertility for both mother and child.

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A. Lopez-Sebastian, A. Gomez-Brunet, A. W. Lishman, S. K. Johnson and E. K. Inskeep

Five experiments were conducted with the objective of developing a method to induce superovulation in ewes with a single i.m. injection of FSH. This was achieved by injection of 10 mg FSH-P in propylene glycol at the same time as luteolysis was induced by cloprostenol on day 13 of the oestrous cycle (day 0 = oestrus). Experiments 1, 2, 3 and 5 were conducted in a single flock of Manchega ewes in Spain during the breeding season. Ovulation rates were determined at laparoscopy. In Expt 1, FSH-P was diluted in saline, and neither 5 mg FSH on day 1 nor 5 or 10 mg FSH-P on day 13 changed the ovulation rate after cloprostenol treatment on day 13. In Expt 2, FSH-P was diluted in propylene glycol and data were collected over 2 years. Ten milligrams FSH-P, on day 13 only, increased (P < 0.01) the mean number of corpora lutea to 5.5 compared with a control value of 1.5. Five milligrams FSH-P on day 13 only had no effect; however, 5 mg FSH-P on day 1 reduced the mean number of corpora lutea formed in ewes receiving 10 mg FSH-P on day 13 to 2.6 (P < 0.01). Saline and propylene glycol, as vehicles for 10 mg FSH-P, were compared directly at two times of injection in Expt 3. FSH-P increased the mean number of corpora lutea when injected on day 13 in propylene glycol (4.7) but not in saline (2.5; P < 0.5). Ovulation rate did not differ between diluents when FSH-P was injected 24 h before cloprostenol (1.3; day 12). Experiments 4 and 5 were conducted to examine possible mechanisms by which propylene glycol improved the response to FSH. In Expt 4, conducted during anoestrus in a crossbred flock in West Virginia, concentrations of FSH in plasma were measured for 58 h after injection of 10 mg FSH-P in saline or propylene glycol. Propylene glycol did not delay the time of maximum concentration of FSH in plasma after i.m. injection (2.7 ± 0.7 h) when compared with saline injection (3.6 ± 0.5 h). Maximum concentration was reached later when FSH-P was injected s.c. in propylene glycol (7.6 ± 0.7 h; P < 0.05). In Expt 5, ovulation rate was greater (P < 0.05) in ewes treated with 10 mg FSH-P in propylene glycol than in ewes treated with FSH-P in saline and an injection of propylene glycol at a separate site. The number of corpora lutea did not differ in ewes treated with FSH-P in saline and in ewes treated with FSH-P in saline and propylene glycol at a separate site. Thus neither delayed absorption nor an augmentation effect could account for the benefit of propylene glycol as a vehicle for delivery of FSH to superovulate ewes.

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J. F. Nelson, K. Karelus, L. S. Felicio and T. E. Johnson

Summary. Studies in C57BL/6J, DBA/2J and C3H/HeJ mice and in two F1 hybrid strains (B6D2F1 and B6C3HF1) 2–5 months old revealed marked genotypic differences among inbred strains. C57 mice had three times as many regular (3–6 days) cycles as DBA and C3H mice, due largely to fewer pseudopregnant-like (7–14 day) cycles. C57 had longer regular cycles than DBA and C3H mice. Although the frequencies of regular cycles of DBA and C3H mice were similar, the cycles of C3H mice were shorter than those of DBA mice. The results indicated that the genetic determinants of the frequency of regular cycles differ from those specifying cycle length. Frequency of regular cycles of F1 hybrids was either intermediate between the parent strains (B6D2F1) or similar to the C57 strain (B6C3HF1), suggesting that regular cycle frequency shows additive genetic variation in the former crosses, but mostly dominant variance in the latter background. Regular cycles were either shorter than in both parent strains (B6D2F1) or similar to one of them (B6C3HF1), indicating heterosis and dominance for genes specifying short cycles. Although the lack of reciprocal crosses meant that maternal effects and possible genomic imprinting effects could not be assessed, these results reveal marked genetic influences on cycle length and frequency and suggest that some of the genes specifying these two traits differ.

Keywords: genotype; oestrous cycle; pregnancy; mouse

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L. D. Johnson, D. F. Albertini, L. K. McGinnis and J. D. Biggers

Changes in chromatin organization, meiotic status and the development of meiotic competence in oocytes retained within mouse ovarian follicles from day 0 to day 6 in culture were examined. The effects of exposure for 24 h to human luteinizing hormone (hLH) during the last day in culture was also determined. Preantral follicles from 22- to 24-day-old (prepubertal) mice develop antra and undergo significant growth from day 0 to day 4 in culture, after which the growth rates slow. The statistical significance of meiotic progression was examined using exact logistical regression analysis, which is particularly useful when the data are sparse and unbalanced. The transition from rimmed to unrimmed germinal vesicle stages was found to occur between day 2 and day 4 of follicle culture and was not influenced by exposure to hLH. Treatment with hLH caused a significant increase in the proportion of intrafollicular oocytes resuming meiosis. Assays of meiotic competence performed in vitro in oocytes retrieved from cultured follicles demonstrated that the transition from an unrimmed to a rimmed state is closely coincident with the acquisition and expression of meiotic competence. Forty-six per cent of competent oocytes from follicle cultures at day 3 progressed to metaphase II. These results indicate that the follicle culture system used in these studies supports the transformation of enclosed oocytes from a precompetent to a competent state and can maintain meiotic arrest for up to 6 days in culture. However, an increasing proportion of oocytes exhibit abnormal meiotic progression with continued follicle culture beyond 4 days.