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P. S. Burgoyne, S. Mahadevaiah and U. Mittwoch

Summary. A quantitative histological analysis of ovaries from 3- and 5-day-old female mice heterozygous for the male-sterile reciprocal autosomal translocation, T(11;19)42H, revealed a marked reduction (by 65%) in the number of oocytes as compared to controls. These findings call into question the widely held view that chromosomal anomalies causing spermatogenic failure have no effect on oogenesis. It is suggested that during meiosis in males and females there is a mechanism operating which tends to eliminate cells which had incomplete chromosome pairing at the pachytene stage.

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P. S. Burgoyne and T. G. Baker

Summary. The total number of oocytes (healthy and atretic) in XO and XX mice at ages ranging from 12 to 200 days after birth was determined using a quantitative histological technique. For each oocyte the stage of the enveloping follicle was recorded. The results demonstrated that throughout the period studied XO mice had approximately half the number of oocytes found in XX mice. No increase in the incidence of atresia was detected in XO ovaries. In XO mice at 12 days of age the establishment of the population of growing follicles was clearly retarded.

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P. S. Burgoyne and T. G. Baker

Summary. Postnatally, XO mice have approximately half as many oocytes as their XX sisters. A quantitative histological analysis of XO and XX ovaries throughout oogenesis (14½24½ days post coitum) revealed that this oocyte deficiency in XO mice is due to excess atresia of oocytes at the late pachytene stage (19½ days post coitum). Female mice heterozygous for a large X inversion (In(X)/X mice) were also found to have excess atresia at late pachytene. It was suggested that in XO mice it is the presence of an unpaired X chromosome, and in In(X)/X mice, the incompleteness of X chromosome pairing, which leads to this excess oocyte atresia.

A new quantitative histological procedure which was developed for the analysis of perinatal mouse ovaries is also described.

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P. S. Burgoyne, P. P. L. Tam and E. P. Evans

Summary. The growth and development of XO and XX mice were compared from 7¼ to 18½ days of gestation. The 7¼-day XO egg cylinders were retarded in development (and consequently small) when compared with XX egg cylinders, and this lag in development remained until 10½ days. By 12¼ days there was a considerable degree of 'catch-up', but this was not fully maintained. A subgroup of very severely retarded XO fetuses were preferentially located near the cervix. The placentas of XO fetuses were of normal size until 18½ days when they were significantly larger than those of XX controls.

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P. S. Burgoyne, E. P. Evans and Karen Holland

Summary. The growth of XO mice and their XX sisters was followed from the day of birth up to 15 weeks post partum. XO mice were underweight at birth, and grew more slowly than XX mice in the preweaning period. Some, but not all, of this decrease in growth rate was attributable to an effect of the reduced birth weight.

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T. G. Baker, S. Challoner and P. S. Burgoyne

Summary. Random bred Schofield albino mice were unilaterally ovariectomized or sham-operated at 6 weeks of age and were killed 4, 8, 16 or 32 weeks later. Ovarian weights were recorded before quantitative histological analysis of the total number of oocytes per ovary, the number of follicles per size class, and the proportion of oocytes undergoing atresia. Ovarian weights were consistently higher in the unilaterally ovariectomized mice than in coeval controls and by the 8th week after surgery the weight of the remaining ovary approached the combined weights of the two ovaries in controls. On a per ovary basis, follicular 'pool' size was reduced and the number of growing follicles increased after unilateral ovariectomy, these changes occurring within the first 4 weeks after surgery. Thereafter, oocyte depletion occurred at the control rates. There was no change in the rate of atresia after surgery.

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S. K. Mahadevaiah, R. Lovell-Badge and P. S. Burgoyne

In this paper we have compared the breeding performance of Tdy-negative XY, XXY and XYY females to assess the relative importance of the lack of a second X chromosome compared with the presence of a Y chromosome, in reducing fertility. The XY females were of poor fertility, although five of twelve produced at least one offspring. The XXY females had larger, more frequent litters, and a longer reproductive lifespan, implicating the lack of a second X as the major cause of the poor fertility of XY females. Nevertheless, XYY females appeared to be more seriously affected than the XY females, suggesting that the presence of the Y may be a contributory factor. Pachytene analysis demonstrated that the Y is a very inefficient pairing partner for the X during female meiosis. In XY females only 11% of pachytene cells had the X and Y paired; in XXY females the two X chromosomes paired and the Y was almost always a univalent, while in XYY females the X paired with a Y in only 15% of pachytene cells. The presence of unpaired sex chromosomes has previously been implicated as a cause of oocyte loss during pachytene, and the proportion of cells with unsynapsed sex chromosomes decreased as pachytene proceeded, suggesting that they were progressively eliminated. Significantly, protection against elimination was afforded not only by synapsis between sex chromosomes, but also by self-synapsis if a sex chromosome remained as a univalent. It is concluded that sex chromosome univalence leading to pachytene oocyte failure is responsible for the postnatal oocyte deficiency seen in XY females. A separate study has shown that XXY females have a similar level of oocyte deficiency. It is suggested that the presence of a second X chromosome improves the fertility of XXY females, compared with XY females, by improving oocyte quality and by eliminating the production of lethal XY and OY zygotes. The genotype frequencies for the offspring of XY and XXY females differed from those predicted from the pachytene data. The XY females showed a marked deficiency of XO offspring compared with XXY and XYY aneuploid offspring, whereas the XXY females had fewer than expected XXY and XYY aneuploid offspring.

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P. S. Burgoyne, R. M. Borland, J. D. Biggers and C. P. Lechene

Summary. Electron probe microanalysis was used to determine the concentrations of Na, Cl, K, Ca, Mg, S and P in samples of follicular fluid, ovarian vein serum and peripheral venous serum obtained from virgin rabbits at 2-h intervals up to 10 h after injection of hCG. Throughout this 10-h period the elemental composition of follicular fluid was essentially the same as that of blood serum. However, there was a significant drop in follicular fluid Ca relative to blood during the 10-h period which may reflect Ca involvement in the regulation of oocyte maturation. Significant differences were also found between follicles within rabbits for K and P concentrations.