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T. G. BAKER

Summary.

Thirteen pre-pubertal and fifteen sexually mature rhesus monkeys (Macaca mulatta) were subjected to 1000 to 12,000 r unilateral X-irradiation; the contralateral ovary served as a control. The ovaries were removed at intervals of 6 hr to 4 weeks later, and the numbers of normal and atretic oocytes at each stage of follicular maturation were counted in every 40th section of both treated and control gonads.

Doses of 4000 to 12,000 r induced cytoplasmic eosinophilia in oocytes, pyknosis in many granulosa cells and the destruction of most of the multi-layered follicles. The changes became more marked as the post-irradiation interval was extended.

The numbers of primordial oocytes were unaffected by exposure to less than 3000 r; such oocytes were, however, almost totally eliminated 7 days after treatment with 12,000 r. The LD50/30 for germ cells at this developmental stage is about 5000 r. Oocytes enveloped by more than one layer of granulosa cells were more radio-sensitive, only a small proportion surviving for 4 days after exposure to 4000 r. Ovaries recovered 25 days following irradiation with 5000 to 12,000 r contained few multi-layered follicles ; most of the germ cells were atretic and the granulosa cells pyknotic.

The primordial oocyte of the rhesus monkey is markedly resistant to radiation-induced cell death (following the terminology of Mandl, 1964), and contrasts sharply with the primordial oocytes of other mammalian species.

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P. NEAL and T. G. BAKER

Summary.

A series of experiments was carried out to assess the effects of HCG in vivo and in organ culture, on mouse oocyte and follicle maturation following PMSG treatment in vivo. Preovulatory maturation and its relationship to time of injection and duration of hormone treatment was assessed using quantitative histological techniques.

The general pattern and timing of meiotic changes was similar in organ culture to that occurring in the intact animal, although fewer oocytes reached metaphase II and ovulation was a rare event. This deficiency in the cultured ovaries was related to the destruction of Graafian follicles during the preparation of explants.

The stage of meiosis reached by the oocyte was largely a factor of critical time intervals, but the size of the follicle also had an important limiting effect. Thus, medium-sized and small antral follicles contained oocytes whose meiotic maturation was seemingly `blocked' at metaphase I, while Graafian follicles either reached metaphase II or persisted at the dictyate stage. These results are discussed in the light of current knowledge of oocyte/follicle cell interactions and the mode of action of gonadotrophic hormones.

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T. G. BAKER and ANNE McLAREN

Summary.

Pregnant Q strain mice received seven injections of tritiated thymidine (4, 40 or 400 μCi/dose) between the 13th and 16th days of pregnancy, and the ovaries of their female progeny were studied. Radioactive label was detected by autoradiography in a high proportion of the oocyte nuclei on the 17th day of gestation, and up to 1 month after birth. The total number of oocytes in the ovaries was reduced. The reduction was proportional to the dose of isotope used, and affected primordial follicles more than multilayered follicles. By 5 months of age, the numbers of oocytes in the 40-μCi and 4-μCi groups were reduced to about 1 % and 50% of the control values, respectively. Body weight, fertility, ovarian weight and the capacity of the ovary to undergo compensatory hypertrophy after unilateral ovariectomy were all affected in the 400-μCi group. We conclude that mouse oocytes are highly sensitive to internal β-radiation from [3H]thymidine incorporated during embryonic life.

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P. NEAL and T. G. BAKER

Summary.

A series of experiments was carried out to assess the effects of HCG in vivo and in organ culture on oocyte and follicle maturation following treatment with PMSG in vivo. Quantitative histological procedures were used to assess the proportion of follicles in which the oocytes had resumed meiosis beyond the germinal vesicle (diffuse diplotene) stage in response to different doses of hormones.

The injection of 3 to 5 i.u. PMSG followed 48 hr later by treatment with 1 to 2 i.u. HCG induced the maximum preovulatory response in ovaries of Schofield albino mice both in vivo and in organ culture. The proportion of large antral follicles with oocytes at metaphase II, however, was much lower in culture than in the intact animal. By contrast, the number of pre-antral and small Graafian follicles increased to a level far higher than in vivo and thus the overall total number of oocytes that progressed beyond the germinal vesicle stage was higher in organ culture.

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P. NEAL and T. G. BAKER

The response of the ovary to exogenous gonadotrophins is known to be dependent upon the age of the animal at the time of treatment (Zarrow & Wilson, 1961; McLaren, 1967). The present experiments were carried out to reinvestigate this phenomenon in intact mice and to determine whether this relationship also holds when mouse ovaries are treated with PMSG and HCG in organ culture.

For the in-vitro studies, mice of the Schofield albino strain aged 14, 21, 28, 35 or 49 days post partum (p.p.) were killed by dislocation of the neck and their ovaries were cleanly dissected free from adnexa. The technique for organ culture has been described in full in our earlier papers (Baker & Neal, 1969, 1972). At the onset of culture, PMSG ('Gestyl': Organon Laboratories, London)

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Morag G. Hunter and T. G. Baker

Summary. Human corpora lutea of various ages were minced and incubated in the presence of hCG (10 i.u./ml), cAMP (10 mm) or FSH (20 mu/ml) and production of progesterone and oestradiol was measured. Cyclic AMP and hCG stimulated progesterone and oestradiol production during at least the mid- and late luteal phases, but FSH stimulated only oestradiol production during the early and mid-luteal phases and had no effect on progesterone production. This demonstrates that progesterone and oestradiol synthesis by the human corpus luteum can be independently controlled.

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J. S. G. Biggs and T. G. Baker

Summary. Human CL removed during the menstrual cycle were cultured for up to 5 days. Production of progesterone, measured by radioimmunoassay of the culture fluid, continued throughout each experiment. Addition of hCG to the culture medium stimulated progesterone production in a biphasic manner.

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B. M. Hobson and T. G. Baker

Summary. The effect of ovarian X-irradiation of the rhesus monkey upon menstrual cycle length, duration of menstrual bleeding, excretion of gonadotrophin in the urine, concentration of gonadotrophin in the pituitary gland, ovarian histology, and breeding performance was investigated.

Doses below 4000 R had no significant effect upon the above criteria. By contrast, doses of 4000 to 7000 R rapidly induced amenorrhoea in most animals, but unexpectedly these animals did not excrete greater than normal amounts of gonadotrophin. The pituitary content of gonadotrophin in cyclic and non-cyclic animals was not significantly different. Some births occurred in the animals which had been X-irradiated.

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O Wai-sum and T. G. Baker

Studies of fetal ovaries maintained in organ culture have shown that meiosis can proceed in vitro (Martinovitch, 1938; Borghese & Venini, 1956; Baker & Neal, 1973). However, if the ovaries are removed from the fetus before the onset of meiosis, the germ cells fail to undergo normal meiosis in culture and subsequently degenerate (mouse: Wolff, 1952; sheep: Mauléon, 1973; hamster: Challoner, 1975a). In the hamster, meiosis fails to occur if the ovaries are removed before the 15th day of gestation, even if gonadotrophic hormones or maternal serum are added to the cultures, suggesting that an 'inductor' is required to initiate meiosis (Challoner, 1975a). However, the meiosis inducing factor has not been investigated further. Byskov (1974) grafted fetal ovaries into nude mice and showed that the external rete ovarii was necessary to 'trigger' the induction of meiosis in the germ cells. In the present study, the effect of the external rete ovarii on the initiation of meiosis in the hamster was studied in organ culture.

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T. G. BAKER and L. L. FRANCHI

It is well known that the sequence of nuclear changes which takes place during the differentiation of the oocyte is interrupted for a relatively lengthy period towards the end of meiotic prophase. This generally occurs with the onset of the diplotene phase, although in the primordial oocytes of some mammals it is superseded by the dictyate phase (see Baker & Franchi, 1967b). The meiotic division is normally only resumed shortly before ovulation, which indicates that for a particular oocyte the `resting' period may last from a few weeks to many years (Brambell, 1956; Franchi, Mandl & Zuckerman, 1962).

On the basis of histological observations, however, certain authors have claimed that the oocytes in the cow `rest' in the pachytene stage (Henricson & Rajakoski, 1959; Rajakoski, 1965; Erickson, 1966). They consider that the diplotene stage does not appear until the