In ovaries of immature rats the following parameters were estimated from autoradiographs prepared after pulse labelling with tritiated thymidine: 1) The time it takes follicles to grow from one stage of development to another. This could be derived from the total number of granulosa cells in these stages and from their doubling times. The doubling time of granulosa cells was determined from their labelling index and the duration of their DNA-synthesis phase.2) The number of follicles present in the ovary at different ages.3) The number of follicles, which start on their development at different ages.It was found, that more follicles start to grow in 8 and 16 days old rats (2.0/h) than in 28 days old ones (1.0/h). Moreover, the follicles grow somewhat faster earlier in life than later. The development from a follicle with one layer of granulosa cells to one with several layers and antrum formation takes about 15 days in the first half of the period of immaturity while it takes about 17 days as the animal approaches maturity. Pedersen (1969Pedersen ( , 1970aPedersen ( ,«, 1972 introduced granulosa cell kinetics, using [3H]-thymidine ([;iH]T), to time follicle growth. His studies in mice indicated that the time it takes a 3 b follicle (20 granulosa cells in the largest cross section) to develop to an antral one changes with age and is about 16 days in the 21 days old mouse. So far, follicle growth rates in the rat, the species in which the reproductive system has been studied most extensively, have not been determined. Therefore, 375
Follicle growth in the ovaries of cyclic rats was studied by autoradiography after pulse labelling with [3H]thymidine: growth rates and numbers were determined for follicles of the types 3b to 6 (type 3b: follicles with 1 \p=n-\2layers of granulosa cells; type 6: follicles with a diameter of 200 \g=m\m and usually with a single antrum). The data indicate a total time of follicle development, from type 3b to ovulation, of 22 days. Growth of follicles of the types 3b to 6 in these cyclic rats was roughly similar to that in late prepubertal animals, previously studied. However, some changes of the growth of these types of follicles, related to the stages of the cycle, could be noted.
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