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A number of authors (Brambell, 1927; Engle, 1931; Barraclough, 1955;Ben-Or, 1963; Peters, 1969; Byskov, Pedersen & Peters, 1970; Stegner & Onken, 1971) have described various aspects of the normal development of infantile and juvenile mouse ovaries. Pedersen (1969, 1970) examined the growth rate of mouse ovarian follicles of various sizes and the size distribution of the follicle population at 7, 14, 21, 28 and 35 days after birth. The present work is concerned with the changes between the ages of 16 and 20 days in the population of mouse ovarian follicles that have at least three layers of granulosa cells. Foster litters of ten female CFW mice (nine sub-lines of the outbred CFW strain were obtained from the M.R.C. Laboratory Animals Centre, Carshalton, Surrey in 1966 and have since been systematically crossbred to maintain heterozygosity) were set up on the day of birth. Two mice from each of two such litters were killed 16, 17, 18, 19 and 20 days after birth and their ovaries were fixed in Bouin's aqueous fixative. As these mice were removed from the cages, they were replaced with males of the same age to maintain a constant litter size. The ovaries were serially sectioned and stained with Weigert's haematoxy¬ lin and eosin. Each section was examined and all follicles with three or more layers of granulosa cells were recorded. The mean diameters of those with four or more layers were also determined. The three-layered follicles corresponded roughly to Type 5a, the four- and five-layered follicles to Type 5b and those with more than five layers to Type 6 of Pedersen & Peters (1968). Table 1 and Text-fig. 1 summarize the results. The Text-figure includes, for comparison, data for 15-day-old mice obtained in a separate experiment (Ryle, 1971). The number of follicles with three or more layers increased steadily up to Day 18 but remained constant thereafter. The mean diameters of four- and five-layered follicles did not change with age. Considerable recruitment into these classes occurred between 17 and 20 days. The mean follicle size distribu¬ tion was similar at 17 and 18 days but the variation between individual ovaries was greater at 17 days than at any other age. For example, the standard error of the mean number of follicles per ovary in Columns 2 and 3 (Table 1) was maximal at that age. Six-layered, as well as five-layered, follicles first appeared at 17 days. Between 18 and 20 days, the numbers of both medium and larger follicles increased rapidly while the number with only three layers of cells declined. The mean diameter of follicles with six or more layers increased appreciably up to Day 19, mainly due to the development of large antral

CHANGES IN THE OVARIAN FOLLICLE POPULATION OF MICE AGED 16 TO 20 DAYS