FOLLICULAR-GROWTH PATTERN IN CYCLIC RATS FROM LATE PRO-ESTRUS TO EARLY ESTRUS

Adult cyclic rats were studied from 16:00 h on pro-oestrus to 07:00 h on oestrus to relate the cyclic hormonal changes to the proliferative activity and growth pattern of growing follicles. The proliferative ac tivity was studied by 5-bromodeoxyuridine (BrdU) labelling and by the presence of mitoses. Small growing follicles (less than 275 mu m in di ameter) were divided into five classes: multilaminar classes a (Ma, up to 75 mu m in diameter), b (Mb, 76-150 mu m), c (Mc, 151-200 mu m) an d d (Md, 201-274 mu m) and follicles measuring greater than or equal t o 275 mu m in diameter were considered as greater than or equal to cla ss I, following previous classifications. LH concentrations were maxim al at 18:30 h on pro-oestrus, and this was coincident with an increase in FSH, prolactin and progesterone concentrations, whereas oestradiol and testosterone concentrations were decreased. From 02:00 h on oestr us the concentrations of all hormones, except those of FSH, were decre ased. The number of Ma, Mb and Mc follicles did not change during pro- oestrus-oestrus, whereas an increase in the number of follicles greate r than or equal to class I was found at 07:00 h on oestrus. This appea rs to be a consequence of the increased proliferative activity of Md f ollicles, evidenced by the increase in the BrdU labelling and mitotic index of this follicle class, found from 02:00 to 07:00 h on oestrus, together with a decrease in the percentage of early atretic follicles greater than or equal to class I at 07:00 h on oestrus. This study pro vides an improved classification of small growing follicles into discr ete classes and delineates a size class of follicles (Md follicles) th at is responsive to the cyclic hormonal changes on early oestrus.