Follicular growth pattern in cyclic rats from late pro-oestrus to early oestrus.

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 activity was studied by 5-bromodeoxyuridine (BrdU) labelling and by the presence of mitoses. Small growing follicles (less than 275 microns in diameter) were divided into five classes: multilaminar classes a (Ma, up to 75 microns in diameter), b (Mb, 76-150 microns), c (Mc, 151-200 microns) and d (Md, 201-274 microns) and follicles measuring > or = 275 microns in diameter were considered as > or = class 1, following previous classifications. I.H concentrations were maximal 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 oestrus the concentrations of all hormones, except those of FSH, were decreased. The number of Ma, Mb and Mc follicles did not change during pro-oestrus-oestrus, whereas an increase in the number of follicles > or = class 1 was found at 07:00 h on oestrus. This appears to be a consequence of the increased proliferative activity of Md follicles, 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 > or = class 1 at 07:00 h on oestrus. This study provides an improved classification of small growing follicles into discrete classes and delineates a size class of follicles (Md follicles) that is responsive to the cyclic hormonal changes on early oestrus.