FOLLICULAR FUNCTION IN THE DOMESTIC CAT AS DETERMINED BY ESTRADIOL-17-BETA CONCENTRATIONS IN PLASMA - RELATION TO ESTROUS BEHAVIOR AND CORNIFICATION OF EXFOLIATED VAGINAL EPITHELIUM

Follicular function was estimated by radioimmunoassay of estradiol-17β in jugular vein plasma of female cats. The mean base level of estradiol-17β in plasma was 11.7 pg/ml (SD = 4.8; n = 106). The follicular phase, defined as the time interval when estradiol-17β levels were >20 pg/ml plasma, was 7.4 days (SD = 2.3; n = 108), range 3-16 days. The duration of follicular function was not affected by the occurrence of coitus or ovulation. Estradiol-17β fell from peak levels to <20 pg/ml plasma within a mean of 2.6 days (SD = 1.1; n = 105). The interval between follicular phases was 8.1 days (SD = 3.1; n = 41), ranging from 3-15 days. Proestrus was not observed with regularity and was seen in only 27/168 cycles, averaging 1.2 days (SD = 0.8). Duration of estrous behavior without regard for coitus was 7.4 days (SD = 3.7; n = 168; range 2-19 days). Duration of estrus in cats that experienced coitus and ovulation was 8.6 days (SD = 4.1; n = 32), in those that had coital contact but failed to ovulate, it was 8.3 days (SD = 4.3; n = 43), but in those without coital contact, it was 6.2 days (SD = 2.9; n = 93) (coitus vs no coitus, P<0.01). Estrous behavior was seen in 8, 36, 52, 80, and 85% of cats on follicular phase Days 1-5, respectively. All cats were in estrus by follicular phase Day 6. The interval between estrous periods was 9.0 days (SD = 7.6; n = 162; range 4-22 days) if ovulation did not occur. Ovulation and the subsequent pseudopregnancy prolonged return to estrus for 45.0 days (SD = 10.3; n = 32). No conspicuous changes in size and appearance of external genitalia were observed. Clearing of the vaginal smear (absence of noncellular debris) was the earliest sign of follicular activity. Vaginal epithelial changes were characterized as follows: Anuclear cells (fully cornified) increased from ~5% to ~40% of total cell population and intermediate cells (partially cornified with intact nucleus) decreased from ~45% to 6% progressively from the first to the fourth day of the follicular phase; parabasal cells (noncornified), generally low throughout a cycle (1-6%), were absent on follicular phase Days 4-7 and the following day; superficial cells (partially cornified with signs of degeneration of the nucleus) did not change much during the cycle (range, 40-50%) except for a slight increase during the follicular phase (range, 50-60%).