ULTRASONOGRAPHIC MONITORING OF FOLLICULAR DEVELOPMENT, OVULATION AND THE EARLY LUTEAL-PHASE IN THE BITCH

Follicular development, ovulation and the early luteal phase in 15 oes trous cycles of 13 individual bitches were monitored daily by ultrason ography using a 7.5 MHz sector scanner transducer. These findings were compared with vaginal cytology and changes in the peripheral plasma c oncentrations of oestradiol, luteinizing hormone (LH) and progesterone . Both ovaries were imaged throughout the period of study. Follicles w ere identified as anechoic spherical structures as early as day 1 of p ro-oestrus in two cases and on days 3-4 in 13 cases. The average folli cular size increased from 3.7 +/- 0.6 mm to 6.9 +/- 0.7 mm (P < 0.001) between day - 5 and the LH surge (day 0). The anechoic structures the n increased in size to 7.5 +/- 0.7 mm (P < 0.05) until day 2 after the LH surge; their diameters continued to increase during corpus luteum development, up to 8.1 +/- 0.6 mm (P < 0.001) on the first day of dioe strus. The average number of follicles counted imaged on the day of th e LH surge was 3.3 +/- 0.4 on the left ovary and 3.7 +/- 0.7 on the ri ght ovary. The rapid disappearance of the anechoic antrum, correspondi ng to ovulation, was detected in only two bitches. In the remaining ca ses follicular rupture was not detected; however, starting at the LH s urge, a gradual thickening of the antral wall occurred. This correspon ded to a progressive obliteration of the anechoic region and was chara cteristic of the postovulatory corpora lutea. These changes led to a s ignificant decrease (P < 0.05) in the number of anechoic structures to 2.4 +/- 0-5 on the left ovary and 3.3 +/- 0-5 on the right ovary by d ay 1 of dioestrus. During the follicular phase, the mean increase in o estradiol concentration was correlated with follicular size (P < 0.001 ). From the LH surge until day 1 of dioestrus, the average increase in the diameter of the luteinizing follicles and developing corpora lute a was significantly correlated with the mean increase in progesterone concentration (P < 0.001).