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Ultrasonic characterization of ovulatory follicular evacuation and luteal development in heifers

The characteristics of follicle evacuation during ovulation and the development of the corpus luteum until day 5 (day 0 = ovulation) were studied in seven nulliparous Holstein heifers using real-time ultrasonography. Ovulation was induced and synchronized with a single injection of PGF₂ followed in 36 h by GnRH. Continuous scanning and videotaping was performed from apparent stigma formation until antral fluid was no longer detected. The beginning of follicular evacuation (second 0) was defined, retrospectively, after the antral area decreased 10% or more in 1 s. The completion of evacuation was defined as the inability to detect the antrum (the beginning of luteal development, 0 h). Corpora lutea development was monitored at 0, 4 and 20 h, and every 24 h thereafter until day 5. Changes in the maximal cross-sectional area of the antrum, luteal tissue, and central luteal cavities and in the pixel intensity of luteal tissue were determined using a computerized image program. The initial antral fluid evacuation occurred in two patterns that could be readily separated: (1) rapid, means of 58 and 89% evacuation in 1 and 4 s, respectively (four heifers); and (2) slow, means of 17 and 35% in 1 and 4 s, respectively (three heifers). The initial loss that distinguished the two patterns involved about 4 and 20 s for rapid and slow evacuation, respectively. Thereafter, the loss patterns were similar for the two types. The time from the beginning to the completion of evacuation ranged from 6 s to 14.5 min. Mean luteal tissue area increased (P < 0.05) between completion of evacuation (91.2 ± 6.5 mm²) and day 3 (164.4 ± 13.7 mm²) and between day 3 and day 4 (263.4 ± 26.6 mm²). The growth rate of the luteal tissue area between day 3 and day 4 (103.2 ± 16.0 mm² day^–1) was greater (P < 0.05) than that between day 2 and day 3 (41.9 ± 12.4 mm² day^–1) and between day 4 and day 5 (49.7 ± 22.0 mm² day^–1). In contrast to increasing luteal tissue area, mean pixel intensity decreased (P < 0.05) progressively between the completion of evacuation (78.4 ± 6.3) and day 2 (60.4 ± 2.5) and did not change significantly thereafter. In conclusion, initial follicular fluid loss during ovulation occurred in two patterns, involving about 4 and 20 s, respectively. The most intensive luteal tissue growth occurred between day 3 and day 4, and the echogenicity of the luteal tissue decreased between day 0 and day 2.