Ultrasonic Morphology of the Corpora Lutea and Central Luteal Cavities during Selection of Recipients for Embryo Transfer

Abstract

Contents The present experiment was designed to investigate the significance of the diameter of the luteal cavities in the corpora lutea on pregnancy rate after embryo transfer of bovine recipients. Transrectal ultrasonography was performed once on 266 nulliparous Holstein heifer recipients on day 7 of the oestrous cycle (standing oestrus = day 0) to evaluate the area and diameter of corpora lutea (CL), central luteal cavities and luteinized tissue area. Heifers were synchronized using an intravaginal progesterone-releasing device (CIDR-B) followed 2 days later by an intramuscular injection of 5 mg estradiol-17β. CIDR-B devices were removed after 9 days and heifers received 500 µg cloprostenol intramuscularly at that time. Oestrus detection was performed every 6 h by observing standing behaviour for 1–4 days after device removal. Ultrasonographic examinations of ovarian structures were performed using a B-mode scanner equipped with a 7.5 MHz linear-array transducer by the same operator. Each ovary was scanned in several planes to identify the presence of a CL. A cross-sectional image of the CL was frozen at its maximal diameter and area of the CL and cavity, and the area of luteal tissue was calculated using a mathematical formula. A blood sample was also collected from each heifer for determination of plasma progesterone concentration. Selection of recipients before transfer was based on the presence of a CL of ≥ 10 mm in diameter (detected by ultrasound) and a plasma progesterone concentration of > 1 ng/ml. Heifers that did not comply with these selection criteria were rejected. Regardless of CL size after examination, each of the selected recipients received a single DNA-microinjected IVF-derived embryo after 7 or 8 days in culture. Each embryo was transferred nonsurgically into the uterine horn ipsilateral to the ovary containing the CL either on the day of selection (day 7) or the next day (day 8). Pregnancy status was determined by transrectal ultrasonography 28 days after embryo transfer. Luteal cavities detected at selection were used retrospectively to group the CL into four categories according to cavity diameter: none ( 10 mm). The data was analysed statistically by χ2 and analysis of variance. Selection of recipients before transfer resulted in 88.7% (236/266) of heifers with a CL of at least ≥ 10 mm in diameter (detected by ultrasound) and a plasma progesterone concentration of > 1 ng/ml. This selection criterion resulted in a recipient rejection rate of only 11.3%. A luteal cavity was present in 40.7% of the CLs. Pregnancy rate did not differ among the different categories and an overall pregnancy rate of 42.4% was obtained. In contrast to nonpregnant heifers, the diameter and area of luteal cavities was significantly smaller (p < 0.05) in pregnant heifers in all three categories. Mean diameter of the CL (20.0 ± 0.3 versus 20.6 ± 0.2 mm), mean diameter of the central cavity (8.6 ± 0.7 versus 10.6 ± 0.7 mm), mean luteal tissue area (292.9 ± 8.9 versus 293.4 ± 7.3 mm2) and progesterone concentration (3.9 ± 0.2 versus 4.1 ±0.2 ng/ml) were not significantly different between pregnant and nonpregnant recipients, respectively. This data suggests that regardless of the presence and size of the luteal cavity, recipients can achieve and maintain pregnancy after embryo transfer.