Ultrasonic evaluation of the corpus luteum of the mare

Two distinct luteal morphologies were observed in the ovaries of mares studied by daily ultrasound examinations. Luteal glands that formed after 48.5% of 95 ovulations were uniformly echogenic over 90 to 100 percent of the area of the image of the gland throughout the period of detectability. The remaining luteal structures (51.5%) exhibited a centrally located nonechogenic area. The nonechogenic area was first detected on day 0 (28%), day 1 (62%), day 2 (6%) or day 3 (4%) postovulation. Glands classified as centrally nonechogenic were echogenic over 80 to 100 percent of the area of the image of the gland on day 0; mean percentages of echogenic tissue decreased to 45 percent by day three then gradually increased to 95 percent before the glands became unidentifiable. The echogenic portion of the luteal glands of both morphologies had a bright echogenicity (gray-scale zone 4.5 to 5) on day 0. The echogenicity decreased (zones 3 to 3.5) by day 8 and was maintained at approximately that level until day 12. Mean gray-scale values tended to increase (zone 4 to 4.5) prior to the time the luteal glands became ultrasonically unidentifiable. These changes in grayscale values may have reflected changes in luteal hemodynamics. The nonechogenic area of centrally nonechogenic glands was attributed to clotted blood (corpus hemorrhagicum). The formation of a corpus hemorrhagicum was apparently not functionally important because it was present in only one half of the luteal glands. In addition, the mean length of time that the luteal gland was identifiable (17 days) or the mean length of the interovulatory interval (21 days) was not significantly different between the two luteal morphologies. Therefore, the hypothesis that the formation of a corpus hemorrhagicum is a necessary step in luteogenesis was not supported.