Corpus luteum development and its morphology after aspiration of a preovulatory follicle is related to size and steroid content of the follicle in dairy cows

Secretion of adequate levels of progesterone from a proper corpus luteum (CL), which develops out of the cells of a healthy preovulatory follicle, is a key-factor for establishment of a pregnancy. The aim of this study was to investigate the relationship between morphological and secretory characteristics of preovulatory follicles and their corresponding corpus luteum with regard to the post-partum period in high-yielding dairy cows. Therefore, ultrasound-guided aspirations of preovulatory follicles were performed repeatedly, using 20 first lactating cows between 26 and 121 days after parturition. Heat was induced with a PGF analogon followed by administration of a GnRH analogon. The dominant follicle was aspirated 21 h after administration of the GnRH analogon. The diameters of the follicles were estimated at aspiration and the morphology of the resulting luteal tissue was examined on day 14 after follicle aspiration using ultrasonographic examinations. Concentrations of progesterone (P4) and 17-beta-oestradiol (E2) were determined in the follicular fluids (FF) and P4 concentration was estimated at the time of CL examination in plasma. A CL development occurred in 82% after dominant follicle aspiration. The interval of time between parturition and follicle aspiration did not affect the investigated variables. The diameter of the aspirated preovulatory follicle was positively correlated to the cross-section area of the developed luteal tissue (R = 0.60; P < 0.01) as well as to the plasma P4 concentration on day 14 after follicle aspiration (R = 0.47; P < 0.05). Also, E2 concentrations in FF were positively correlated to cross-section area of the luteal tissue (R = 0.54; P < 0.05). Comparing the FF of the follicles that gave rise to a CL after aspiration to follicles that did not, both types had comparable P4, but the former type harboured higher E2 concentrations. In conclusion, preovulatory follicle diameter as well as steroid concentrations in the follicular fluid could be used prospectively to identify cows which will have well-developed CLs and high plasma P4 levels later. On the other hand, CL development after follicle aspiration can be used as a retrospective quality parameter of dominant follicles. These results will help to identify suitable animals for breeding or recipients for embryo transfer.