Morphology of porcine cumulus-oocyte-complexes depends on the stage of preovulatory maturation

Abstract

The aim of this investigation was to determine the relationship between the morphology of the cumulus-oocyte-complexes (COCs) and the meiotic configuration of oocytes as an LH peak mimicked by hCG. Estrus was synchronized in a total of 29 crossbred Landrace gilts by feeding Regumate® for 15 d and administering 1000 IU PMSG. The LH peak was simulated by treatment with 500 IU hCG at 80 h after PMSG. Endoscopic oocyte recovery was carried out 2 h before and 10, 22 and 34 h after hCG. Only macroscopically healthy follicles with a diameter of more than 5 mm were punctured. Altogether, 410 follicles from 57 ovaries were punctured and 251 COCs were aspirated. Oocyte recovery rate increased from 48.5 % (P < 0.01) of the early, not yet preovulatory follicles (2 h before hCG) to 80.8 % of late preovulatory follicles (34 h after hCG). Cumulus morphology in COCs recovered 2 h before and 10 h after hCG was heterogeneous, with most (72.9 to 57.4 %; P<0.01) showing a compact or slightly expanded cumulus. Starting at about 22 h after hCG, COC morphology changed dramatically (86.7 % of COCs with expanded cumulus; P<0.01), and 34 h after hCG, 98.3 % of the COCs had only an expanded cumulus. The percentage of oocytes with a mature meiotic configuration increased (11.2; 7.1; 41.4 and 70.2 %, respectively, n=238 oocytes; P<0.01) as the interval post hCG increased (−2, 10, 22, 34 h, respectively). Meiotic configuration was related to COC morphology: compact COCs − 88.9 % diplotene, expanded COCs − 53.8 % metaphase II (M-II), and denuded oocytes − 69.2 % degenerated chromatin. These results indicate that there is a relationship between oocyte recovery rate, COC morphology, and meiotic configuration and preovulatory follicle maturation after the application of hCG.