Effects of Grade of Oocyte–Cumulus Complex and the Interactions Between Grades on the Production of Blastocysts in the Cow, Ewe and Lamb

Abstract

There is an association between the size of the cumulus investment and the in vitro developmental ability of the oocyte-cumulus complex (OCC) that provides a basis for the selection of OCCs. However, the value of selection is confounded by humoral interactions between OCCs that influence the development of OCCs of other grade(s). This study examined the effect of size of the cumulus investment (OCC grade) and the interactions between grades on the developmental ability of oocytes collected from the cow, ewe and lamb. OCCs were classified into A, B and C grades on visual assessment of the number of cumulus cell layers or left unselected (Unselected). In the cow, there were 669 +/- 228 to 4763 +/- 228 cells per OCC whereas comparable figures in the ewe and lamb were 593 +/- 252 to 3716 +/- 252 and 366 +/- 228 to 3263 +/- 228 respectively (A > Unselected > B > C; Experiment 1). In Experiment 2, OCCs were made to mature within grade and the efficiency of blastocyst production and blastocyst quality was compared with that obtained in the Unselected group. Grade was associated with significant (p < 0.05) differences in cleavage rate, blastocyst production rate and the mean number of nuclei per embryo (generally A > B > C across animal types). However, the performance of A grade OCCs in the cow and lamb did not differ significantly from that obtained in the Unselected group whereas in the ewe, A grade OCCs were significantly (p < 0.05) better. Furthermore, the performance of the Unselected group was significantly (p < 0.05) better than that of the combined grades (A + B + C) in the cow but there were no differences in either the ewe or lamb. It is concluded that (i) interactions between OCCs of different grade influence the developmental ability of OCCs in the cow and, to a lesser extent, the lamb, (ii) selection of OCCs in the cow and lamb would lead to the exclusion of many OCCs that have the ability to develop into blastocysts and (iii) selection in the ewe would improve the efficiency of blastocyst production although its value is limited by the low percentage of A grade OCCs.