Effect of follicle size on bovine oocyte quality and developmental competence following maturation, fertilization, and culture in vitro

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Abstract
The aim of the present series of experiments was to investigate the effect of the size of follicle from which the oocytes originate on their subsequent in vitro developmental ability. Ovarian follicles were isolated and grouped according to size (2–6 mm, >6 mm). Primary oocytes were carefully liberated and grouped according to morphology into one of five categories: denuded; expanded; with two or three layers of cumulus; with four or five layers; and with many (six or more) layers. Following in vitro maturation (IVM), fertilization (IVF), and culture (IVC), more oocytes with many layers of cumulus (P < 0.01, 70.2%, 73/104 vs. 46.8%, 87/186, respectively) and a higher proportion of blastocysts were obtained from follicles > 6 mm compared to 2–6 mm follicles (P < 0.01, 65.9%, 60/91 from >6 mm follicles vs. 34.3%, 34/99 from 2–6 mm follicles, respectively). Use of follicular fluid (BFF) from follicles of different sizes in the IVM medium did not significantly increase the cleavage rate or blastocyst yield compared to controls. Administration of procine folliclestimulating hormone (pFSH) to donors prior to slaughter was investigated as a possible means of increasing the number of larger sized follicles in the ovaries and, thereby, the quality of the recovered oocytes. It was found that administration of six injections of pFSH beginning 3 days prior to slaughter resulted in a significant increase (P < 0.001) in the proportion of follicles >6 mm in diameter (31.6%) compared to that in nontreated controls (6.6%) and to animals that received only four injection groups (9.4%). The blastocyst yield from oocytes originating from >6 mm follicles, whether from unstimulated or from pFSH‐treated animals, was approximately double that of oocytes from 2–6 mm follicles (P < 0.01; 42.9%, 24/56 for >6 mm follicles vs. 22.8%, 21/92 for 2–6 mm follicles, respectively, for the 6 pFSH group; P > 0.05; 62.5%, 5/8 for >6 mm follicles vs. 32.8%, 22/67 for 2–6 mm follicles, respectively, for the control).