In several species, the developmental competence of the oocyte is acquired progressively
during late follicular growth, after the acquisition of the competence to resume and
complete meiosis. In the pig, full meiotic competence of the oocyte is reached in
ovarian follicles with a diameter of 3 mm or more. However, there is no information
about developmental competence acquisition. We analyzed the ability of oocytes from
three foll icular size classes to resume and complete meiosis, to be fertilized, and
to develop in vitro to the blastocyst stage. A total of 941 follicles were dissected
from slaughterhouse gilt ovaries and classified as small (<3 mm, n = 330), medium
(3-5 mm, n = 373), or large (>5 mm, n = 238). The cumulus-oocyte complexes recovered
from these follicles were submitted to in vitro maturation for 44 h in TCM199 supplemented
with 10 ng/ml EGF, 400 ng/ml pFSH and 570 microM cysteamine; in vitro fertilized for
18 h in mTBM with 10(5) frozen-thawed percoll-selected sperms/ml; and developed for
7 days in mSOF. Samples of oocytes or presumptive zygotes were fixed and stained at
the end of maturation and fertilization. Groups of oocytes were cultured for 3 h in
the presence of 35S-methionine before or after maturation for SDS-PAGE analysis of
protein neosynthesis. More oocytes originating from medium and large follicles were
competent for maturation than oocytes from small follicles (77 and 86% of metaphase
II, respectively, versus 44%, P < 0.05). More oocytes from medium and large follicles
werepenetratedby spermatozoa during in vitro fertilization, resulting in significantly
more oocytes presenting two or more pronuclei at the end of fertilization (73 and
77% for medium and large follicles, respectively, versus 53% for small follicles,
P < 0.05). More oocytes from medium and large follicles developed to the blastocyst
stage (14 and 23%, respectively) than those from small follicles (3%, P < 0.05), even
if the development rates were corrected by the maturation or fertilization rates.
It is concluded that a high proportion of oocytes harvested from follicles of less
than 3 mm in the pig are not fully competent for meiosis and are cytoplasmically deficient
for development.