In livestock, most of the follicles on the ovarian surface are small
follicles. A procedure that supports the in vitro growth and
maturation of these small follicle-derived oocytes may offer a new
source of useable oocytes for both biotechnological and fundamental
research purposes. The objective of the current study was to
test the hypothesis that providing a more growth-supporting and
less maturation-promoting environment during the first phase of
small follicle-derived oocyte maturation may improve oocyte
competence for meiosis and embryo development upon activation.
In our small follicle-derived oocyte growth-maturation system
(SGM group), cumulus-oocyte complexes (COCs) from small
follicles (1–3 mm) were first cultured in oocyte growth medium
for 24 h, then in oocyte maturation medium for 20 h. As controls,
COCs from small (SM group) and large (LM group) follicles were
cultured using a conventional in vitro maturation (IVM) approach
in which they were directly cultured in oocyte maturation medium.
At 24 h of culture, the percentage of small follicle-derived oocytes
that underwent germinal vesicle breakdown (GVBD) in the SGM
group was comparable to that of large follicle-derived oocytes (LM
group) but was significantly higher than that of the SM group (P <
0.05). At 44 h of culture, compared to 36% in the SM group,
55% of the SGM group oocytes reached metaphase II (MII; P <
0.05). In addition, the level of cyclin B in oocytes of the SGM group
was comparable to that of oocytes from LM group and was
significantly higher than that of oocytes from the SM group (P <
0.05).When activated and in vitro fertilized (IVF), 7.3 and 9.0 times
more parthenogenetic and IVF embryos developed to blastocyst
stage in the SGM group than in the SM group (P < 0.05). The mRNA
expression levels of three developmentally important genes—
DNA-methyltransferase 1, Pou domain class 5 transcription factor
1, and Fibroblast growth factor receptor 2—in embryos of the SGM
group were comparable to those of embryos developed from the
LM group, whereas they were significantly lower in those of the SM
group (P < 0.05). Our data suggest that the oocyte growthmaturation
system facilitates the final stage of oocyte growth and
thus resulted in better oocyte nuclear, cytoplasmic maturation, and
developmental competency compared with the conventional
direct oocyte maturation system.