Wh. Wang et al., QUANTIFIED ANALYSIS OF CORTICAL GRANULE DISTRIBUTION AND EXOCYTOSIS OF PORCINE OOCYTES DURING MEIOTIC MATURATION AND ACTIVATION, Biology of reproduction, 56(6), 1997, pp. 1376-1382
Polyspermy is one of the unresolved problems that exist regarding pig
oocytes matured and inseminated in vitro. Quantitative study of the ch
anges in the cortical granule (CC) population in oocytes is essential
for understanding the mechanism of how oocytes block polyspermic penet
ration and for developing the optimum conditions for in vitro maturati
on (IVM) and in vitro fertilization (IVF). The present study was condu
cted to quantify the CC distribution in pig oocytes during IVM and IVF
by using fluorescein isothiocyanate-labeled peanut agglutinin with la
ser confocal microscopy. The results indicate that CCs are distributed
in the cortex cytoplasm of oocytes at the germinal vesicle (CV) stage
with a mean number of 33.8 +/- 7.3 CGs/100 mu m(2) of cortex. As nucl
ear maturation proceeded to metaphase I and metaphase II, CCs migrated
to the cortex and formed a continuous monolayer under the oolemma. No
distinct CC-free domain was observed in oocytes during maturation. Th
e migration of CCs to the cortex continued during maturation, with an
increased CC density after the CV stage. All oocytes penetrated by spe
rmatozoa were activated and released CCs from ooplasm with an average
residual number of 3.5 +/- 4.6 CGs/100 mu m(2) of cortex at 18 h after
insemination. Complete CC exocytosis was observed in 45% of oocytes.
Calcium ionophore did not induce oocyte nuclear activation, but CCs we
re released from oocytes with an average of 7.1 +/- 4.5 CGs/100 mu m(2
) of cortex still present when examined 18 h after treatment. An elect
rical pulse induced 89% of nuclear activation in matured oocytes, and
CC exocytosis was observed only in nuclear-activated oocytes with an a
verage residual number of 6.4 +/- 9.4 CGs/100 mu m(2) of cortex. Compl
ete CC exocytosis was induced by ionophore and electrical pulse in 10%
and 25% of the oocytes, respectively. These results indicate that CCs
migrate to the cortex in pig oocytes during IVM and that the matured
oocytes obtained under these maturation conditions possess the ability
to release CCs upon sperm penetration, ionophore treatment, and elect
rical pulse. However, a functional block to polyspermic penetration in
oocytes after CC exocytosis was not fully established in these studie
s. The present methods and results provide the approach for further in
vestigation of the reasons for polyspermy in pig oocytes matured and i
nseminated in vitro.