It is not known how the fertilizing sperm elicits the release of Ca2+ from
the oocyte's intracellular stores. We investigated whether a crude extract
isolated from boar sperm could induce the Ca2+ release and trigger subseque
nt early and late activation events upon injection into matured porcine ooc
ytes. The sperm extract induced an immediate rise in the intracellular free
Ca2+ concentration in all oocytes tested, which was followed by repetitive
Ca2+ transients in 11 out of 14 oocytes. Heat or trypsin treatment of the
extract totally abolished the Ca2+ releasing activity of the sperm factor.
The injected oocytes showed cortical granule exocytosis, they resumed meios
is and entered first interphase: pronuclei were formed in 89.2% (132/148) o
f the cases. Pronuclear formation was accompanied by the appearance of a ne
w 22 kDa protein as normally seen at fertilization. Of the successfully inj
ected oocytes 51.7% (105/203) cleaved and 2.0% (4/203) developed to the bla
stocyst stage after being cultured for 7 days in NCSU 23 medium. Injection
of the carrier medium could not trigger these changes. The results indicate
that the sperm might activate porcine oocytes by introducing a soluble fac
tor into the oocyte's cytoplasm after gamete fusion. Mel. Reprod. Dev. 57:2
90-295, 2000. (C) 2000 Wiley-Liss, Inc.