Аннотация
Mouse sperm-egg fusion was examined using two-photon and confocal
microscopy. A delay of several minutes occurred between the first
observable event of fusion (which was the diffusion of Ca$^2+$-sensitive
dyes from egg into sperm) and any change in egg cytoplasmic Ca$^2+$.
When indo-1 dextran was used to obtain ratiometric two-photon images,
there was no detectable local increase in egg cytoplasmic Ca$^2+$
near the site of sperm fusion. However, the sperm underwent a Ca$^2+$
transient which appeared to be coincident with the egg cytoplasm
Ca$^2+$ transient, which suggested that there was a high permeability
pathway for Ca$^2+$ between egg and sperm. To exclude this pathway
from providing trigger Ca$^2+$ for the egg transient, we reduced
bathing Ca$^2+$ to approx. 18 microM and 13nM (with EGTA).
In these conditions the first egg Ca$^2+$ transient was not prevented,
which makes an obligatory role for extracellular Ca$^2+$ in the
initiation of the egg Ca$^2+$ transient unlikely. Both FITC-albumin
(70 kDa) and 10 kDa dextran-linked Ca$^2+$ indicators were able
to diffuse into the sperm from the egg. In addition, phycoerythrin
(240 kDa) rapidly diffused into the sperm shortly after fusion (but
before any changes in Ca$^2+$ occurred). This suggests that the
'pore(s)' that form during sperm-egg fusion must be at least 8 nm
in diameter. These data are compatible with the idea that a diffusible
sperm protein could trigger the observed changes in intracellular
Ca$^2+$ in the egg, but do not exclude the possibility that other
second messengers are generated during sperm-egg fusion.
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