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Time course of cortical and zona reactions of pig oocytes upon intracellular calcium increase induced by thimerosal

Published online by Cambridge University Press:  05 April 2001

Wei-Hua Wang ,
Zoltan Macháty ,
Lalantha R. Abeydeera ,
Randall S. Prather  and
Billy N. Day
Wei-Hua Wang
Affiliation:
Department of Animal Science, University of Missouri-Columbia, Columbia, Missouri, USA
Zoltan Macháty
Affiliation:
Department of Animal Science, University of Missouri-Columbia, Columbia, Missouri, USA
Lalantha R. Abeydeera
Affiliation:
Department of Animal Science, University of Missouri-Columbia, Columbia, Missouri, USA
Randall S. Prather
Affiliation:
Department of Animal Science, University of Missouri-Columbia, Columbia, Missouri, USA
Billy N. Day
Affiliation:
Department of Animal Science, University of Missouri-Columbia, Columbia, Missouri, USA
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Abstract

Our previous study indicated that thimerosal is one of the most effective artificial activators to mimic sperm-induced increases in the intracellular free calcium concentration ([Ca2+]i) and other activation events in pig oocytes (Macháty et al., 1997). The present study was conducted to examine the temporal relationship between intracellular calcium transients, cortical granule (CG) exocytosis and the zona reaction induced by thimerosal. When pig oocytes matured in vitro were exposed to 200 μM thimerosal the first intracellular calcium transient, with a mean peak ratio of 4.97 ± 1.14, was observed 509.64 ± 122.03 s after addition of thimerosal. The density of CGs fell significantly from 63.3 ± 11.7 CGs/100 μm2 of cortex in control oocytes to 25.7 ± 19.2 CGs/100 μm2 of cortex (59.4% release) at 2 min after the first intracellular calcium transient. At 5 min after the calcium transient the residual CG density had been reduced to 10.7 ± 10.4 CGs/100 μm2 of cortex (83.1% release). This degree of CG exocytosis was the same as that in oocytes penetrated by sperm (9.5 ± 5.1 CGs/100 μm2 of cortex). No further decrease in residual CG density was observed at 10 min (10.3 ± 14.8 CGs/100 μm2 of cortex). Whereas 77.4% (120/155) of control oocytes were penetrated by spermatozoa only 1.4% (2/144) of thimerosal-treated oocytes were penetrated. Further experimental results obtained by in vitro fertilisation of oocytes with preincubated (capacitated) spermatozoa suggested that the zona block to sperm penetration in thimerosal-treated oocytes occurred within 35 min after CG exocytosis and 40 min after the first calcium transient. These results indicate that polyspermic penetration of pig oocytes inseminated in vitro is not due to delayed or incomplete CG exocytosis but more likely to a delayed zona reaction and/or simultaneous sperm penetration.

Keywords

Cortical reactionIntracellular calcium transientPigPolyspermyZona block
Type
Research Article
Information
Zygote , Volume 7 , Issue 1 , February 1999 , pp. 79 - 86
Copyright
1999 Cambridge University Press

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