Heated spermatozoa: effects on embryonic development and epigenetics

Sperm chromatin is highly condensed and relatively resistant to chemical and physical treatments. The purpose of this study was to explore the highest temperature that sperm can tolerate and still produce live offspring. Mouse sperm were heated in a water bath at 50, 65, 80 or 95C for 30 min before they were microinjected into mouse oocytes. Fertilization, embryo development and 1-cell embryo karyotypes were evaluated. Epigenetic reprogramming including DNA methylation and histone H3K4-trimethylation were evaluated by immunofluorescent staining. The ability of mouse sperm to activate the egg after ICSI was heat sensitive; only 20 of eggs were activated by sperm that had been heated to 50C and none was activated by sperm heated to 80C. However, if eggs were activated artificially, mouse sperm subjected to 80C for 30 min were able to produce live offspring, while 95C treatment disabled sperm decondensation after ICSI. Once the heat-treated sperm nucleus had developed into a pronucleus, sperm chromatin was able to undergo normal active DNA demethylation and histone methylation. Aberrant chromosome rates increased from 16.3 to 100 when the temperature was raised from 50 to 95C. Heat treatment destroys integrity of sperm chromatin in a temperature-dependent manner. Eighty degree Celsius was the highest temperature that mouse sperm could withstand and still produce live offspring.