Effects of Hypothermia and Hyperthermia on Fertilization in Rat Eggs

Abstract

The chilling of rats to body temperatures down to 4-7° C. before and during the time of ovulation had little effect upon the subsequent penetration of the eggs by spermatozoa. With a hypothermia of 0-1° C, however, there was almost complete and apparently permanent inhibition of spermatozoon entry. There seemed to be no reason to ascribe this effect to direct influence of cold on eggs or spermatozoa. Probably the failure of penetration arose largely from delay of ovulation, coupled perhaps with some derangement of the tubal environment. Hypothermia after ovulation but before and during the time of spermatozoon entry into the eggs temporarily but completely inhibited penetration when the body temperature was kept below 33° C. The likely explanation appears to be that ‘capacitation’ is prevented or greatly slowed by such temperatures. Hyperthermia, induced before and during the time of ovulation, somewhat reduced the mean number of eggs ovulated (from 10.5 to 7.5) and the proportion penetrated by spermatozoa (from 97 to 66%). The proportion of eggs penetrated by two or more spermatozoa was increased (from 19 to 28%), but the frequency of polyspermic fertilization did not change significantly. Hyperthermia after ovulation but before and during the time of spermatozoon entry also led to a greater proportion of eggs containing two or more spermatozoa (31%), but in addition it increased the incidence of polyspermy (from 2.5 to21.2%). Larger rats (170-200 g.) subjected to hyperthermia showed a higher frequency of polyspermy (34%) than smaller rats (100-130 g.; 13% polyspermy). Evidently the processes involved in the ‘zona reaction’ are reduced in efficiency by hyperthermia whether applied before or after ovulation. On the other hand, the block to polyspermy depends on processes that are much more sensitive to heat after ovulation. It is considered that the slowing down of the block to polyspermy is associated with ageing of the egg, that the ageing is accelerated by hyperthermia and that this effect is promoted in older rats.