Embryonic development, conception rate, ovarian function and structure in pregnant rabbits heat-stressed before or during implantation

Rabbits were heat-stressed during two definite stages of pregnancy: 1. during early stages of blastocyst formation and rapid growth of the corpus luteum (Days 3-5), and 2. during implantation period (Days 6-8). Hyperthermia was created (+ 1.2°C) in which food intake was reduced to 1/3 of control. Both treatments, H(3-5) and H(6-8), reduced conception rate to 59% from 74% of the control. The development of the remaining live embryos was retarded in the H(3-5) group only. Plasma progesterone concentration in the H(3-5) group was significantly lower than in the control, immediately following termination of heat exposure. In the H(6-8) group, progesterone was only slightly changed, as compared to control. Histological examination of the ovarian corpus luteum indicated that size and number of the large luteal cells were reduced in both heat-stressed groups, as compared with the control. These results indicate that: (a) rabbit embryos are more susceptible to heat stress during blastocyst formation than during implantation, (b) ovarian function impairment is more pronounced during the rapid linear growth stage of the corpus luteum (Days 3-5), and (c) a possible link between impaired ovarian function and reduced reproductive performance can be suggested.