Volume changes during the preimplantation stages of mouse egg development.

Abstract

A study of the preimplantation changes of mouse eggs is presented. Data on the largest and smallest diameters of the inner and outer surface of the zona pellucida were analyzed by computer. Measurements were made during the following stages of egg growth: germinal vesicle, ovulated but unfertilized egg, ovulated and fertilized egg, 2-cell embryo on Day 2 of pregnancy, 4- to 8-cell embryos on Day 3 of pregnancy, and the morulablastocyst on Day 4 of pregnancy. The diameter of the vitellus of the ovarian oocyte was the largest of any stage measured, whereas the diameter of the zona pellucida of the oocyte was the smallest. The total volume of the oocyte increases considerably by the time of ovulation and increases only slightly after fertilization. It then decreases until the 4- to 8-cell stage of development. The volumes of the entire egg, intrazonal cavity, zona pellucida perivitteline space, and the vitellus were measured during the initial 18 hours of development following ovulation and fertilization. 6 hours after ovulation, the total fertilized egg is reduced in volume as is the volume enclosed by the zona pellucida. The egg remains relatively constant in volume for 6 more hours and then shrinks again. No changes occurred in the total volume and intrazonal cavity of unfertilized eggs 6 hours after ovulation. The unfertilized egg reduces slightly until 9 hours, when it suddenly expands beyond the size of the fertilized egg. The perivitteline space in fertilized and unfertilized eggs increases with time, while the zona pellucida gradually decreases in the fertilized egg but increases in the unfertilized egg prior to the 1st cleavage. The vittelus of the fertilized egg shrinks at a relatively constant rate, whereas transient swelling was observed in the unfertilized tubal egg. Temperature differences had no effect on the size of fertilized and unfertilized eggs. The results give further evidence of an increased metabolic capability of the mouse embryo between the 2-cell stage and the 8-cell stage. Possible reasons for the observed changes are discussed in relation to the literature.