Pre-ovulatory changes in the oocyte of the adult rat

Abstract

The sequence of nuclear changes in the oocyte of the adult rat has been investigated, with particular emphasis on early pre-ovulatory stages. The condition of the chromosomes during the so-called dictyate phase is discussed, and an attempt made to determine the time at which diakinesis occurs. The relationship between the onset of the successive stages of the maturation division and the diurnal rhythm of light and darkness (‘natural’ and artificial) was determined. The first clear-cut change (condensation of chromatin) occurs at about 5 p. m. under ‘natural’ light conditions, and within the first 1 h of the onset of darkness under artificial illumination (12 h light: 12 h darkness). Ovulation occurs some 9 to 10 h later. An approximate estimate was made of the duration of each stage of the division up to second metaphase. The variability in the onset of the various stages of the division was greater between individual rats kept under ‘natural’ light conditions than between those kept in light-controlled rooms.