Determination of the Ovulation Time in the Laboratory Rats (Rattus norvegicus)

Abstract

The rat has been elected as the main animal model in different studies involving reproduction. However, there are scarce and conflicting data related to its estrous cycle. The aim of the experiment was to determine the time of ovulation in the primiparous laboratory rats (Rattus Norvegicus) by counting the number of graafian follicles present in the ovary at the time of oestrus, determining the percentage of these follicles that eventually ovulate and determining the 'spread' of ovulation during the oestrus period. Fifty (50) albino laboratory rats were observed in oestrus with the help of males to determine the time the female first stood to be mounted. This time was considered the onset of oestrus and used as a landmark for timing of oestrus period. From this onset of oestrus, the period was divided into 10 x 1-hour intervals into which the female rats were grouped. Each hour interval had five rat members. Ovaries of rats harvested at the end of their group-hour intervals were studied histologically, for functional structures. Observed structures were counted for the calculation of ovulation rates. All oestrus rats had far shorter periods, the ovulation in this study were found to be widespread from the onset to the tenth hour peaking maximally at the sixth and seventh hour of oestrous. Follicular activity was found to be more in the left ovaries while ovulatory activity was more in the right ovaries. It was concluded that rats presented far shorter oestrus periods and all ovulations in the albino rats used in our laboratory occurred at the onset of oestrus and that the mechanism responsible for the first "stance for mounting" is responsible for the ovulation and that ovulation is induced in these rats.