Observations on nuclear size and form in living rat and mouse eggs

Abstract

Figures are presented for the mean nuclear volume, the mean total nucleolar volume, and the mean number of nucleoli, in the pronuclei of polyspermic rat eggs, in the nuclei of artificially activated 1-cell and 2-cell rat eggs, in the pronuclei and 2-cell nuclei of fertilized mouse eggs, and in the nuclei of artificially activated 1-cell and 2-cell mouse eggs. From a consideration of these data, and of information previously published, the following conclusions have been reached for the eggs of rats and mice: 1. 1. To fulfil the requirements of pronuclear growth, material is drawn from cytoplasmic stores in the egg and is available in just sufficient quantity for the development of pronuclei of normal size and number. Limited availability of material thus constitutes a normal restriction to the growth of the pronuclei. 2. 2. The pronuclei compete for the formative material and the male pronucleus appears to have the greater affinity for it, since it maintains about twice the size of the female pronucleus throughout development. 3. 3. There is, at least in the female pronucleus, also an inherent limit to size; this is not normally operative but becomes so when a female pronucleus develops in the absence of competition from a male. The restriction affects nuclear volume more than total nucleolar volume. 4. 4. Nuclear volume and total nucleolar volume in 2-cell eggs is apparently unrelated to ploidy, since haploid, diploid and triploid eggs have nuclei that differ little from each other. Similarly, the number of nucleoli is evidently no guide to chromosome number. 5. 5. Although mouse and rat eggs have the same vitelline volume (about 200,000 cu.μ), the pronuclei and nuclei are smaller in the mouse egg. It seems likely, however, that the nucleo-cytoplasmic ratio is in reality the same for both species and that the apparent difference in ratio should be imputed to the presence of more yolk in the mouse egg.