Early Mouse Embryo Intracisternal Particle: Fourth Type of Retrovirus-Like Particle Associated With the Mouse2

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Abstract
Electron microscopy combined with a quantitative analysis was used to study the virus-like particles appearing in oocytes and preimplantation embryos of nine mouse strains: inbred AKR, NZB, BALB/c, and C3H/He; randombred Swiss; inbred DDK; noninbred Peru; and randombred Q and MF1. Main findings were as follows: 1) The omnipresence was noted in mice of a fourth type of retrovirus-like particle (referred to as ε-particle) clearly distinguishable from the known A-, B-, and C-type particles and resembling more closely the R-type particles of the hamster. 2) ε-Particles occur transiently within the endoplasmic reticulum of early cleavage-stage embryos and have not yet been observed in either oocytes or later embryos, in normal adult tissues, or in tumors. 3) Intracisternal A-particles, identical to those seen generally in mouse tumors, also occurred in oocytes and early embryos but by far in smaller numbers than did ε-particles. 4) The production of ε- and A-particles exhibited exactly inverse patterns of developmental stage dependence. These patterns were basically similar, regardless of the mouse strain examined. 5) In contrast, the mode of occurrence of C-type particles was subject to a great strain-dependent variation.