Differentiation of endoplasmic reticulum in the developing oocyte of the golden hamster (Mesocricetus auratus).

The differentiation of the endoplasmic reticulum (ER) in the developing oocyte of the golden hamster is accompanied by changes in susceptibility to impregnation with a zinc iodide-osmium tetroxide (ZnOs) mixture. The staining of two of the three categories of oocyte ER is first seen at or about the time when rapid oocyte growth is initiated. Staining reaches a peak before antrum formation, then declines. A third category of ER remains unstained at all stages. Aberrant reactivity to ZnOs is seen on oocytes which become atretic, and differs with the stage of oocyte development at which atresia occurs. Relationships between the three categories are described, and an attempt made to relate changes in form and distribution to developmental processes. The frequent contact/continuity between ER and mitochondria is discussed with regard to its possible role in lipid metabolism.