Changes in the distribution of endoplasmic reticulum (ER) in porcine oocytes during meiotic maturation were examined. In the Germinal Vesicle (GV) stage oocyte, there were many bright clusters of ER present in the peripheral cytoplasm, and some of them formed a mass. This mass was absent in the Prometaphase I (Pro-MI) and Metaphase I (MI) stage oocytes, and the clusters became fewer than in the GV stage oocyte. ER accumulations that appear to be generated by scattering of the cluster or the mass were uniformly distributed in the peripheral cytoplasm and formed a thin layer beneath the plasma membrane. In the Metaphase II (MII) stage oocyte, the layer became more evident and wider, and a space with a relatively sparser distribution of ER was observed beneath the layer. From these results, it is clear that the distribution in the ER of porcine oocytes is changing during miotic maturation.

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