We injected DiI in oil into mouse oocytes in order to label the ER. In immature oocytes, the ER was relatively uniform. However, in mature oocytes, membrane clusters had developed in the vegetal cortex (side away from the site of polar body formation). Since fertilization normally occurs in the vegetal half of the mouse egg, the development of these clusters may be necessary to permit the large release of Ca at fertilization.
Low magnification view of immature oocyte (upper) and mature oocyte (lower). The immature oocyte has a large germinal vesicle (nucleus). The mature oocyte is arrested at second meiotic metaphase; the meiotic spindle is located at the top of the egg (animal pole). membrane clusters are present in the cortex on the opposite side of the meiotic spindle (vegetal half).
High magnification view of the cortex of an immature oocyte (upper) and a mature oocyte (lower), showing the membrane clusters in the mature oocyte.