An electron microscope study of the nuclear envelope of amphibian oocytes

Abstract

Immature oocytes from Triturus viridescens of two stages, both prior to yolk formation, were studied by electron microscopy. In the transition from the first to the second of these stages, two nuclear transformations, namely, undulation of the nuclear envelope and nucleolar shredding, apparently take place simultaneously. The nuclear outpocketings are usually bulbous. Infrequently, invaginations of the nuclear envelope occur. The nuclear envelope is composed of two membranes, each about 75 Å thick, which are separated by a perinuclear space which is 150 Å in width. The envelope is interrupted by small circular discontinuities, 800–1000 Å in diameter, which result from the union of the two membranes at regular intervals. It has been established that there are tubes within the discontinuities which are probably not interrupted by the nuclear envelope. Each tube is about 1000 Å in diameter and has a total height of 1500 Å. The annuli seen in tangential sections are considered to represent cross-sections of the tubes. Within or making up the tube walls are about 8 microcylinders which are seen in cross-section as subannuli. Many of the annuli were seen to contain a central granule about 150 Å in diameter. The nucleoli, which increase in size during the transition from one stage to another, are made up of granules about 150 Å in diameter. A new approach to the geometric interpretation of structures has been presented. X-rays of lucite models embedded in paraffin have been studied in order to determine that the conclusions regarding the structural relationships seen in electron micrographs were not optical illusions.