The nuclear envelope: Its breakdown and fate in mammalian oogonia and oocytes

The dissolution of the nuclear envelope (NE) has been studied ultrastructurally in mouse oocytes maturing in vitro. NE breakdown begins with the undulation of the nuclear envelope followed by the disappearance of nuclear pores. Subsequently, extensive NE convolutions, possibly caused by impinging microtubules, result in the apposition of adjacent NE segments, the appearance of breaks and the formation of NE doublets. Doublets may also be formed by the apposition of intranuclear cisternae to the inner leaflet of the NE. Nuclear envelope breakdown in mouse oocytes takes approximately three and one‐half hours in vitro. The fate of the resulting nuclear envelope fragments has been followed in both mouse oocytes and human oogonia. In the mouse, NE doublets separate into individual cisternae, indistinguishable from endoplasmic reticulum, by the time of the first meiotic metaphase. A different fate for NE doublets is suggested in human oogonia which retain NE doublets throughout the division cycle. The nuclear envelope of the daughter cells reforms, at least in part, from NE doublets by separation of the apposed cisternae into single units and the re‐establishment of chromosomal contact. Thus, at least some of the NE appears to be conserved in actively dividing human oogonia.