Differentiation of nuclear structure during the sexual cycle in Tetrahymena thermophila: II. Degeneration and autolysis of macro- and micronuclei

Abstract

During the postzygotic period of the sexual phase (conjugation) in the protozoon, Tetrahymena, the parental somatic macronucleus and one of the presumptive germinal micronuclei degenerate and eventually disintegrate within the cells. Electron microscopy was applied to reveal the fine-structural details of the degeneration and autolysis of these macro- and micronuclei. From detailed analysis of our data, we divided the different processes involved into several distinct cytological stages. An initial event of degeneration is the strong condensation of the nuclei, referred to as nuclear pyknosis. This process also involves transformation of the nuclear envelope, including the loss of pore complexes, attached ribosomes, and continuity with the endoplasmic reticulum. Following this event, large, flattened cisternae can occasionally be seen in the process of enveloping pyknotic nuclei ( autophagosome formation). A later stage is reached when the autophagosome has closed and its inner membrane has dissolved ( autophagosome type II according to the classification of Nilsson). At this stage, small, electron-lucent vesicles can be seen fusing with and/or ‘pinching off’ from the phagosome membrane around old macronuclei. Very dense old macronuclei are sometimes encountered in which the surface layer has become so compressed that the residual nuclear envelope underneath the autophagosome membrane is obscured ( compact autophagosome stage). In a later typical stage, the residual nuclear envelope reappears and separates from the autophagosome membrane ( ‘sickle’ stage). During this stage, there is the first clear morphological indication of chromatin digestion. The last stage that we were able to observe is characterized by the presence of a large autophagosome and a light ‘halo’ around the enclosed pyknotic nucleus ( ‘halo’ stage). During this stage, lysosomes fuse with the autophagosome, creating an auto(phago)lysosome. Within these organelles, progressive digestion and destruction of the pyknotic nuclei are observable.