Stratification within centrifuged amoeba nuclei

Summary Two species of large, fresh water amoebae were ultracentrifuged and studied with the electron microscope. Emphasis was placed on the stratification of the nucleoplasm, including nucleoli, within the confines of the nuclear envelope during interphase. Three major strata were found in the nuclei of both amoeba species, namely the centripetal nucleoplasm, the middle chromatin stratum, and the centrifugal nucleolar mass. In the highly radioresistant A. proteus, the nucleolar mass separated into a centripetal electron-opaque layer and a centrifugal electron-lucent layer. The latter layer appears to be missing from the radiosensitive P. illinoisensis. The nature of these nucleolar layers and their possible relationship to differences in radiosensitivity between the two species of amoebae is discussed. The contents of the heavier of the two nucleolar layers in A. proteus might be resistant to radiation damage and may possess radiorestorative capacity.Work supported by U.S. Atomic Energy Commission. A part of the work was reported at the 17th Meeting of the Society of Protozoologists, Boulder, Colorado in 1964.