Cajal Bodies, Nucleoli, and Speckles in the Xenopus Oocyte Nucleus Have a Low-Density, Sponge-like Structure

Abstract

Nuclear organelles, unlike many cytoplasmic organelles, lack investing membranes and are thus in direct contact with the surrounding nucleoplasm. Because the properties of the nucleoplasm and nuclear organelles influence the exchange of molecules from one compartment to another, it is important to understand their physical structure. We studied the density of the nucleoplasm and the density and permeability of nucleoli, Cajal bodies (CBs), and speckles in the Xenopus oocyte nucleus or germinal vesicle (GV). Refractive indices were measured by interferometry within intact GVs isolated in oil. The refractive indices were used to estimate protein concentrations for nucleoplasm (0.106 g/cm 3 ), CBs (0.136 g/cm 3 ), speckles (0.162 g/cm 3 ), and the dense fibrillar region of nucleoli (0.215 g/cm 3 ). We determined similar protein concentrations for nuclear organelles isolated in aqueous media, where they are no longer surrounded by nucleoplasm. To examine the permeability of nuclear organelles, we injected fluorescent dextrans of various molecular masses (3-2000 kDa) into the cytoplasm or directly into the GV and measured the extent to which they penetrated the organelles. Together, the interferometry and dextran penetration data show that organelles in the Xenopus GV have a low-density, sponge-like structure that provides access to macromolecules from the nucleoplasm.

Investigating the spectral characteristics of backscattering from heterogeneous spherical nuclei using broadband finite-difference time-domain simulations

“…29 The average refractive index of the nucleolus and chromatin condensation for normal cells was set at 1.39. 8,28 For dysplastic cells, darker appearance of Feulgenthionin-stained nuclei in histology slides indicates higher concentrations of nucleic acids and nucleoproteins, which leads to higher refractive indices.…”

Experimental evidence for the influence of molecular crowding on nuclear architecture

“…This so-called 'excluded volume effect' promotes segregation, assembly and alignment of macromolecules and particulate matter depending on their size and shape (Ellis, 2001;Herzfeld, 1996;Minton, 2001). Two features render the nucleoplasm a crowded environment: the high macromolecular content above 10% w/v (Handwerger et al, 2005) and the abundance of large, particulate compounds such as the ribonucleoprotein particles. Model calculations of the excluded volume effect involved in replication, polymerase I and polymerase II transcription, allowed the prediction of the presence of replication factories, nucleoli and transcription foci as a consequence of macromolecular crowding (Marenduzzo et al, 2006b).…”

The nucleolus: reviewing oldies to have new understandings

“…Inside the nucleus, the nucleolus is the most prominent structure. Owing to the difference in density between nucleolus and its surrounding nucleoplasm (0.215 g/cm 3 versus 0.106 g/cm 3 ), known for a long time but recently determined by the refractive indices in the nucleus of Xenopus oocytes [1], the nucleolus is easily observed in monolayer culture cells under a phase-contrast microscope ( Figure 1A). It appears as an oval to round shape but varies in sizes and numbers in different types of the cell.…”