Publisher Summary This chapter provides an overview of the principal features of perikaryal responses to axon injury. The neuron is an unusual cell. Its axon terminals may be situated at what in cellular terms is an enormous distance from the cell body (perikaryon); the volume of the latter may be but a small fraction of the total cellular volume. Yet the neuronal processes are maintained and their substance is constantly renewed from the perikaryon. The separation of an axon from its cell body results (in vertebrates) in the degeneration of the separated portion and is followed by a series of morphological changes in the perikarya. The most conspicuous of these is the disintegration, redistribution, and apparent disappearance from the cell body of cytoplasmic basophil material. Changes in axotomized neurons are generally assessed by comparison with the corresponding contralateral neurons of the experimental animal.

The Axon Reaction: A Review of the Principal Features of Perikaryal Responses to Axon Injury

Intracellular ribonucleoprotein (RNP) granules are membrane-less droplet organelles that are thought to regulate posttranscriptional gene expression. While liquid-liquid phase separation may drive RNP granule assembly, the mechanisms underlying their supramolecular dynamics and internal organization remain poorly understood. Herein, we demonstrate that RNA, a primary component of RNP granules, can modulate the phase behavior of RNPs by controlling both droplet assembly and dissolution in vitro. Monotonically increasing the RNA concentration initially leads to droplet assembly by complex coacervation and subsequently triggers an RNP charge inversion, which promotes disassembly. This RNA-mediated reentrant phase transition can drive the formation of dynamic droplet substructures (vacuoles) with tunable lifetimes. We propose that active cellular processes that can create an influx of RNA into RNP granules, such as transcription, can spatiotemporally control the organization and dynamics of such liquid-like organelles.

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Reentrant Phase Transition Drives Dynamic Substructure Formation in Ribonucleoprotein Droplets

In Vivo Formation of Vacuolated Multi-phase Compartments Lacking Membranes

2 – The Nucleolus and Nucleolar DNA

Introduction Quiescent and dormant embryos, germination: definitions Nuclear characteristics of the cell population in quiescent embryos Distribution of quiescent embryo cell nuclei in different phases of the cell cycle How is the high G\\ cell level of quiescent embryos established? What are the mechanisms controlling G\\ cell accumulation? Controversial observations on the high G\\ accumulation in quiescent embryos Asynchrony of the first mitotic cycle The problem of dry tissue fixation Ultrastructure and cytochemistry of the nucleus in quiescent and early germinating embryos Shape of the nucleus Chromatin in quiescent embryos Possible factors responsible for chromatin condensation in quiescent embryos Dehydration Ions HistoneHl Poly(ADP-ribose) polymerase Chromatin during early germination Possible relationship between chromatin dispersion and its activity Transcription DNA replication and DNA repair RNP translocation Nucleoplasmic ribonucleoproteins Are nucleoplasmic RNPs the storage form of long-lived mRNAs? The nucleolus Ultrastructure Vacuolation Possible significance of nucleolar RNP loss during early germination

Nuclear activation during early germination of the higher plant embryo.

Previously it has been found that in tobacco callus cells nucleolar vacuoles repeatedly form and contract. In this study, nucleolar vacuoles were investigated by using radioautography, actinomycin D, and electron microscopy. It was found, from grain counts of nucleoli labeled with uridine-3H, that nucleoli containing vacuoles had more than three times as many grains/µ2 of nucleolar substance as did nucleolei without vacuoles. Treatment of tobacco callus cells with various concentrations of actinomycin D caused the percentage of cells containing nucleolar vacuoles to decrease; with the highest concentration the percentage of these cells dropped from the normal level of about 70% to less than 10%. However, after removal of actinomycin D the cells regained nucleolar vacuoles up to the control level. When radioautography was used with actinomycin D, it was found that the actinomycin D inhibited the uptake of uridine-3H, i.e. inhibited RNA synthesis, in those nucleoli which lost their nucleolar vacuoles. In addition, after removal of the cells from actinomycin D, it was found that as the cells regained nucleolar vacuoles the nucleoli also began to incorporate uridine-3H. Electron micrographs showed the nucleoli to be composed of a compact, finely fibrous central portion surrounded by a layer of dense particles 100–150 A in diameter. Nucleolar vacuoles occurred in the fibrous central portion. Dense particles similar to those in the outer layer of the nucleoli were found scattered throughout the vacuoles and in a dense layer at their outer edge. These data suggest that in cultured tobacco callus cells the formation and contraction of nucleolar vacuoles is closely related to RNA synthesis in the nucleolus.

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J. Morris Johnson

Papers

A study of nucleolar vacuoles in cultured tobacco cells using radioautography, actinomycin d, and electron microscopy