Effect of cell population density on G2 arrest in Tetrahymena

Cell quiescence in planarian stem cells, interplay between p53 and nutritional stimuli

Cell quiescence appeared early in evolution as an adaptive response to adverse conditions (i.e. nutrient depletion). In metazoans, quiescence has been involved in additional processes like tissue homeostasis, which is made possible by the presence of adult stem cells (ASCs). Cell cycle control machinery is a common hub for quiescence entrance, and evidence indicates a role for p53 in establishing the quiescent state of undamaged cells. Mechanisms responsible for waking up quiescent cells remain elusive, and nutritional stimulus, as a legacy of its original role, still appears to be a player in quiescence exit. Planarians, rich in ASCs, represent a suitable system in which we characterized a quiescent population of ASCs, the dorsal midline cord (DMC) cells, exhibiting unique transcriptional features and maintained quiescent by p53 and awakened upon feeding. The function of DMC cells is puzzling and we speculate that DMC cells, despite retaining ancient properties, might represent a functional drift in which quiescence has been recruited to provide evolutionary advantages.

Quiescence Entry, Maintenance, and Exit in Adult Stem Cells

Cells of unicellular and multicellular eukaryotes can respond to certain environmental cues by arresting the cell cycle and entering a reversible state of quiescence. Quiescent cells do not divide, but can re-enter the cell cycle and resume proliferation if exposed to some signals from the environment. Quiescent cells in mammals and humans include adult stem cells. These cells exhibit improved stress resistance and enhanced survival ability. In response to certain extrinsic signals, adult stem cells can self-renew by dividing asymmetrically. Such asymmetric divisions not only allow the maintenance of a population of quiescent cells, but also yield daughter progenitor cells. A multistep process of the controlled proliferation of these progenitor cells leads to the formation of one or more types of fully differentiated cells. An age-related decline in the ability of adult stem cells to balance quiescence maintenance and regulated proliferation has been implicated in many aging-associated diseases. In this review, we describe many traits shared by different types of quiescent adult stem cells. We discuss how these traits contribute to the quiescence, self-renewal, and proliferation of adult stem cells. We examine the cell-intrinsic mechanisms that allow establishing and sustaining the characteristic traits of adult stem cells, thereby regulating quiescence entry, maintenance, and exit.

The cell biology of quiescent yeast – a diversity of individual scenarios

Most cells, from unicellular to complex organisms, spend part of their life in quiescence, a temporary non-proliferating state. Although central for a variety of essential processes including tissue homeostasis, development and aging, quiescence is poorly understood. In fact, quiescence encompasses various cellular situations depending on the cell type and the environmental niche. Quiescent cell properties also evolve with time, adding another layer of complexity. Studying quiescence is, above all, limited by the fact that a quiescent cell can be recognized as such only after having proved that it is capable of re-proliferating. Recent cellular biology studies in yeast have reported the relocalization of hundreds of proteins and the reorganization of several cellular machineries upon proliferation cessation. These works have revealed that quiescent cells can display various properties, shedding light on a plethora of individual behaviors. The deciphering of the molecular mechanisms beyond these reorganizations, together with the understanding of their cellular functions, have begun to provide insights into the physiology of quiescent cells. In this Review, we discuss recent findings and emerging concepts in Saccharomyces cerevisiae quiescent cell biology.

Cell-cycle quiescence maintains Caenorhabditis elegans germline stem cells independent of GLP-1/Notch

Many types of adult stem cells exist in a state of cell-cycle quiescence, yet it has remained unclear whether quiescence plays a role in maintaining the stem cell fate. Here we establish the adult germline of Caenorhabditis elegans as a model for facultative stem cell quiescence. We find that mitotically dividing germ cells--including germline stem cells--become quiescent in the absence of food. This quiescence is characterized by a slowing of S phase, a block to M-phase entry, and the ability to re-enter M phase rapidly in response to re-feeding. Further, we demonstrate that cell-cycle quiescence alters the genetic requirements for stem cell maintenance: The signaling pathway required for stem cell maintenance under fed conditions--GLP-1/Notch signaling--becomes dispensable under conditions of quiescence. Thus, cell-cycle quiescence can itself maintain stem cells, independent of the signaling pathway otherwise essential for such maintenance.

Proliferation/Quiescence: When to start? Where to stop? What to stock?

The cell cycle is a tightly controlled series of events that ultimately lead to cell division. The literature deciphering the molecular processes involved in regulating the consecutive cell cycle steps is colossal. By contrast, much less is known about non-dividing cellular states, even if they concern the vast majority of cells, from prokaryotes to multi-cellular organisms. Indeed, cells decide to enter the division cycle only if conditions are favourable. Otherwise they may enter quiescence, a reversible non-dividing cellular state. Recent studies in yeast have shed new light on the transition between proliferation and quiescence, re-questioning the notion of cell cycle commitment. They also indicate a predominant role for cellular metabolic status as a major regulator of quiescence establishment and exit. Additionally, a growing body of evidence indicates that environmental conditions, and notably the availability of various nutrients, by impinging on specific metabolic routes, directly regulate specific cellular re-organization that occurs upon proliferation/quiescence transitions.

Cell cycles in the male accessory glands of mealworm pupae

During the pupal stage of Tenebrio molitor, the accessory reproductive glands of males grow by cell division. Within the secretory epithelium of the bean-shaped accessory glands (BAGs), cell numbers triple. In the tubular accessory glands (TAGs), the increase is 14-fold. There are two mitotic maxima in each gland. The first maximum occurs at 1-2 days while the second is at 4-5 days. The second maximum coincides with the major ecdysteroid peak described by Delbecque et al. [Dev. Biol. 64, 11-30 (1978)]. Nuclei were isolated from TAGs during the pupal mitotic bouts and during mitotic inactivity in the adult. After Feulgen or propidium iodide staining, the DNA content of these nuclear populations was measured by absorption cytophotometry or by fluorescence flow cytometry, respectively. The proportion of cells in each phase of the cycle was calculated using an iterative model. After mitoses have ended in the late pupa, the cells were arrested in G2. [3H]Thymidine was injected into 1- and 4-day pupae to pulse-label cells of the TAGs. After allowing various periods from 4 to 60 hr for cells to progress through G2 to reach mitosis, fractions of labelled mitoses were determined by autoradiography. From the combined cytometric and autoradiographic data, the duration of each phase of the cell cycle was calculated assuming the population was in exponential growth. Cell cycles in 4-day pupal TAGs take 48 hr. G1, S, G2, and, M lasted 13, 14, 17, and 4 hr, respectively.

A quantitative lifespan study of changes in cell number, cell division and cell death in various regions of the mouse forebrain

A quantitative study of changes in total cell number was carried out in the indusium griseum and anterior commissure from fetal life to old age in the mouse brain. The changes in the number of mitotic and pyknotic cells were recorded in the indusium griseum, anterior commissure, subependymal and ependymal layers over the same period. The number of neurons which are produced and which migrate to the indusium griseum are in excess of the number eventually required and the surplus neurons are lost by cell death in late gestation and early postnatal life while synaptogenesis and neuronal differentiation is taking place. This neuronal loss is associated with a rapid turnover of glia. Most first generation glia, or their immediate precursors, are produced prenatally, in parallel but one day behind neurons. There is no large burst of mitotic activity in the postnatal brain which gives rise to the myelination gliosis which is probably largely a migratory phenomenon. Cell division continues throughout life in all parts of the brain studied. The greatest mitotic activity is centred in the subependymal layer where mitotic cells substantially outnumber pyknotic ones. There is a gradual decrease in mitotic activity in the subependymal layer up to 9 months of age with fairly constant mitotic activity thereafter. Mitotic activity in the indusium griseum levels out at 3 months postnatum with mitotic and pyknotic cells present in roughly equal numbers thereafter. Mitotic activity in all parts of the anterior commissure levels out at 6 months postnatum and remains constant thereafter. Mitotic and pyknotic cells are present in similar numbers except for a peak in pyknotic cells at 9 months. Cell number in the indusium griseum and anterior commissure is fairly constant between 3 and 9 months, but glial number begins to decrease in all parts of the anterior commissure from 12 to 22 months. In the indusium griseum the number of glia increased slightly between 6 and 22 months. The number of neurons fluctuated during the first week after birth then remained constant until 18 months. There was a significant decrease in the number of neurons between 18 and 22 months.

Transformation in Tetrahymena pyriformis: description of an inducible phenotype

Transformation of Tetrahymena pyriformis to a rapid-swimming (presumably dispersal) form can be induced by washing cells and suspending them in distilled H2O, Dryl's solution or 10 mM Tris. Transformation is possible with high efficiency in mass cultures of axenically grown cells within approximately 5 h at 30 C. The radically different phenotype produced during transformation is characterized by a more elongate body form, increased numbers of somatic basal bodies and cilia, a long caudal cilium and oral membranelles positioned beneath the cell surface. DNA quantities characteristic of G1, S, and G2 cells are found in these transformed ciliates, suggesting that achievement of a particular stage in the DNA-division cycle is not a prerequisite for transformation. Preliminary observations on cells belonging to syngens 2-12 indicate that they also have a capacity to form a caudal cilium, but that the amicronucleate strain GL-C does not. Possible relevance of the transformed phenotype for taxonomy of Tetrahymena is discussed.