True divergent differentiation in a cellular slime mold, Polysphondylium pallidum.

Three abstract models of divergent cell differentiation to multiple cell types are presented. These differ primarily in the proportion of developmental events common to the pathways leading to each cell type. Two experimental approaches are outlined to determine which type best describes divergent differentiation occurring in a particular organism. The first technique is to describe and compare changes in labeling of specific polypeptides which characterize development to the several cell types. The second is to observe the ability of mutants which are blocked in one pathway to develop along alternate pathways. These approaches are applied to the case of Polysphondylium pallidum, where amebae develop into stalk cells, spores, or microcysts. It is concluded that cell differentiation in P. pallidum is of the truly divergent type in which developing cells show identical sequences of events until a branch point, and thereafter very different sequences of events.     

The molecular basis of pattern formation in the developing compound eye of Drosophila

Throughout metazoan development cells select pathways of specialization that lead to the differentiation of specific cell types. Differential gene activation converts initially homogeneous populations of cells into spatial arrangements of diverse cell types. As discussed in other articles in this issue, the signals specifying divergent pathways can be encoded in a cell's lineage, its environment, or a combination of both. This article reviews recent analyses of the developing Drosophila compound eye which have focussed upon the mechanisms by which cells assess environmental information in order to determine their fate. More specifically, it examines the molecular mechanisms used by cells to communicate signals which instruct the developmental pathways of other cells.     

Developmental predetermination of the structural and molecular polarization of photoreceptor cells

Through mechanisms still unknown, the apparently homogeneous neuroepithelium of the embryonic optic cup differentiates into such divergent cell types as photoreceptors, glia, and various subsets of neurons. Questions that still remain unanswered in this field include the timing and mechanism of action of the "instructive" events directing each neuroepithelial cell to undergo the sequence of phenotypic changes necessary to develop into a specific retinal cell type. This laboratory is investigating some of these questions using cultures in which dissociated neural retina cells, obtained before the onset of overt photoreceptor differentiation, develop at low density in the absence of glia and pigment epithelium. The cultures initially are a morphologically homogeneous population of process-free, round cells. Some cells retain this morphology throughout the first week in vitro, while others develop either as photoreceptors or as multipolar neurons. Photoreceptors elongate and become very asymmetric as they do in vivo, with characteristic compartments orderly arranged along their longitudinal axis (an outer segment-like process, inner segment, cell body, and a characteristically short, single neurite). Cell polarization can also be observed in the distribution of opsin immunoreactive materials and some cytoskeletal elements. Thus, certain precursor cells present in the embryonic retina seem to be programmed to differentiate into photoreceptors even when developing in the absence of contacts with other retinal cells. However, interactions with other constituents of the retina/pigment epithelium complex are probably necessary to ensure final photoreceptor maturation, including further growth of the opsin-rich outer segment process.     

Repetitive cellular patterns in the secondary phloem of conifer and dicot trees,and a hypothesis for their development

Abstract

The radial fusiform cell files of the secondary phloem of conifers and dicots are composed of different cell types?–?fibres, parenchyma and sieve cells (in conifers), or sieve elements plus companion cells (in dicots). These cell types are arranged in characteristic, species-specific sequences along the radii of the files. The sequences are replicated in adjacent files and this leads to tangential bands of similar cell type. Moreover, the sequences are developed repetitively so that a sequence found in one year's growth increment of phloem is repeated in the next increment. In some species, many repetitions of the same sequence occur within one annual increment. A general hypothesis has been developed to account for the radial sequences of cell types. It is proposed that there is a gradient of a phloem-promoting morphogen, a series of morphogen thresholds for the determination of each phloem cell type, and a particular spatio-temporal pattern of periclinal cell division in the phloem domain of the vascular cambium that generates a corresponding pattern of cell displacement through the morphogen gradient in the immediately post-mitotic zone of cell determination. The feasibility of the hypothesis was supported by means of simulation which, using a constant set of initial conditions, could reproduce very nearly all the radial sequences of cell types found in the secondary phloem of a range of species of conifers and woody dicots. The tangential banding of the various cell types suggests that cell production and cell determination are events which occur synchronously across the radial files. The repeating blocks of cell types may constitute functional modules of phloem tissue, and the constituent cells probably have particular patterns of symplasmic connections and mechano-structural properties.     

Potential of embryonic stem cells

Embryonic stem (ES) cells are pluripotent cell lines established from undifferentiated embryonic cells characterized by nearly unlimited self-renewal and differentiation capacity. During differentiation in vitro, ES cells were found to be able to develop into specialized somatic cells types and to recapitulate processes of early embryonic development. These properties allow to use ES cells as model system for studying early embryonic development by gain- or loss-of-function approaches, or to investigate the effects of drugs and environmental factors on differentiation and cell function in embryotoxicity and pharmacology. Now, ES cells derived of human blastocysts may be used for the generation of somatic precursor or differentiated cells in cell and tissue therapy. The review presents data of mouse ES cell differentiation and gives an outlook on future perspectives and problems of using human ES cells in regenerative medicine.     

A Mathematical Model of Cancer Stem Cell Driven Tumor Initiation: Implications of Niche Size and Loss of Homeostatic Regulatory Mechanisms

Hierarchical organized tissue structures, with stem cell driven cell differentiation, are critical to the homeostatic maintenance of most tissues, and this underlying cellular architecture is potentially a critical player in the development of a many cancers. Here, we develop a mathematical model of mutation acquisition to investigate how deregulation of the mechanisms preserving stem cell homeostasis contributes to tumor initiation. A novel feature of the model is the inclusion of both extrinsic and intrinsic chemical signaling and interaction with the niche to control stem cell self-renewal. We use the model to simulate the effects of a variety of types and sequences of mutations and then compare and contrast all mutation pathways in order to determine which ones generate cancer cells fastest. The model predicts that the sequence in which mutations occur significantly affects the pace of tumorigenesis. In addition, tumor composition varies for different mutation pathways, so that some sequences generate tumors that are dominated by cancerous cells with all possible mutations, while others are primarily comprised of cells that more closely resemble normal cells with only one or two mutations. We are also able to show that, under certain circumstances, healthy stem cells diminish due to the displacement by mutated cells that have a competitive advantage in the niche. Finally, in the event that all homeostatic regulation is lost, exponential growth of the cancer population occurs in addition to the depletion of normal cells. This model helps to advance our understanding of how mutation acquisition affects mechanisms that influence cell-fate decisions and leads to the initiation of cancers.     

Cellular Plasticity Among Axolotl Neural Crest-Derived Pigment Cell Lineages

Many of the factors and mechanisms guiding the migration/differentiation of neural crest cells that give rise to a number of distinguishable cell types, including all dermal and epidermal pigment cells, remain unknown. The axolotl possesses three pigment cell types that differentiate according to specific developmentally programmed sequences and contribute to pigment pattern in the adult. A single lineage of the crest that becomes restricted to one of three pigment cell types gives us the opportunity to examine the existence of a neural crest stem cell population and the potential for transdifferentiation events. Interpretations of experiments involving drug-treated and mutant axolotls implicate cellular plasticity leading to observed phenotypes. We present results from recent in vitro studies designed to identify parameters influencing differentiation events of individual neural crest-derived pigment cell lineages. We demonstrate that the differentiation of xanthophores is enhanced, while that of the melanophores are inhibited in guanosine-supplemented neural crest cell cultures. Data suggest that the increase in one pigment cell population is at the expense of another, indicative of cellular plasticity. Videomicroscopy used in this study agrees with an abundance of correlative evidence supporting the hypothesis of transdifferentiation events among neural crest-derived pigment cell populations. The embryonic neural crest-derived pigment cell system is an ideal model to study differentiation of multipotential stem cells that play critical roles in patterning.     

Patterned cell determination in a plant tissue: the secondary phloem of trees

The secondary vascular tissues (xylem and phloem) of woody plants originate from a vascular cambium and develop as radially oriented files of cells. The secondary phloem is composed of three or four cell types, which are organised into characteristic recurrent cellular sequences within the radial cell files of this tissue. There is a gradient of auxin (indole acetic acid) across both the cambium and the immediately postmitotic cells within the xylem and phloem domains, and it is believed that this morphogen, probably in concert with other morphogenic factors, is closely associated with the determination and differentiation of the different cells types in each tissue. A hypothesis is developed that, in conjunction with the positional values conferred by the graded radial distribution of morphogen, cell divisions at particular positions within the cambium are sufficient to determine not only each of the phloem cell types but also their recurrent pattern of differentiation within each radial cell file.     

Transgenic stem cells in Hydra reveal an early evolutionary origin for key elements controlling self-renewal and differentiation

Little is known about stem cells in organisms at the beginning of evolution. To characterize the regulatory events that control stem cells in the basal metazoan Hydra, we have generated transgenics which express eGFP selectively in the interstitial stem cell lineage. Using them we visualized stem cell and precursor migration in real-time in the context of the native environment. We demonstrate that interstitial cells respond to signals from the cellular environment, and that Wnt and Notch pathways are key players in this process. Furthermore, by analyzing polyps which overexpress the Polycomb protein HyEED in their interstitial cells, we provide in vivo evidence for a role of chromatin modification in terminal differentiation. These findings for the first time uncover insights into signalling pathways involved in stem cell differentiation in the Bilaterian ancestor; they demonstrate that mechanisms controlling stem cell behaviour are based on components which are conserved throughout the animal kingdom.     

Sphingomyelin metabolites in vascular cell signaling and atherogenesis

The atherosclerotic lesion most probably develops through a number of cellular events which implicate all vascular cell types and include synthesis of extracellular proteins, cell proliferation, differentiation and death. Sphingolipids and sphingolipid metabolizing enzymes may play important roles in atherogenesis, not only because of lipoprotein alterations but also by mediating a number of cellular events which are believed to be crucial in the development of the vascular lesions such as proliferation or cell death. Exogenous sphingolipids may mediate various biological effects such as apoptosis, mitogenesis or differentiation depending on the cell type. Moreover, several molecules present in the atherogenic lesion, such as oxidized LDL, growth factors or cytokines, which activate intracellular signaling pathways leading to vascular cell modifications, can stimulate sphingomyelin hydrolysis and generation of ceramide (and other metabolites as sphingosine-1-phosphate). Here we review the potential implication of the sphingomyelin/ceramide cycle in vascular cell signaling related to atherosclerosis, and more generally the role of sphingolipids in the events observed during the atherosclerotic process as cell differentiation, migration, adhesion, retraction, proliferation and death.     

Stem cell death and survival in heart regeneration and repair

Cardiovascular diseases are major causes of mortality and morbidity. Cardiomyocyte apoptosis disrupts cardiac function and leads to cardiac decompensation and terminal heart failure. Delineating the regulatory signaling pathways that orchestrate cell survival in the heart has significant therapeutic implications. Cardiac tissue has limited capacity to regenerate and repair. Stem cell therapy is a successful approach for repairing and regenerating ischemic cardiac tissue; however, transplanted cells display very high death percentage, a problem that affects success of tissue regeneration. Stem cells display multipotency or pluripotency and undergo self-renewal, however these events are negatively influenced by upregulation of cell death machinery that induces the significant decrease in survival and differentiation signals upon cardiovascular injury. While efforts to identify cell types and molecular pathways that promote cardiac tissue regeneration have been productive, studies that focus on blocking the extensive cell death after transplantation are limited. The control of cell death includes multiple networks rather than one crucial pathway, which underlies the challenge of identifying the interaction between various cellular and biochemical components. This review is aimed at exploiting the molecular mechanisms by which stem cells resist death signals to develop into mature and healthy cardiac cells. Specifically, we focus on a number of factors that control death and survival of stem cells upon transplantation and ultimately affect cardiac regeneration. We also discuss potential survival enhancing strategies and how they could be meaningful in the design of targeted therapies that improve cardiac function.     

Expression of the v-crk oncogene product in PC12 cells results in rapid differentiation by both nerve growth factor- and epidermal growth factor-dependent pathways.

The transforming gene of the avian sarcoma virus CT10 encodes a fusion protein (p47gag-crk or v-Crk) containing viral Gag sequences fused to cellular sequences consisting primarily of Src homology regions 2 and 3 (SH2 and SH3 sequences). Here we report a novel function of v-Crk in the mammalian pheochromocytoma cell line, PC12, whereby stable expression of v-Crk induces accelerated differentiation, as assessed by induction of neurites following nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) treatment compared with the effect in native PC12 cells. Surprisingly, however, these cells also develop extensive neurite processes after epidermal growth factor (EGF) stimulation, an event which is not observed in native PC12 cells. Following EGF or NGF stimulation of the v-CrkPC12 cells, the v-Crk protein itself became tyrosine phosphorylated within 1 min. Moreover, in A431 cells or TrkA-PC12 cells, which overexpress EGF receptors and TrkA, respectively, a GST-CrkSH2 fusion protein was indeed capable of binding these receptors in a phosphotyrosine-dependent manner, suggesting that v-Crk can directly couple to receptor tyrosine kinase pathways in PC12 cells. In transformed fibroblasts, v-Crk binds to specific tyrosine-phosphorylated proteins of p130 and paxillin. Both of these proteins are also complexed to v-Crk in PC12 cells, as evidenced by their coprecipitation with v-Crk in detergent lysates, suggesting that common effector pathways may occur in both cell types. However, whereas PC12 cellular differentiation can occur solely by overexpression of the v-Src or oncogenic Ras proteins, that induced by v-Crk requires a growth factor stimulatory signal, possibility in a two-step process.     

Asymmetric cell divisions in flowering plants - one mother, "two-many" daughters

Plant development shows a fascinating range of asymmetric cell divisions. Over the years, however, cellular differentiation has been interpreted mostly in terms of a mother cell dividing mitotically to produce two daughter cells of different fates. This popular view has masked the significance of an entirely different cell fate specification pathway, where the mother cell first becomes a coenocyte and then cellularizes to simultaneously produce more than two specialized daughter cells. The "one mother - two different daughters" pathways rely on spindle-assisted mechanisms, such as translocation of the nucleus/spindle to a specific cellular site and orientation of the spindle, which are coordinated with cell-specific allocation of cell fate determinants and cytokinesis. By contrast, during "coenocyte-cellularization" pathways, the spindle-assisted mechanisms are irrelevant since cell fate specification emerges only after the nuclear divisions are complete, and the number of specialized daughter cells produced depends on the developmental context. The key events, such as the formation of a coenocyte and migration of the nuclei to specific cellular locations, are coordinated with cellularization by unique types of cell wall formation. Both one mother - two different daughters and the coenocyte-cellularization pathways are used by higher plants in precise spatial and time windows during development. In both the pathways, epigenetic regulation of gene expression is crucial not only for cell fate specification but also for its maintenance through cell lineage. In this review, the focus is on the coenocyte-cellularization pathways in the context of our current understanding of the asymmetric cell divisions. Instances where cell differentiation does not involve an asymmetric division are also discussed to provide a comprehensive account of cell differentiation.     

The Ras/Raf/ERK signalling pathway drives Schwann cell dedifferentiation

Schwann cells are a regenerative cell type. Following nerve injury, a differentiated myelinating Schwann cell can dedifferentiate and regain the potential to proliferate. These cells then redifferentiate during the repair process. This behaviour is important for successful axonal repair, but the signalling pathways mediating the switch between the two differentiation states remain unclear. Sustained activation of the Ras/Raf/ERK cascade in primary cells results in a cell cycle arrest and has been implicated in the differentiation of certain cell types, in many cases acting to promote differentiation. We therefore investigated its effects on the differentiation state of Schwann cells. Surprisingly, we found that Ras/Raf/ERK signalling drives the dedifferentiation of Schwann cells even in the presence of normal axonal signalling. Furthermore, nerve wounding in vivo results in sustained ERK signalling in associated Schwann cells. Elevated Ras signalling is thought to be important in the development of Schwann cell-derived tumours in neurofibromatosis type 1 patients. Our results suggest that the effects of Ras signalling on the differentiation state of Schwann cells may be important in the pathogenesis of these tumours.     

Roles for Dicer1 in the patterning and differentiation of the optic cup neuroepithelium

The embryonic ocular neuroepithilium generates a myriad of cell types, including the neuroretina, the pigmented epithelium, the ciliary and iris epithelia, and the iris smooth muscles. As in other regions of the developing nervous system, the generation of these various cell types requires a coordinated sequence of patterning, specification and differentiation events. We investigated the roles of microRNAs (miRNAs) in the development of optic cup (OC)-derived structures. We inactivated Dicer1, a key mediator of miRNA biosynthesis, within the OC in overlapping yet distinct spatiotemporal patterns. Ablation of Dicer1 in the inner layer of the OC resulted in patterning alteration, particularly at the most distal margins. Following loss of Dicer1, this region generated a cryptic population of cells with a mixed phenotype of neuronal and ciliary body (CB) progenitors. Notably, inactivation of Dicer1 in the retinal progenitors further resulted in abrogated neurogenesis, with prolongation of ganglion cell birth and arrested differentiation of other neuronal subtypes, including amacrine and photoreceptor cells. These alterations were accompanied by changes in the expression of Notch and Hedgehog signaling components, indicating the sensitivity of the pathways to miRNA activity. Moreover, this study revealed the requirement of miRNAs for morphogenesis of the iris and for the regulation of CB cell type proliferation and differentiation. Together, analysis of the three genetic models revealed novel, stage-dependent roles for miRNAs in the development of the ocular sub-organs, which are all essential for normal vision.     

Calcium signaling in lizard red blood cells

The ion calcium is a ubiquitous second messenger, present in all eukaryotic cells. It modulates a vast number of cellular events, such as cell division and differentiation, fertilization, cell volume, decodification of external stimuli. To process this variety of information, the cells display a number of calcium pools, which are capable of mobilization for signaling purposes. Here we review the calcium signaling on lizards red blood cells, an interesting model that has been receiving an increasing notice recently. These cells possess a complex machinery to regulate calcium, and display calcium responses to extracellular agonists. Interestingly, the pattern of calcium handling and response are divergent in different lizard families, which enforces the morphological data to their phylogenetic classification, and suggest the radiation of different calcium signaling models in lizards evolution.