Quantitative cellular analysis of growth and reproduction in freshwater planarians (Turbellaria; Tricladida). I. A cellular description of the intact organism

Non-senescent species are not immortal: Stress and decline in two planaria species

Potential immortality is observed in several species (e.g. prickly pear cactus, hydra and flatworms) and is indicative of their negligible or even negative senescence rates. Unlike in senescent species, which experience reduced individual performance with age due to physiological degradation, species with negligible or negative senescence display mortality rates that remain constant or decline with age, respectively. These rates vary across taxa and are correlated with life history traits. Yet, the extent to which variable resource availability, a key driver of variation in life history traits, impacts species that show negligible or negative senescence is currently unknown. Here, we examine whether and how variation in the quantity, quality and feeding interval of resources impact population structure, population performance and life history trait trade-offs in two long-lived planaria that do not senesce: Schmidtea mediterranea and Dugesia tahitiensis. In a full factorial design, different combinations of resource quantity (reduced intake, standard intake and high intake) and quality (high and low quality) were provided in two different feeding intervals (7-day and 14-day intervals) for 19 weeks. We show that variability in resource availability, via decreases in quantity, quality and frequency of resources, does not diminish population viability in either species but does result in suboptimal conditions of stress in S. mediterranea. The high population viability we report can be attributed to two different mechanisms: increased reproduction or increased investment into maintenance at the expense of reproduction. Moreover, which mechanism was responsible for said high population viability was context-dependent and modulated by the specific life history strategy of the two planaria species. We show that suboptimal conditions can cause stress responses that have significant impacts on non-senescent species. The context-dependent response we observe suggests that species that do not senesce but are subject to suboptimal conditions of stress may ultimately exhibit declines in performance and ultimately die. A clearer understanding of the impact of suboptimal conditions of resource availability on non-senescent species is needed to determine the extent of stress experienced and ultimately whether a species can truly be immortal.

Characterization of the dynamics and variability of neuronal subtype responses during growth, degrowth, and regeneration of Nematostella vectensis

BACKGROUND

The ability to regenerate body parts is a feature of metazoan organisms and the focus of intense research aiming to understand its basis. A number of mechanisms involved in regeneration, such as proliferation and tissue remodeling, affect whole tissues; however, little is known on how distinctively different constituent cell types respond to the dynamics of regenerating tissues. Preliminary studies suggest that a number of organisms alter neuronal numbers to scale with changes in body size. In some species with the ability of whole-body axis regeneration, it has additionally been observed that regenerates are smaller than their pre-amputated parent, but maintain the correct morphological proportionality, suggesting that scaling of tissue and neuronal numbers also occurs. However, the cell dynamics and responses of neuronal subtypes during nervous system regeneration, scaling, and whole-body axis regeneration are not well understood in any system. The cnidarian sea anemone Nematostella vectensis is capable of whole-body axis regeneration, with a number of observations suggesting the ability to alter its size in response to changes in feeding. We took advantage of Nematostella's transparent and "simple" body plan and the NvLWamide-like mCherry fluorescent reporter transgenic line to probe the response of neuron populations to variations in body size in vivo in adult animals during body scaling and regeneration.

RESULTS

We utilized the previously characterized NvLWamide-like::mCherry transgenic reporter line to determine the in vivo response of neuronal subtypes during growth, degrowth, and regeneration. Nematostella alters its size in response to caloric intake, and the nervous system responds by altering neuronal number to scale as the animal changes in size. Neuronal numbers in both the endodermal and ectodermal nerve nets decreased as animals shrunk, increased as they grew, and these changes were reversible. Whole-body axis regeneration resulted in regenerates that were smaller than their pre-amputated size, and the regenerated nerve nets were reduced in neuronal number. Different neuronal subtypes had distinct responses during regeneration, including consistent, not consistent, and conditional increases in number. Conditional responses were regulated, in part, by the size of the remnant fragment and the position of the amputation site. Regenerates and adults with reduced nerve nets displayed normal behaviors, indicating that the nerve net retains functionality as it scales.

CONCLUSION

These data suggest that the Nematostella nerve net is dynamic, capable of scaling with changes in body size, and that neuronal subtypes display differential regenerative responses, which we propose may be linked to the scale state of the regenerating animals.

Nutrient availability contributes to a graded refractory period for regeneration in Xenopus tropicalis

Xenopus tadpoles are a unique model for regeneration in that they exhibit two distinct phases of age-specific regenerative competence. In Xenopus laevis, young tadpoles fully regenerate following major injuries such as tail transection, then transiently lose regenerative competence during the "refractory period" from stages 45-47. Regenerative competence is then regained in older tadpoles before being permanently lost during metamorphosis. Here we show that a similar refractory period exists in X. tropicalis. Notably, tadpoles lose regenerative competence gradually in X. tropicalis, with full regenerative competence lost at stage 47. We find that the refractory period coincides closely with depletion of maternal yolk stores and the onset of independent feeding, and so we hypothesized that it might be caused in part by nutrient stress. In support of this hypothesis, we find that cell proliferation declines throughout the tail as the refractory period approaches. When we block nutrient mobilization by inhibiting mTOR signaling, we find that tadpole growth and regeneration are reduced, while yolk stores persist. Finally, we are able to restore regenerative competence and cell proliferation during the refractory period by abundantly feeding tadpoles. Our study argues that nutrient stress contributes to lack of regenerative competence and introduces the X. tropicalis refractory period as a valuable new model for interrogating how metabolic constraints inform regeneration.

Body size-dependent energy storage causes Kleiber's law scaling of the metabolic rate in planarians

Kleiber's law, or the 3/4 -power law scaling of the metabolic rate with body mass, is considered one of the few quantitative laws in biology, yet its physiological basis remains unknown. Here, we report Kleiber's law scaling in the planarian Schmidtea mediterranea. Its reversible and life history-independent changes in adult body mass over 3 orders of magnitude reveal that Kleiber's law does not emerge from the size-dependent decrease in cellular metabolic rate, but from a size-dependent increase in mass per cell. Through a combination of experiment and theoretical analysis of the organismal energy balance, we further show that the mass allometry is caused by body size dependent energy storage. Our results reveal the physiological origins of Kleiber's law in planarians and have general implications for understanding a fundamental scaling law in biology.

Regenerative tissue remodeling in planarians - The mysteries of morphallaxis

Biologists have long marveled at the ability of planarian flatworms to regenerate any parts of their bodies in just a little over a week. While great progress has been made in deciphering the mechanisms by which new tissue is formed at sites of amputation, we know relatively little about the complementary remodeling response that occurs in uninjured tissues to restore anatomical scale and proportion. This review explores the mysterious biology of this process, first described in hydra by the father of experimental zoology, Abraham Trembley, and later termed 'morphallaxis' by the father of experimental genetics, Thomas Hunt Morgan. The perceptive work of these early pioneers, together with recent studies using modern tools, has revealed some of the key features of regenerative tissue remodeling, including repatterning of the body axes, reproportioning of organs like the brain and gut, and a major increase in the rate of cell death. Yet a mechanistic solution to this longstanding problem in the field will require further study by the next generation of planarian researchers.

BERGMANN SIZE CLINES: A SIMPLE EXPLANATION FOR THEIR OCCURRENCE IN ECTOTHERMS

In general ectothermic organisms grow larger at both lower temperatures and higher latitudes. Adult size in the soil nematode Caenorhabditis elegans reared at 10°C was approximately 33% greater than worms grown at 25°C. Nematode egg size and fish red blood cell size showed similar size increases at lower temperatures. These results indicate that body size differences in many ectotherms may simply be a consequence of developmental processes that cause cells to grow larger at lower temperatures. This would provide a general explanation for the increased size of ectotherms at lower temperatures independent of species-specific ecology.

The EGFR signaling pathway controls gut progenitor differentiation during planarian regeneration and homeostasis

The planarian Schmidtea mediterranea maintains and regenerates all its adult tissues through the proliferation and differentiation of a single population of pluripotent adult stem cells (ASCs) called neoblasts. Despite recent advances, the mechanisms regulating ASC differentiation into mature cell types are poorly understood. Here, we show that silencing of the planarian EGF receptor egfr-1 by RNA interference (RNAi) impairs gut progenitor differentiation into mature cells, compromising gut regeneration and maintenance. We identify a new putative EGF ligand, nrg-1, the silencing of which phenocopies the defects observed in egfr-1(RNAi) animals. These findings indicate that egfr-1 and nrg-1 promote gut progenitor differentiation, and are thus essential for normal cell turnover and regeneration in the planarian gut. Our study demonstrates that the EGFR signaling pathway is an important regulator of ASC differentiation in planarians.

Wnt/Notum spatial feedback inhibition controls neoblast differentiation to regulate reversible growth of the planarian brain

Mechanisms determining final organ size are poorly understood. Animals undergoing regeneration or ongoing adult growth are likely to require sustained and robust mechanisms to achieve and maintain appropriate sizes. Planarians, well known for their ability to undergo whole-body regeneration using pluripotent adult stem cells of the neoblast population, can reversibly scale body size over an order of magnitude by controlling cell number. Using quantitative analysis, we showed that after injury planarians perfectly restored brain:body proportion by increasing brain cell number through epimorphosis or decreasing brain cell number through tissue remodeling (morphallaxis), as appropriate. We identified a pathway controlling a brain size set-point that involves feedback inhibition between wnt11-6/wntA/wnt4a and notum, encoding conserved antagonistic signaling factors expressed at opposite brain poles. wnt11-6/wntA/wnt4a undergoes feedback inhibition through canonical Wnt signaling but is likely to regulate brain size in a non-canonical pathway independently of beta-catenin-1 and APC. Wnt/Notum signaling tunes numbers of differentiated brain cells in regenerative growth and tissue remodeling by influencing the abundance of brain progenitors descended from pluripotent stem cells, as opposed to regulating cell death. These results suggest that the attainment of final organ size might be accomplished by achieving a balance of positional signaling inputs that regulate the rates of tissue production.

Cell death: a program to regenerate

Recent studies in Drosophila, Hydra, planarians, zebrafish, mice, indicate that cell death can open paths to regeneration in adult animals. Indeed injury can induce cell death, itself triggering regeneration following an immediate instructive mechanism, whereby the dying cells release signals that induce cellular responses over short and/or long-range distances. Cell death can also provoke a sustained derepressing response through the elimination of cells that suppress regeneration in homeostatic conditions. Whether common properties support what we name "regenerative cell death," is currently unclear. As key parameters, we review here the injury proapoptotic signals, the signals released by the dying cells, the cellular responses, and their respective timing. ROS appears as a common signal triggering cell death through MAPK and/or JNK pathway activation. But the modes of ROS production vary, from a brief pulse upon wounding, to repeated waves as observed in the zebrafish fin where ROS supports two peaks of cell death. Indeed regenerative cell death can be restricted to the injury phase, as in Hydra, Drosophila, or biphasic, immediate, and delayed, as in planarians and zebrafish. The dying cells release in a caspase-dependent manner a variety of signaling molecules, cytokines, growth factors, but also prostaglandins or ATP as recorded in Drosophila, Hydra, mice, and zebrafish, respectively. Interestingly, the ROS-producing cells often resist to cell death, implying a complex paracrine mode of signaling to launch regeneration, involving ROS-producing cells, ROS-sensing cells that release signaling molecules upon caspase activation, and effector cells that respond to these signals by proliferating, migrating, and/or differentiating.

Flies developed smaller cells when temperature fluctuated more frequently

Changes in cell size might be an important component of adaptation to thermal heterogeneity. Although Drosophila melanogaster develops smaller cells at fluctuating temperatures, we do not know whether this response depends on the frequency or amplitude of thermal change. In a laboratory experiment, we exposed flies to either frequent or infrequent fluctuations between 17 and 27 °C, while controlling the total exposure to each temperature. Flies emerged from these treatments with similar body sizes, but flies at more frequent fluctuations emerged earlier and had smaller epidermal cells for a given body size. Tissue built from small cells has more nuclei for transcription, shorter distances between cell compartments, and a larger surface area for transport across membranes. Therefore, we hypothesize that physiological effects of small cells reduce lags in metabolic activity and enhance performance of flies during warming. For plasticity of cell size to confer a fitness advantage, this hypothetical benefit must outweigh the cost of maintaining a greater area of plasma membrane.

Flies developed small bodies and small cells in warm and in thermally fluctuating environments

Although plasma membranes benefit cells by regulating the flux of materials to and from the environment, these membranes cost energy to maintain. Because smaller cells provide relatively more membrane area for transport, ectotherms that develop in warm environments should consist of small cells despite the energetic cost. Effects of constant temperatures on cell size qualitatively match this prediction, but effects of thermal fluctuations on cell size are unknown. Thermal fluctuations could favour either small or large cells; small cells facilitate transport during peaks in metabolic demand whereas large cells minimize the resources needed for homeoviscous adaptation. To explore this problem, we examined effects of thermal fluctuations during development on the size of epidermal cells in the wings of Drosophila melanogaster. Flies derived from a temperate population were raised at two mean temperatures (18 and 25°C), with either no variation or a daily variation of ±4°C. Flies developed faster at a mean temperature of 25°C. Thermal fluctuations sped development, but only at 18°C. An increase in the mean and variance of temperature caused flies to develop smaller cells and wings. Thermal fluctuations reduced the size of males at 18°C and the size of females at 25°C. The thorax, the wings and the cells decreased with an increase in the mean and in the variance of temperature, but the response of cells was the strongest. Based on this pattern, we hypothesize that development of the greater area of membranes under thermal fluctuations provides a metabolic advantage that outweighs the greater energetic cost of remodelling membranes.

Why are organisms usually bigger in colder environments? Making sense of a life history puzzle

Environmental effects on body size are of widespread ecological and economic importance but our understanding of these effects has been obscured by an apparent paradox. Life history analysis suggests that it is adaptive for adults to emerge smaller if reared in conditions that slow down juvenile growth. However, whereas smaller adults emerge if growth is limited by food availability, the reverse is usually observed if growth is limited by temperature. The resolution of this apparent paradox may be that the response of adult size to temperature is adaptive, but is constrained by a trade-off that can be understood in terms of von Bertalanffy's classic theory of growth. Alternatively, the response may be the unavoidable consequence of a fundamental relationship between cell size and temperature.

EGFR signaling regulates cell proliferation, differentiation and morphogenesis during planarian regeneration and homeostasis

Similarly to development, the process of regeneration requires that cells accurately sense and respond to their external environment. Thus, intrinsic cues must be integrated with signals from the surrounding environment to ensure appropriate temporal and spatial regulation of tissue regeneration. Identifying the signaling pathways that control these events will not only provide insights into a fascinating biological phenomenon but may also yield new molecular targets for use in regenerative medicine. Among classical models to study regeneration, freshwater planarians represent an attractive system in which to investigate the signals that regulate cell proliferation and differentiation, as well as the proper patterning of the structures being regenerated. Recent studies in planarians have begun to define the role of conserved signaling pathways during regeneration. Here, we extend these analyses to the epidermal growth factor (EGF) receptor pathway. We report the characterization of three epidermal growth factor (EGF) receptors in the planarian Schmidtea mediterranea. Silencing of these genes by RNA interference (RNAi) yielded multiple defects in intact and regenerating planarians. Smed-egfr-1(RNAi) resulted in decreased differentiation of eye pigment cells, abnormal pharynx regeneration and maintenance, and the development of dorsal outgrowths. In contrast, Smed-egfr-3(RNAi) animals produced smaller blastemas associated with abnormal differentiation of certain cell types. Our results suggest important roles for the EGFR signaling in controlling cell proliferation, differentiation and morphogenesis during planarian regeneration and homeostasis.

Autophagy and apoptosis in planarians

Adult planarians are capable of undergoing regeneration and body remodelling in order to adapt to physical damage or extreme environmental conditions. Moreover, most planarians can tolerate long periods of starvation and during this time, they shrink from an adult size to, and sometimes beyond, the initial size at hatching. Indeed, these properties have made them a classic model to study stem cells and regeneration. Under such stressful conditions, food reserves from the gastrodermis and parenchyma are first used up and later the testes, copulatory organs and ovaries are digested. More surprisingly, when food is again made available to shrunken individuals, they grow back to adult size and all their reproductive structures reappear. These cycles of growth and shrinkage may occur over long periods without any apparent impairment to the individual, or to its future maturation and breeding capacities. This plasticity resides in a mesoderm tissue known as the parenchyma, which is formed by several differentiated non-proliferating cell types and only one mitotically active cell type, the neoblasts, which represent approximately 20-30% of the cells in the parenchyma. Neoblasts are generally thought to be somatic stem-cells that participate in the normal continuous turnover of all cell types in planarians. Hence, planarians are organisms that continuously adapt their bodies (morphallaxis) to different environmental stresses (i.e.: injury or starvation). This adaptation involves a variety of processes including proliferation, differentiation, apoptosis and autophagy, all of which are perfectly orchestrated and tightly regulated to remodel or restore the body pattern. While neoblast biology and body re-patterning are currently the subject of intense research, apoptosis and autophagy remain much less studied. In this review we will summarize our current understanding and hypotheses regarding where and when apoptosis and autophagy occur and fulfil an essential role in planarians.

Cell death and tissue remodeling in planarian regeneration

Many long-lived organisms, including humans, can regenerate some adult tissues lost to physical injury or disease. Much of the previous research on mechanisms of regeneration has focused on adult stem cells, which give rise to new tissue necessary for the replacement of missing body parts. Here we report that apoptosis of differentiated cells complements stem cell division during regeneration in the planarian Schmidtea mediterranea. Specifically, we developed a whole-mount TUNEL assay that allowed us to document two dramatic increases in the rate of apoptosis following amputation-an initial localized response near the wound site and a subsequent systemic response that varies in magnitude depending on the type of fragment examined. The latter cell death response can be induced in uninjured organs, occurs in the absence of planarian stem cells, and can also be triggered by prolonged starvation. Taken together, our results implicate apoptosis in the restoration of proper anatomical scale and proportion through remodeling of existing tissues. We also report results from initial mechanistic studies of apoptosis in planarians, which revealed that a S. mediterranea homolog of the antiapoptotic gene BCL2 is required for cell survival in adult animals. We propose that apoptosis is a central mechanism working in concert with stem cell division to restore anatomical form and function during metazoan regeneration.

A Caenorhabditis elegans wild type defies the temperature-size rule owing to a single nucleotide polymorphism in tra-3

Ectotherms rely for their body heat on surrounding temperatures. A key question in biology is why most ectotherms mature at a larger size at lower temperatures, a phenomenon known as the temperature-size rule. Since temperature affects virtually all processes in a living organism, current theories to explain this phenomenon are diverse and complex and assert often from opposing assumptions. Although widely studied, the molecular genetic control of the temperature-size rule is unknown. We found that the Caenorhabditis elegans wild-type N2 complied with the temperature-size rule, whereas wild-type CB4856 defied it. Using a candidate gene approach based on an N2 x CB4856 recombinant inbred panel in combination with mutant analysis, complementation, and transgenic studies, we show that a single nucleotide polymorphism in tra-3 leads to mutation F96L in the encoded calpain-like protease. This mutation attenuates the ability of CB4856 to grow larger at low temperature. Homology modelling predicts that F96L reduces TRA-3 activity by destabilizing the DII-A domain. The data show that size adaptation of ectotherms to temperature changes may be less complex than previously thought because a subtle wild-type polymorphism modulates the temperature responsiveness of body size. These findings provide a novel step toward the molecular understanding of the temperature-size rule, which has puzzled biologists for decades.

Structure and function of primitive immunoglobulin superfamily neural cell adhesion molecules: a lesson from studies on planarian

Precise wiring and proper remodeling of the neural network are essential for its normal function. The freshwater planarian is an attractive animal in which to study the formation and maintenance of the neural network due to its high regenerative capability and developmental plasticity. Although a recent study revealed that homologs of netrin and its receptors are required for regeneration and maintenance of the planarian central nervous system (CNS), the roles of cell adhesion in the formation and maintenance of the planarian neural network remain poorly understood. In the present study, we found primitive immunoglobulin superfamily cell adhesion molecules (IgCAMs) in a planarian that are homologous to vertebrate neural IgCAMs. We identified planarian orthologs of NCAM, L1CAM, contactin and DSCAM, and designated them DjCAM, DjLCAM, DjCTCAM and DjDSCAM, respectively. We further confirmed that they function as cell adhesion molecules using cell aggregation assays. DjCAM and DjDSCAM were found to be differentially expressed in the CNS. Functional analyses using RNA interference revealed that DjCAM is partly involved in axon formation, and that DjDSCAM plays crucial roles in neuronal cell migration, axon outgrowth, fasciculation and projection.

The power of regeneration and the stem-cell kingdom: freshwater planarians (Platyhelminthes)

The great powers of regeneration shown by freshwater planarians, capable of regenerating a complete organism from any tiny body fragment, have attracted the interest of scientists throughout history. In 1814, Dalyell concluded that planarians could "almost be called immortal under the edge of the knife". Equally impressive is the developmental plasticity of these platyhelminthes, including continuous growth and fission (asexual reproduction) in well-fed organisms, and shrinkage (degrowth) during prolonged starvation. The source of their morphological plasticity and regenerative capability is a stable population of totipotent stem cells--"neoblasts"; this is the only cell type in the adult that has mitotic activity and differentiates into all cell types. This cellular feature is unique to planarians in the Bilateria clade. Over the last fifteen years, molecular studies have begun to reveal the role of developmental genes in regeneration, although it would be premature to propose a molecular model for planarian regeneration. Genomic and proteomic data are essential in answering some of the fundamental questions concerning this remarkable morphological plasticity. Such information should also pave the way to understanding the genetic pathways associated with metazoan somatic stem-cell regulation and pattern formation.

Limitation of size by hypoxia in the fruit fly Drosophila melanogaster

The size of an organism is of fundamental importance in all biological processes. It dictates many of the critical interactions and physical factors that delimit the envelope within which an organism can grow. We investigated the effects of reduced oxygen on size and development in the fruit fly Drosophila melanogaster, and showed that limiting the oxygen in the environment limits both whole animal and cell size. When oxygen levels were reduced from 20% in nitrogen to 15%, 10% and 7.5%, there was a linear decrease in both male and female mass. Both cell size and cell number decreased in low oxygen, but changes in cell size accounted for a larger proportion of the overall change in fly size. Cell numbers decreased by a maximum of 11% between flies reared in 20% oxygen and those reared in 7.5% oxygen, whereas cell surface area decreased by 17%. Low oxygen levels increased development time and mortality, but reduced fecundity. Reducing the level of oxygen available significantly slowed development times, with flies reared in 10% oxygen emerging on average 1.5 days later than those in 20% oxygen. The effect of oxygen on size is reversible during embryonic and larval development up to the pupal stage, when final size is set.

From cells to colonies: at what levels of body organization does the 'temperature-size rule' apply?

An inverse relationship between temperature during ontogeny and final body size is widespread in ectotherms, but poorly understood. Evidence suggests that within organs, this "temperature-size rule" (TSR) may also apply to cell size with no change in numbers. So how closely do reductions in size and number of cells and other repeated structures correlate with size reduction at higher levels of organization? We examine this in the context of a proposal that size and/or number changes at various organizational levels are adaptive responses to temperature- and size-dependent oxygen supply. We subjected two clones of the modular colonial bryozoan, Celleporella hyalina, to orthogonal combinations of two temperatures and two oxygen concentrations during ontogeny, observing effects on sizes of colonies and larvae, and sizes and numbers of cells, tentacles, and modules (autozooids). We found that the size:number responses varied among cell types and among structures at different levels of organization, with the inverse temperature-size relationship applying only to larval parenchymal cells and colony modules. Using our findings and other evidence we propose a unifying adaptive hypothesis that predicts how temperature affects the sizes of mitochondria, cells, organs, modules and organisms, and their relationships with processes that determine the functional capacity of aerobic metabolism.

An in situ hybridization protocol for planarian embryos: monitoring myosin heavy chain gene expression

The monitoring of gene expression is fundamental for understanding developmental biology. Here we report a successful experimental protocol for in situ hybridization in both whole-mount and sectioned planarian embryos. Conventional in situ hybridization techniques in developmental biology are used on whole-mount preparations. However, given that the inherent lack of external morphological markers in planarian embryos hinders the proper interpretation of gene expression data in whole-mount preparations, here we used sectioned material. We discuss the advantages of sectioned versus whole-mount preparations, namely, better probe penetration, improved tissue preservation, and the possibility to interpret gene expression in relation to internal morphological markers such as the epidermis, the embryonic and definitive pharynges, and the gastrodermis. Optimal fixatives and embedding methods for sectioning are also discussed.

The embryonic development of the triclad Schmidtea polychroa

Triclad flatworms are well studied for their regenerative properties, yet little is known about their embryonic development. We here describe the embryonic development of the triclaty 120d Schmidtea polychroa, using histological and immunocytochemical analysis of whole-mount preparations and sections. During early cleavage (stage 1), yolk cells fuse and enclose the zygote into a syncytium. The zygote divides into blastomeres that dissociate and migrate into the syncytium. During stage 2, a subset of blastomeres differentiate into a transient embryonic epidermis that surrounds the yolk syncytium, and an embryonic pharynx. Other blastomeres divide as a scattered population of cells in the syncytium. During stage 3, the embryonic pharynx imbibes external yolk cells and a gastric cavity is formed in the center of the syncytium. The syncytial yolk and the blastomeres contained within it are compressed into a thin peripheral rind. From a location close to the embryonic pharynx, which defines the posterior pole, bilaterally symmetric ventral nerve cord pioneers extend forward. Stage 4 is characterized by massive proliferation of embryonic cells. Large yolk-filled cells lining the syncytium form the gastrodermis. During stage 5 the external syncytial yolk mantle is resorbed and the embryonic cells contained within differentiate into an irregular scaffold of muscle and nerve cells. Epidermal cells differentiate and replace the transient embryonic epidermis. Through stages 6-8, the embryo adopts its worm-like shape, and loosely scattered populations of differentiating cells consolidate into structurally defined organs. Our analysis reveals a picture of S. polychroa embryogenesis that resembles the morphogenetic events underlying regeneration.

Not your father's planarian: a classic model enters the era of functional genomics

Freshwater planarians were a classic model for studying the problems of development and regeneration. However, as attention shifted towards animals with more rigid developmental processes, the planarians, with their notoriously plastic ontogeny, declined in significance as a model system. This trend was exacerbated with the introduction of genetic and molecular approaches, which did not work well in planarians. More recently, the heightened interest in stem-cell biology, along with the successful application of molecular, cellular and genomic approaches in planarians, is re-establishing these fascinating organisms as models for studying regeneration and developmental plasticity.

Dissecting planarian central nervous system regeneration by the expression of neural-specific genes

The planarian central nervous system (CNS) can be used as a model for studying neural regeneration in higher organisms. Despite its simple structure, recent studies have shown that the planarian CNS can be divided into several molecular and functional domains defined by the expression of different neural genes. Remarkably, a whole animal, including the molecularly complex CNS, can regenerate from a small piece of the planarian body. In this study, a collection of neural markers has been used to characterize at the molecular level how the planarian CNS is rebuilt. Planarian CNS is composed of an anterior brain and a pair of ventral nerve cords that are distinct and overlapping structures in the head region. During regeneration, 12 neural markers have been classified as early, mid-regeneration and late expression genes depending on when they are upregulated in the regenerative blastema. Interestingly, the results from this study show that the comparison of the expression patterns of different neural genes supports the view that at day one of regeneration, the new brain appears within the blastema, whereas the pre-existing ventral nerve cords remain in the old tissues. Three stages in planarian CNS regeneration are suggested.

Not your father's planarian: a classic model enters the era of functional genomics

Freshwater planarians were a classic model for studying the problems of development and regeneration. However, as attention shifted towards animals with more rigid developmental processes, the planarians, with their notoriously plastic ontogeny, declined in significance as a model system. This trend was exacerbated with the introduction of genetic and molecular approaches, which did not work well in planarians. More recently, the heightened interest in stem-cell biology, along with the successful application of molecular, cellular and genomic approaches in planarians, is re-establishing these fascinating organisms as models for studying regeneration and developmental plasticity.

Temperature modulates epidermal cell size in Drosophila melanogaster

Most ectotherms show increased body size at maturity when reared under colder temperatures. In principle, temperature could produce this outcome by influencing growth, proliferation and/or death of epidermal cells. Here we investigated the effects of rearing temperature on the cell size and cell number in the wing blade, the basitarsus of the leg and the cornea of the eye of Drosophila melanogaster from two populations at opposite ends of a South American latitudinal cline. We found that, in both strains of D. melanogaster and in both sexes, a decrease in rearing temperature increases the size of the wings, legs and eyes through an effect on epidermal cell size, with no significant change in cell number. Our results indicate that temperature has a consistent effect on cell size in the Drosophila epidermis and this may also apply to other cell types. In contrast, the evolutionary effects of temperature on the different organs are not consistent. We discuss our findings in the context of growth control in Drosophila.