Electron microscopic studies of planarian regeneration. IV. Cell division of neoblasts inDugesia dorotocephala

Planarian stem cells sense the identity of the missing pharynx to launch its targeted regeneration

In order to regenerate tissues successfully, stem cells must detect injuries and restore missing cell types through largely unknown mechanisms. Planarian flatworms have an extensive stem cell population responsible for regenerating any organ after amputation. Here, we compare planarian stem cell responses to different injuries by either amputation of a single organ, the pharynx, or removal of tissues from other organs by decapitation. We find that planarian stem cells adopt distinct behaviors depending on what tissue is missing to target progenitor and tissue production towards missing tissues. Loss of non-pharyngeal tissues only increases non-pharyngeal progenitors, while pharynx removal selectively triggers division and expansion of pharynx progenitors. By pharmacologically inhibiting either mitosis or activation of the MAP kinase ERK, we identify a narrow window of time during which stem cell division and ERK signaling produces pharynx progenitors necessary for regeneration. These results indicate that planarian stem cells can tailor their output to match the regenerative needs of the animal.

Expression of Piwi Genes during the Regeneration of Lineus sanguineus (Nemertea, Pilidiophora, Heteronemertea)

The transposon silencer piwi genes play important roles in germline determination and maintenance, gametogenesis, and stem-cell self-renewal, and the expression of certain piwi genes is indispensable for regeneration. Knowledge about piwi genes is needed for phylum Nemertea, which contains members (e.g., Lineus sanguineus) with formidable regeneration capacity. By searching the L. sanguineus genome, we identified six Argonaute genes including three ago (Ls-Ago2, Ls-Ago2a, and Ls-Ago2b) and three piwi (Ls-piwi1, Ls-piwi2, and Ls-piwi3) genes. In situ hybridization revealed that, in intact females, Ls-piwi2 and Ls-piwi3 were not expressed, while Ls-piwi1 was expressed in ovaries. During regeneration, Ls-piwi1 and Ls-pcna (proliferating cell nuclear antigen) had strong and similar expressions. The expression of Ls-piwi1 became indetectable while Ls-pcna continued to be expressed when the differentiation of new organs was finished. During anterior regeneration, expression signals of Ls-piwi2 and Ls-piwi3 were weak and only detected in the blastema stage. During posterior regeneration, no expression was observed for Ls-piwi2. To date, no direct evidence has been found for the existence of congenital stem cells in adult L. sanguineus. The "pluripotent cells" in regenerating tissues are likely to be dedifferentiated from other type(s) of cells.

No head regeneration here: regeneration capacity and stem cell dynamics of Theama mediterranea (Polycladida, Platyhelminthes)

Research on the regeneration potential of flatworms (Platyhelminthes) has been mainly undertaken with planarians (Tricladida), where most species can regenerate a head and no proliferation takes place in the blastema, i.e. the early undifferentiated regenerative tissue. Only few studies are available for an early-branching group within the Platyhelminthes, the Polycladida. Head regeneration in polyclads is not possible, with a single exception from a study performed more than 100 years ago: Cestoplana was reported to be able to regenerate a head if cut a short distance behind the brain. Here, we show that 'Cestoplana' was misdetermined and most likely was the small interstitial polyclad Theama mediterranea. We revisited regeneration capacity and dynamics of T. mediterranea with live observations and stainings of musculature, nervous system, and proliferating and differentiating stem cells. In our experiments, after transversal amputation, only animals retaining more than half of the brain could fully restore the head including the brain. If completely removed, the brain was never found to regenerate to any extent. Different from planarians, but comparable to other free-living flatworms we detected cell proliferation within the posterior regeneration blastema in T. mediterranea. Similar to other free-living flatworms, proliferation did not occur within, but only outside, the differentiating organ primordia. Our results strongly imply that brain regeneration in the absence of the latter is not possible in any polyclad studied so far. Also, it appears that proliferation of stem cells within the regeneration blastema is a plesiomorphy in flatworms and that planarians are derived in this character.

Stem Cells, Patterning and Regeneration in Planarians: Self-Organization at the Organismal Scale

The establishment of size and shape remains a fundamental challenge in biological research that planarian flatworms uniquely epitomize. Planarians can regenerate complete and perfectly proportioned animals from tiny and arbitrarily shaped tissue pieces; they continuously renew all organismal cell types from abundant pluripotent stem cells, yet maintain shape and anatomy in the face of constant turnover; they grow when feeding and literally degrow when starving, while scaling form and function over as much as a 40-fold range in body length or an 800-fold change in total cell numbers. This review provides a broad overview of the current understanding of the planarian stem cell system, the mechanisms that pattern the planarian body plan and how the interplay between patterning signals and cell fate choices orchestrates regeneration. What emerges is a conceptual framework for the maintenance and regeneration of the planarian body plan on basis of the interplay between pluripotent stem cells and self-organizing patterns and further, the general utility of planarians as model system for the mechanistic basis of size and shape.

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.

Cellular dynamics during regeneration of the flatworm Monocelis sp. (Proseriata, Platyhelminthes)

BACKGROUND

Proseriates (Proseriata, Platyhelminthes) are free-living, mostly marine, flatworms measuring at most a few millimetres. In common with many flatworms, they are known to be capable of regeneration; however, few studies have been done on the details of regeneration in proseriates, and none cover cellular dynamics. We have tested the regeneration capacity of the proseriate Monocelis sp. by pre-pharyngeal amputation and provide the first comprehensive picture of the F-actin musculature, serotonergic nervous system and proliferating cells (S-phase in pulse and pulse-chase experiments and mitoses) in control animals and in regenerates.

RESULTS

F-actin staining revealed a strong body wall, pharynx and dorsoventral musculature, while labelling of the serotonergic nervous system showed an orthogonal pattern and a well developed subepidermal plexus. Proliferating cells were distributed in two broad lateral bands along the anteroposterior axis and their anterior extension was delimited by the brain. No proliferating cells were detected in the pharynx or epidermis. Monocelis sp. was able to regenerate the pharynx and adhesive organs at the tip of the tail plate within 2 or 3 days of amputation, and genital organs within 8 to 10 days. Posterior pieces were not able to regenerate a head. The posterior regeneration blastema was found to be a centre of cell proliferation, whereas within the pharynx primordium, little or no proliferation was detected. The pharynx regenerated outside of the blastema and was largely, but not solely formed by cells that were proliferating at the time of amputation.

CONCLUSIONS

Our findings suggest that proliferating cells or their offspring migrated to the place of organ differentiation and then stopped proliferating at that site. This mode of rebuilding organs resembles the mode of regeneration of the genital organs in another flatworm, Macrostomum lignano. Pharynx regeneration resembles embryonic development in Monocelis fusca and hints at the vertically directed pharynx being plesiomorphic in proseriates. Proliferation within the regeneration blastema has been detected in anterior and posterior blastemas of other flatworms, but is notably missing in triclads. The phylogenetic relationships of the flatworms studied indicate that proliferation within the blastema is the plesiomorphic condition in Platyhelminthes.

Stem cell systems and regeneration in planaria

Planarians are members of the Platyhelminthes (flatworms). These animals have evolved a remarkable stem cell system. A single pluripotent adult stem cell type ("neoblast") gives rise to the entire range of cell types and organs in the planarian body plan, including a brain, digestive-, excretory-, sensory- and reproductive systems. Neoblasts are abundantly present throughout the mesenchyme and divide continuously. The resulting stream of progenitors and turnover of differentiated cells drive the rapid self-renewal of the entire animal within a matter of weeks. Planarians grow and literally de-grow ("shrink") by the food supply-dependent adjustment of organismal turnover rates, scaling body plan proportions over as much as a 50-fold size range. Their dynamic body architecture further allows astonishing regenerative abilities, including the regeneration of complete and perfectly proportioned animals even from tiny tissue remnants. Planarians as an experimental system, therefore, provide unique opportunities for addressing a spectrum of current problems in stem cell research, including the evolutionary conservation of pluripotency, the dynamic organization of differentiation lineages and the mechanisms underlying organismal stem cell homeostasis. The first part of this review focuses on the molecular biology of neoblasts as pluripotent stem cells. The second part examines the fascinating mechanistic and conceptual challenges posed by a stem cell system that epitomizes a universal design principle of biological systems: the dynamic steady state.

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.

Smed-SmB, a member of the LSm protein superfamily, is essential for chromatoid body organization and planarian stem cell proliferation

Planarians are an ideal model system to study in vivo the dynamics of adult pluripotent stem cells. However, our knowledge of the factors necessary for regulating the 'stemness' of the neoblasts, the adult stem cells of planarians, is sparse. Here, we report on the characterization of the first planarian member of the LSm protein superfamily, Smed-SmB, which is expressed in stem cells and neurons in Schmidtea mediterranea. LSm proteins are highly conserved key players of the splicing machinery. Our study shows that Smed-SmB protein, which is localized in the nucleus and the chromatoid body of stem cells, is required to safeguard the proliferative ability of the neoblasts. The chromatoid body, a cytoplasmatic ribonucleoprotein complex, is an essential regulator of the RNA metabolism required for the maintenance of metazoan germ cells. However, planarian neoblasts and neurons also rely on its functions. Remarkably, Smed-SmB dsRNA-mediated knockdown results in a rapid loss of organization of the chromatoid body, an impairment of the ability to post-transcriptionally process the transcripts of Smed-CycB, and a severe proliferative failure of the neoblasts. This chain of events leads to a quick depletion of the neoblast pool, resulting in a lethal phenotype for both regenerating and intact animals. In summary, our results suggest that Smed-SmB is an essential component of the chromatoid body, crucial to ensure a proper RNA metabolism and essential for stem cell proliferation.

Spoltud-1 is a chromatoid body component required for planarian long-term stem cell self-renewal

Freshwater planarians exhibit a striking power of regeneration, based on a population of undifferentiated totipotent stem cells, called neoblasts. These somatic stem cells have several characteristics resembling those of germ line stem cells in other animals, such as the presence of perinuclear RNA granules (chromatoid bodies). We have isolated a Tudor domain-containing gene in the planarian species Schmidtea polychroa, Spoltud-1, and show that it is expressed in neoblast cells, germ line cells and central nervous system, and during embryonic development. Within the neoblasts, Spoltud-1 protein is enriched in chromatoid bodies. Spoltud-1 RNAi eliminates protein expression after 3 weeks, and abolishes the power of regeneration of planarians after 7 weeks. Neoblast cells are eliminated by the RNAi treatment, disappearing at the end rather than gradually during the process. Neoblasts with no detectable Spoltud-1 protein are able to proliferate and differentiate. These results suggest that Spoltud-1 is required for long term stem cell self renewal.

DjCBC-1, a conserved DEAD box RNA helicase of the RCK/p54/Me31B family, is a component of RNA-protein complexes in planarian stem cells and neurons

The stem cells of planarians, known as neoblasts, can give rise to all cell types in planarians. Neoblasts can be identified by electron microscopy as cells with electron-dense chromatoid bodies, which are large RNP (ribonucleoprotein) complexes, in their cytoplasm. However, the components and function of chromatoid bodies are still relatively unknown. Here we identified a DEAD box RNA helicase gene of the RCK/p54/Me31B family from a planarian EST database and showed the localization of its product in chromatoid bodies by immunoelectron microscopy. We named this gene Djcbc-1 (Dugesia japonica chromatoid body component 1). Djcbc-1 was also strongly expressed in the brain and in the germline stem cells of sexualized planarians. We observed chromatoid body-like electron-dense bodies in brain neurons, where DjCBC-1 was also expressed. These observations suggest that common molecular components of RNP complexes may be involved in the regulation of somatic and germline stem cells, and neurons in planarians.

Effects of neuropeptide F on regeneration in Girardia tigrina (Platyhelminthes)

The effects of neuropeptide F (NPF; from Moniezia expansa) on the regeneration of Girardia tigrina were studied. The animals were decapitated and incubated in water (control) or NPF. The dynamics of the proliferation of the neoblasts in the developing tissue were studied during the course of regeneration by monitoring the mitotic index (MI). The effects of incubation in FMRFamide and GYIRFamide on the MI were also tested. The course of cephalic regeneration was followed with in vivo computer-assisted morphometry for up to 7 days. The development of the regenerating nervous system and the musculature was visualised by immunostaining with a primary antiserum to the C-terminal decapeptide of NPF (YFAIIGRPRFa) and tetramethylrhodamine-isothiocyanate-conjugated phalloidin, which stains F-actin in muscle filaments. The study showed that NPF had a stimulatory effect on the mitotic activity of the neoblasts. FMRFamide and GYIRFamide did not have this effect. NPF also stimulated the growth of the regenerating head and the growing nervous system and musculature. NPF is postulated to have a morphogenetic action in the regenerating animals.

Characterization and categorization of fluorescence activated cell sorted planarian stem cells by ultrastructural analysis

Planarians have regenerative ability made possible by pluripotent stem cells referred to as neoblasts. Classical ultrastructural studies have indicated that stem cells can be distinguished by a unique cytoplasmic structure known as the chromatoid body and their undifferentiated features, and they are specifically eliminated by X-ray irradiation. Recently, by using fluorescence activated cell sorting (FACS), planarian cells were separated into two X-ray-sensitive fractions (X1 and X2) and an X-ray-insensitive fraction (XIS) according to DNA content and cytoplasmic size. Here we analyzed the fractionated cells by transmission electron microscopy (TEM). First, we found that both undifferentiated cells (stem cells) and regenerative cells (differentiating cells) were concentrated in the X1 fraction containing the S/G2/M phase cells. The regenerative cells were considered to be committed stem cells or progenitor cells, suggesting that some stem cells may maintain proliferative ability even after cell fate-commitment. Second, we succeeded in identifying a new type of stem cells, which were small in size with few chromatoid bodies and a heterochromatin-rich nucleus. Interestingly, they were concentrated in the X2 fraction, containing G0/G1 phase cells. These results suggest that planarian stem cells are not homogeneous, but may consist of heterogeneous populations, like mammalian stem cells.

Identification and origin of the germline stem cells as revealed by the expression of nanos-related gene in planarians

The planarian's remarkable regenerative ability is thought to be supported by the stem cells (neoblasts) found throughout its body. Here we report the identification of a subpopulation of neoblasts, which was revealed by the expression of the nanos-related gene of the planarian Dugesia japonica, termed Djnos. Djnos-expressing cells in the asexual planarian were distributed to the prospective ovary or testes forming region in the sexual planarian. During sexualization, Djnos-expressing cells produce germ cells, suggesting that in the asexual state these cells were kept as germline stem cells for the oogonia and spermatogonia. Interestingly, the germline stem cells were indistinguishable from the neoblasts by morphology and X-ray sensitivity and did not seem to contribute to the regeneration at all. Germline stem cells initially appear in the growing infant planarian, suggesting that germline stem cells are separated from somatic stem cells in the planarian. Thus, planarian neoblasts can be classified into two groups; somatic stem cells for regeneration and tissue renewal, and germline stem cells for production of germ cells during sexualization. However, Djnos-positive cells appeared in the newly formed trunk region from the head piece, suggesting that somatic stem cells can convert to germline stem cells.

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.

Ultrastructural localization of RNA in the chromatoid bodies of undifferentiated cells (neoblasts) in planarians by the RNase-gold complex technique

Undifferentiated cells of planarians (Platyhelminthes, Turbellaria), also called neoblasts, are totipotent stem cells, which give rise to all differentiated cell types, while maintaining their own density by cell proliferation. Neoblasts are the only somatic cells of planarians bearing chromatoid bodies in their cytoplasm; these organelles disappear as differentiation takes place. Studies on germinal cells of several groups of organisms have shown that chromatoid bodies contain substantial amounts of RNA. To test its presence in neoblasts, we have used an RNase-gold technique. We found chromatoid bodies labeled with RNase-gold particles. Heterogeneity in the density of the label, may be correlated with the functionality and complexity of these organelles. The gold marker was also present over the nucleus and rough endoplasmic reticulum, but mitochondria, secretory granules, and the extracellular space were devoid of label. This specific localization of RNA in planarian chromatoid bodies supports earlier findings on germ cells and embryonic cells in a variety of organisms, indicating that chromatoid bodies are information-storage structures, essential during the process of cell differentiation. © 1993 Wiley-Liss, Inc.

Distribution of the stem cells (neoblasts) in the planarian Dugesia japonica

It has been postulated that the high regeneration ability of planarians is supported by totipotent stem cells, called neoblasts. There have been a few reports showing the distribution of neoblasts in planarians. However, the findings were not completely consistent. To determine the distribution of neoblasts, we focused on proliferating cell nuclear antigen (PCNA), which is present in proliferative cells. We cloned and sequenced the cDNA of PCNA from the planarian Dugesia japonica and produced an antiserum recognizing the gene product. X-ray irradiation caused rapid loss of all PCNA-positive cells and loss of the neoblasts (which were morphologically defined by the presence of the chromatoid body), strongly suggesting that all PCNA-positive cells were true neoblasts. Using the antiserum, we were successful in identifying the neoblasts more clearly than any previous work. In addition to their dispersed distribution in the dorsal and ventral mesenchyme, the neoblasts were distributed as clusters along the midline and bilateral lines in the dorsal mesenchyme. We also examined the behavior of the neoblasts after decapitation. Decapitation did not seem to affect the migration of neoblasts far from the wound. We demonstrated here that DjPCNA is a powerful tool for identifying planarian neoblasts.

Flatworm asexual multiplication implicates stem cells and regeneration

The phenomenon of asexual multiplication is rare in the animal kingdom, but it occurs in all main flatworm taxa. In the present paper, we review data regarding the presence of different forms of asexual multiplication in flatworms and argue that the presence of a population of totipotent or pluripotent stem cells, "neoblasts", is a primitive feature of decisive importance for the developing potential of flatworms. Next we present information on the role of stem cells in fission, head regeneration, and pharynx regeneration of planarians. Furthermore, the tracing of neoblasts in lower flatworms and cestodes is presented, and the results indicating heterogeneity of the neoblast pool are discussed. Finally, the mode by which the neoblasts are stimulated to divide, migrate, and differentiate and the nature of the interactions are discussed. We focus on (i) biogenic amines and neuropeptides, (ii) the role of neuropeptides in the early stage of regeneration, (iii) the evidence for the influences of growth factors and nitric oxide, and (iv) the influence of weak electromagnetic fields. We discuss the pattern in which a gradient system of morphogens and (or) a hierarchical system of inductions is expressed in development.

The planarian HOM/HOX homeobox genes (Plox) expressed along the anteroposterior axis

In the freshwater planarian Dugesia japonica, five cDNAs for HOM/HOX homeobox genes were cloned and sequenced. Together with sequence data on HOM/HOX homeobox genes of platyhelminthes deposited in databases, comparison of the deduced amino acid sequences revealed that planarians have at least seven HOM/HOX homeobox genes, Plox1 to Plox7 (planarian HOM/HOX homeobox genes). Whole-mount in situ hybridization and RT-PCR revealed that Plox4 and Plox5 were increasingly expressed along a spatial gradient in the posterior region of intact animals. During regeneration, Plox5 was expressed only in the posterior region of regenerating body pieces, suggesting that the gene is involved in the anteroposterior patterning in planarians. Plox5 was not found to be expressed in a blastema-specific manner, which contradicts a previous report (J. R. Bayascas, E. Castillo, A. M. Muños-Mármol, and E. Saló. Development 124, 141-148, 1997). X-ray irradiation experiments showed that Plox5 was expressed at least in some cells other than neoblasts, but that the induction of Plox5 expression during regeneration might require neoblasts.

Identification of a receptor tyrosine kinase involved in germ cell differentiation in planarians

To investigate external signals involved in germ cell differentiation from somatic stem cells, we have tried to identify protein kinases whose expression is regulated during the process of sexualization of asexual-state planarians. It is known that in planarians germ cells differentiate from totipotent somatic stem cells called "neoblasts" during sexualization. As a first step, we have isolated twelve protein kinase genes from cDNAs of sexual-state planarians, including three non-receptor tyrosine kinases, three receptor-tyrosine kinases and three non-receptor serine/threonine kinases, and then analyzed their expression patterns during sexualization. One of them, the DjPTK1 gene, is specifically expressed in germ cells of sexual-state planarians. DjPTK1-positive cells were also detected in the mesenchymal space during the process of sexualization, and it appears that these cells migrate to the dorsal side and then differentiate into spermatogonia/spermatocytes in testis. Sequence analysis indicated that the DjPTK1 gene encodes a receptor protein tyrosine kinase belonging to the FGFR/PDGF family. These results suggest that a receptor tyrosine kinase system may be involved both at an early stage of germ cell differentiation and in a step of germ cell maturation in planarians.

Neoplastic transformation in the planarian: I. Cocarcinogenesis and histopathology

Although several investigators have reported that exposure to mammalian carcinogens induces abnormal tumorlike growths and teratogenic remodeling in planarians, there is no general agreement that these, or comparable responses in any other invertebrates, model mammalian carcinogenesis. To investigate this question, freshwater planarians of the species Dugesia dorotocephala were exposed to culture water containing an initiator and a promoter, either alone or in combination. Cadmium, a potent carcinogen, was used as an initiator in the protocol. Treatment with sublethal concentrations of cadmium sulfate produced a benign, but persistent, tumor in a small percentage of the planarians. The addition of 12-O-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester and well-known promoter, to the cadmium-containing solutions resulted in the induction of a progressive, potentially lethal, transplantable tumor in a large proportion of the treated flatworms. Light and electron microscopy revealed this particular tumor to be composed both of immature cells and of a single mature cell type: newly differentiated, but transformed, reticular cells. Further examination of the infiltrating tissue formations elucidated the profile of differentiation, from a population of mitotically active transformed stem cells through the transitional stages in the associated reticuloma. These results suggest that 1) the freshwater planarian displays the major phenomenology of mammalian cocarcinogenesis and that 2) the planarian reticuloma models several important features of a neoplastic stem cell disease.

Stimulation of cellular proliferation and differentiation in the intact and regenerating planarian Dugesia(G) tigrina by the neuropeptide substance P

The neuropeptide substance P (SP) is shown to be a potent mitogen for intact and regenerating planarians. At nanomolar concentrations, SP markedly enhances cellular proliferation causing an increase in the mitotic index and in the number of blastema cells. Moreover, albeit to a lower extent, SP enhances cellular differentiation as shown by the increases in eye and pharynx length in regenerating organisms. On the basis of these observations, we hypothesize that SP may be one of the postulated growth factors necessary for the stimulation of proliferation, and to a lesser extent differentiation of cells in intact and regenerating planarians.