Ultrastructural evidence for the presence of nerve cells in the gastrodermis of Hydra

Summary Two types of nerve cells, namely, neurosensory and neurosecretory cells have been identified and described in the gastrodermis of Hydra pseudoligactis. The morphological criteria used for the identification of gastrodermal nerves are based on those presented previously for epidermal nerves. The third type of nerve cell in the epidermis, ganglionic cells, was not observed in these studies. The distribution, function and origin of gastrodermal nerve cells are discussed briefly.With the technical assistance of Linda M. Bookman.     

Ultrastructure of the calcium-sequestering gastrodermal cell in the hydroid Hydractinia symbiolongicarpus (Cnidaria, Hydrozoa)

Large, free-floating crystals of calcium carbonate occur in vacuoles of gastrodermal cells of the hydroid Hydractinia symbiolongicarpus. Here, morphological details about the process by which these cells accumulate and sequester calcium are provided by a cytochemical method designed to demonstrate calcium at the ultrastructural level. Electron-dense material presumably indicative of the presence of calcium was EGTA-sensitive and was shown by parallel electron energy loss spectroscopy (EELS) and energy spectroscopic imaging (ESI) to contain calcium. Calcium occurred in only one cell type, the endodermally derived gastrodermal cell. In these cells, the electron-dense material appeared first as a fine precipitate in the cytosol and nucleus and later as larger deposits and aggregates in the vacuole. During the life cycle, gastrodermal cells of the uninduced planula and the planula during metamorphic induction sequestered calcium. In primary polyps and polyps from established colonies, gastrodermal cells sequestered calcium, but the endodermal secretory cells did not. Our observations support the hypothesis that gastrodermal cells function as a physiological sink for calcium that enters the organism in conjunction with calcium-requiring processes such as motility, secretion, and metamorphosis.     

Ultrastructural observations on gastrodermal regeneration in the planarian Dugesia japonica (Turbellaria)

The earliest detectable change during regeneration of the gastrodermis in Dugesia japonica was an aggregation of regenerative cells underneath the gastrodermis remaining at the wound margin. The gastrodermal cells in experimental regenerates retained some of their original characters and presented no indication of cell dedifferentiation. The regenerative cells came into contact with the basal surface of gastrodermal cells, forming stratified cell layers. Differentiation of these cells into gastrodermal cells was initiated by the development of synthetic organelles within their cytoplasm. These differentiating cells gave rise to two different types of gastrodermal cells, namely phagocytic cells and sphere cells. In later stages, there was an apparent movement of differentiated gastrodermal cells towards the parenchyma.     

Cnidarian Primary Cell Culture as a Tool to Investigate the Effect of Thermal Stress at Cellular Level

In the context of global change, symbiotic cnidarians are largely affected by seawater temperature elevation leading to symbiosis breakdown. This process, also called bleaching, is triggered by the dysfunction of the symbiont photosystems causing an oxidative stress and cell death to both symbiont and host cells. In our study, we wanted to elucidate the intrinsic capacity of isolated animal cells to deal with thermal stress in the absence of symbiont. In that aim, we have characterized an animal primary cell culture form regenerating tentacles of the temperate sea anemone Anemonia viridis. We first compared the potential of whole tissue tentacle or separated epidermal or gastrodermal monolayers as tissue sources to settle animal cell cultures. Interestingly, only isolated cells extracted from whole tentacles allowed establishing a viable and proliferative primary cell culture throughout 31 days. The analysis of the expression of tissue-specific and pluripotency markers defined cultivated cells as differentiated cells with gastrodermal origin. The characterization of the animal primary cell culture allowed us to submit the obtained gastrodermal cells to hyperthermal stress (+?5 and +?8 °C) during 1 and 7 days. Though cell viability was not affected at both hyperthermal stress conditions, cell growth drastically decreased. In addition, only a +?8 °C hyperthermia induced a transient increase of antioxidant defences at 1 day but no ubiquitin or carbonylation protein damages. These results demonstrated an intrinsic resistance of cnidarian gastrodermal cells to hyperthermal stress and then confirmed the role of symbionts in the hyperthermia sensitivity leading to bleaching.     

Histochemical and ultrastructural observations on feeding and digestion in Reighardia sternae (Pentastomida: Cephalobaenida)

Histological, histochemical and ultrastructural methods have been used to study gut structure and function in the cephalobaenid pentastomid Reighardia sternae (Diesing, 1864).
R. sternae feeds exclusively on blood. Haemolysis of ingested erythrocytes is thought to be initiated by non-specific esterase secreted by cells lining the posterior oesophagus. Only one other enzyme, acid phosphatase, was demonstrated; its activity is confined to the microvillar layer of the gastrodermis.
Digestion appears to be largely extracellular, as large volumes of haematin form in the gut lumen after haemolysis. However, this is supplemented by a significant amount of intracellular digestion: accumulations of iron-positive granules occur in approximately 20% of gastrodermal cells.
All gastrodermal cells are alike, and all can undertake the same intracellular digestive process. This is characterized by a number of morphologically distinct stages which have been studied using the electron microscope. The iron-positive particles distinguished under the light microscope were easily identifiable ultrastructurally. Spherical, weakly iron-positive bodies form within the cisternae of the endoplasmic reticulum. Phagosomes containing haemoglobin, or some breakdown product of haemoglobin, apparently bud off from the termini of a labyrinth system of tubules under the microvilli and fuse with the ER to form these phago-lysosomes. Intensely iron-positive heterolysosomes, also present in the cytoplasm of these cells, accumulate haemosiderin particles which are thought to be released into the cytoplasm during phago-lysosomal digestion.
The ensuing complex changes in cell morphology are, apparently, related to the elimination of spent phago-lysosomes. It has been suggested that the purpose of intracellular digestion is to provide a specific metabolite which the extracellular degradation of haemoglobin cannot supply.     

Nuclear transfer of adult bone marrow mesenchymal stem cells: developmental totipotency of tissue-specific stem cells from an adult mammal

Recent studies have demonstrated that somatic stem cells have a flexible potential greater than previously expected when they are transplanted into different tissues. On the other hand, recent studies also have revealed that these potentials might occur because of spontaneous cell fusion with recipient cells. The nuclei of somatic cells could have been reprogrammed when they were artificially or spontaneously fused with mouse embryonic stem (ES) cells. The resultant hybrid cells acquired a developmental pluripotency that the original somatic cells did not have but that ES cells did. LaBarge and Blau (Cell 2002; 111:589-601) demonstrated that adult bone marrow-derived cells contributed to muscle tissue in a stepwise biological progression. This means that bone marrow-derived cells became satellite cells of mononucleate muscle stem cells after the first irradiation-induced damage to the mouse, and after the second irradiation-induced damage, multinucleate myofibers appeared from the bone marrow-derived cells. Considered together, the differentiation potential of the somatic stem cell nucleus itself remains unclear. Although the pluripotency of somatic stem cell populations has been evaluated, the developmental totipotency of the nuclei of somatic stem cells, whether or not they fused with other cells, has not been shown, except in only one study concerning fetal neural cells (never in adult stem cells). Here, we showed the developmental totipotency of adult bovine mesenchymal stem cells by nuclear transfer.     

Differentiation capabilities of human germ cell tumors

It is well known that human germ cell tumors are an excellent model to study not only differentiation capacity of tumor cells but also human normal somatic cell differentiation. A variety of polyclonal and monoclonal antibodies were developed against cell surface antigens of murine embryos and teratocarcinomas. Accumulated data has revealed that these antigens are sequentially expressed on embryonic cells in a well-programmed manner. They have also been shown to be useful markers to investigate somatic cell differentiation in fetal and adult tissue. In humans, however, little is known about the cellular differentiation mechanism in early embryos and whether they could be studied, i.e. whether they occur in human germ cell tumors. In present review, we discussed newly established monoclonal antibodies which were raised from human embryonal carcinoma cells. We have been studying differentiation capacity of human germ cell tumor cells by using these antibodies. Some of these antibodies clearly indicates their usefulness to specify the developmental stage of normal tissue.     

Immunogold-labeled S-phase neoblasts, total neoblast number, their distribution, and evidence for arrested neoblasts in Macrostomum lignano (Platyhelminthes, Rhabditophora)

Neoblasts in Platyhelminthes are the only cells to proliferate and differentiate into all cell types. In Macrostomum lignano, the incorporation of 5'-bromo-2'-deoxyuridine (BrdU) in neoblasts confirmed the distribution of S-phase cells in two lateral bands. BrdU labeling for light and for transmission electron microscopy (TEM) identified three populations of proliferating cells: somatic neoblasts located between the epidermis and gastrodermis (mesodermal neoblasts), neoblasts located within the gastrodermis (gastrodermal neoblasts), and gonadal S-phase cells. In adults, three stages of mesodermal neoblasts (2, 2-3, and 3) defined by their ultrastructure were found. Stage 1 neoblasts where only seen in hatchlings. These stages either were phases within the S-phase of one neoblast pool or were subsequent stages of differentiating neoblasts, each with its own cell cycle. Regular TEM and immunogold labeling provided the basis for calculating the total number of neoblasts and the ratio of labeled to non-labeled neoblasts. Somatic neoblasts represented 6.5% of the total number of cells. Of these, 27% were labeled in S-phase. Of this fraction, 33% were in stage 2, 46% in stage 2-3, and 21% in stage 3. Immunogold labeling substantiated results concerning the differentiation of neoblasts into somatic cells. Non-labeled stage 2 neoblasts were present, even after a 2-week BrdU exposure. Double labeling of mitoses and FMRF-amide revealed a close spatial relationship of mesodermal neoblasts with the nervous system. Immunogold-labeled sections showed that nearly 70% of S-phase cells were in direct contact or within 5 microm from nerve cords.     

Ultrastructure of neurons and synapses in the tentacle gastrodermis of the sea anemone Calliactis parasitica

Little is known about gastrodermal neurons and synapses in the tentacles of sea anemones. Using transmission electron microscopy of serial thin sections of Calliactis parasitica, we have identified both a sensory cell and a ganglion cell with granular vesicles originating from the Golgi complex and have identified four types of synapses in the tentacular gastrodermal nerve plexus. The sensory cell has a recessed apical cilium with a basal body and a perpendicularly oriented centriole, below which are several strands of striated rootlets surrounded by mitochondria. The ganglion cell lacks a cilium and resembles a bipolar neuron, with oppositely directed processes lying parallel to the basally located circular smooth muscle. Both one-way and two-way interneuronal synapses are present with 60- to 90-nm granular vesicles of various densities aligned at the paired electron-dense membranes and fine cross filaments in the intervening 13-nm cleft. Two types of neuroeffector synapses have been located. Dense granular vesicles are present at neuromuscular synapses, whereas less dense vesicles are present at neuroglandular synapses. Most of the synaptic vesicles range from 60 to 120 nm in diameter. Two types of nerve cells and a variety of synaptic loci provide morphological substrates for the spontaneous SS2 conduction pulses in the tentacular gastrodermis of C. parasitica. J Morphol 231:217–223, 1997. © 1997 Wiley-Liss, Inc.     

The Ultrastructure of the Gastrodermal Gland Cells in the Freshwater Planarian Dugesia gonocephala s.l.

Light and transmission electron microscopy have been used to study the gastrodermal gland cells of the triclad Dugesia gonocephala s.l. The events involved in the ultrastructural transformation and the secretion process in these cells were followed at four different stages in both fasted and fed animals. During the feeding stage their secretory granules are directly discharged into the intestinal lumen by means of a secretion process of the holocrine type that is described in this paper. It is suggested that such secretions contribute to extracellular digestion and that disintegration of the gland cells is accompanied by a differentiation of neoblasts into new gland cells, reflecting a turnover of gland cells during the triclad digestive stages.     

Ultrastructural features of the trophonema and oogenesis in the starlet sea anemone, Nematostella vectensis (Edwardsiidae)

Abstract. The starlet sea anemone, Nematostella vectensis Stephenson 1935, is a burrowing, estuarine species that has become a model organism for fundamental studies of cnidarian and metazoan development. During early oogenesis, oocytes appear in the basal region of the gastrodermis in the reproductive mesenteries and gradually bulge into the adjacent connective tissue space (mesoglea) where the majority of oocyte growth and vitellogenesis occurs. However, oocytes do not physically contact the cellular and amorphous matrix of the mesogleal compartment due to a thin, intervening basal lamina. Oocytes retain limited contact with the basal gastrodermal epithelium via groups of ultrastructurally modified gastrodermal cells called trophocytes. Trophocytes are monociliated accessory cells of somatic origin that collectively form a structure called the trophonema, a unique accessory cell/oocyte association not observed outside the Cnidaria. The trophonema consists of 50–60 trophocytes that maintain contact with <1% of the oocyte surface and forms a circular, bowel‐shaped depression on the luminal surface of the gastrodermis as they sink into the mesoglea with the oocyte. The oocyte remains highly polarized throughout oogenesis with the germinal vesicle positioned near the trophonema and presumably representing the future animal pole of the embryo. Contact between the trophonema and the oocyte is restricted to cell junctions connecting peripheral trophocytes and narrow extensions from the oocyte. Previous studies suggest that the trophonema plays a role in transport of extracellular digestive products from the gastrovascular cavity to the oocyte, and the ultrastructural features described in this study are consistent with that view. Vitellogenesis is described for the first time in a sea anemone. Yolk synthesis involves both autosynthetic and heterosynthetic processes including the biosynthetic activity of the Golgi complex and the uptake of extraoocytic yolk precursors via endocytosis, respectively.     

Isolation of murine and porcine fetal stem cells from somatic tissue

Adult stem cells have been previously isolated from a variety of somatic tissues, including bone marrow and the central nervous system; however, contribution of these cells to the germ line has not been shown. Here we demonstrate that fetal somatic explants contain a subpopulation of somatic stem cells (FSSCs), which can be induced to display features of lineage-uncommitted stem cells. After injection into blastocysts, these cells give rise to a variety of cell types in the resultant chimeric fetuses, including those of the mesodermal lineage; they even migrate into the genital ridge. In vitro, FSSCs exhibit characteristics of embryonic stem cells, including extended self-renewal; expression of stem cell marker genes, such as Pou5f1 (Oct4), Stat3, and Akp2 (Tnap) and growth as multicellular aggregates. We report that fetal tissue contains somatic stem cells with greater potency than previously thought, which might form a new source of stem cells useful in somatic nuclear transfer and cell therapy.     

Cell Death and Acid Phosphatase Activity in the Regenerating Planarian Polycelis tenuis Iijima

A combination of microscopical, cytochemical, and biochemical techniques have been employed to study the changes occurring during the first seven days of blastema formation and regeneration after decapitation in adult Polycelis tenuis worms. Fine structural data reveal evidence of cell fragmentation, selective cell deletion, and phagocytosis at and below the wound surface. Initially, (0–12 h regeneration) cell debris is phagocytosed by intact parenchymal and gastrodermal cells near the cut surface which is later sealed (24 h) by a stretching of marginal epidermal cells. Wound sealing is followed by a migration of newly differentiated rhabdite cells into the epithelium. Morphological evidence of a selective cell autolysis precedes evidence of an accumuluation of lipid and glycogen reserves in the parenchymal and gastrodermal cells and the later (48 h regeneration time) aggregation of undifferentiated mitotically active neoblasts beneath the wound.
Biochemical data reveal an early period of high acid phosphatase (p-nitrophenyl phosphatase and sodium-β-glycerophosphatase) activity 3–12 h after injury, followed by a further intense period of activity at 44–48 h after decapitation. The coincident cytochemical data show an increased level of acid phosphatase activity associated with cell lysis and death in the wound and blastema zone and also with the digestion of phagocytosed cell debris.     

Conversion of midbodies into germ cell intercellular bridges

Whereas somatic cell cytokinesis resolves with abscission of the midbody, resulting in independent daughter cells, germ cell cytokinesis concludes with the formation of a stable intercellular bridge interconnecting daughter cells in a syncytium. While many proteins essential for abscission have been discovered, until recently, no proteins essential for mammalian germ cell intercellular bridge formation have been identified. Using TEX14 as a marker for the germ cell intercellular bridge, we show that TEX14 co-localizes with the centralspindlin complex, mitotic kinesin-like protein 1 (MKLP1) and male germ cell Rac GTPase-activating protein (MgcRacGAP) and converts these midbody matrix proteins into stable intercellular bridge components. In contrast, septins (SEPT) 2, 7 and 9 are transitional proteins in the newly forming bridge. In cultured somatic cells, TEX14 can localize to the midbody in the absence of other germ cell-specific factors, suggesting that TEX14 serves to bridge the somatic cytokinesis machinery to other germ cell proteins to form a stable intercellular bridge essential for male reproduction.     

Quantitative analysis of cell types during growth,degrowth and regeneration in the planarians Dugesia mediterranea and Dugesia tigrina

A method of tissue maceration (dissociation) of planarian tissues into single cells was used to characterize the basic cell types in the planarians Dugesia mediterranea and Dugesia tigrina, and to determine the total cell number and distribution of cell types during growth, degrowth and regeneration.Using this method, 13 basic cell types have been determined for both species. The total number of cells increases with body length and volume whereas the distribution of cell types is only slightly affected. Growth and degrowth occur mainly through changes in total cell number leaving cell distribution only moderately affected. During regeneration, an increase in neoblast density in the blastema followed later on by increases in nerve cells are the more significant changes detected.These results are discussed in relation to mechanisms of cell renewal, blastema formation and maintenance of tissue polarity.Abbreviations nb neoblasts - nv nerve cells - ep epidermal cells - fp fixed parenchyma cells - g gastrodermal cells     

Uncoupling of pathways that promote postmitotic life span and apoptosis from replicative immortality of Caenorhabditis elegans germ cells

A dichotomy exists between germ and somatic cells in most organisms, such that somatic cell lineages proliferate for a single generation, whereas the germ cell lineage has the capacity to proliferate from one generation to the next, indefinitely. Several theories have been proposed to explain the unlimited replicative life span of germ cells, including the elimination of damaged germ cells by apoptosis or expression of high levels of gene products that prevent aging in somatic cells. These theories were tested in the nematode Caenorhabditis elegans by examining the consequences of eliminating either apoptosis or the daf-16, daf-18 or sir-2.1 genes that promote longevity of postmitotic somatic cells. However, germ cells of strains deficient for these activities displayed an unlimited proliferative capacity. Thus, C. elegans germ cells retain their youthful character via alternative pathways that prevent or eliminate damage that accumulates as a consequence of cell proliferation.     

Molecular mechanisms controlling germline and somatic stem cells: similarities and differences

Germline and somatic stem cells are distinct types of stem cells that are dedicated to reproduction and somatic tissue homeostasis, respectively. Extensive studies on these two stem cell types in different organisms over the past few years have revealed some commonalities in the mechanisms controlling their self-renewal and differentiation. Furthermore, germline or somatic cells in various organisms and sexes also exhibit their own unique ways of regulating stem cell function. By understanding these similarities and differences we might gain a better insight into how stem cells are regulated in general and how germline and somatic stem cell types are regulated differently.