Stem cells in a basal bilaterian

Abstract

In Platyhelminthes, totipotent stem cells (neoblasts) are supposed to be the only dividing cells. They are responsible for the renewal of all cell types during development, growth, and regeneration, a unique situation in the animal kingdom. In order to further characterize these cells, we have applied two immunocytochemical markers to detect neoblasts in different stages of the cell cycle in the acoel flatworm Convolutriloba longifissura: (1) the thymidine analog 5'-bromo-2'-deoxyuridine (BrdU) to identify cells in S-phase, and (2) an antibody to phosphorylated histone H3 to locate mitosis. BrdU pulse-chase experiments were carried out to follow differentiation of neoblasts. We demonstrate the differentation into four labeled, differentiated cell types. S-phase cells and mitotic cells showed a homogenous distribution pattern throughout the body of C. longifissura. Two different types of S-phase cells could be distinguished immunocytochemically by their pattern of incorporated BrdU in the nuclei. Transmission electron microscopy was used to study ultrastructural characters of neoblasts and revealed two different stages in maturation of neoblasts, each with a characteristic organization of heterochromatin. The stem-cell pool of C. longifissura is an important prerequisite for the extraordinary mode of asexual reproduction and the high capacity of regeneration. A comparison of the stem-cell pool in Acoela and higher platyhelminth species can provide evidence for the phylogenetic relationships of these taxa.

Bacterial Symbiosis Maintenance in the Asexually Reproducing and Regenerating Flatworm Paracatenula galateia

“…In general, cell renewal strategies of an animal taxon should be indicative for bacteriocyte formation and maintenance in their respective symbiotic tissues. Also acoelomorph flatworms, whose phylogenetic placement is highly disputed [47], [48], [49], [50], have a totipotent stem cell system with comparable features [49], [51], [52]. It would therefore be interesting to study the formation process of the symbiont-housing cells in the acoels Symsagittifera roscoffensis (formerly Convoluta roscoffensis ) Graff 1891 [53], [54] or Convolutriloba longifissura Bartolomaeus & Balzer 1997 [55], [56].…”

Acoel development supports a simple planula-like urbilaterian

“…Acoelomorphs have tremendous capacities for regeneration and asexual reproduction through the actions of multipotent, mesodermally derived stem calls called neoblasts. pluripotent cells in other bilaterian taxa, such as Planaria, is debatable (Gschwentner et al 2001). The cleavage pattern of acoels appears to be a unique 'spiral duet' cleavage programme (Henry et al 2000) that is different from any other metazoan, including nemertodermatids ).…”

Dynamics of cell proliferation and apoptosis reflect different life strategies in hydrothermal vent and cold seep vestimentiferan tubeworms

“…Migration of these differentiated cells (with large coccoid-shaped symbionts) to the periphery (p) is followed by the fully differentiated cells entering apoptosis including the elimination of symbionts platyhelminths Sanchez Alvarado 2000, 2002;Nimeth et al 2004), hydroids (Bosch and David 1984;Muller et al 2004), crinoids (Carnevali et al 2001), molluscs (Martynova and Bystrova 2002), crustaceans (Harrison et al 2000), and insects (Calvi and Lilly 2004;Baonza and Freeman 2005). For example, about 15% of the stem cells in Convolutriloba longifissura (Acoela, Platyhelminthes; Gschwentner et al 2001) and only about 2% in adult Macrostomum sp. (Rhabditophora, Platyhelminthes;Ladurner et al 2000) are labeled after 30 min, in contrast to 21% in R. pachyptila.…”

“…To identify proliferating and mitotically active cells, all semithin sections per specimen were treated as described by Gschwentner et al (2001) with anti-BrdU (mouse IgG, monoclonal PRB-1; Alexa-Fluor-488 conjugate; Molecular Probes) and the mitosis marker anti-Phosphohistone H3 (anti-PH3, rabbit polyclonal IgG; Upstate Biotechnology) with slight modifications: preincubation with 0.1 M phosphate-buffered saline (PBS) pH 7.2 and 0.1% Triton X-100 for 5 h followed by incubation with proteinase K for 20 min, and rinsing with 0.1 N HCl on ice; denaturation with 2 N HCl at 37°C for 60 min and rinses with PBS; incubation with a blocking solution (PBS, Triton X, 0.25% bovine serum albumin) for 30 min; incubation with 50µl anti-BrdU Alexa-Fluor-488 conjugate (20-fold diluted in PBS) and anti-PH3 (dilution 1:200) at room temperature overnight. Following the washing procedure the next day, slides were incubated with a secondary antibody (aminomethylcoumarin acetate [AMCA]-conjugated goat antirabbit IgG; Vector Laboratories; dilution 1:100) to detect anti-PH3 for 1 h at room temperature, washed again, and air-dried.…”