Ultrastructure of neoblasts in microturbellaria: significance for understanding stem cells in free-living Platyhelminthes

Platyhelminths possess a unique stem cell system that is claimed to be toti potent. It is supposed to be competent for the renewal of all cell types, i ncluding germ cells, during postembryonic development and regeneration. A c onnection to stem cells in the embryo has been postulated repeatedly. This cell type is now most frequently termed "neoblast'. Light microscopy can re veal only a few neoblast characters, and ultrastructural studies have shown additional characters for discriminating possible types and/or stages. Whi le some progress has been made in this respect for triclad turbellarians (f reshwater planarians), rather little is known about the microturbellarians. We have investigated the fine structure of neoblasts of hatchlings and adu lts of Macrostomum hystricinum marinum, a member of a primitive taxon in th e "Turbellaria"-Rhabditophora (a paraphyletic group giving rise to the main parasitic flatworm taxa). In Macrostomum, one population of neoblasts is l ocated in lateral bands along the main longitudinal nerve cords within the body cavity. Another population is found in the gastrodermis in a basi-epit helial position. Based on their cytoplasmic and nuclear organization, three stages in neoblast differentiation have been distinguished. The first and second stages are characterized by cytoplasm lacking organelles except free ribosomes and scattered mitochondria, a finding identical with the picture known from the "classical" planarian neoblast. In the first stage, heteroc hromatin is scattered over the nucleus in isolated clumps in a typical spec kled (checkerboard) appearance; a nuclear lamina is weakly developed. In st age 2 the heterochromatin forms strands and clumps connected to each other. In stage 3 the nucleus is characterized by more prominent heterochromatin strands and by heterochromatin attachments to the well developed nuclear la mina. In this last stage a rough endoplasmic reticulum (rER) and Golgi comp lex are also present, indicating the entrance into cytoplasmic differentiat ion. Early epidermal replacement cells are located baso-epithelially, which show a nuclear organization similar to stage 3 neoblasts. Observations of stem cells in regenerating specimens and on isolated neoblasts are reported briefly. The data show that from the three types of differentiating cells distinguished recently in regenerative blastemas of planarians, the first s tage ("undifferentiated cells") resembles stage 2 neoblasts described here for postembryonic development. The results are compared with observations t hat have been published for neoblasts in other free-living platyhelminths.