Differentiation during asexual reproduction and regeneration in a microturbellarian

Abstract

Autoradiography, immunocytochemistry, and microscopy were employed to study stem cells and their fates in experimentally decapitated and in asexually reproducing specimens of Microstomum lineare (O. F. Müller). Differentiation of the nervous system in developing zooids was monitored by the appearance of immunoreactivity to the neuropeptide FMRF/RF-amide and to the biogenic amine 5-HT. Labelling with [³H]T (tritiated thymidine) provided a means to follow the proliferation of stem cells and their differentiation into nerve and other cell types both in the regenerating head end and in more posterior zooids produced by asexual reproduction. Initiation of development of a new zooid was characterized by immunoreactivity for 5-HT in the postpharyngeal commissure and in a cell group at its branching point with the parental lateral nerve cords; initiation of regeneration, on the other hand, was characterized by increased immunoreactivity for FMRF/RF-amide in the cut nerve ends. Antigenicity to FMRF/RF-amide antibody appeared before that to 5-HT in the developing brain as well as in the pharyngeal plexus. FMRF/RF-amide-positive neurons were not, however, found among differentiating nerve cells until dense-core vesicles appeared in their cytoplasm. The only cells incorporating tritiated thymidine ([³H]T) were neoblasts in the mesenchyme and gastrodermis, which proliferate continuously, replenishing the population of stem cells available for growth, asexual reproduction, and regeneration. Two forms of the proliferative cells arose, one with and one without ciliary basal bodies in the cytoplasm. Even in completely regenerated specimens there were many labelled undifferentiated cells (i.e., 20–45 h after exposure to [³H]T). I propose that neoblasts function both as stem cells for all differentiated cell types and as reserve cells with a determined life time.