Cytological approach to morphogenesis in the planarian blastema. III. Ultrastructure and regeneration of the acoel turbellarian Convoluta naikaiensis

Fine structure and regeneration of the acoel turbellarian Convoluta naikien sis were studied using light and electron microscope. The epidermis of the intact worm consists of a typical insunk type of epidermal cells. Cell memb ranes at their apical portion are interdigitated to varying degrees and are connected with septate junctions. The apical cytoplasm is abundant in secr etory granules and mitochondria. Nuclei belonging to the epidermal cells ar e situated deeply in the parenchyma. Various types of free parenchymal cell s such as gland cells and regenerative cells are seen in the parenchyma. Th ere is no evidence indicating syncytial tissues. When decapitated, wound cl osure is completed within a few days by migration of the epidermal cells fr om the non-injured region. In the following stage, the initial thin layer o f the wound epidermis is recruited by epidermal-replacement cells, which st are cell differentiation at the subepidermal region. The regeneration blast ema is at first formed by regenerative cells which later begin cell differe ntiation. Although the regenerative cells have no chromatoid bodies around che nucleus, they can divide mitotically and differentiate into some tissue cells. When the decapitated worms are maintained in neuropeptide-containin g seawater, the process of blastema development is remarkably accelerated. Early stages of the acoel regeneration are compared with those of planarian regeneration.