PhD thesis - Biologisk Institut - Københavns Universitet

31 Resultsarea of the developing tentacles of post metamorphic stages. However, no S-phasecells can be detected in the posterior tip of the ventral trunk region anymore.General discussionImmunocytochemistry and F-actin labelling in conjunction with confocal microscopyand 3D reconstruction software has proven to be an excellent tool to characterise theneuromuscular system as well as the expression pattern of certain neurotransmitters,thus allowing for comparative analyses of neural and muscular characters (Wanninger2009, Richter et al. 2010). Using these methods, the main goal of the present PhDthesis was to characterise nervous and muscle system formation as well as thedistribution of proliferating cells during sipunculan development. Additional studieson myo- and neurogenesis in other sipunculans supplement this comparative approach(Wanninger et al. 2005a, Schulze and Rice 2009). So far, eight species representingfour families and three different developmental modes have been investigated by theabove mentioned methods.Early neurogenesis in TrochozoaIn all investigated sipunculan species neurogenesis begins in the trochophore larvae atthe apical pole, from which two neurites grow posteriorly, forming a scaffold for thefuture ventral nervous system (Wanninger et al. 2005a; chapters I and II herein).Moreover, the investigated species herein, Phascolosoma agassizii, Themistepyroides, and Thysanocardia nigra, all show initially two and later up to fourserotonergic and FMRFamidergic flask-shaped cells within the apical organ (chaptersI-III), whereas Phascolion strombi lacks serotonergic apical cells (Wanninger et al.2005a). The most common developmental pathway in sipunculans is via alecithotrophic trochophore and a planktotrophic pelagosphera (Fig. 1), which mightremain pelagic for several weeks or even months (Scheltema and Hall 1975, Rice1981, 1985). Compared to other sipunculans, Phascolion strombi has a relativelyshort larval development (approx. 3 days) (Åkesson 1958, Wanninger et al. 2005a).Accordingly, the lack of serotonergic expression in the apical organ might be asecondary condition. However, the above described pattern of early neurogenesisfound in sipunculans is remarkably similar to that reported for the early trochophorelarvae of the polychaetes Polygordius lacteus, Pomatoceros lamarkckii, Sabellaria