Ort: Hörsaal, Museum Koenig
Zeit: 24.11.2011, 17:15h
On the contractility of Porifera and Placozoa: functional morphology of epithelia in basal Metazoa
The early evolution of muscle cells has been discussed for over a century. Still, the emergence of contractile muscle filaments is an unresolved enigma. While the Cnidaria show the first muscle cells, the Porifera and Placozoa are regarded as muscleless, but both display remarkable body contractility. However, the effector cells are still debated. Two competing hypotheses on sponge contractile cells were postulated: (1) mesohyl contraction mediated by smooth muscle-like actinocytes ('myocytes'); (2) epidermal contraction by pinacocytes. Neither of these cell types has been directly shown to contract, only indirect evidence has been presented. Comprehensive new data on sponge contractility including all four major sponge taxa (Demospongiae, Hexactinellida, Calcarea and Homoscleromorpha) will be presented. In addition, the contractile behaviour of Placozoa is addressed. A modern functional morphology approach based on in vivo and 3D imaging techniques provides evidence for epithelial contractility.
In the Placozoa, the mesenchymal fibre cell syncytium has been shown to be contractile. However, our recent morphological study which considered dynamic states of contractions demonstrates prominent morphological changes of the upper epithelia of T. adhaerens during contraction. Shape changes functionally identical to those in contracting sponge pinacocytes could be demonstrated, raising the question of epithelial contractility in placozoans, too.
In conclusion, epithelial contractility might be a character of the metazoan ground pattern. Most likely, such early contractile epithelia lacked functional compartmentalization like in typical epithelio-muscle cells. The evolutionary scenario emerging from our studies suggests that the transition from a general cytoskeleton contractility (also found in unicellular Opistokonta) to the muscle specific, filament based contractility, occurred early within the metazoan stem group. The contractile epithelia in basal Metazoa might be derived, but likely share a functional root with muscle cells on the gene and protein level. This hypothesis will have to be tested on genomic data and gene expression level.
In the Placozoa, the mesenchymal fibre cell syncytium has been shown to be contractile. However, our recent morphological study which considered dynamic states of contractions demonstrates prominent morphological changes of the upper epithelia of T. adhaerens during contraction. Shape changes functionally identical to those in contracting sponge pinacocytes could be demonstrated, raising the question of epithelial contractility in placozoans, too.
In conclusion, epithelial contractility might be a character of the metazoan ground pattern. Most likely, such early contractile epithelia lacked functional compartmentalization like in typical epithelio-muscle cells. The evolutionary scenario emerging from our studies suggests that the transition from a general cytoskeleton contractility (also found in unicellular Opistokonta) to the muscle specific, filament based contractility, occurred early within the metazoan stem group. The contractile epithelia in basal Metazoa might be derived, but likely share a functional root with muscle cells on the gene and protein level. This hypothesis will have to be tested on genomic data and gene expression level.
