RESUMEN
This study reveals the peculiar in vivo cell kinetics and cell turnover of the marine sponge Halisarca caerulea under steady-state conditions. The tropical coral reef sponge shows an extremely high proliferation activity, a short cell cycle duration and massive cell shedding. Cell turnover is predominantly confined to a single cell population, i.e. the choanocytes, and in this process apoptosis only plays a minor role. To our knowledge, such fast cell kinetics under steady-state conditions, with high turnover by shedding in the absence of apoptosis, has not been observed previously in any other multicellular organism. The duration of the cell cycle in vivo resembles that of unicellular organisms in culture. Morphological and histochemical studies demonstrate compartmentalization of choanocytes in the sponge tissue, which corresponds well with its remarkable cellular kinetics. Coral reef cavity sponges, like H. caerulea, inhabit low nutrient tropical waters, forcing these organisms to filter large volumes of water and to capture the few nutrients efficiently. Under these oligotrophic conditions, a high cell turnover may be considered as a very useful strategy, preventing permanent damage to the sponge by environmental stress. Halisarca caerulea maintains its body mass and keeps its food uptake system up to date by constantly renewing its filter system. We conclude that studies on cell kinetics and functional morphology provide new and essential information on the growth characteristics and the regulation of sponge growth in vivo as well as in vitro and the role of choanocytes in tissue homeostasis.