Loomis Stephen H
Department of Biology, Connecticut College, New London, CT 06320, USA.
Prog Mol Subcell Biol. 2010;49:231-43. doi: 10.1007/978-3-642-02421-4_11.
Sponges can be found in fresh or saltwater habitats. As part of their life cycle, many sponges produce gemmules as a means of surviving environmental challenge. In most sponges, the gemmules contain cells that are initially in a state of metabolic arrest that is controlled by endogenous factors. This state is known as diapause. Following a period of exposure to unfavorable conditions, the cells in the gemmule transit from diapause into a state known as quiescence in which metabolic depression is controlled by environmental factors. When favorable conditions return, the gemmules germinate and produce a new sponge. Production of gemmules is triggered by environmental factors such as decreased temperature or desiccation and involves cell aggregation of thesocytes and the laying down of the gemmule coat. Thesocytes contain yolk platelets as an energy store and high concentrations of polyols that maintain high osmotic concentration in the cells of the gemmules. The high osmotic concentration maintains metabolic depression and turns off cell division. It is the inability to reduce the osmotic concentration that maintains the gemmules in diapause. Transition to quiescence requires the ability of the cells in the gemmules to convert the polyols to glycogen, and thus reduce the osmotic concentration. At this stage, the cells are able to reduce osmotic concentration but do not until favorable conditions return. Early in the germination process, the polyols are converted to glycogen, reducing the osmotic pressure and releasing the inhibition of cell division and metabolic rate. Both cell division and metabolic rate increase eventually leading to germination of the gemmules and production of a new sponge.
海绵动物可以在淡水或咸水栖息地中找到。作为其生命周期的一部分，许多海绵动物会产生芽球作为应对环境挑战的一种生存方式。在大多数海绵动物中，芽球包含的细胞最初处于由内源性因素控制的代谢停滞状态。这种状态被称为滞育。在经历一段不利条件后，芽球中的细胞从滞育转变为一种被称为静止的状态，在这种状态下，代谢抑制由环境因素控制。当有利条件恢复时，芽球萌发并产生一个新的海绵动物。芽球的产生由温度降低或干燥等环境因素触发，涉及储细胞的细胞聚集和芽球包被的形成。储细胞含有作为能量储存的卵黄小板以及高浓度的多元醇，这些多元醇在芽球细胞中维持高渗透压浓度。高渗透压浓度维持代谢抑制并关闭细胞分裂。正是无法降低渗透压浓度使芽球处于滞育状态。向静止状态的转变需要芽球中的细胞将多元醇转化为糖原的能力，从而降低渗透压浓度。在这个阶段，细胞能够降低渗透压浓度，但直到有利条件恢复才会这样做。在萌发过程的早期，多元醇被转化为糖原，降低渗透压并解除对细胞分裂和代谢率的抑制。细胞分裂和代谢率最终都会增加，最终导致芽球萌发并产生一个新的海绵动物。
