Desouky Mahmoud M A
Zoology Department, Faculty of Science, Zagazig University, Egypt.
Aquat Toxicol. 2006 May 1;77(2):143-52. doi: 10.1016/j.aquatox.2005.11.009. Epub 2006 Jan 18.
The present study was undertaken to elucidate the cellular mechanisms, which govern metal sequestration and detoxification in gastropods. For this purpose the pond snail, Lymnaea stagnalis was exposed to environmentally relevant concentrations of three species of metals (Al, Zn and Cd) for 30 days and the localization and fate of these metals were followed in different tissues of the snails. The measurement of relative distribution of metals between tissues revealed that the digestive gland and kidney account for most of the accumulated metals. Al and Cd (non-essential metals) were redistributed to the digestive gland, possibly because of the presence of specific binding entities in the digestive glands of the herein species. This study focuses on the role of intracellular metal-containing granules on metal sequestration. Three main types of granules were identified in the digestive gland cells namely small, green and yellow granules. The morphological examination and the progressive accumulation of elements within these granules revealed that they are developmental stages with the yellow granule being the mature one. The total number of these granules was found to be significantly increased upon exposure of the snails to Al only. This increase may be a response to the large amount of Al that is accumulated through feeding route of this grazing snail. X-ray microanalysis (XRMA) revealed that metals were localized in all three types of digestive gland granules. The increased amount of ligands (P and S) in the granules may give evidence for their role in metal sequestration. Levels of Al and P were positively correlated in the digestive gland granules. It is possible that aluminium is bound to phosphorus to render it insoluble and so to both immobilize it within the lysosome and to be excreted in a highly insoluble form. On the other hand, both Zn and Cd induced marked upregulation of S in mature (yellow) granules by 26- and 11-folds, respectively. The lysosomal codeposition of S and either Cd or Zn in the lysosomal granules in addition to the increase in RER cisternae may indicate that the exposure to these metals could induce metallothionein synthesis in the cells. The microscopical examinations in the present study revealed that metal detoxification from the digestive gland cells may occur via faeces or via basal exocytosis towards hemocytes dispersed by the connective tissue in the visceral mass. In the kidney, one type of granules, the excretory concretions, was identified in the nephrocytes. The significant increase in the number of these concretions in the snail L. stagnalis upon exposure to metals may give further evidence for their role in metal excretion.
本研究旨在阐明腹足纲动物中控制金属螯合和解毒的细胞机制。为此，将椎实螺暴露于环境相关浓度的三种金属（铝、锌和镉）中30天，并追踪这些金属在蜗牛不同组织中的定位和归宿。组织间金属相对分布的测量结果显示，消化腺和肾脏积累了大部分金属。铝和镉（非必需金属）被重新分配到消化腺，可能是因为本物种消化腺中存在特定的结合实体。本研究聚焦于细胞内含金属颗粒在金属螯合中的作用。在消化腺细胞中鉴定出三种主要类型的颗粒，即小颗粒、绿色颗粒和黄色颗粒。对这些颗粒的形态学检查以及元素在其中的逐步积累表明，它们处于不同的发育阶段，黄色颗粒为成熟颗粒。仅在蜗牛暴露于铝后，发现这些颗粒的总数显著增加。这种增加可能是对通过这种植食性蜗牛的摄食途径积累的大量铝的一种反应。X射线微分析（XRMA）显示，金属定位于所有三种类型的消化腺颗粒中。颗粒中配体（磷和硫）含量的增加可能证明它们在金属螯合中的作用。消化腺颗粒中铝和磷的含量呈正相关。铝可能与磷结合使其不溶，从而将其固定在溶酶体内并以高度不溶的形式排出。另一方面，锌和镉分别使成熟（黄色）颗粒中的硫上调26倍和11倍。硫与镉或锌在溶酶体颗粒中的共沉积以及粗面内质网池的增加可能表明，暴露于这些金属会诱导细胞中金属硫蛋白的合成。本研究中的显微镜检查显示，消化腺细胞中的金属解毒可能通过粪便或通过向由内脏团结缔组织分散的血细胞进行基底胞吐作用来实现。在肾脏中，在肾细胞中鉴定出一种颗粒，即排泄性结石。蜗牛暴露于金属后，这些结石数量的显著增加可能进一步证明它们在金属排泄中的作用。
