Calcium and its localization in human lens fibres: an electron tomographic study

The ultrastructural distribution of calcium was studied in human lens fibres with the oxalate pyroantimonate technique. In the intermediate cortex precipitates were found perimembranous and occasionally in the fibre cytoplasm. The improved resolution of electron tomography revealed (a) that the perimembranous precipitates as described earlier by Vrensen et al. (1995) are restricted to the intercellular space; no indications were found of an increased intracellular submembranous calcium level, and (b) the existence of an intracellular pool of small protein attached precipitates in the fibre cytoplasm not observed with conventional electron microscopy. It is concluded that in the intermediate and deep cortex, where in the mature fibres all cellular calcium pools have disappeared, two calcium pools exist: (a) a pool of free calcium ionically bound to the negatively charged phospholipids of the external face of the fibre membrane and (b) a cytoplasmic pool of protein associated calcium. Possible candidates for this cytoplasmic calcium binding are discussed.