Localization artefacts in ultracytochemical ion precipitation reactions

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.

Elemental and cytochemical localization of calcium in rat cheek epithelium

Energy-dispersive X-ray microanalysis, combined with scanning electron microscopy, was applied to 18 microns thick freeze-dried cryosections to determine the level of calcium in successive layers of the epithelium. This indicated low levels of calcium in basal cells, high in spinous cells, moderate in granular cells, lowest in the inner keratin and highest in the outer keratin layers. The potassium pyroantimonate technique was used to study the cytochemical distribution of Ca2+. The reaction product, calcium pyroantimonate (Ca-PA), was generated from cellular and intercellular calcium by perfusion of the anaesthetized rat with a solution of potassium pyroantimonate in glutaraldehyde. Ca-PA was localized in nuclei, mitochondria, lysosomal-type bodies and membrane-coating granules. A denser Ca-PA distribution was found between upper spinous and lower granular cells. The proposed identification of Ca2+ as a major component in Ca-PA was confirmed by EDTA decalcification and X-ray microprobe analysis of the reaction product. Thus X-ray microanalysis in combination with cytochemistry can localize Ca2+ in a growing and differentiating tissue such as stratified squamous epithelium.

Regulatory mechanism of contraction in the proboscis retractor muscle of a sipunculid worm, Phascolosoma scolops

Regulatory mechanism of contraction in the proboscis retractor muscle of Phascolosoma scolops was studied by physiological measurements and cytochemical electron microscopy. The magnitude of K+-contracture was dependent on external Ca2+ concentration and the contracture disappeared in Ca2+-free solution. The K+-contracture was suppressed by application of procaine and Mn2+. Caffeine induced contracture even when external Ca2+ was absent. Ultrastructural observations of the retractor muscle cells showed the presence of a large number of vesicles (subsarcolemmal vesicles), corresponding to the sarcoplasmic reticulum in vertebrate skeletal muscle, underneath the plasma membrane. For the cytochemical electron microscopy, the muscle fibers were fixed with 1% OsO4 solution containing 2% K-pyroantimonate. In the relaxed fibers, pyroantimonate precipitates were localized along the inner surface of plasma membrane and in the subsarcolemmal vesicles. In the contracting fibers, the precipitates were uniformly distributed in the myoplasm. The X-ray microanalysis revealed that the precipitates contained Ca. These results suggest that the contractile system is activated by the influx of extracellular Ca2+ as well as by the release of Ca2+ from the intracellular structures such as the inner surface of the plasma membrane and subsarcolemmal vesicles.

Ultrastructural phosphatase histochemistry of submandibular and parotid salivary glands of man

Thiamine pyrophosphatase was demonstrated in the Golgi complex and acid phosphatase in the GERL of acinar cells of submandibular and parotid glands and were previously demonstrated in cells of intercalary ducts. Thiamine pyrophosphatase was also demonstrated in the Golgi complex of cells of striated and excretory ducts and myoepithelial cells. Acid phosphatase was also demonstrated in lysosomes. Alkaline phosphatase was rarely demonstrated light microscopically at luminal surfaces of striated and excretory ducts and electron microscopically in luminal vesicles in cells of striated ducts. The demonstration of the phosphatases in Golgi complexes and GERLs indicates that investigations on these structures in experimental animals are relevant to human salivary glands and supports the opinion that ductal cells as well as acinar cells secrete organic material. The presence of alkaline phosphatase at luminal surfaces of striated and excretory ducts suggests that resorption as well as secretion may occur in them.