Parathyroid cell variants may be provoked during immersion fixation

Enhanced resolution of membranes in cultured cells by cryoimmobilization and freeze-substitution

Investigations of cellular processes demand immediate arresting of the process at any given time and excellent retention of cellular material and excellent visibility of membranes. To achieve this goal we used cryofixation to arrest cellular processes instantly and tested diverse freeze-substitution protocols. Madin-Darby kidney cells and Vero cells were grown on carbon-coated sapphire disks. For cryofixation the sapphire disks covered with a cell monolayer were injected with the aid of a guillotine into liquid propane or ethane or a mixture of both cooled by liquid nitrogen. Freezing of the cryogen was prevented by using a partially insulated cylinder and by vigorous stirring that results in a substantial decrement of the freezing point of the cryogen. Cell monolayers can be cryofixed successfully using the guillotine in a safety hood at ambient temperature and humidity or at 37 degrees C and 45% humidity. The freezing unit can also be placed in a laminar flow for working under biohazard conditions. For visualizing cell membranes at high contrast and high resolution, cells were substituted in the presence of various concentrations of glutaraldehyde and osmium tetroxide and the temperature was raised to diverse final temperatures. Substitution for 4 hours at -90 degrees C in anhydrous acetone containing 0.25% anhydrous glutaraldehyde and 0.5% osmium tetroxide followed by a temperature rise of 5 degrees C/hour to 0 degrees C and final incubation for 1 hour at 0 degrees C resulted in high contrast and excellent visibility of subcellular components at the level of the membrane bilayer. The high spatial and temporal resolution makes this methodology an excellent tool for studying cell membrane-bound processes, such as virus-cell interactions.

Ultrastructure of human parathyroid cells in health and disease

Parathyroid glands (n = 271) removed from 130 patients were examined by light and electron microscopy. A standardized method of tissue processing was employed and morphometry was performed. The aim of the paper is to provide a description of the human parathyroid chief cell ultrastructure in health and disease, with quantitative evaluation of structures involved in secretion of parathyroid hormone in a large case series, and to discuss their role in current diagnostic histopathology. The patients were euparathyroid (n = 10), or affected by primary (n = 97), secondary (n = 8), or tertiary (n = 15) hyperparathyroidism. In normal glands, solid parenchyma was composed of chief cells, large clear cells, transitional-oxyphil cells, and oxyphil cells. Chief cell hyperplasia, pseudo-adenomatous hyperplasia, adenoma, water-clear cell hyperplasia, and carcinoma were the most usual forms of parathyroid disease responsible for primary hyperparathyroidism. In chief cell hyperplasia, all the parathyroid glands were enlarged and the chief cells were in an active state of hormone secretion, with a large Golgi complex, abundant rough endoplasmic reticulum (RER), small lipid droplets, and tortuous plasma membrane. In pseudo-adenomatous hyperplasia, one gland was enlarged and the others displayed a normal size; however, electron microscopic examination and morphometric analysis showed that all the glands had active cells. Adenomas displayed a pattern similar to those of pseudo-adenomatous hyperplasia, with one gland enlarged and the others of normal size. However, ultrastructural examination and morphometry showed that the normal-size glands were hypo-active. Water-clear cell hyperplasia showed cells filled with cytoplasmic vacuoles. In these cells, structures with intermediate features between secretory granules and vacuoles were visible. Nucleo-cytoplasmic atypias were frequently visible in parathyroid carcinoma cells. In secondary and tertiary hyperplasia, active chief cells were regularly mixed with oxyphil or transitional-oxyphil cells. The tertiary hyperplasia was characterized by RER-associated structures that were not found in the normal or other pathological conditions. These results demonstrate that electron microscopy and morphometry represent useful tools in parathyroid histopathology.

Mammalian parathyroids: morphological and functional implications

Fixation with aldehydes is achieved either by immersion or perfusion. The parenchyma of parathyroid glands fixed by immersion consists of dark cells containing a lot of membranes of these organelles which are concerned with hormone secretion, light cells which are poor in these organelles, intermediate forms between the two, and multinuclear syncytial cells. They have been attributed to represent different functional stages of secretory activity, the dark cell being in an active form, the light cell in a resting form. Studies of the parathyroids of mice, rats, rabbits, cats, dogs, pigs, cattle, sheep, goats, and horses employing various fixation protocols clearly demonstrate that light cell variants and multinuclear syncytial cells are formed during improper immersion fixation as a result of membrane disintegration. Parathyroids fixed by perfusion or by immersion in an appropriate fixation medium comprise only one cell type which correspond to the dark chief cell. Parathyroid cells are polar cells bearing some of the rough endoplasmic reticulum in the basal pole, the rest of it, the Golgi complex, and secretory granules in the apical pole. The secretory product is released by exocytosis at the apicolateral domain of the plasma membrane into the intercellular space. Secretory activity can be altered experimentally, leading to drastic changes in the amount of cell membrane related to hormone synthesis, intracellular transport, exocytic release, and secretion coupled membrane retrieval. The sensitive reaction of parathyroid cells to both the mode of fixation and to fixation media demands careful evaluation of the fixation protocol. This and the polarity of parathyroid cells have to be borne in mind for estimating secretory activity on the basis of morphological criteria.

Quantitative and three-dimensional aspects of the rat parathyroid gland in normo-, hypo-, and hypercalcemia

The ultrastructure of the rat parathyroid has been under study for more than 35 years, but controversies still exist, especially regarding structure-function relationships. The present review focuses on recent morphological parathyroid research on rats under normal conditions and in various states of disturbed calcium metabolism. To facilitate discussions on functional aspects, current biochemical data, particularly those dealing with the regulation of parathyroid hormone synthesis and release, are also considered. Our results from quantitative studies and from investigations employing serial sectioning form the basis for the discussions. A central issue is whether the parathyroid secretory cells undergo secretory cycles. Prompted by results obtained from improved fixation procedures and serial sectioning, we question the basis for the theory of secretory cycles. Since the rat parathyroid secretory cell is polar, a single section is not an appropriate sample for estimating functional activity and for comparing the structure and distribution of intracellular components of adjacent cells. The heterogeneity in ultrastructural appearance of intracellular vesicles calls for the use of specific markers in relating the structure of the vesicular compartment to intracellular processing of hormone. The importance of unbiased quantitative techniques is illustrated in discussions on cell number and size for estimating the response of the parathyroid gland to different functional states or disorders demanding changes in secretion of parathyroid hormone, e.g., hyper- and hypocalcemia.

Parathyroid glands in primary hyperparathyroidism: an ultrastructural morphometric study of 25 cases

In parathyroid glands removed from patients with primary hyperparathyroidism (pHPT), hyperplasias and adenomas cannot be distinguished from one another by light microscopy when only one gland is available for examination. When a second gland is available, it is necessary to establish whether it is normal, suppressed, or hyperplastic. This distinction may be difficult, and the main criterion is the amount of cytoplasmic lipid in the parenchymal cells. If the lipid is abundant, the gland is considered normal or suppressed, and if it is scanty, the gland is interpreted as hyperplastic. We have performed a morphometric ultrastructural study to test the reliability of this criterion. Twenty-five adenomatous glands removed from patients with pHPT, when compared with glands of normal size from euparathyroid patients, showed a significant increase in the parameters indicative of metabolic activity, namely the size of the Golgi apparatus, the amount of rough endoplasmic reticulum, and the length of plasmalemmas. In addition, the amount of cytoplasmic lipid was significantly reduced. Furthermore, 25 glands of normal size removed from the same patients with pHPT showed an amount of lipid similar to that of normal glands from euparathyroid patients. However, all the parameters indicative of metabolic activity were significantly higher than those in glands from euparathyroid patients and comparable to those found in adenomatous glands. These results suggest that in pHPT, normal-size glands are as active as adenomatous glands, regardless of a higher lipid content.