The parathyroid glands in the rat as seen by ultrathin step and serial sectioning

Parathyroid cells exhibit ultrastructural heterogeneity. This has been interpreted as an indication that the cells are undergoing secretory cycles, varying in synthetic activity. By electron microscopy we examined step sections at 1 and 2 micron intervals and serial sections of parathyroids from normocalcemic rats of three different age groups. The secretory cells in all three age groups exhibit structural polarity: nuclei and secondary lysosomes are generally located towards the interstitial tissue, whereas the Golgi is confined mainly to regions in the interior of the cell sheets. Cell surfaces facing the interstitial tissue are smooth, whereas plasma membranes in the interior of the cell sheets form tortuous areas. Consequently, an individual cell profile may show quite different appearance depending on section level. Our results do not support the concept that the observed heterogeneity is necessarily due to differences in organelle contents between the cells. The present investigation thus offers an alternative explanation to the ultrastructural variability among secretory parathyroid cells in the rat.