
the ultrastructure, histopathology, and
histochemistry of the parathyroid glands of pregnant
and nonpregnant cows fed a high level of vitamin d.
the parathyroid glands of pregnant and
nonpregnant cows were studied by light and
electron microscopy following the daily
feeding of 30 million units of vit d for 30
days and compared to untreated controls.
in both the pregnant and nonpregnant cows
the parathyroid glands became markedly
atrophic with a diminution in the amount of
parenchyma compared to the untreated controls.
after vit d feeding the number of
actively secreting dark and intermediate
chief cells was reduced and the percentage
of inactive (light) and atrophic chief cells
was increased.  the parathyroid glands
of the vit d-fed, nonpregnant cows were
dominated by severely atrophic chief cells.
these cells were characterized by their
highly irregular shape, shrunken nucleus
and cytoplasm, widened intercellular spaces
traversed by numerous cytoplasmic
processes, increased electron density, few
secretory granules, inactive ergastoplasm
and golgi complexes.  atrophic chief
cells were not present in the untreated
controls.  the predominating cell type in
the parathyroids of the vit d-fed pregnant
cows were the inactive chief cells.  in contrast
to the atrophic chief cells, they were
larger and more regular in shape with
relatively straight cell membranes.  they
were comparatively electron-transparent,
containing more glycogen and fewer
secretory granules.  organelles concerned
with protein synthesis and packaging into
secretory units were poorly developed.
inactive chief cells were observed only
occasionally in the parathyroid glands of
untreated pregnant cows where the actively
secreting dark and intermediate chief cells
predominated.
