Abstract
The parafollicular cells of the thyroid gland of the dog were studied by electron microscopy, and the following results were obtained.
Parafollicular cells were larger than ordinary follicle cells and have large round to oval, sometimes irregular-shaped nuclei. They were located on the follicle walls or wedged in between follicle cells but usually did not border the follicle cavity directly. They were round to oval, but when in groups they were polygonal and gathered together as buds and plates surrounded in a set with the follicles by argyrophilic fibers and capillaries. The parafollicular cells could be classified into two kinds, clear and dark cells. The latter were found to be less in number. Thyroid stimulating hormone produced no significant changeno increase of colloid dropletsin them.
At the electron microscopic level, the parafollicular cells showed the same findings as by light microscopy, but revealed the following fine structure.
1. The majority of parafollicular cells were less electron dense in the cytoplasm than follicle cells.2. Rough-surfaced endoplasmic reticulum was poorly developed and dispersed in small numbers in the form of small vesicles and sometimes as a few thin lamellar forms together with the vesicular forms. This was one of the characteristics of this kind of cell.3. The mitochondria were smaller in size and number than of follicle cells. They showed round to oval, partly branched profiles with ruffled outline, and had irregularly arranged cristae mitochondriales.4. The Golg is complex was well-developed and consisted of thin lamellar sacs, vesicles and vacuoles.5. Secretory granules were found in small numbers and different in size and electron density. They were small near the Golg is complex, probably taking their origin in the complex.6. No desmosomes nor complicated interdigitations were found between themselves or in between them and the follicle cell.7. Thyroid stimulating hormone produced no significant change in their cytoplasm, also at the electron microscopic level.8. In both the control series and experimental series no intermediate forms between the follicle cells and parafollicular cells appeared.9. The parafollicular cells may be considered to be an independent cell species different in origin, structure and function from the follicle cells.
