Colloid resorption in thyroid glands as revealed by scanning electron microscopy.

Colloid resorption from thyroid follicles in control and in calcitonin-treated rabbits was studied with the scanning electron microscope. Thus, an intense cell surface activity with several vesicles (exocytotic vesicles) and large pseudopods, sometimes cylindroids, is predominant on the thyroid follicular cells of calcitonin-treated rabbits. The pseudopods are irregular, mostly elongated, cup or dome-shaped and operate by phagocytosis or engulfment of colloid droplets (endocytosis). Only microvilli are visible on the cell surface of thyroid follicles in control rabbits. Comparative ultrastructural studies also revealed the occurrence of small exocytotic vesicles and large pseudopods in thyroid glands stimulated by calcitonin. The present findings demonstrate that the scanning electron microscopy better visualizes the colloid resorption and provides more details for the role of follicular cell surface in thyroid physiology.