Light and electron microscopic observations of thyroid parafollicular cells of the opossum

Abstract

The thyroid parafollicular cells from adult opossums Didelphis virginiana are described by light and electron microscopy. In the light microscope, ovoid parafollicular cells are observed in abundance throughout the upper two-thirds of each lobe of the thyroid lobe. Under high magnification, parafollicular cells are observed to contain abundant lead hematoxylin and phosphotungstic acid-hematoxylin-stained secretory granules. In sections of Zenker formol-fixed tissue, parafollicular cells also exhibited differential staining with lead hematoxylin and particularly with phosphotungstic acid-hematoxylin. Occasionally, tightly knit clusters of cells which are lead hematoxylin positive are observed among the thyroid follicles. Serial sections taken through these cell clusters reveal that they represent solid masses of cells rather than thyroid follicles which are sectioned in the tangential plane. On the basis of observations made in the electron microscope, thyroid parafollicular cells of the opossum are confirmed to be parafollicular in terms of the established morphological criteria for that cell type. Parafollicular cells of the opossum lack contact with the lumen of the follicle, and they contain and abundance of round secretory granules in their cytoplasm. The granules average approximately 300–500 nm in diameter and vary markedly in their electron opacity in the same cell and between different parafollicular cells. Light microscopy of thick plastic-embedded sections stained with lead hematoxylin is used to correlate the information obtained by conventional light and electron microscopy. Those preparations indicate that parafollicular cells exhibiting dark lead hematoxylin reactivity contained granules which by electron microscopy showed mostly moderate to heavy electron opacity.