In order to define the fine structure of the parafollicular cells in the squirrel thyroid gland, small blocks of the thyroid tissue were fixed in the mixture of 2% paraformalde￢hyde-2.5% glutaraldehyde (phosphate buffer; pH 7.4) and post-fixed in 1% osmium tetroxide (phosphate buffer, pH 7.4). The tissue were embedded in Epon 812, ultrathin￢sectioned with the LKB-III ultramicrotome. The sections contrasted with uranyl acetate and lead citrate were observed with a JEM 100B electron microscope.
Observed results were as follows:
1. The parafollicular cells of the squirrel were observed in the intra-, para-and inter￢follicular positions. They were not contact with the follicular lumen.
2. The parafollicular cells with few indented nucleus contain moderately developed cell organelles, such as Golgi complex, granular endoplasmic reticulum, ribosomes, etc. The laminae of granular ´endoplasmic reticulum occasionally arranged with the lamellar structures.
3. The secretory granules with tight limiting membrane were generally spherical with a diameter of 100-350 nm. The electron density of the core texture varied as granules, coarse with lower density and homogeneous with higher density.
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