Unusual Cytoplasmic Inclusion Bodies in Medullary Carcinoma of the Thyroid Gland

Abstract

Medullary carcinoma of the thyroid from a 43‐year‐old patient showed 4 types of unusual cytoplasmic inclusion bodies ultrastructurally. Type 1 inclusion is composed of rod and doughnut‐shaped bodies aggregated In the cytoplasma and not enclosed by a limiting membrane. Typical neurosecretory granules are intermingled with the rod and doughnut‐shaped bodies. The doughnut‐shaped bodies often have an electron dense core which is similar to the neurosecretory granules. The rod‐shaped bodies are bounded by trilaminar unit membrane and have a slightly electron dense matrix. A centriole is often found associated with this type of inclusion. Type 2 inclusion is composed of small secretory granules and clear vesicles. These inclusions can be found either with or without surrounding membrane. Type 3 inclusion is a large osmiophilic body, or secondary lysosome, in which some degenerated organelles are discernible. Type 4 inclusion is a fragmented nucleus with condensation and margination of the chromatin, so‐called karyorrhexis. Routine hematoxylin and eosin sections did not show each type of inclusion but semithin sections stained with Paragon suggested 4 different types of inclusions. Type 3 and 4 inclusions represent forms of degeneration and cell death in the tumor cells and probably represent a remote effect of radiation. Type 1 inclusion is not commonly associated with cell death and may represent a unique organellar aberration in these tumor cells. Type 2 inclusion is simply an aggregation of secretory granules and clear vesicles.