Four horses are described with rupture of the aortic arch and pulmonary trunk which resulted in a fistula between these vessels. Ruptures were located near the ligamentum arteriosum. Histological examination of the vessel walls revealed media necrosis which was probably due to intimal thickening and/or medial fibrosis of the vasa vasorum.

A 12-year-old Dutch warmblood mare showed a large firm swelling on the left side of the throat region at the spot of the thyroid lobe. Because no other clinical abnormalities were noticed, the tumor was surgically extirpated. Based on histological findings the tumor was thought to be a medullary thyroid carcinoma which was confirmed by the immunohistochemical demonstration of calcitonin and the presence of characteristic ultrastructural features of the tumor cells. Amyloid was not detected in the tumor.Medullary thyroid carcinoma (MTC) is a well-known tumor derived from the C cells of the thyroid gland.4 After its recognition as a special primary thyroid tumor in man in 195 1 , 6 histological and clinical features were first described in 1959,5 and the term medullary carcinoma was used. The tumor also has been described in animal species including bulls, dogs, mice, mouflons and rats.lo Histological features of MTC are a fairly uniform granular eosinophilic cell type arranged in sheets and lobules of varying sizes and shapes separated by irregular and sometimes very thin fibrovascular stroma in which amyloid can be demon~trated.~ These criteria differentiate MTC from follicular or papillary and undifferentiated thyroid carcinomas. This paper presents the first descriptions of a case of medullary thyroid carcinoma in a horse.A 12-year-old Dutch warmblood mare was admitted to the Large Animal Clinic for examination of a swelling on the left side of the throat region. The swelling had been noticed 3 months before and had increased gradually. Abnormal behavior or compromised performance ability was not observed. On examination, a round firm tissue mass, about 15 cm in diameter, was palpable under the skin on the left side of the most cranial portion of the trachea. It was only loosely attached to the surrounding tissues. Based upon its location and consistency, the mass was suspected to be a tumorous thyroid gland. No abnormality of the right thyroid gland was palpable. Further clinical examination of the mare revealed no other abnormalities. Hematological values for hemoglobin, packed cell volume, leucocyte count and differentiation, and serum protein, calcium, magnesium and phosphate were normal. Removal of the tissue (i.e., left thyroid gland) was decided upon. Surgery was carried out with the mare in right lateral recumbency under general anesthesia. The thyroid gland was dissected bluntly from the surrounding connective tissue and removed. Post-operatively no complications were observed. Six months later the owner reported the mare in good health and performing normally. There had been no recurrence of the paralaryngeal swelling to date.Some fragments of the enlarged thyroid lobe were available for post-mortem examination. Tissues were fixed in 4% buffered formalin and routinely processed. Paraffin-embedded tissues were sectioned at 5 pm and stained with hematoxylin and eosin (HE). Additional stains used on selected sections included periodic acid-Schiff (PAS), Congo red, and the Grimelius' silver nitrate proce...

Was studied in vitro. The(109)Cd-labeled protein was isolated by gel filtration and incubated with a lysosomal extract from the same source. No degradation was seen when the pH of the incubation medium was 5 or higher, whereas the degradation of Cd-thionein was completed in 2 h at a pH of 4.5. Dissociation of Cd(2+) from thionein, which takes place to a fair extent at a pH below 5, appears necessary before the enzymatic breakdown of the thionein moiety can take place.