ON THE OCCURRENCE OF CYSTS AT THE CRANIAL PART OF THYMUS IN PIGS

Specimens of cysts, located at the cranial part of thymus in 14 pigs w ere investigated by dissection, light and electron microscopy to deter mine their origin. The pigs were of different age and sex. Most of the m were Edelschwein/Pietrain hybrids, two were Edelschwein and one was a Pietrain. Similar cysts were observed in 4 % of necropsies in the ye ars 1992 and 1993. The cysts were located medial to the mandibular gla nd and the hypoglossal nerve, varied in size and were filled with clea r, aqueous fluid. Cytological examination of smears of this fluid was of no diagnostic value. Light microscopic examination of serial transv erse sections showed an epithelial lining of the cystic cavity. Theref ore, a salivary origin was excluded. Electron microscopic examination of the lining cells showed a diverse population of cells. Some possess ed secretory granules, some exhibited an apical ciliated border while others were poorly differentiated. Because of the similarity of these cells with those of thymic cysts in other animal species it was obviou s to classify them as thymic cysts. This is supported by the fact that both third pharyngeal pouch and ectodermal precervical duct contribut e to development of cranial part of thymus in pigs. In this area, wher e both germ layers are fusing, a considerable cavity was described alr eady in embryos. Moreover, it is possible that this is a vulnerable sp ot during development of thymus.