A light and electron microscopical study of intranuclear cytoplasmic invaginations in interstitial cell tumours of dogs

Abstract

Intranuclear cytoplasmic invaginations, so-called “pseudoinclusions”, were seen in all 7 interstitial cell tumours identified among 32 consecutive canine testicular tumours examined. The proportion of cells with intranuclear cytoplasmic invaginations varied from less than 1 per cent to 15 per cent. Similar “pseudoinclusions” were not seen in any other canine testicular tumours examined; this finding might be helpful in differential diagnosis, especially in less differentiated tumours. Histologically, the cytoplasmic invaginations appeared as round, eosinophilic, clearly demarcated intranuclear inclusions and stained with periodic acid-Schiff. Affected nuclei were enlarged. The process of progressive cytoplasmic invaginations into the nucleus and replacement of most of the nuclear volume of the neoplastic interstitial cells with cytoplasmic material was examined by transmission electron microscopy. Two or more cytoplasmic invaginations were sometimes present within a single nucleus. The nuclear membrane could be followed continuously around the cytoplasmic invaginations and nuclear pores were present in the membrane covering invaginations. The invaginations contained smooth and rough endoplasmic reticulum, vesicles, lipid vacuoles, myelin figures and disrupted membranous profiles. Bundles of interwoven cytoplasmic filaments were among the organelles seen in the invaginations in one tumour. Possible reasons for the formation of intranuclear cytoplasmic inclusions are discussed and maintenance of the normal ratio of nuclear surface to nuclear volume is suggested as the mechanism promoting formation of inclusions in interstitial cell tumours.