Spontaneous Ectopic Choroid Plexus with Sclerosis in Adult Beagle Dogs.

Microscopic examination of the brain of adult Beagle dogs from four different general toxicity studies revealed the presence of ectopic choroid plexus tissue in six individual dogs (4 females and 2 males) with ages ranging from 12 to 18 months. In each dog, this finding was characterized by a well-circumscribed mass localized to a region above and along the corpus callosum without any apparent compression of adjacent brain tissue. Each mass was composed of columnar ependymal cells forming tubular structures surrounded by variable amounts of fibrovascular connective tissue and had the appearance of small rests of ependymal cells that had been penetrated by the leptomeninges during neural development. There were no associated clinical signs or macroscopic correlates. Based on morphologic appearance, a diagnosis of spontaneous ectopic choroid plexus with secondary sclerosis was made. To the authors' knowledge, ectopic choroid plexus has not been reported in Beagle dogs and is rare in humans and horses.