An atypical spinal meningioma in a dog.

proliferating component of this tumor type, whereas the vac-uolated cells are believed to be the older and degenerating cell population derived from the stellate cells. This interpretation is supported by our study which revealed the ultrastructural features of an actively synthesizing cell in the stellate cells, while the cells with large cytoplasmic vacuoles had lost most of these features. The nature of the material produced in the stellate cells is not clear. The fine structure of the rough endoplasmic retic-ulum together with the abundance of fibrils in the matrix suggest that primitive cartilaginous components are being synthesized. This interpretation is supported by recent biochemical studies which have identified the presence of chon-droitin 4-and 6-sulfate. keratan sulfate and hyaluronate in a proportion similar to that found in immature intervertebral discs.' Meningiomas are neoplasms derived from the dura mater, arachnoid, or pia mater and characterized morphologically by the presence of a whorling or twisting pattern. The tumor is composed of various populations of two cell types: the epithelial-like meningothelium and fibroblasts. Previous reports in the dog have described the epithelioid form in terms of the following types: endotheliomatous," syncytia1,'-" psammomatous'. I' and tran~itional.~.~.~ Tumors which primarily exhibit a fibroblastic appearance also have been described .x.'Z as have cases with mixtures of the above types-these are referred to only as rneningioma.'.'".'' In man, up to nine morphologically distinct types have been described with approximately 22 variant forms.' The vaned appearance has been attributed to the mesodermal origin of the meningothe-lial cell. This report describes the cytologic and histologic appearance of a meningeal tumor without significant whorling patterns. In addition, the presence of a homogeneous population of histiocyte-like cells was recognized as the prominent feature of this neoplasm. A seven-year-old female mixed breed dog was presented with a two-year history of neck pain and front leg paresis. During this time, temporary improvement resulted from dex-amethasone therapy. On its last presentation to the clinic, the dog was nonambulatory in all four limbs but voluntary movements were retained. All reflexes appeared to be of an upper motor neuron nature. Myelograms were taken twice during this time and revealed a slowly progressive extradural mass on the right side of the C34 vertebral space. Exploratory surgery revealed a large discrete reddish-gray, gritty mass, approximately 1 x 1.5 x 0.5 cm, compressing the spinal cord from right to left. The mass was extradural with expansive compression of the spinal …