EXTRAMEDULLARY HEMATOPOIESIS IN THE CHOROID-PLEXUS OF 5 DOGS

Five dogs euthanatized because of refractory seizures were found to ha ve hematopoietic elements in the interstitium of the choroid plexus at the level of the fourth ventricle. None of the dogs had significant h ematologic or cerebrospinal fluid abnormalities. The extramedullary he matopoiesis was confined to the central nervous system and consisted o f megakaryocytes, immature granulocytes, and rubricytes in two dogs an d of one predominant cell population in each of the other three dogs. These findings are unique, and factors possibly contributing to the fo rmation of a hematopoietic inductive microenvironment in the choroid p lexus are cytokine-neurokine homologies, locally altered vascular supp ly, and aberrant functioning of bone marrow-derived central nervous sy stem macrophages.