HETEROTOPIC GLIAL NESTS IN THE SUBARACHNOID SPACE: HISTOPATHOLOGIC CHAR11CTERISTICS, MODE OF ORIGIN AND RELATION TO MENINGEAI.J GI.JIOl\1AS*

The occurrence, in the subarachnoid space, of heterotopic nests composed of neuroglial tissue has been recognized since the original report, in 1907, of a case by Wolbach (1) whose description of heterotopic glial nests was substantiated by O. T. Bailey (2) in 1936. The essential anatomic features of the discrete structures reported by these authors are: 1) they are composed of mature astrocytes whose nuclei tend to be grouped in the center of the nest, 2) they are, invested by a delicate connective tissue sheath, 3) the circumferential neuroglia! fibers are arranged in an arched manner similar to the marginal glia of the spinal cord, and 4) they may contain ependymal-lined canals. The origin of these heterotopic glial nests has generally been ascribed to embryonic neurogenic elements which mature after wandering into the subarachnoid space, this hypothesis having been supported by Wolbach (1), O. T. Bailey (2) and P. Bailey and Bucy (3). Oberling (4), however, has suggested that glial tissue in the meninges is derived from meningoblasts which are capable of differentiation into glial elements. These hypotheses have not adequately accounted for the presence of a sheath about the neuroglial nests, the central distribution of nuclei within the nests, nor the architectural arrangement of glial fibers. Gliomatous tumors in the meninges have been reported by many investigators (2, 4-8). O. T. Bailey has said that primary gliomas of the meninges make up a definite group of meningeal tumors, and has postulated that the neoplasms arise from heterotopic glial tissue in the subarachnoid space (2). This explanation for the origin of primary extramedullary gliomas, without any apparent attachment to the brain or spinal cord, has been supported by reports of other investigators (5, 8~10).