1985 Volume 25 Issue 5 Pages 387-392
The capsule of a retrocerebellar glio-ependymal cyst, which was incidentally found in a 54-year-old asymptomatic female, was studied by both light and electron microscopy.
Microscopically, the capsule measured 25-400 μm in thickness and consisted of four layers. The external covering was an arachnoid cell layer abutting on the thick connective tissue with scattered vessels, whereas the luminal lining was a layer of ependymal cells abutting on the glial tissue. The ependymal cells were cuboidal in shape and some, but not all, were ciliated. The glial tissue underneath the ependymal lining contained numerous corpora amylacea, especially in its abluminal side. Electronmicroscopically, the arachnoid cell layer consisted of both arachnoid epithelium and the underlying arachnoid cells. The former showed a relatively higher electron density than the latter. There was a distinct basement membrane between them. In the former, a moderate number of desmosomes and intermediate junctions assured contacts between the arachnoid cells. In the latter, there were many extracellular clefts containing numerous collagen fibrils and microfibrils. Neighboring cytoplasmic processes often formed interdigitations. The luminal surface of ependymal cells displayed variable numbers of microvilli with or without cilia, whereas the abluminal surface abutted directly on the astrocytic processes of the glial layer. There were some interdigitations of the plasmalemma of adjacent cells, with surface specializations consisting of fasciae adherentes joined by gap junctions. Two or more adjacent cells contributed to the development of microrosettes, packed with both numerous surface microvilli and several profiles of cilia. The corpora amylacea varied 5-15 μm in diameter and consisted of randomly oriented filaments with fine osmiophilic granules. The fibrillar elements within the processes of the fibrillary astrocytes swept around the corpora amylacea or impinged upon the latter. Although their outline was usually smooth and clearly demarcated, no membrane or space intervened between the corpora amylacea and the surrounding structures. It is most probable that the present glio-ependymal cyst arose from the displaced segment of the wall of the neural tube, which corresponds to the sites from which the tela chorioidea forms.
