SCANNING AND TRANSMISSION ELECTRON-MICROSCOPIC STUDY ON A MULTILAYERED BASEMENT-MEMBRANE IN THE PINEAL ORGAN OF THE AYU PLECOGLOSSUS-ALTIVELIS

The pineal organ possesses highly fenestrated capillaries, and is devo id of the so-called blood-brain barrier. The present study indicated t hat the pineal epithelium of the teleost fish, ayu Plecoglossus altive lis, possesses an unusually thick and convoluted basement membrane (2. 2-2.4 mu m in width) which is visible even under the light microscope. This pineal basement membrane was observed by scanning and transmissi on electron microscopy and its detailed composition and relationships with the fenestrated capillaries and the perivascular space were inves tigated. As the basement membrane was composed of three to eight layer s of basal laminae interspersed with laminae lucidae, we termed it the ''multilayered basement membrane''. In consideration of our previous demonstration that macromolecules such as HRP are trapped by the basem ent membrane, it is suggested that this multilayered basement membrane may prevent foreign substances from reaching the pineal epithelium.