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 devoid of the so-called blood-brain barrier. The present study indicated that the pineal epithelium of the teleost fish, ayu Plecoglossus altivelis, possesses an unusually thick and convoluted basement membrane (2.2-2.4 microm in width) which is visible even under the light microscope. This pineal basement membrane was observed by scanning and transmission electron microscopy and its detailed composition and relationships with the fenestrated capillaries and the perivascular space were investigated. As the basement membrane was composed of three to eight layers 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 basement membrane, it is suggested that this multilayered basement membrane may prevent foreign substances from reaching the pineal epithelium.