Electron microscopy of healthy nuclei pulposi indicates that the intercellular matrix should be regarded as a three-dimensional lattice gel system, containing a dense network of poorly differentiated collagenous fibrils and an amorphous interfibrillar substance. During aging, this system becomes subject to an irregular disappearance of the amorphous mucoid material, whereby the fibrils become partly unmasked. The mucoid interfibrillar material is probably responsible for the high water content and water-binding capacity.