Interlamellar tight junctions of central myelin. II. A freeze fracture and cytochemical study on their arrangement and composition.

The interlamellar tight junctions (ITJ) of central myelin (white matter from the parietal lobe and the medulla oblongata of the rat) were analyzed electron microscopically, making use of a wide range of different preparatory techniques. Freeze-fracture observations indicate that the ITJ are composed of rows of particulate subunits in glutaraldehyde-fixed or formaldehyde-fixed material, and in the unfixed state. The particulate subunits of the ITJ are preferentially associated with the protoplasmic (P) face in the aldehyde-fixed state, and no shift in the binding characteristics of the particles was observed after omission of aldehyde fixation. Tracer studies in conjunction with the dissociated appearance of the junctional globules suggest that the ITJ represent a leaky type of zonula occludens. It is assumed that the ITJ particles represent an "integral-type protein" that preferentially serves as a mechanical device maintaining the structural integrity of the central myelin sheath. By means of cytochemical experiments, the proteinaceous character of the ITJ subunits is established. An attempt is made, based on results from lipid extraction and protein digestion, to define certain cytochemical parameters of the ITJ proteins and to compare them with the current collection of chemically identified proteins of central myelin.