A comparison of cerebrosides, proteolipid proteins, and cholesterol as indices of myelin in the architecture of rat cerebrum

Abstract— —Serial frozen section sampling and microspectrophotometric assays were used to compare the fidelity of cerebrosides, proteolipid proteins (PLP) and cholesterol as indices of myelinated fibres in the somatosensory region of rat cerebrum. Cerebrosides (including sulphatides) were a satisfactory index regardless of the mode of expression. Per unit dry wt., they were lowest in layer II (4 per cent) and increased with the subpial depth to 16 percent in white matter. Elevations were seen at the locations of: the outer tangential plexus (1); the stripe of Kaes-Bechterew (2); the outer and inner stripes of Baillarger (3,4); and the inner tangential plexus (5), respectively. Expression in terms of residue protein, total lipid or DNA gave somewhat sharper profiles of the myeloarchitecture. PLP per unit dry wt. were lowest in layers I, II and III (2 per cent) and increased to 5·5 per cent in white matter. Peaks occurred near (3), (4) and (5). The PLP per cell and per unit residue protein rose six-fold, from the upper layers of cortex to white matter, and depicted the myeloarchitecture more faithfully. In general outline, PLP paralleled cerebrosides in distribution, in contrast with previous findings in human prefrontal cortex. Human cortex may contain more non-myelin PLP in the upper cortical layers owing to its greater wealth of neuropil and synapses. Cholesterol per unit dry weight was a poor index of the myeloarchitecture; it was lowest in layer II (5 per cent) and increased to 13-5 per cent in white matter without revealing the five horizontal bands of myelinated fibres. Expression of cholesterol in terms of total lipid or DNA improved its value as an index, but it was less satisfactory than cerebrosides.