A comparative study of the immunohistochemical localization of basic protein to myelin and oligodendrocytes in rat and chicken brain

Abstract

Antisera to highly purified basic protein (BP) from rat and chicken brain were prepared and their purity and specificity demonstrated by double immunodiffusion and cross-immunoadsorption. These antisera were used for immunohistochemical localization of BP in the brains of adult and developing rat and chick. Myelin basic protein was exclusively localized to myelin or the myelin forming elements of the CNS. It was present in high concentrations in white matter and absent in areas free of myelin. Neuronal parikarya and dendrites were negative as were axons cut in cross section and at Nodes of Ranvier. The latter was best observed in cross sections of human spinal cord demonstrating also the immunoreactivity of the antibodies with human BP. The internodal distance in a fine (1.5 micrometer) rat cortical fiber was determined to be approximately 45 micrometers. Myelin basic protein was shown to extend into cranial roots, in contrast to myelin proteolipid protein which abruptly lose fluorescence as the nerves emerged from the brain. During development, BP was first observed on the fourteenth day of incubation in chick and at birth in the rat. The protein appeared in oligodendrocytes and in association with fibers near these cells. Fluorescent processes were frequently observed connecting the oligodendrocytes with the fibers. As myelination progressed, the intensity of the immunohistochemical reaction decreased in the oligodendrocytes while the brightness in fibers increase. Eventually, the oligodendrocytes became undetectable. Fibers with immature myelin exhibited a beaded or varicosed appearance with the highest concentration of immunofluorescence in the outer portion of the varicosities. The varicosities were postulated to represent dilations in the newly forming sheath between intervals of compaction along the axon undergoin myelination. These dilations might represent areas of increased cytoplasmic volume which could serve as channels for transport and/or storage sites for myelin proteins prior to incorporation into the membrane. The varicosities became less prominent with the thickening of the myelin sheath and mature myelinated fibers became smooth. The process of synthesis of BP, transport of the protein to the varicosed fibers, and maturation of the myelin sheath was seen to progress in a more or less caudal to rostral direction as myelination of the CNS takes place. In the rat, this was accomplished over approximately a 30-day period starting near the time of birth. In the chick, most of the myelination was accomplished in the three or four days immediately before hatching. At this time, innumerable oligodendrocytes were observed producing BP simultaneously in the major white fiber tracts. It is postulated that in chick some degree of oligodendrocytic cell death occurs normally during myelination.

Developmental regulation of myelin basic protein in dispersed cultures

“…During myelin formation in vivo MBP gradually disappears from the perikarya of the oligodendrocytes as the myelin sheath is formed (5,6) and accumulates in the myelin membrane. In contrast, in culture the MBP remained distributed throughout the oligodendrocytes for up to 60 days.…”

“…in. culture an oligodendrocyte accumulates MBP at the rate of 0.2 fmol/day, and therefore acquires its full complement of MBP (1 fmol per cell) inless than 5 days. This suggests that the accumulation of MBP'in the culture between 20 and35 days is due primarily to an increase in the number of oligodendrocytes containing MBP rather than to continued increases in the amount of MBP per cell.…”

“…In rodents no MBP can be found at birth, but MBP accumulates rapidly between 10 and 30 days after birth. MBP is found in both the cell body and processes of the oligodendrocytes prior to myelination, but upon myelination MBP is rapidly incorporated into the myelin membrane and disappears from the perikaryon of the cell (5,6). There are four forms of MBP in rodent myelin with molecular weights of 21,500, 18,500, 17,000, and 14,000 (7); they are designated 21.5 kilodalton (kDal), 18.5 kDal, 17 kDal, and 14 kDal, respectively.…”

Even in culture, oligodendrocytes myelinate solely axons.

“…1A) but failed to consistently demonstrate the presence of a myelin sheath (data not shown). In contrast, anti-MBP clearly immunolabeled myelin sheaths but poorly stained the oligodendrocyte cell bodies, because MBP migrates out of the cell bodies of the myelinating oligodendrocytes (17,18). Preparations were therefore double-labeled with both anti-GalC and anti-MBP.…”

The effects of electromagnetic fields to whole protein, myelin basic protein, neuron specific enolase profiles and nitric oxide levels in rat brains

“…Myelin basic protein (MBP) is localized in myelin of central nervous system (CNS), in the elements that produce myelin, and in oligodentrocytes and plays a role in the process of myelinization as a structural element (6).…”