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The amount of myelin basic protein (MBP) was quantified in human fetal spinal cords from 12 to 24 gestational weeks (GW). MBP expression was determined by Northern blot, quantitative immunoblot, and immunocytochemistry. The development of compact myelin was analyzed by electron microscopy. Thirty-eight human fetal spinal cords were obtained after elective termination of intrauterine pregnancies from healthy women. Northern blot analysis showed a 15.8-fold increase in MBP mRNA between 12 and 18 GW. From 18 to 24 GW, MBP mRNA increased by 2.2-fold. The mRNA data paralleled immunoblot results that showed a 90.5-fold increase in MBP (0.147 ng/mg to 13.3 ng/mg tissue) between 12 and 18 GW and an approximately 11.5-fold increase between 18 and 24 GW (13.3 ng/mg to 154 ng/mg tissue). Immunocytochemical analysis also showed increased staining for MBP with advancing gestational age. At 12 GW, MBP immunoreactivity was observed in all three spinal cord funiculi. By 18 GW, MBP was expressed throughout the spinal cord white matter with the exception of the lateral corticospinal tracts and in the rostral levels of the fasciculus gracilis. With respect to myelin, at 12 GW, rare, noncompacted myelin lamellae were observed by electron microscopy. By 18 GW, discrete areas of compact myelin were observed in areas that showed MBP immunoreactivity, and at 24 GW, compact myelin was prominent throughout the white matter of the spinal cord. This study demonstrates a quantitative increase in MBP expression that is associated with myelin formation during the second trimester of human gestation. This information may provide normative data that can aid in the diagnosis of myelin disorders of the preterm, neonatal, and pediatric spinal cord. |
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