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P2, P1, and P0 myelin protein expression in developing rat sixth nerve: A quantitative immunocytochemical study

Myelination and the expression of myelin proteins P2, P1, and P0 were studied quantitatively in the rat sixth cranial nerve during development. The postnatal development and growth of all myelin sheaths in this nerve have been studied morphometrically in a companion paper. Epon‐embedded blocks with closely matched topography in the transverse plane were selected from rats perfused at ages 1–4, 8, 15, and 20 days. From each block, serial semithin sections were cut, etched, and immunostained according to the peroxidase‐antiperoxidase method with well‐characterized polyclonal antisera that reacted specifically with P0 glycoprotein and the basic proteins P1 and P2. The immunoreactivities of individual myelin sheaths were measured by densitometry. Numbers of compact myelin lamellae, myelin spiral lengths, and axon diameters were determined on electronmicrographs of adjacent thin sections. At birth anti‐ P0 immunoreactivity was found on sheaths with two and more compact lamellae; neither P1 nor P2 immunoreactivity was observed. On day 2, myelin sheaths with five and eight lamellae were stained respectively by anti‐ P1 and anti‐ P2. On day 3 the percentages of myelin sheaths stained were substantially higher: P0 95%, P1 78%, P2 15%. By day 4, anti‐ P0 and anti‐ P1 immunoreactivity was present in 95% of myelin sheaths; 35% were stained by anti‐ P2. For P2, staining intensity and percentage of myelin sheaths stained continued to increase and by day 20, 85% were anti‐ P2‐positive.