In early development of the rat mRNA for the major myelin protein P₀ is expressed in nonsensory areas of the embryonic inner ear, notochord, enteric nervous system, and olfactory ensheathing cells

The myelin protein P₀ has a major structural role in Schwann cell myelin, and the expression of P₀ protein and mRNA in the Schwann cell lineage has been extensively documented. We show here, using in situ hybridization, that the P₀ gene is also activated in a number of other tissues during embryonic development. P₀ mRNA is first detectable in 10-day-old embryos (E10) and is at this time seen only in cells in the cephalic neural crest and in the otic placode/pit. P₀ expression continues in the otic vesicle and at E12 P₀ expression in this structure largely overlaps with expression of another myelin gene, proteolipid protein. In the developing ear at E14, P₀ expression is complementary to expression of serrate and c-ret mRNAs, which later are expressed in sensory areas of the inner ear, while expression of bone morphogenetic protein (BMP)-4 and P₀, though largely complementary, shows small areas of overlap. P₀ mRNA and protein are detectable in the notochord from E10 to at least E13. In addition to P₀ expression in a subpopulation of trunk crest cells at E11/E12 and in Schwann cell precursors thereafter, P₀ mRNA is also present transiently in a subpopulation of cells migrating in the enteric neural crest pathway, but is down-regulated in these cells at E14 and thereafter. P₀ is also detected in the placode-derived olfactory ensheathing cells from E13 and is maintained in the adult. No signal is seen in cells in the melanocyte migration pathway or in TUJ1 positive neuronal cells in tissue sections. The activation of the P₀ gene in specific tissues outside the nervous system was unexpected. It remains to be determined whether this is functionally significant, or whether it is an evolutionary relic, perhaps reflecting ancestral use of P₀ as an adhesion molecule.