In early development of the rat mRNA for the major myelin protein P-0 is expressed in nonsensory areas of the embryonic inner ear, notochord, entericnervous system, and olfactory ensheathing cells

The myelin protein P-0 has a major structural role in Schwann cell myelin, and the expression of P-0 protein and mRNA in the Schwann cell lineage has been extensively documented. We show here, using in situ hybridization, tha t the P-0 gene is also activated in a number of other tissues during embryo nic development. P-0 mRNA is first detectable in 10-day-old embryos (E10) a nd is at this time seen only in cells in the cephalic neural crest and in t he otic placode/pit. P-0 expression continues in the otic vesicle and at E1 2 P-0 expression in this structure largely overlaps with expression of anot her myelin gene, proteolipid protein. In the developing ear at E14, P-0 exp ression is complementary to expression of serrate and c-ret mRNAs, which la ter are expressed in sensory areas of the inner ear, while expression of bo ne morphogenetic protein (BMP)-4 and P-0, though largely complementary, sho ws small areas of overlap. P-0 mRNA and protein are detectable in the notoc hord from E10 to at least E13. In addition to P-0 expression in a subpopula tion of trunk crest cells at E11/E12 and in Schwann cell precursors thereaf ter, P-0 mRNA is also present transiently in a subpopulation of cells migra ting in the enteric neural crest pathway, but is down-regulated in these ce lls at E14 and thereafter. Po is also detected in the placode-derived olfac tory en. sheathing cells from E13 and is maintained in the adult. No signal is seen in cells in the melanocyte migration pathway or in TUJ1 positive n euronal cells in tissue sections. The activation of the P-0 gene in specifi c tissues outside the nervous system was unexpected. It remains to be deter mined whether this is functionally significant, or whether it is an evoluti onary relic, perhaps reflecting ancestral use of P-0 as an adhesion molecul e. (C) 2001 Wiley-Liss, Inc.