Receptor tyrosine kinases are selectively expressed at the nodal regions of myelinating Schwann cells. A, Low-magnification image of erbB2 expression in rat sciatic nerves. Teased preparations of rat sciatic nerve were immunostained with three different antibody preparations targeted to erbB2 (see Materials and Methods). All three antibodies decorate the nodal regions of the axons. In occasional cells, some perinuclear staining is also seen (*). Scale bar, 10 μm. B, Nodal staining pattern reflects erbB2 in Schwann cells. Deconvolved images of sequential 0.4 μm optical sections (as numbered from top left) of a nodal region of a teased nerve immunostained with antibodies to sodium channel (green) and erbB2 (red). The erbB2 immunofluorescence surrounds the sodium channel, indicating that erbB2 is localized in the nodal region of the myelinating Schwann cell. Scale bar, 5 μm. C, Within Schwann cells, erbB2 is localized to the nodes rather than the paranodes. A teased nerve preparation immunostained with antibodies to erbB2 and Caspr. Expression of erbB2 is flanked by Caspr expression in the paranodes. D, Within the nodes, erbB2 is localized to the microvilli of myelinating Schwann cells. A frozen section of rat sciatic nerve was immunostained with antibodies to erbB2 (red) and Ezrin (green). Expression of erbB2 colocalizes with that of Ezrin, indicating that erbB2 protein is expressed on microvilli of Schwann cells. E, The PDGFβR is also localized to the microvilli [as in D except antibodies were to Ezrin (red) and PDGFβR (green)]. Scale bars: C-E, 15 μm. F, A nutrient transport receptor (transferrin) is targeted to the perinuclear regions of myelinating Schwann cells [as in D and E except that antibodies are directed to the transferrin receptor (green) or erbB2 (red)]. A nucleus of myelinating Schwann cells are indicated by DAPI staining (blue). G, Diffuse expression of erbB2 in nonmyelinating Schwann cells. Frozen sections of rat sciatic nerve were immunostained with antibody against GFAP (green) to visualize nonmyelinating Schwann cells. In GFAP-positive nonmyelinating Schwann cells, erbB2 expression (red) is distributed along the cytoplasm. Nodal expression of erbB2 in myelinating Schwann cells is also shown (arrows). Scale bars: F, G, 30 μm