Fig. 2. Immunofluorescence images of developing vibrissae (v), vibrissa follicles (f), nerve bundles (large arrows) and individual Merkel-related axons (small arrows) in rat mystacial pads on E15.5 and the caudal region of rat mystacial pads on E19.5. White arrowheads indicate labeled Merkel endings and cells. Open arrowheads indicate just labeled Merkel endings. Inserts in A,C,E,G show Merkel endings at twice the magnification as shown in white rectangles. e, epidermis. Asterisks indicate sites of non-neuronal labeling. (A,B) Development proceeds along a caudal-to-rostral gradient (right to left). Developing axons completely engulf more immature rostral follicles but caudally have become restricted to the mid-follicle level. Merkel innervation begins to penetrate caudal follicles around E14.5 (not shown). Anti-PGP labels the Merkel endings and begins to label Merkel cells a day or two later. Labeled Merkel endings and cells are shown on E19.5. (C,D) Anti-trkA labeling is clear on axons at E15.5 and is faint on axons and endings by E19.5. By comparison, axons (curved arrow) ascending to innervate the epidermis are more intensely labeled. Labeling at E19.5 has been digitally enhanced in comparison with that at E15.5. TrkA is not clearly expressed on Merkel cells. (E,F) Anti-trkC immunoreactivity is intensely expressed on the developing follicle cells as well as on axons and Merkel endings at E15.5 and is present at low levels on dermal cells immediately surrounding the follicles. Follicle labeling rapidly diminishes over the next day and is lacking on E19.5, while trkC is still evident on axons and Merkel endings. (G,H) Anti-p75 labels axons and Schwann cells within the growing nerves and is present on individual axons at the level where they form Merkel endings. On E15.5 intense p75 immunoreactivity is expressed on dermal cells completely surrounding the follicles. On E19.5, anti-p75 labeling is present on Merkel endings and cells as well as other cells both within and adjacent to the follicles. Scale bar: 50 µm.