Figure 1.
Key structural elements of primary endings in muscle spindles and lanceolate endings on hair follicles.
Schematic diagrams of a muscle spindle primary ending, annulo-spiral in form (A, C) and of a lanceolate ending forming a palisade-like structure around a hair follicle (E, K). Representative planes of section of subsequent figures and of the remaining parts of this figure are shown on the diagrams. Immunoreactivity for synaptophysin is apparently restricted to the sensory terminals of both spindle primary (B) and lanceolate (arrows in G) endings. In the muscle spindle, the very large sensory terminals (arrowheads in D) are wound around specialised intrafusal muscle fibres. Surrounding, but not in contact with the sensory ending are inner capsule cells (arrows in D). In the lanceolate ending, the much smaller individual terminals (arrowheads in L) are sandwiched closely between processes of accessory glial cells (asterisks in L). The glial cells, both processes (arrows in H) and cell bodies (arrowheads in H) are immunoreactive for S100. Glial cell processes and lanceolate terminals are so small that in longitudinal section anti-synaptophysin and anti-S100 antibodies can appear to colocalise in a single confocal plane (G and H merged in F). Imaging techniques: B, single plane confocal laser scanning image, scale 50 µm; D, transmission electron micrograph, scale 10 µm; F–H, single plane confocal laser scanning images, scale 20 µm; L, transmission electron micrograph, scale 1 µm.
Table 1.
Colocalisation analysis using Manders’ coefficients for double-labelled immunofluorescence in muscle spindles and lanceolate endings.
Figure 2.
Evidence for the expression of SK2 in muscle spindle sensory endings.
Muscle-spindle sensory endings double labelled with anti-synaptophysin (SYN, green channel) and anti-SK2 (red channel) antibodies. In one ending the green and red channels are shown separately (A and B), as well as merged (C). In a second ending only the merged channels are shown (D). Partial colocalisation of SYN and SK2 immunoreactivity is evident in the sensory terminals (arrowheads), and in what are probably unmyelinated preterminal branches of the parent sensory nerve fibre (arrows). SK2 immunoreactivity can also be detected in inner capsule cells (ic). Scale = 10 µm.
Figure 3.
Evidence for the expression of SK2 in sensory terminals and glial cells of lanceolate endings.
Lanceolate endings double labelled with anti-synaptophysin (SYN, green channel) and anti-SK2 (red channel) antibodies (A–C) and with anti-S100 (green channel) and anti-SK2 (red channel) antibodies (D–F). In each case the green (A, D) and red (B, E) channels are shown separately, as well as merged (C, F). Partial colocalisation of both SYN and S100 with SK2 immunoreactivity indicates that SK2 is present in sensory terminals and satellite glial cells (SGCs). Scale (in A, D) = 10 µm.
Figure 4.
Further evidence for the expression of SK2 in lanceolate sensory terminals and glial cells.
An oblique section of a lanceolate ending double labelled with anti-synaptophysin (SYN, green channel) and anti-SK2 (red channel) antibodies in a merged image. Note especially the satellite glial cell (SGC) body labelled only with anti-SK2 antibody (arrow), and the double labelled branched axon and terminals (arrowheads), confirming the expression of SK2 in both sensory terminals and SGCs. Scale = 10 µm.
Figure 5.
SK2 may be expressed in circumferential as well as lanceolate endings.
A lanceolate ending double labelled with anti-S100 (green channel) and anti-SK2 (red channel) antibodies in a merged image. The structure indicated by an arrow may be part of a circumferential ending. Scale = 5 µm.
Figure 6.
Evidence for the expression of SK3 in satellite glial cells of lanceolate endings.
Lanceolate endings double labelled with anti-synaptophysin (SYN, green channel) and anti-SK3 (red channel) antibodies (A–C) and with anti-S100 (green channel) and anti-SK3 (red channel) antibodies (D–F). In each case the green (A, D) and red (B, E) channels are shown separately, as well as merged (C, F). Partial colocalisation of S100 with SK3 immunoreactivity, but only close association of SYN with SK3 antibodies, indicates that SK3 is present in satellite glial cells (SGCs), especially where they adjoin sensory terminals. Scale (in A, D) = 10 µm.
Figure 7.
SK3 may be absent from some satellite glial cells in lanceolate endings.
Lanceolate ending double labelled with anti-S100 (green channel) and anti-SK3 (red channel) antibodies shown separately and merged (A–C, respectively). SK3 immunoreactivity is clearly present in some satellite glial cells (SGC)s and their processes (arrows), whereas it is not apparent in others (arrowheads). Scale (in A) = 10 µm.
Figure 8.
Partial colocalisation of ASIC2 and S100 reflects close association between sensory terminals and glial cells.
Lanceolate endings double labelled with anti-synaptophysin (SYN, green channel) and anti-ASIC2 (red channel) antibodies (A–C) and with anti-S100 (green channel) and anti-ASIC2 (red channel) antibodies (D–F). In each case the green (A, D) and red (B, E) channels are shown separately, as well as merged (C, F). Since ASIC2 is not expressed in satellite glial cell (SGC) bodies, partial colocalisation, or close association, of both SYN and S100 with ASIC2 immunoreactivity indicates that ASIC2 is present in sensory terminals at, or close to their interface with SGCs. Scale (in A, D) = 10 µm.
Figure 9.
Example of SK1 expression in unidentified structure close to lanceolate ending.
Lanceolate ending double labelled with anti-S100 (green channel) and anti-SK1 (red channel) antibodies. Immunoreactivity to S100 is present in processes of satellite glial cells (SGCs), and to SK1 in unidentified structures. Although the latter resemble axons it seems unlikely that is what they are, as there are no associated glial cell processes close to them. Scale = 10 µm.