Periarticular Proprioception: Analyzing the Three-Dimensional Structure of Corpuscular Mechanosensors in the Dorsal Part of the Scapholunate Ligament

Abstract

Introduction: Sensory nerve endings transmit mechanical stimuli into afferent neural signals and form the basis of proprioception, giving rise to the self-perception of dynamic stability of joints. We aimed to analyze the three-dimensional structure of periarticular corpuscular sensory nerve endings in a carpal ligament to enhance our understanding of their microstructure.

Methods: Two dorsal parts of the scapholunate ligament were excised from two human cadaveric wrist specimens. Consecutive cryosections were stained with immunofluorescence markers protein S100B, neurotrophin receptor p75, protein gene product 9.5 (PGP 9.5), and 4',6-diamidino-2-phenylindole. Three-dimensional images of sensory nerve endings were obtained using confocal laser scanning microscopy, and subsequent analysis was performed using Imaris software.

Results: Ruffini endings were characterized by a PGP 9.5-positive central axon, with a median diameter of 4.63 μm and a median of 25 cells. The p75-positive capsule had a range in thickness of 0.94 μm and 15.5 μm, consisting of single to three layers of lamellar cells. Ruffini endings were significantly smaller in volume than Pacini corpuscles or Golgi-like endings. The latter contained a median of three intracorpuscular structures. Ruffini endings and Golgi-like endings presented a similar structural composition of their capsule and subscapular space. The central axon of Pacini corpuscles was surrounded by S100-positive cells forming the inner core which was significantly smaller than the outer core, which was immunoreactive for p75 and PGP 9.5.

Conclusion: This study reports new data regarding the intricate outer and intracorpuscular three-dimensional morphology of periarticular sensory nerve endings, including the volume, number of cells, and structural composition. These results may form a basis to differ between normal and pathological morphological changes in periarticular sensory nerve endings in future studies.

Introduction: Sensory nerve endings transmit mechanical stimuli into afferent neural signals and form the basis of proprioception, giving rise to the self-perception of dynamic stability of joints. We aimed to analyze the three-dimensional structure of periarticular corpuscular sensory nerve endings in a carpal ligament to enhance our understanding of their microstructure.

Methods: Two dorsal parts of the scapholunate ligament were excised from two human cadaveric wrist specimens. Consecutive cryosections were stained with immunofluorescence markers protein S100B, neurotrophin receptor p75, protein gene product 9.5 (PGP 9.5), and 4',6-diamidino-2-phenylindole. Three-dimensional images of sensory nerve endings were obtained using confocal laser scanning microscopy, and subsequent analysis was performed using Imaris software.

Results: Ruffini endings were characterized by a PGP 9.5-positive central axon, with a median diameter of 4.63 μm and a median of 25 cells. The p75-positive capsule had a range in thickness of 0.94 μm and 15.5 μm, consisting of single to three layers of lamellar cells. Ruffini endings were significantly smaller in volume than Pacini corpuscles or Golgi-like endings. The latter contained a median of three intracorpuscular structures. Ruffini endings and Golgi-like endings presented a similar structural composition of their capsule and subscapular space. The central axon of Pacini corpuscles was surrounded by S100-positive cells forming the inner core which was significantly smaller than the outer core, which was immunoreactive for p75 and PGP 9.5.

Conclusion: This study reports new data regarding the intricate outer and intracorpuscular three-dimensional morphology of periarticular sensory nerve endings, including the volume, number of cells, and structural composition. These results may form a basis to differ between normal and pathological morphological changes in periarticular sensory nerve endings in future studies.

Keywords: Histology; Ligament; Morphology; Proprioception; Sensory nerve endings.

Fig. 1.

Ruffini ending. A 3D visualization of a Ruffini ending displays the distribution of S100 (a), PGP 9.5 (b), p75 (c), DAPI (d), and all immunofluorescent markers (e) simultaneously. The central axon is surrounded by S100-positive Schwann cells (arrow in a). The central axon is precisely visualized with PGP 9.5 (arrow in b) and branches out into dendritic terminal nerve endings. The capsule shows strong immunoreactivity for p75 (c); scale bar 50 μm.

Fig. 2.

Histogram Ruffini ending. A section of a Ruffini ending is displayed as the deconvolved CLSM image (a) and as a reconstructed 3D visualization (b). The histogram (c) graphically represents the antibody intensity along the white lines in a and b. The distribution indicates that p75 is mainly found in the capsule, S100 is distributed in the inner part of the corpuscle and in the capsule, whereas PGP 9.5 is located in the inner part exclusively; scale bar 15 μm.

Fig. 3.

Microstructure Ruffini ending. a The p75-positive capsule is rendered transparent to visualize the underlying PGP 9.5-positive dendritic nerve endings. b The central axon divides into two main branches, with a diameter of 5.14 μm and 3.77 μm, respectively. The capsule is cropped in the z-axis to visualize its circular arrangement around the dendritic nerve endings (c) with a subscapular space of about 4 μm (d). Scale bar 50 μm (a), 20 μm (b), 40 μm (c), 10 μm (d).

Fig. 4.

Capsule Ruffini ending. a The capsule of a Ruffini ending is cropped in the z-axis to display the inner core of the corpuscle with a globular arrangement of nuclei. The capsule is multi-layered (white arrows in b) with flattened cells and has bud-like projections and a subscapular space (star in b). The capsule may extend with projections into the corpuscle (red arrow in b). Scale bar 35 μm (a), 15 μm (b).

Fig. 5.

Pacini corpuscle. The 3D visualization of a Pacini corpuscle shows immunostaining with S100 (a), PGP 9.5 (b), p75 (c), DAPI (d), and all immunofluorescent markers (e) simultaneously. The parent axon shows immunoreactivity for PGP 9.5 and S100 (arrows in a, b) and is covered by p75-positive perineurium (arrow in c). The inner core shows immunoreactivity mainly for S100 (arrowhead in a). The lamellae are immunoreactive for p75 and PGP 9.5 (arrowhead b, c). Scale bar 50 μm.

Fig. 6.

Central axon Pacini corpuscle. The 3D microscopic image (a) is used to render a 3D visualization (b) of a Pacini corpuscle. Two markers are set across the central axon of the Pacini corpuscle (arrow in b), and a histogram (c) displays the intensity values of the antibodies across the marked positions in b. The central axon mainly shows immunofluorescence for PGP 9.5 and to a lesser extent S100 and p75. Scale bar 15 μm (a), 20 μm (b).

Fig. 7.

Golgi-like ending. A Golgi-like ending is displayed as a 3D visualization showing S100 (a), PGP 9.5 (b), p75 (c), DAPI (d), and all immunofluorescent markers (e) simultaneously. The Golgi-like ending contains multiple smaller corpuscles which are concentric in shape and show immunoreactivity for S100, p75, and PGP 9.5 (arrows in a–c). Scale bar 50 μm.

Fig. 8.

Histogram Golgi-like ending. A section of an inner corpuscle of a Golgi-like ending is displayed as the deconvolved CLSM image (a) and as a reconstructed 3D visualization (b). The histogram (c) graphically represents the antibody intensity along the white lines in a, b and shows that p75 is mainly located on the surface of the inner corpuscle, S100 is found on the surface, and PGP 9.5 is also found in the inner part of the corpuscle. Scale bar 30 μm.

Fig. 9.

Volume measurements of corpuscular sensory nerve endings. The ovoid volume of a Ruffini (a), the globular volume of a Pacini corpuscle (b), and the fusiform volume of a Golgi-like ending (c) are shown. Scale bar 20 μm.