Morphology and chemical characteristics of subepithelial laminar nerve endings in the rat epiglottic mucosa

Abstract

The laminar nerve endings are distributed in the laryngeal mucosa, and described as sensory receptors evoked by laryngeal pressure changes. The present study aimed to determine detailed morphological characteristics of the laryngeal laminar endings of the rat. Immunohistochemistry for Na(+)-K(+)-ATPase, α(3) subunit, showed that laminar endings were distributed in the entire laryngeal surface of the epiglottis. The parent axons of the endings were thick in diameter, and they were branched and continued to the endings. In some cases, several endings from different parent axons fused into a large complex structure of 500 μm in width. The laminar endings were also immunoreactive for vesicular glutamate transporter 1 (vGLUT1) and vGLUT2, but not for P2X(3) purinoceptor. Around the laminar endings, terminal Schwann cells with immunoreactivity for S-100 protein were closely associated with axon terminals. Use of scanning electron microscopy with alkaline maceration method showed that the terminal Schwann cells consisted of a rounded perinuclear region and lamellar cytoplasmic processes. Ultrastructurally, axon terminals with numerous mitochondria were partly covered with Schwann cell sheath, and some terminals intruded into the epithelial layer. Clear vesicles of 50 nm in diameter were also observed especially in small cytoplasmic processes of 400 nm to 1 μm in size. The results in the present study suggested that the laminar endings in epiglottic mucosa have morphological characteristics of slowly adapting mechanoreceptors and contribute to sensation of laryngeal pressure via mucosal tension.

Distribution and morphology of P2X3‐immunoreactive subserosal afferent nerve endings in the rat gastric antrum

“…P2X3‐immunoreactive lamellar processes of subserosal nerve endings extended in every direction on the surface of longitudinal smooth muscle layers, unlike muscle spindle afferents and vagal afferent IMAs that are arranged in axes with intrafusal muscle fibers and gastric smooth muscle layers, respectively (Powley et al, 2013; Schoultz & Swett, 1974). Furthermore, two‐dimensionally extended net‐like terminal structures formed by multiple parent axons of P2X3‐immunoreactive nerve endings have been shown to resemble mechanosensory endings such as the laminar endings in the laryngeal mucosa (Soda & Yamamoto, 2012), and sensory endings in airway smooth muscle (Brouns et al, 2006) and visceral pleura (Pintelon et al, 2007). Since antral web endings were formed by a subset (41%) of vagal afferents forming IMAs in the LM layers, as shown by anterograde tracing into the rat NG (Powley et al, 2012; Powley et al, 2016), P2X3‐immunoreactive subserosal nerve endings may be a subtype of vagal mechanoreceptors in the distal antrum of the lesser curvature.…”

“…Previous immunohistochemical studies have revealed that terminal Schwann cells tightly surround axon terminals of mechanoreceptors, such as Ruffini endings in the periodontal ligaments (Maeda et al, 1999; Takahashi‐Iwanaga et al, 1997), laminar endings in the laryngeal mucosa (Soda & Yamamoto, 2012), and arterial baroreceptor endings in the aortic arch and carotid sinus (Krauhs, 1979; Yokoyama et al, 2019). In P2X3‐immunoreactive subserosal nerve endings, axon terminals surrounded these cells and projected pleomorphic and thread‐like protrusions to the surrounding tissues.…”

“…These P2X3-immunoreactive subserosal nerve endings may be the morphological subtype of the antral web endings. (Soda & Yamamoto, 2012), and sensory endings in airway smooth muscle (Brouns et al, 2006) and visceral pleura (Pintelon et al, 2007). Since antral web endings were formed by a subset (41%) of vagal afferents forming IMAs in the LM layers, as shown by anterograde tracing into the rat NG (Powley et al, 2012;Powley et al, 2016), P2X3-immunoreactive subserosal nerve endings may be a subtype of vagal mechanoreceptors in the distal antrum of the lesser curvature.…”

Morphology of P2X3‐immunoreactive nerve endings in the rat tracheal mucosa

“…Furthermore, P2X3‐immunoreactive axonal twigs from different axons interdigitated to form a large complex of nerve endings. The interdigitating arrangement of P2X3‐immunoreactive nerve endings resembles the anatomical arrangement of subepithelial laminar endings in the epiglottis (Soda & Yamamoto, ) and one of the vagal afferent endings in the pyloric sphincter, intramuscular arrays (IMAs; Powley et al, ). These nerve endings widely covered the epiglottic mucosa and pyloric sphincter, respectively, by multiple nerve endings from different axons.…”

“…Morphological differences between nerve endings immunoreactive to P2X3 and CGRP have also been reported in the skin (Taylor, Peleshok, & Ribeiro-da-Silva, 2009), intrapulmonary airways (Brouns et al, 2009), and laryngeal mucosa (Soda & Yamamoto, 2012;Takahashi et al, 2016). It is well-known that CGRP is one of the immunohistochemical markers for unmyelinated C-fiber nociceptors activated by various endogenous and exogeneous noxious stimuli, that is, hydrogen ions, reactive oxygen species, and capsaicin via TRPV1 channels (review see, Lee & Yu, 2014).…”