Sensory Innervation of the External Genital Tract of Female Guinea Pigs and Mice

Abstract

The structural and neurochemical characterization of the sensory innervation of the external genitalia of females is poorly known. To immunohistochemically map the sensory innervation of external genitalia and surrounding structures of female guinea pigs and mice. Large-diameter sensory fibers, presumably mechanoreceptors, were identified by their immunoreactivity to neuron-specific enolase (NSE) or vesicular glutamate transporter 1 (VGluT1). Peptidergic sensory fibers, presumably unmyelinated nociceptors, were identified by their immunoreactivity to calcitonin gene-related peptide (CGRP), substance P, or both. Multiple-labelled tissues were examined with high-resolution confocal microscopy. Microscopic identification of sensory endings, including potential nociceptors, characteristic of the external genitalia. Large complex nerve endings immunoreactive for NSE and VGluT1 were abundant in dermal papillae of the clitoris. Each large ending was accompanied by one or two fine fibers immunoreactive for CGRP but neither substance P nor VGluT1. More simple NSE-immunoreactive endings occurred within dermal papillae in non-hairy skin of the labia and anal canal but were rare in pudendal or perineal hairy skin. Fine intra-epithelial fibers immunoreactive for NSE but not CGRP were abundant in hairy skin but rare in non-hairy genital skin and the clitoris. Only fine varicose fibers immunoreactive for both CGRP and substance P occurred in connective tissue underlying the mucosal epithelium of cervix and endometrium. Compared with surrounding tissues, the sensory innervation of the clitoris is highly specialized. The coactivation of nociceptors containing CGRP but not substance P within each mechanoreceptor complex could be the explanation of pain disorders of the external genitalia.