Multiscale analysis of the adult human superior hypogastric plexus and hypogastric nerve

The superior hypogastric plexus (SHP) and hypogastric nerves (HN) are components of the autonomic nervous system required for sympathetic regulation of the pelvic viscera. Despite their functional importance, the three-dimensional (3D) distribution and immunohistochemical composition of neurons in these structures remain poorly characterized. In this study, we used a multiscale imaging approach to generate a 3D anatomical and immunohistochemical characterization of the SHP and HN in adult humans. Both embalmed (body donor program) and unembalmed paraformaldehyde (PFA)-fixed specimens (organ donor program) were cleared using a modified Adipo-Clear/iDISCO protocol and imaged by light sheet fluorescence microscopy. Thousands of neuronal cell bodies were identified within the HN, demonstrating that this nerve does not function solely as a conduit for axons. In one HN sample analyzed along its whole length by immunohistochemistry, more than 90% of the neurons were tyrosine hydroxylase-immunoreactive (TH-IR), so presumed to be noradrenergic. Neuronal cell bodies in both the SHP and HN were arranged in clusters of diverse size, embedded within nerve tracts rather than in discrete ganglia. High-resolution confocal microscopy of cryosections confirmed the presence of numerous TH-IR (presumed noradrenergic) neuronal cell bodies and axons in the SHP and HN. Non-noradrenergic axonal populations were also abundant. Putative afferent axons (calcitonin gene-related peptide- and substance P-immunoreactive) traversed the SHP and HN and occasionally encircled individual ganglion neurons, raising the possibility of direct sensory-motor communication at these sites. Collectively, these findings provide a new 3D anatomical and immunohistochemical characterization of the SHP and HN in the adult human. This has important implications for understanding normal pelvic autonomic function and pathophysiology of genitourinary disorders. These data and further application of our imaging approach will inform the improvement of nerve-sparing surgical techniques in the pelvic region and the development of targeted neuromodulation approaches.