Cranial Pair 0: The Nervus Terminalis

Smelling TNT: Trends of the Terminal Nerve

There is very little knowledge regarding the terminal nerve, from its implications in the involvement and pathogenesis of certain conditions, to its embryological origin. With this review, we try to summarize the most important evidence on the terminal nerve, aiming to clarify its anatomy and the various functions attributed to it, to better interpret its potential involvement in pathological processes. Recent studies have also suggested its potential role in the control of human reproductive functions and behaviors. It has been hypothesized that it plays a role in the unconscious perception of specific odors that influence autonomic and reproductive hormonal systems through the hypothalamic-pituitary-gonadal axis. We used the PubMed database and found different articles which were then selected independently by three authors. We found 166 articles, of which, after careful selection, only 21 were analyzed. The terminal nerve was always thought to be unimportant in our body. It was well studied in different types of animals, but few studies have been completed in humans. For this reason, its function remains unknown. Studies suggest a possible implication in olfaction due to the anatomical proximity with the olfactive nerve. Others suggest a more important role in reproduction and sexual behaviors. New emerging information suggests a possible role in Kallmann syndrome and COVID-19.

Illuminating the terminal nerve: Uncovering the link between GnRH-1 neuron and olfactory development

During embryonic development, the olfactory placode (OP) generates migratory neurons, including olfactory pioneer neurons, cells of the terminal nerve (TN), gonadotropin-releasing hormone-1 (GnRH-1) neurons, and other uncharacterized neurons. Pioneer neurons from the OP induce olfactory bulb (OB) morphogenesis. In mice, GnRH-1 neurons appear in the olfactory system around mid-gestation and migrate via the TN axons to different brain regions. The GnRH-1 neurons are crucial in controlling the hypothalamic-pituitary-gonadal axis. Kallmann syndrome is characterized by impaired olfactory system development, defective OBs, secretion of GnRH-1, and infertility. The precise mechanistic link between the olfactory system and GnRH-1 development remains unclear. Studies in humans and mice highlight the importance of the prokineticin-2/prokineticin-receptor-2 (Prokr2) signaling pathway in OB morphogenesis and GnRH-1 neuronal migration. Prokr2 loss-of-function mutations can cause Kallmann syndrome (KS), and hence the Prokr2 signaling pathway represents a unique model to decipher the olfactory/GnRH-1 connection. We discovered that Prokr2 is expressed in the TN neurons during the critical period of GnRH-1 neuron formation, migration, and induction of OB morphogenesis. Single-cell RNA sequencing identified that the TN is formed by neurons distinct from the olfactory neurons. The TN neurons express multiple genes associated with KS. Our study suggests that the aberrant development of pioneer/TN neurons might cause the KS spectrum.

The olfactory striae: A historical perspective on the inconsistent anatomy of the bulbar projections

Central olfactory pathways (i.e., projection axons of the mitral and tufted cells), and especially olfactory striae, lack common terminology. This is due to their high degree of intra- and interindividual variability, which has been studied in detail over the past century by Beccari, Mutel, Klass, Erhart, and more recently, by Duque Parra et al. These variations led to some confusion about their number and anatomical arrangement. Recent advances in fiber tractography have enabled the precise in vivo visualization of human olfactory striae and the study of their projections. However, these studies require their algorithms to be set up according to the presumed anatomy of the analyzed fibers. A more precise definition of the olfactory striae is therefore needed, not only to allow a better analysis of the results but also to ensure the quality of the data obtained. By studying the various published works on the central olfactory pathways from the first systematic description by Soemmerring to the present, I have traced the different discussions on the olfactory tracts and summarized them here. This review adopts a systematic approach by addressing each stria individually and tracing the historical background of what was known about it in the past, compared to the current knowledge. The chronological and organized approach used provides a better understanding of the anatomy of these essential structures of the olfactory system.

Illuminating the Terminal Nerve: Uncovering the Link between GnRH-1 and Olfactory Development

During embryonic development, the olfactory placode (OP) generates migratory neurons, including olfactory pioneer neurons, cells of the terminal nerve (TN), Gonadotropin-releasing hormone-1 (GnRH-1) neurons, and other uncharacterized neurons. Pioneer neurons from the olfactory placode induce olfactory bulb morphogenesis. In mice, GnRH-1 neurons appear in the olfactory system around mid-gestation and migrate via the terminal nerve axons to different brain regions. The GnRH-1 neurons are crucial in controlling the hypothalamic-pituitary-gonadal (HPG) axis. Kallmann syndrome is characterized by impaired olfactory system development, defective olfactory bulbs, defective secretion of GnRH-1, and infertility. The precise mechanistic link between the olfactory system and GnRH-1 development remains unclear. Studies in humans and mice highlight the importance of the Prokineticin-2/Prokineticin-Receptor-2 (Prokr2) signaling pathway in olfactory bulb morphogenesis and GnRH-1 neuronal migration. Prokr2 loss-of-function mutations can cause Kallmann syndrome, and hence the Prokr2 signaling pathway represents a unique model to decipher the olfactory/GnRH-1 connection. We discovered that Prokr2 is expressed in the TN neurons during the critical period of GnRH-1 neuron formation, migration, and induction of olfactory bulb morphogenesis. Single-cell RNA sequencing identified that the TN is formed by neurons that are distinct from the olfactory neurons. The TN neurons express multiple genes associated with KS. Our study suggests that the aberrant development of pioneer/TN neurons might cause the KS spectrum.

Key Points

1) Pioneer or terminal nerve neurons play a crucial role in initiating the development of the olfactory bulbs. We found that the Prokineticin Receptor-2 gene, associated with Kallmann syndrome, is expressed by the olfactory pioneer/terminal nerve neurons.2) We genetically traced, isolated, and conducted Single-cell RNA sequencing on terminal nerve neurons of rodents. This analysis revealed a significant enrichment of gene expression related to Kallmann syndrome.3) Our study indicates that the investigation of Pioneer/terminal nerve neurons should be a pivotal focal point for comprehending developmental defects affecting olfactory and GnRH-1 systems.

Transient expression of NF200 by fibers in the nasal septum and rostral telencephalon of developing opossums (Monodelphis domestica)

Marsupials are born very immature and crawl on their mother's belly to attach to teats. Sensory information is required to guide the newborn and to induce attachment to the teat. Olfaction has been classically proposed to influence neonatal behaviors, but recent studies suggest that the central olfactory structures are too immature to account for them. In the newborn opossum, we previously described a fascicle of nerve fibers expressing neurofilament-200 (NF200, a marker of fiber maturity) from the olfactory bulbs to the rostral telencephalon. The course of these fibers is compatible with that of the terminal nerve that, during development, is characterized by the presence of neurons synthetizing gonadotropin hormones (GnRH). To evaluate if these fibers are related to the terminal nerve and if they play a role in precocious behaviors in opossums, we used immunohistochemistry against NF200 and GnRH. The results show that NF200-labeled fibers are present between P0 and P11, but do not reach much further caudally than the septal region. Only a few NF200-labeled fibers were found near the olfactory and vomeronasal epitheliums and they did not penetrate the olfactory bulbs. NF200-labeled fibers follow the same path as fibers labeled for GnRH. In contrast to the latter, NF200-labeled fibers are no longer visible at P15. These results suggest that these fibers are neither from the olfactory nor from the vomeronasal nerves but may be part of the terminal nerve. Their limited caudal extension does not support a role in the sensorimotor behaviors of the newborn opossum.

Development of the gonadotropin-releasing hormone system

This review summarizes the current understanding of the development of the neuroendocrine gonadotropin-releasing hormone (GnRH) system, including discussion on open questions regarding (1) transcriptional regulation of the Gnrh1 gene; (2) prenatal development of the GnRH1 system in rodents and humans; and (3) paracrine and synaptic communication during migration of the GnRH cells.

A comprehensive structural, lectin and immunohistochemical characterization of the zebrafish olfactory system

Fish chemosensory olfactory receptors allow them to detect a wide range of water-soluble chemicals, that mediate fundamental behaviours. Zebrafish possess a well-developed sense of smell which governs reproduction, appetite, and fear responses. The spatial organization of functional properties within the olfactory epithelium and bulb are comparable to those of mammals, making this species suitable for studies of olfactory differentiation and regeneration and neuronal representation of olfactory information. The advent of genomic techniques has been decisive for the discovery of specific olfactory cell types and the identification of cell populations expressing vomeronasal receptors. These advances have marched ahead of morphological and neurochemical studies. This study aims to fill the existing gap in specific histological, lectin-histochemical and immunohistochemical studies on the olfactory rosette and the olfactory bulb of the zebrafish. Tissue dissection and microdissection techniques were employed, followed by histological staining techniques, lectin-histochemical labelling (UEA, LEA, BSI-B4) and immunohistochemistry using antibodies against G proteins subunits αo and αi2, growth-associated protein-43, calbindin, calretinin, glial-fibrillary-acidic-protein and luteinizing-hormone-releasing-hormone. The results obtained enrich the available information on the neurochemical patterns of the zebrafish olfactory system, pointing to a greater complexity than the one currently considered, especially when taking into account the peculiarities of the nonsensory epithelium.

Mechanisms underlying pre- and postnatal development of the vomeronasal organ

The vomeronasal organ (VNO) is sensory organ located in the ventral region of the nasal cavity in rodents. The VNO develops from the olfactory placode during the secondary invagination of olfactory pit. The embryonic vomeronasal structure appears as a neurogenic area where migratory neuronal populations like endocrine gonadotropin-releasing hormone-1 (GnRH-1) neurons form. Even though embryonic vomeronasal structures are conserved across most vertebrate species, many species including humans do not have a functional VNO after birth. The vomeronasal epithelium (VNE) of rodents is composed of two major types of vomeronasal sensory neurons (VSNs): (1) VSNs distributed in the apical VNE regions that express vomeronasal type-1 receptors (V1Rs) and the G protein subunit Gαi2, and (2) VSNs in the basal territories of the VNE that express vomeronasal type-2 receptors (V2Rs) and the G subunit Gαo. Recent studies identified a third subclass of Gαi2 and Gαo VSNs that express the formyl peptide receptor family. VSNs expressing V1Rs or V2Rs send their axons to distinct regions of the accessory olfactory bulb (AOB). Together, VNO and AOB form the accessory olfactory system (AOS), an olfactory subsystem that coordinates the social and sexual behaviors of many vertebrate species. In this review, we summarize our current understanding of cellular and molecular mechanisms that underlie VNO development. We also discuss open questions for study, which we suggest will further enhance our understanding of VNO morphogenesis at embryonic and postnatal stages.

Recent advances in understanding and managing Kallmann syndrome

Many of the recent advances in our understanding of human reproductive biology and its genetic basis have arisen directly via the genetic investigation of patients with Kallmann syndrome and their families. The disease is characterised by the association of an isolated defect in the secretion (or, less commonly, action) of gonadotropin-releasing hormone (GnRH) and consequent infertility, with anosmia and potentially other associated non-reproductive features. GnRH-producing neurons are located in the hypothalamic brain region after a peculiar migration during embryonic life. To date, different genes affecting GnRH neuron development/migration have so far been implicated in Kallmann syndrome, but our knowledge of the genetic basis of the syndrome remains incomplete. From a clinical point of view, the disease has suffered from a lack of definitive diagnosis and treatment, and although progress has been made in terms of timely diagnosis and evidence-based treatment of patients, implementation remains inconsistent. These aspects will be discussed in this review, which examines new strategies for arriving at more evidence-based and patient-centred medical practice in Kallmann syndrome.

The human olfactory system in two proteinopathies: Alzheimer's and Parkinson's diseases

Alzheimer's and Parkinson's diseases are the most prevalent neurodegenerative disorders. Their etiologies are idiopathic, and treatments are symptomatic and orientated towards cognitive or motor deficits. Neuropathologically, both are proteinopathies with pathological aggregates (plaques of amyloid-β peptide and neurofibrillary tangles of tau protein in Alzheimer's disease, and Lewy bodies mostly composed of α-synuclein in Parkinson's disease). These deposits appear in the nervous system in a predictable and accumulative sequence with six neuropathological stages. Both disorders present a long prodromal period, characterized by preclinical signs including hyposmia. Interestingly, the olfactory system, particularly the anterior olfactory nucleus, is initially and preferentially affected by the pathology. Cerebral atrophy revealed by magnetic resonance imaging must be complemented by histological analyses to ascertain whether neuronal and/or glial loss or neuropil remodeling are responsible for volumetric changes. It has been proposed that these proteinopathies could act in a prion-like manner in which a misfolded protein would be able to force native proteins into pathogenic folding (seeding), which then propagates through neurons and glia (spreading). Existing data have been examined to establish why some neuronal populations are vulnerable while others are resistant to pathology and to what extent glia prevent and/or facilitate proteinopathy spreading. Connectomic approaches reveal a number of hubs in the olfactory system (anterior olfactory nucleus, olfactory entorhinal cortex and cortical amygdala) that are key interconnectors with the main hubs (the entorhinal-hippocampal-cortical and amygdala-dorsal motor vagal nucleus) of network dysfunction in Alzheimer's and Parkinson's diseases.

Cranial Nerves: Morphology and Clinical Relevance

This Special Issue entitled "Cranial Nerves: phylogeny, ontogeny, morphology and clinical significance" has been divided into two consecutive volumes. This second volume is devoted to morphology and clinical relevance. Articles in this volume examine these topics from a macroscopic point of view and with a surgical interest. This volume includes articles on oculomotor nerves III, IV, and VI and their course in the orbit; intracranial and extracranial views of the V and VII pairs; and branching patterns of IX, X, XI, and XII pairs with medical significance. Together, these articles provide a general overview of cranial nerves' gross anatomical organization, as well as improving on the knowledge necessary for clinical approaches. Anat Rec, 302:555-557, 2019. © 2019 Wiley Periodicals, Inc.

Nervus terminalis and nerves to the vomeronasal organ: a study using human fetal specimens

The human nervus terminalis (terminal nerve) and the nerves to the vomeronasal organ (VNON) are both associated with the olfactory nerves and are of major interest to embryologists. However, there is still limited knowledge on their topographical anatomy in the nasal septum and on the number and distribution of ganglion cells along and near the cribriform plate of the ethmoid bone. We observed serial or semiserial sections of 30 fetuses at 7-18 weeks (crown rump length [CRL], 25-160 mm). Calretinin and S100 protein staining demonstrated not only the terminal nerve along the anterior edge of the perpendicular lamina of the ethmoid, but also the VNON along the posterior edge of the lamina. The terminal nerve was composed of 1-2 nerve bundles that passed through the anterior end of the cribriform plate, whereas the VNON consisted of 2-3 bundles behind the olfactory nerves. The terminal nerve ran along and crossed the posterior side of the nasal branch of the anterior ethmoidal nerve. Multiple clusters of small ganglion cells were found on the lateral surfaces of the ethmoid's crista galli, which are likely the origin of both the terminal nerve and VNON. The ganglions along the crista galli were ball-like and 15-20 µm in diameter and, ranged from 40-153 in unilateral number according to our counting at 21-µm-interval except for one specimen (480 neurons; CRL, 137 mm). An effect of nerve degeneration with increasing age seemed to be masked by a remarkable individual difference.