Immunolocalization of VEGF/VEGFR system in human fetal vomeronasal organ during early development

The vomeronasal system (VNS) is an accessory olfactory structure present in most mammals adhibited to the detection of specific chemosignals implied in social and reproductive behavior. The VNS comprises the vomeronasal organ (VNO), vomeronasal nerve and accessory olfactory bulb. VNO is characterized by a neuroepithelium constituted by bipolar neurons and supporting and stem/progenitor cells. In humans, VNO is present during fetal life and is supposed to possess chemoreceptor activity and participate in gonadotropin-releasing hormone neuronal precursor migration toward the hypothalamus. Instead, the existence and functions of VNO in postnatal life is debated. Vascular endothelial growth factor (VEGF) and its receptors (VEGFRs) have been demonstrated to play fundamental roles in various neurogenic events. However, there are no data regarding the localization and possible function of VEGF/VEGFRs in human fetal VNO. Therefore, this study was conceived to investigate the expression of VEGF/VEGFRs in human VNO in an early developmental period (9-12 weeks of gestation), when this organ appears well structured. Coronal sections of maxillofacial specimens were subjected to peroxidase-based immunohistochemistry for VEGF, VEGFR-1 and VEGFR-2. Double immunofluorescence for VEGF, VEGFR-1 or VEGFR-2 and the neuronal marker protein gene product 9.5 (PGP 9.5) was also performed. VEGF expression was evident in the entire VNO epithelium, with particularly strong reactivity in the middle layer. Strongly VEGF-immunostained cells with aspect similar to bipolar neurons and/or their presumable precursors were detected in the middle and basal layers. Cells detaching from the basal epithelial layer and detached cell groups in the surrounding lamina propria showed moderate/strong VEGF expression. The strongest VEGFR-1 and VEGFR-2 expression was detected in the apical epithelial layer. Cells with aspect similar to bipolar neurons and/or their presumable precursors located in the middle and basal layers and the detaching/detached cells displayed a VEGFR-1 and VEGFR-2 reactivity similar to that of VEGF. The basal epithelial layer exhibited stronger staining for VEGFRs than for VEGF. Cells with morphology and VEGF/VEGFR expression similar to those of the detaching/detached cells were also detected in the middle and basal VNO epithelial layers. Double immunofluorescence using anti-PGP 9.5 antibodies demonstrated that most of the VEGF/VEGFR-immunoreactive cells were neuronal cells. Collectively, our findings suggest that during early fetal development the VEGF/VEGFR system might be involved in the presumptive VNO chemoreceptor activity and neuronal precursor migration.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

Human Neural Cells Transiently Express Reelin during Olfactory Placode Development

Reelin, an extracellular glycoprotein is essential for migration and correct positioning of neurons during development. Since the olfactory system is known as a source of various migrating neuronal cells, we studied Reelin expression in the two chemosensory olfactory systems, main and accessory, during early developmental stages of human foetuses/embryos from Carnegie Stage (CS) 15 to gestational week (GW) 14. From CS 15 to CS 18, but not at later stages, a transient expression of Reelin was detected first in the presumptive olfactory and then in the presumptive vomeronasal epithelium. During the same period, Reelin-positive cells detach from the olfactory/vomeronasal epithelium and migrate through the mesenchyme beneath the telencephalon. Dab 1, an adaptor protein of the Reelin pathway, was simultaneously expressed in the migratory mass from CS16 to CS17 and, at later stages, in the presumptive olfactory ensheathing cells. Possible involvements of Reelin and Dab 1 in the peripheral migrating stream are discussed.

Epithelial membrane transporters expression in the developing to adult mouse vomeronasal organ and olfactory mucosa

To contribute clarifying mechanisms operating in nose chemosensory epithelia and their developmental patterns, we analyzed the expression of different epithelial membrane transporters as well as the Clara cell secretory protein, CC26 in the olfactory, vomeronasal organ (VNO), and respiratory epithelia of embryonic (E13-E19) and postnatal (P1-P60) mice by means of immunohistochemistry and reverse transcriptase-polymerase chain reaction. Results showed that CC26, cAMP-activated chloride channel (CFTR), and the water channel protein aquaporin 2, 3, 4, and 5 (AQP2, AQP3, AQP4, and AQP5) are expressed in developing to adult chemosensory epithelia with differential timing; moreover, their pattern of expression is not identical in VNO and olfactory epithelia as well as the corresponding associated glands; co-localization experiments using olfactory marker protein showed that CFTR, CC26, and AQP4 are not expressed in olfactory neurones. CFTR is expressed in sustentacular cells of the VNO and olfactory epithelium as well as blood vessels of the underlying mucosa, and VNO (but not Bowman's) glands; a similar pattern (excluding blood vessels) is present for AQP2; AQP4 is found in the two chemosensory epithelia and in Bowman's glands. AQP3 is expressed in the olfactory epithelium and the associated Bowman's glands, but not in the VNO chemosensory epithelium and glands. AQP5 is expressed in the olfactory epithelium and both Bowman's and VNO glands. These results indicate that water/ions handling as well as antioxidant mechanisms operating at the surface and/or inside the nose chemosensory epithelia start developing in utero and are maintained up to sexual maturity.

[Immunohistochemical study of human fetal vomerona structures the nasal septum, by applying neuron-specific beta3-tubulin antibodies]

The functioning of Jacobson's or vomeronasal organ (VNO) in man is the subject-matter of discussion today. It is generally taken that VNO as an anatomic structure also remains in the adult; however, its receptor apparatus still degenerates in the fetal stage of ontogenesis. Nevertheless, the data available in the literature on the time and specific features of degenerative changes in the human fetal VNO are conflicting and ambiguous. The authors examined the human fetal nasal septum from the 8th week of development to birth, by applying the traditional histological procedures and neuron-specific beta3-tubulin antibodies. An immunohistochemical study could first show the receptor apparatus of the human fetal VNO at weeks 8-26 of development. The immunohistochemical study on a series of sections could reveal the regularities of spatial receptor distribution depending on the time of fetal development. In addition, the developed human fetal vomeronasal nerve and ganglion at weeks 8-26 were described, in human fetuses at weeks 8-26. The neuron-specific marker test has shown the nerve fibers departing directly from the VNO wall, which is inconsistent with the data available in the literature on vomeronasal nerve degeneration in this sign just after the 18th week of development.

Robo-2 controls the segregation of a portion of basal vomeronasal sensory neuron axons to the posterior region of the accessory olfactory bulb

The ability of sensory systems to detect and process information from the environment relies on the elaboration of precise connections between sensory neurons in the periphery and second order neurons in the CNS. In mice, the accessory olfactory system is thought to regulate a wide variety of social and sexual behaviors. The expression of the Slit receptors Robo-1 and Robo-2 in vomeronasal sensory neurons (VSNs) suggests they may direct the stereotypic targeting of their axons to the accessory olfactory bulb (AOB). Here, we have examined the roles of Robo-1 and Robo-2 in the formation of connections by VSN axons within the AOB. While Robo-1 is not necessary for the segregation of VSN axons within the anterior and posterior regions of the AOB, Robo-2 is required for the targeting of some basal VSN axons to the posterior region of the AOB but is dispensable for the fasciculation of VSN axons. Furthermore, the specific ablation of Robo-2 expression in VSNs leads to mistargeting of a portion of basal VSN axons to the anterior region of the AOB, indicating that Robo-2 expression is required on projecting VSN axons. Together, these results identify Robo-2 as a receptor that controls the targeting of basal VSN axons to the posterior AOB.

Fibroblast growth factor 8 signaling through fibroblast growth factor receptor 1 is required for the emergence of gonadotropin-releasing hormone neurons

GnRH neurons are essential for the onset and maintenance of reproduction. Mutations in both fibroblast growth factor receptor (Fgfr1) and Fgf8 have been shown to cause Kallmann syndrome, a disease characterized by hypogonadotropic hypogonadism and anosmia, indicating that FGF signaling is indispensable for the formation of a functional GnRH system. Presently it is unclear which stage of GnRH neuronal development is most impacted by FGF signaling deficiency. GnRH neurons express both FGFR1 and -3; thus, it is also unclear whether FGFR1 or FGFR3 contributes directly to GnRH system development. In this study, we examined the developing GnRH system in mice deficient in FGF8, FGFR1, or FGFR3 to elucidate the individual contribution of these FGF signaling components. Our results show that the early emergence of GnRH neurons from the embryonic olfactory placode requires FGF8 signaling, which is mediated through FGFR1, not FGFR3. These data provide compelling evidence that the developing GnRH system is exquisitely sensitive to reduced levels of FGF signaling. Furthermore, Kallmann syndrome stemming from FGF signaling deficiency may be due primarily to defects in early GnRH neuronal development prior to their migration into the forebrain.

Distribution of Notch1-expressing cells and proliferating cells in mouse vomeronasal organ

Vomeronasal receptor neurons (VRNs) proliferate and differentiate continuously in the vomeronasal organ (VNO) throughout life. In adult mice, new VRNs are generated mainly in the marginal region, located in the boundary region between sensory and nonsensory epithelia. The Notch signaling pathway is involved in differentiation in the developing nervous system. To understand the Notch signaling pathway involved in generating VRNs, we focused on the relationship between the expression pattern of Notch1 and the localization of proliferating cells in both developing and regenerating mice VNO, and examined the Notch signaling pathway involved in the development of VNO by in situ hybridization of Notch1 and immunocytochemistry of 5-bromo-2'-deoxyuridine. During embryonic and neonatal development, proliferating cells and Notch1-expressing (+) cells were observed evenly throughout VNO. A large number of proliferating cells and Notch1 (+) cells were observed in embryonic VNO, but gradually decreased during development. The localization of proliferating cells was similar to that of Notch1 (+) cells at each developmental stage. In adult VNO, there are a few proliferating cells and Notch1 (+) cells, which were only in the marginal region of VNO. Seven days after removal of the accessory olfactory bulb (AOB), VRNs proliferated throughout VNO. Although the number of Notch1 (+) cells also increased in VNO, the majority of these were concentrated in the dorsal region of VNO, suggesting that it has two types of differentiating cell. These results suggest that Notch1 plays a role in the differentiation of VRNs during development and regeneration of VRNs after removal of AOB.

Lactosamine modulates the rate of migration of GnRH neurons during mouse development

Gonadotropin-releasing hormone (GnRH) neurons are derived from progenitor cells in the olfactory placodes and migrate from the vomeronasal organ (VNO) across the cribriform plate into the forebrain. At embryonic day (E)12 in the mouse most of these neurons are still in the nasal compartment but by E15 most GnRH neurons have migrated into the forebrain. Glycoconjugates with carbohydrate chains containing terminal lactosamine are expressed by neurons in the main olfactory epithelium and in the VNO. One of the key enzymes required to regulate the synthesis and expression of lactosamine, beta1,3-N-acetylglucosaminyltransferase-1 (beta3GnT1), is strongly expressed by neurons in the olfactory epithelium and VNO, and on neurons migrating out of the VNO along the GnRH migratory pathway. Immunocytochemical analysis of lactosamine and GnRH in embryonic mice reveals that the percentage of lactosamine+-GnRH+ double-labeled neurons decreases from > 80% at E13, when migration is near its peak, to approximately 30% at E18.5, when most neurons have stopped migrating. In beta3GnT1-/- mice, there is a partial loss of lactosamine expression on GnRH neurons. Additionally, a greater number of GnRH neurons were retained in the nasal compartment of null mice at E15 while fewer GnRH neurons were detected later in embryonic development in the ventral forebrain. These results suggest that the loss of lactosamine on a subset of GnRH neurons impeded the rate of migration from the nose to the brain.

Expression of pheromone receptor gene families during olfactory development in the mouse: expression of a V1 receptor in the main olfactory epithelium

In the mouse, two large gene families, V1R and V2R, encoding putative pheromone receptors have been described. Studies have suggested a homotypic recognition role for V1Rs and V2Rs during development in the targeting of vomeronasal axons to specific sets of glomeruli in the accessory olfactory bulb (AOB). Analysis of the onset of expression of the V1R and V2R gene families in developing vomeronasal neurons using polymerase chain reaction and in situ hybridization now suggests that a role for these receptors in the organization of axon projections is only likely at the final stages of targeting within the AOB. Surprisingly, our studies reveal expression of a V1Rd receptor in scattered cells within the main olfactory epithelium, suggesting that limited pheromone detection may also take place in this structure. The pheromone sensory neurons of the vomeronasal system and the neuroendocrine gonadotrophin-releasing hormone (GnRH) neurons that regulate fertility both arise from progenitor cells of the nasal placode. The development of these two cell types is intimately linked, and the GnRH neuron population migrates into the forebrain during embryogenesis in close association with a subset of vomeronasal sensory axons; how GnRH neurons recognize this axon subset is unknown. We report selective expression of a V1Ra gene in the clonal NLT GnRH cell line, raising the possibility of a similar role for V1Rs or V2Rs in the directed migration of GnRH neurons. However, no expression of this gene or of other V1Rs and V2Rs is detectable at the cellular level in migrating GnRH neurons in the mouse.

Tyrosine hydroxylase expression in the olfactory/respiratory epithelium in early sheep fetuses (Ovis aries)

Transient expression of tyrosine hydroxylase (TH, the first enzyme in catecholamine synthesis) has been shown in different brain and peripheral structures of various species. TH-immunoreactive neurons have been reported in the nasal region of human and rat fetuses migrating to the forebrain with GnRH neurons during embryogenesis. In the present study, immunohistochemical analysis and in situ hybridization were performed in fetal sheep and in vitro sheep embryo olfactory placode cultures to confirm this population in this species. On embryonic days 33 to 35, TH-immunoreactive cells as well as TH cDNA-hybridized cells were found in the olfactory and respiratory epithelium and were spatially separated from GnRH-immunoreactive neurons. In days 40 to 44 of gestation, TH-immunoreactive neurons were no longer observed in the olfactory epithelium, and TH-immunoreactive fibers were found on the trajectories of the olfactory nerves. At this stage, some TH-immunoreactive fibers were also labeled for GnRH. TH-immunoreactive cells were also found in primary cultures of olfactory placodes of fetal sheep at 10 to 18 days in vitro. Some of them coexpressed GnRH. These results imply that olfactory epithelium is also able to give rise to TH expressing cells in fetal sheep, but this expression is suppressed earlier in ontogenesis than in humans due to some unidentified factors not present in the primary cultures of olfactory placode. The role of TH expression remains unclear as in other previously described examples.

General organization of the perinatal and adult accessory olfactory bulb in mice

The vomeronasal system is currently a topical issue since the dual functional specificity, vomeronasal system-pheromones, has recently been questioned. Irrespective of the tools used to put such specificity in doubt, the diversity of the anatomy of the system itself in the animal kingdom is probably of more importance than has previously been considered. It has to be pointed out that a true vomeronasal system is integrated by the vomeronasal organ, the accessory olfactory bulb, and the so-called vomeronasal amygdala. Therefore, it seems reasonable to establish the corresponding differences between a well-developed vomeronasal system and other areas of the nasal cavity in which putative olfactory receptors, perhaps present in other kinds of mammals, may be able to detect pheromones and to process them. In consequence, a solid pattern for one such system in one particular species needs to be chosen. Here we report on an analysis of the general morphological characteristics of the accessory olfactory bulb in mice, a species commonly used in the study of the vomeronasal system, during growth and in adults. Our results indicate that the critical period for the formation of this structure comprises the stages between the first and the fifth day after birth, when the stratification of the bulb, the peculiarities of each type of cell, and the final building of glomeruli are completed. In addition, our data suggest that the conventional plexiform layers of the main olfactory bulb are not present in the accessory bulb.

FGF signalling controls expression of vomeronasal receptors during embryogenesis

Fibroblast growth factors (FGFs) have been shown to control formation and differentiation of multiple organ systems in the developing vertebrate embryo. The analysis of differential gene expression during embryogenesis is, therefore, a potent tool to identify novel target genes regulated by FGF signalling. Here, we have applied microarray analysis to identify differentially regulated genes in FGF mutant mouse embryos. Surprisingly, transcripts corresponding to vomeronasal receptors (VRs), which so far have been only detected in the vomeronasal organ (VNO), were found to be downregulated in FGF mutant embryos. VR expression was detected in the developing olfactory pit and the anlage of the VNO. Interestingly, several FGFs can be detected in the developing olfactory pit during mouse embryogenesis [Bachler, M., Neubuser, A. 2001. Expression of members of the Fgf family and their receptors during midfacial development. Mech. Dev. 100, 313-316]. FGF signalling may thus control expression of VRs in the olfactory pit and formation of the VNO. Moreover, VR expression was detected in unexpected locations within the developing embryo including retina, dorsal root ganglia and neural tube. The relevance of VR expression in these structures and for formation of the VNO is discussed.

Timing of neuronal intermediate filament proteins expression in the mouse vomeronasal organ during pre- and postnatal development. An immunohistochemical study

Several types of intermediate filament proteins are expressed in developing and mature neurons; they cooperate with other cytoskeletal components to sustain neuronal function from early neurogenesis onward. In this work the timing of expression of nestin, peripherin, internexin, and the neuronal intermediate filament triplet [polypeptide subunits of low (NF-L), medium (NF-M), and high (NF-H) molecular weight] was investigated in the developing fetal and postnatal mouse vomeronasal organ (VNO) by means of immunohistochemistry. The results show that the sequence of expression of intermediate filament proteins is internexin, nestin, and NF-M in the developing vomeronasal sensory epithelium; internexin, peripherin, and NF-M in the developing vomeronasal nerve; and nestin, internexin and peripherin, NF-L, and NF-M in the nerve supply to accessory structures of the VNO. At sexual maturity (2 months) NF-M is only expressed in vomeronasal neurons and NF-M, NF-L and peripherin are expressed in extrinsic nerves supplying VNO structures. The differential distribution of intermediate filament proteins in the vomeronasal sensory epithelium and nerve is discussed in terms of the cell types present therein. It is concluded that several intermediate filament proteins are sequentially expressed during intrauterine development of the VNO neural structures in a different pattern according to the different components of the VNO.

UMODL1/Olfactorin is an extracellular membrane-bound molecule with a restricted spatial expression in olfactory and vomeronasal neurons

The olfactory system provides a unique model for developmental neurobiology. Precise targeting of axonal projections from sensory neurons located in the olfactory epithelium to specific neurons in the olfactory bulb establishes a highly refined spatial sensory map. Distinctively, this process is not restricted to embryonic stages, but continues during the entire life of mammals. A number of secreted and membrane molecules have been implicated in guidance and targeting of olfactory sensory neurons. Here we describe olfactorin, the protein product of the mouse Umodl1 gene, as a potential new element in this process. Olfactorin is a secreted modular protein containing several domains typically present in extracellular matrix proteins (EMI, WAP, FNIII, Ca2+ -binding EGF-like, SEA and ZP domains). By in situ hybridization we find that during embryonic development expression of the Umodl1 gene is detectable only in the olfactory epithelium and vomeronasal organ starting at embryonic day 16.5. At this stage, Umodl1 expression within the olfactory epithelium is punctate, and is restricted to only some of the sensory neurons. At birth and postnatally, expression in these organs continues and involves more neurons. Kallmann syndrome is a genetic disease in which olfactory axons fail to connect to target neurons in the bulb. We tested whether olfactorin might be responsible for an autosomal form of this disease and show that this is not the case. However, based on its domain composition and on the expression in olfactory neurons we suggest that olfactorin may play a role in correct olfactory axon navigation to the brain.

The prenatal maturity of the accessory olfactory bulb in pigs

The morphological development of the accessory olfactory bulb of the fetal pig was studied by classical and histo-chemical methods, and the vomeronasal organ and nasal septum were studied histochemically. Specimens were obtained from an abattoir and their ages estimated from their crown-to-rump length. The accessory olfactory bulb was structurally mature in fetuses of crown-to-rump length 21-23 cm, by which time the lectin Lycopersicum esculentum agglutinin stained the same structures as in adults (in particular, the entire sensory epithelium of the vomeronasal organ, the vomeronasal nerves, and the nervous and glomerular layers of the accessory olfactory bulb). These results suggest that the vomeronasal system of the pig may, like that of vertebrates such as snakes, be functional at birth.

Cholecystokinin modulates migration of gonadotropin-releasing hormone-1 neurons

Expression of the brain-gut peptide cholecystokinin (CCK) in the developing olfactory-gonadotropin-releasing hormone-1 (GnRH-1) neuroendocrine systems was characterized, and the function of CCK in these systems was analyzed both in vivo and in vitro. We present novel data demonstrating that CCK transcript and protein are expressed in sensory cells in the developing olfactory epithelium and vomeronasal organ, with both ligand and receptors (CCK-1R and CCK-2R) found on olfactory axons throughout prenatal development. In addition, migrating GnRH-1 neurons in nasal regions express CCK-1R but not CCK-2R receptors. The role of CCK in olfactory-GnRH-1 system development was evaluated using nasal explants, after assessing that the in vivo expression of both CCK and CCK receptors was mimicked in this in vitro model. Exogenous application of CCK (10(-7) m) reduced both olfactory axon outgrowth and migration of GnRH-1 cells. This inhibition was mediated by CCK-1R receptors. Moreover, CCK-1R but not CCK-2R antagonism caused a shift in the location of GnRH-1 neurons, increasing the distance that the cells migrated. GnRH-1 neuronal migration in mice carrying a genetic deletion of either CCK-1R or CCK-2R receptor genes was also analyzed. At embryonic day 14.5, the total number of GnRH-1 cells was identical in wild-type and mutant mice; however, the number of GnRH-1 neurons within forebrain was significantly greater in CCK-1R-/- embryos, consistent with an accelerated migratory process. These results indicate that CCK provides an inhibitory influence on GnRH-1 neuronal migration, contributing to the appropriate entrance of these neuroendocrine cells into the brain, and thus represent the first report of a developmental role for CCK.

Nicotine inhibition of pulsatile GnRH secretion is mediated by GABAA receptor system in the cultured rat embryonic olfactory placode

In past work, we suggested that nicotine inhibition of in vivo pulsatile LH release is not mediated by opiate receptors known to be involved in the inhibition of LH release. In the present study, we examined whether nicotine inhibits the pulsatile gonadotropin-releasing hormone (GnRH) release, and whether this inhibition of GnRH release by nicotine is mediated by the GABA receptor system, by checking in vitro pulsatile GnRH release from cultured GnRH neurons obtained from olfactory placodes of rat embryos at E13.5. The mean interpulse interval of pulsatile GnRH release into the medium was 34.2+/-2.0 min in the control period and increased to 95.3+/-19.0 min (n=6) in the period of nicotine treatment at a concentration of 500 nM, showing an inhibitory effect of nicotine on pulsatile GnRH release. The GABA(A) receptor antagonist bicuculline used alone at a concentration of 20 microM caused no significant changes in the pulsatile GnRH release, but when used in combination with 500 nM of nicotine, bicuculline blocked the nicotine inhibition of GnRH release. In a separate experiment, nicotine treatment at a concentration of 500 nM significantly increased GABA release. These results suggest that, in the cultured embryonic olfactory placode, nicotine stimulates GABA release, which then inhibits GnRH release through GABA(A) receptor system.

Prenatal morphogenesis of the human incisive canal

The literature describing the formation of the incisive canal is very bizarre. The fusion of the primary and secondary palatal processes leads to formation of a triangular seam, which erroneously has been taken for the future incisive canal. If so, the nasopalatine (incisive) nerve and its accompanying vessels were to run through the primary oronasal cavity, which is not compatible with our biological experience. This study was undertaken to shed light on this region of fusion. We focus on the formation of the incisive canal; the neighboring nasopalatine ducts, which are a transient formation, are mentioned where present. A series of seven horizontal cross-sections of human embryos and fetuses from the 7th to the 24th week of pregnancy (between 25 and 225 mm CRL, crown-rump-length) were examined histologically and partly reconstructed in 3D applying the software analySIS (Soft Imaging Systems, Münster, Germany). The incisive canal did not develop at the junction of the primary and the secondary palate, but within the primary palate rostral to that location. The nasopalatine nerve and the nasopalatine artery are structures that exist before ossification starts in the area of the future incisive canal. The neighboring nasopalatine ducts were found in regions laterally and anterolaterally of the nasopalatine nerve, and it was mostly separated from it by bone. In advanced stages of development, the nasopalatine duct only existed as single epithelial remnants and was prone to obliteration.

[The human vomeronasal organ]

Odors influence human behavior. The perception of so-called pheromones is frequently mentioned in the context of a functional vomeronasal organ. Vomeronasal ducts can be detected in approximately half of the population. Its functionality, still a matter of debate, seems to be unlikely, at least after birth. It is easily conceivable that pheromone-induced changes in behavior are mediated through receptors in the human olfactory epithelium.

Fetal development of vomeronasal system in the goat

Our previous study morphologically revealed that the adult goat vomeronasal (VN) system was different from the rodent and opossum one, and at least two types of VN systems exist in mammals. However, it remains unknown whether the developments in both types of VN systems are ontogenetically distinct and when the goat VN system is established. In this study, we morphologically observed the fetal development of the goat accessory olfactory bulb (AOB) and VN neuron. In the fetus, Gi2-expressing VN terminals terminated at glomeruli throughout the AOB, and no immunoreactivities for Go were detected in the nerve terminals reaching into AOB. The layer structure of AOB rapidly developed in the latter half of gestation. In the VN organ (VNO), at the middle stage of gestation, the dendritic processes of VN neuron were exposed in the VN lumen, and scattered and thin microvilli existed on the protrusion of the VN neuron. In the apical part of dendritic processes, no clear vesicle existed. However, the immunohistochemistry of an olfactory marker protein (OMP) revealed that a few VN neurons with OMP exist in VN sensory epithelium (VSE) before birth, although marked immunoreactivities were detected in adult VSE. Fetal VN neurons appeared to be underdeveloped. These results suggest that the goat VN system is ontogenetically distinct from the rodent and opossum VN systems, and is underdeveloped before birth. The goat VN system will develop and mature during the early postnatal period similar to the rodent and opossum VN systems.

Observations on the vomeronasal organ of prenatal Tarsius bancanus borneanus with implications for ancestral morphology

Adult primates have at least five known phenotypes of vomeronasal organ (VNO), ranging from the typical morphology seen in most other mammals to complete absence. With such morphological disparity, the phylogenetic value and any inferences on ancestral VNO morphology of the primate VNO are left uncertain. The present study investigated the VNO of embryonic and fetal Tarsius bancanus borneanus (n = 4) in comparison with prenatal specimens from four other species of primates in an effort to clarify adult morphological variations. In all except one of the fetal primates, the VNO communicated to the nasopalatine duct. One exception occurred in the largest fetal Tarsius (25 mm crown-rump length), in which the VNO communicated with the nasal cavity alone. The vomeronasal neuroepithelium was well differentiated from a thinner, non-sensory epithelium in all Tarsius and New World monkeys studied, as well as late embryonic and fetal Microcebus myoxinus. In anterior sections, this neuroepithelium was found in a more superior location in Tarsius and New World monkeys compared with Microcebus myoxinus. In all primates, masses of cell bodies were found superior to the VNO, intermingled with nerve fibres. These morphologically resembled luteinizing hormone-releasing hormone neurons described in other mammals, including humans, suggesting that a primitive association of these neurons with the VNO may exist in all primate taxa. The present study revealed that prenatal similarities exist in Tarsius and New World primates in VNO epithelial morphology. However, these are transient stages of morphology. If tarsiers and anthropoids do represent a clade (Haplorhini), then the atypical morphology seen in adult tarsiers and New World monkeys probably represents the adult VNO morphology of a haplorhine common ancestor.

Netrin 1-mediated chemoattraction regulates the migratory pathway of LHRH neurons

Luteinizing hormone-releasing hormone (LHRH) neurons migrate from the vomeronasal organ (VNO) to the forebrain in all mammals studied. In mice, the direction of LHRH neuron migration is dependent upon axons that originate in the VNO, but bypass the olfactory bulb and project caudally into the basal forebrain. Thus, factors that guide this unique subset of vomeronasal axons that comprise the caudal vomeronasal nerve (cVNN) are candidates for regulating the migration of LHRH neurons. We previously showed that deleted in colorectal cancer (DCC) is expressed by neurons that migrate out of the VNO during development [Schwarting et al. (2001) J. Neurosci., 21, 911-919]. We examined LHRH neuron migration in Dcc-/- mice and found that trajectories of the cVNN and positions of LHRH neurons are abnormal. Here we extend these studies to show that cVNN trajectories and LHRH cell migration in netrin 1 (Ntn1) mutant mice are also abnormal. Substantially reduced numbers of LHRH neurons are found in the basal forebrain and many LHRH neurons migrate into the cerebral cortex of Ntn1 knockout mice. In contrast, migration of LHRH cells is normal in Unc5h3rcm mutant mice. These results are consistent with the idea that the chemoattraction of DCC+ vomeronasal axons by a gradient of netrin 1 protein in the ventral forebrain guides the cVNN, which, in turn, determines the direction of LHRH neuron migration in the forebrain. Loss of function through a genetic deletion in either Dcc or Ntn1 results in the migration of many LHRH neurons to inappropriate destinations.

Ontogeny of the nasopalatine duct in primates

Ecological explanations have been put forward to account for the precocious or delayed development of patency in ducts leading to the vomeronasal organ (VNO) in certain mammals. Perinatal function may be related, in part, to the patency or fusion of the vomeronasal and nasopalatine (NPD) ducts. However, few studies have focused on NPD development in primates, which generally have a prolonged period of dependence during infancy. In this study we examined 24 prenatal primates and 13 neonatal primates, and a comparative sample of fetal mice and insectivores. In embryonic and early fetal Microcebus murinus, the NPD was completely fused, whereas in fetuses of later stages the duct was partially fused or completely patent. M. myoxinus of all stages demonstrated some degree of NPD fusion. In all other prenatal primates, the NPD was fused to some extent. Four prenatal insectivores (Tenrec ecaudatus) showed some degree of NPD fusion. In Mus musculus at 19 days gestation, the NPD was patent, although the anatomically separate VNO duct was fused. T. ecaudatus and most of the neonatal primates revealed complete NPD patency. An exception was Saguinus geoffroyi, which exhibited fusion of the NPD near the VNO opening. These observations may relate to differences in perinatal VNO function. The differences noted in our study suggest that M. murinus and M. myoxinus may differ in perinatal VNO functionality and perhaps in related behavior. Observations of neonatal primates suggest that NPD patency may be relatively common at birth and could serve other purposes in addition to being an access route for VNO stimuli.

Development of the vomeronasal receptor epithelium and the accessory olfactory bulb in sheep

The morphological development of the vomeronasal organ (VNO) and accessory olfactory bulb (AOB) of the sheep from anlage to birth were studied by classical and histochemical methods using embryos and fetuses obtained from an abattoir with ages estimated from crown-to-rump length. Both VNO and AOB developed in a biologically logical sequence and completed their morphological development around day 98, at entry into the last third of the gestation period. A lectin with specificity for oligomeric N-acetylglucosamine labeled the sensory epithelium of the VNO, the vomeronasal nerves, and the nervous and glomerular layers of the AOB before birth. These results suggest that the vomeronasal system, which is well developed and functional in adult sheep, may be able to function at or even before birth in these animals (whereas in rodents, for example, this is precluded by the AOB not completing its development until after birth).

Ontogenetic characteristics of the vomeronasal organ in Saguinus geoffroyi and Leontopithecus rosalia, with comparisons to other primates

It has been suggested that the variability of the primate vomeronasal organ (VNO) may be greater than previously thought, especially among New World monkeys. It is not clear to what extent VNO variation reflects ontogenetic, functional, or phylogenetic differences among primates. The present study investigated VNO anatomy in an ontogenetic series of two genera of callitrichid primates, in order to assess recent attempts to develop VNO character states and to examine the evidence for VNO functionality at different life stages. A sample of six Leontopithecus rosalia, one L. chrysomelas, and six Saguinus geoffroyi was serially sectioned and stained using various methods. Two adult Callithrix jacchus were also sectioned for comparative purposes. The VNO of each primate was examined by light microscopy along its entire rostrocaudal extent. VNOs of the tamarins were described to determine whether they fit into 1 of 3 character states recently attributed to various New World monkeys. At birth, the two species of tamarins differed in the nature of communication between the VNO and nasopalatine duct (NPD). Two of 3 neonatal S. geoffroyi exhibited a fused VNO duct in a more dorsal position (adjacent to the nasal cavity) compared to that of L. rosalia. The VNO duct communicated with the NPD and was patent in neonatal L. rosalia. Both species appeared to have an age-related increase in the amount of sensory epithelium in the VNO. Subadult L. rosalia had caudal regions of the VNO that were exceptionally well-developed, similar to those of strepsirhine primates. Compared to subadults, all adult callitrichids appeared to have more ventral communications of the VNO duct directly into the NPD. Adult S. geoffroyi and L. chrysomelas both had VNO sensory epithelium separated by multiple patches of nonsensory epithelium. This contrasted with the VNOs of C. jacchus, which had a nearly continuous distribution of receptors on all surfaces of the VNO. The findings indicate that tamarins have delayed maturation of the VNO epithelium, and that some species have little or no perinatal function. These results also suggest that ontogenetic changes in craniofacial form may alter the position of the VNO in tamarins. The present study supports the use of at least two character states to categorize the VNO of various callitrichids, but it is suggested that one of these, previously called "reduced sensory epithelium" should be instead termed "interrupted sensory epithelium." The distribution of VNO sensory epithelium does not appear to reflect phylogenetic influences; it is more likely a functional characteristic that varies throughout postnatal life. Therefore, this chemosensory system has a high degree of plasticity relating to age and function, which in some instances can confound the use of characteristics as phylogenetic traits. Further study is needed to quantify VNO receptors in various species to determine if functional differences exist and if some species have more precocious VNO function than others.

Differential development of binding sites for four lectins in the vomeronasal system of juvenile mouse: from the sensory transduction site to the first relay stage

Four lectins -the galactose-specific BSI-B(4) (from Bandeiraea simplicifolia), the N-acetyl-galactosamine-specific DBA (from Dolichos biflorus), the L-fucose-specific UEA-I (from Ulex europaeus) and the (oligomeric N-acetylglucosamine)-specific LEA (from Lycopersicum esculentum)- were used to study the vomeronasal organ, vomeronasal nerves and accessory olfactory bulb of the mouse on embryonic days 11, 13, 15, 17 and 19, during the first 3 weeks after birth, at age 25 days, and after reaching maturity. No lectins labelled any structure before the 17th day of gestation, and even on the 19th day staining was sporadic and/or diffuse. During the early postnatal period, the lectin binding patterns differed from those of adults, but the division of the accessory olfactory bulb into anterior, rostral posterior and caudal posterior regions was already present and was shown up by the four lectins in a way that was coherent with the known zone-to-zone correspondence between the apical and basal zones of the sensory epithelium and the anterior and posterior accessory olfactory bulb, respectively. By age 25 days, the staining patterns were essentially those of the adult mouse. BSI-B(4) appears to be specific for the accessory vs. the main olfactory bulb throughout life.

Morphogenesis and growth of the soft tissue and cartilage of the vomeronasal organ in pigs

The morphology of the soft tissue and supporting cartilage of the vomeronasal organ of the fetal pig was studied from early stages to term. Specimens obtained from an abattoir were aged by crown-to-rump distance. Series of transverse sections show that some time before birth all structures--cartilage, connective tissue, blood vessels, nerves, glands and epithelia--are well developed and very similar in appearance to those of the adult. Furthermore, in transmission electron microscopy photomicrographs obtained at this stage the vomeronasal glands exhibit secretory activity.

Increase in the number of tyrosine hydroxylase-containing neurons in a primary culture system of the rat accessory olfactory bulb by co-culture with vomeronasal pockets

Previously, we established a culture system of the accessory olfactory bulb in order to investigate the functional role of each accessory olfactory bulb neurons in pheromonal signal processing. In the present study, we developed a co-culture system of cultured accessory olfactory bulb neurons with partially dissociated cells of the vomeronasal organ. The dissociated cells of the vomeronasal organ form spherical structures surrounding a central cavity in culture, referred to as the vomeronasal pockets. The projection and activity of olfactory receptor neurons affect the differentiation and maturation of main olfactory bulb neurons. It was also reported induction of tyrosine hydroxylase expression in main olfactory bulb neurons when they were co-cultured with explants of the olfactory epithelium. Thus, we investigated the effects of co-culture with vomeronasal pockets on the differentiation and/or maturation of cultured accessory olfactory bulb neurons in relation to tyrosine hydroxylase expression. The number of tyrosine hydroxylase-containing neurons developmentally increased over time in the accessory olfactory bulb culture. This increase was significantly enhanced by coculture with vomeronasal pockets. Interestingly, a significant change in tyrosine hydroxylase expression was not observed when main olfactory bulb neurons were co-cultured with vomeronasal pockets. Moreover, significant changes in tyrosine hydroxylase expression were not observed when accessory olfactory bulb neurons were co-cultured with olfactory epithelium explants, as was previously observed in co-culture of main olfactory bulb neurons and olfactory epithelium explants. These results suggest that the differentiation and/or maturation of accessory olfactory bulb neurons is modified by vomeronasal organ neurons via specific interactions between the sensory organ and its target.

Early development of the olfactory organ in sturgeons of the genus Acipenser: a comparative and electron microscopic study

Formation and morphology of the olfactory organ of vertebrates has been intensely studied in some taxa for more than a century. As a functionally important and complex sensory organ, its ontogenetic development has often been a matter of debate on higher-level craniate evolution. However, sufficient knowledge of structure and development of the olfactory organ in the crucial taxa needed for a serious phylogenetic reasoning is generally not available. This study aims at this essential primary data source, the detailed structure, morphogenesis, and character definition of the olfactory organ in more basal clades of jawed vertebrates (Gnathostomata). Sturgeon fishes (Acipenseriformes) as recent basal actinopterygians are expected to provide insight into archaic characters and character combinations in bony fishes. Thus, the development of the olfactory placodes of the sterlet, Acipenser ruthenus, and the Siberian sturgeon, Acipenser baerii, was followed histologically, by semi-thin serial sections, and by scanning and transmission electron microscopy. Except for the timing, virtually no differences were observed between the two species. The olfactory placodes become two-layered early in embryonic development. Both the superficial epidermal and the subepidermal layer can easily be distinguished and their development followed by ultrastructural properties. There are three different types of receptor cells: ciliated, microvillous, and crypt cells. The development of the ciliated and the less abundant microvillous receptor cells from the subepidermal layer of the placode is demonstrated. The non-sensory cells of the differentiated olfactory epithelium, i.e. ciliated non-sensory cells and supporting cells, exclusively derive from the superficial epidermal layer. In this respect, acipenserids clearly demonstrate close resemblance to the morphogenetic process found in the tetrapod Xenopus (Anura). The only other adequately described mode found in the actinopterygian zebrafish ( Danio rerio), is considered a derived character. In this case, all cells of the differentiated olfactory epithelium derive from one placodal cell layer. The mode of formation of the nasal sac and its ventilatory openings found in the acipenserids examined here, represents a widespread and probably a plesiomorphic condition of osteognathostomes. In both species, differentiation of the basic cellular composition of the olfactory epithelium is far advanced at the time of onset of extrinsic feeding.

Expression of Hes6 and NeuroD in the olfactory epithelium, vomeronasal organ and non-sensory patches

Basic helix-loop-helix transcription factors NeuroD and Hes6 promote neuronal differentiation. The expression of their genes in the olfactory epithelium (OE), vomeronasal organ (VNO) and the non-sensory patches of the posterior nasal cavity of mice was examined. As detected by in situ hybridization, Hes6 was expressed in a basal progenitor layer of the embryonic OE. After birth, the expression of Hes6 was detected in a cell layer above the basement membrane, globose basal cells (GBCs). Expression of NeuroD in the embryonic OE was in agreement with that previously described; and in the postnatal OE, it was detected in cells of GBC layer and cells upper to GBCs. In the VNO, Hes6 was expressed throughout the sensory epithelium (S-VNO) at embryonic day 12, and later became restricted to a single layer of cells in the basal region of the S-VNO, where Hes5-expressing undifferentiated cells were present. NeuroD was expressed throughout the S-VNO during the embryonic stage. After birth, Hes6 and NeuroD expressions were observed in the border between the S-VNO and non-sensory VNO. Immunohistochemistry using anti-NeuroD antibody revealed that NeuroD-positive cells were still present not only at the edges but also in the center of the S-VNO until P3. These findings suggest that Hes6 and NeuroD are expressed in progenitors of chemoreceptor neurons and that the expression of Hes6 precedes that of NEUROD: Moreover, in the regenerating VNO of bulbectomized mice, NeuroD-positive cells were observed both at the edges and in the center of the S-VNO, suggesting that neuronal turnover occurred in both regions. Moreover, in the dorsal fossa of the posterior nasal cavity, several non-sensory patches are formed between postnatal (P) days 10 and 21 because of programmed death of ORNs and GBCs. During embryonic stages, the expression of Hes6 and NeuroD in the OE showed no regional differences. At P3-P7, expression of NeuroD and Hes6 disappeared in the region corresponding to the presumptive non-sensory patches. The loss of these genes may stop the differentiation and may cause apoptosis of GBCs and ORNs.

The human vomeronasal organ. V. An interpretation of its discovery by Ruysch, Jacobson, or Kölliker, with an English translation of Kölliker (1877)

The vomeronasal organs (VNOs) of mammals are highly variable epithelial structures found bilaterally in the mucosa of the nasal septum. Whereas the discovery of the human VNO is traditionally ascribed to Frederick Ruysch (1703, 1724), the organ is named after Ludwig Levin Jacobson (1811, 1813) who described it in nonhuman mammals. We recently have pointed out controversies surrounding the incidence and structure of the enigmatic human VNO, and herein, we provide a historical analysis of its discovery. We present evidence that the honor of discovering the human VNO truly belongs to Kölliker (1877), and not to Ruysch. Ruysch illustrated the lateral view of a 2-year-old infant's nasal septum, and it is unclear whether the right nasal passage, the tubular VNO or its opening, or an unrelated duct is being indicated. Jacobson reported the VNO to be missing in humans. Its discovery in the human embryo can be related in part to later authors, such as Dursy (1869). Our reappraisal of the literature confirms that Kölliker was actually the first among these 18th-689th century investigators to provide evidence of the human VNO as a histologically identifiable structure in the fetus and the adult.

Neuronal nitric oxide synthase expression in the mouse vomeronasal organ during prenatal development

The presence and distribution of immunoreactivity for nitric oxide synthase type I and a panel of regulatory neuropeptides was investigated in the vomeronasal organ (VNO) of mouse embryos. Results show that nitric oxide synthase type I is first expressed in putative extrinsic nerve fibers reaching areas of vascular development at embryonic day 16 and in the vomeronasal nerve at embryonic day 15. Immunoreactivity for vasoactive intestinal peptide appears around developing vessels of the VNO during embryonic day 18. No immunoreactivity for atrial natriuretic peptide, substance P and calcitonin gene-related peptide is present in the VNO. It is concluded that, in the mouse, nitric oxide synthesis is a precocious event in the development of peripheral and central neural vomeronasal structures, representing a very early step in the neurochemical maturation of the VNO.

A comparative immunocytochemical study of development and regeneration of chemosensory neurons in the rat vomeronasal system

Vomeronasal neurons undergo continuous neurogenesis during development and after neuronal injury. We used immunocytochemical methods to compare different stages of the vomeronasal organ development to those of regeneration following vomeronasal nerve transection. At E15 and at 6 to 10 days after injury, nestin-positive cells were observed throughout the sensory epithelium. We did not find nestin immunoreactivity to be localized to the boundary region of the epithelium. The early appearance and wide distribution of nestin-positive cells suggests that they represent chemosensory precursor cells that develop and migrate vertically in the epithelium. Vomeronasal receptor cells degenerated 6 to 8 days after nerve transection, but axon terminals in the accessory olfactory bulb (AOB) continued to show the presence of the chemosensory specific marker (OMP) for up to ten days, a significant finding observed in this study. It is likely that the distance from the site of nerve transection may contribute to differences in the time course of anterograde and retrograde axon degradation. OMP-positive neurons were observed in the normal adult epithelium and to a much lesser extent 10-60 days after recovery from nerve transection. Axons from regenerated receptor cells did not reach the AOB during this time period. This failure to reestablish connections with target cells in the AOB could explain why OMP-positive cells were rarely observed among the regenerated cells in the vomeronasal epithelium.

Recent progress in the neurobiology of the vomeronasal organ

In many terrestrial tetrapodes, a pair of vomeronasal organs (VNOs), which are chemosensory apparatuses, are situated at the base of the nasal septum in the anterior nasal cavity. The purposes of this review are to summarize comparative neuroanatomy and to introduce recent progress in neurobiological studies of the VNO. Five types of VNOs can be identifiable in terms of anatomical organization; snakes possess the most complex one. Sensory cells in the VNO, vomeronasal receptor neurons (VRNs), are located in its neuroepithelium, vomeronassal sensory epithelium. The VRNs retain the characteristic of epithelial cells in that they are born continuously from progenitor cells. They contain two prominent subcellular structures: microvilli and extraordinarily large amounts of smooth endoplasmic reticulum and a few unique glycoconjugates. The VRNs express two types of G-protein -subunits: Gi(alpha2) and Go(alpha) and each of them is coupled with putative pheromone receptors, V1Rs and V2Rs, respectively. Recent physiological and biochemical studies have demonstrated that pheromones depolarize the V1R-Gi(alpha2) and V2R-Go(alpha) VRNs via IP(3)-mediated mechanisms. The VRNs do not show adaptation and are ultrasensitive to putative pheromones. Other than being a chemosensory organ, the VNO and its primordium might play important roles for brain development; hypothalamic neurons that produce gonadotropin-releasing hormone are born in the VNO primordium and a few other neuron-like cells may be born in the VNO primordium and VNO. In human fetuses, anatomical findings strongly suggest that their VNOs contain a neuroepithelium. By contrast, it is unlikely that adult human VNO serves as a chemosensory organ.

Expression of netrin-1 and netrin-1 receptor, DCC, in the rat olfactory nerve pathway during development and axonal regeneration

Netrin-1 is a bifunctional secreted protein that directs axon extension in various groups of developing axonal tracts. The transmembrane DCC (deleted in colorectal cancer) receptor is described as netrin-1 receptor and is involved in the attractive effects of netrin-1. In this study, we examined the spatio-temporal expression patterns of both netrin-1 and DCC in the rat olfactory system at different stages of development and during axonal regeneration following unilateral bulbectomy. High DCC expression was detected on the pioneer olfactory axons as they are extending toward the telencephalon. This expression was transient since from embryonic day 16 onwards, DCC was no longer detected along the olfactory nerve path. From embryonic day 14 until birth, DCC was also expressed within the mesenchyme surrounding the olfactory epithelium. During the same period, netrin-1 protein was detected along the trajectory of olfactory axons up to the olfactory bulb and its expression pattern in the nasal mesenchyme largely overlapped that of DCC. Moreover, netrin-1 continued to be present during the two first post-natal weeks, and a weak protein expression still persisted in the dorso-medial region of the olfactory epithelium in adult rats. While unilateral bulbectomy induced a transient up-regulation of netrin-1 in the lamina propria, particularly in the dorso-medial region of the neuroepithelium, no DCC expression was detected on the regenerating olfactory axons. In the developing olfactory bulb, the extension of mitral cell axons was associated with DCC presence while netrin-1 was absent along this axonal path. DCC was also highly expressed in the newly formed glomeruli after birth, and a weak DCC expression was still detected in the glomerular layer in adult rats. Taken together, these data support the notion that netrin-1, via DCC expressed on axons, may play a role in promoting outgrowth and/or guidance of pioneering olfactory axons toward the olfactory bulb primordium. Moreover, association of netrin-1 with mesenchymal DCC may provide a permissive environment to the growth of both pioneer and later-growing axons. The maintenance of netrin-1 expression in the nasal mesenchyme of adult rats as well as its regional up-regulation following unilateral bulbectomy infer that netrin-1, even in the absence of DCC, may be involved in the process of axonal growth of newly differentiated olfactory receptor neurons probably through the use of other receptors.

Expression and immunohistochemical localization of heparan sulphate proteoglycan N-syndecan in the migratory pathway from the rat olfactory placode

N-syndecan, a membrane-bound heparan sulphate proteoglycan, is abundantly present in the developing nervous system and thought to play important roles in the neurite outgrowth. In the present study, we examined the distribution of N-syndecan in the migratory route from the rat olfactory placode using immunohistochemistry and in situ hybridization. At embryonic day 15, both heparan sulphate and N-syndecan immunoreactivities were localized in and around the migrating cell clusters, which contained luteinizing hormone-releasing hormone (LHRH) and calbindin D-28k. Immunoreactivity for other glycosaminoglycan chains, such as chondroitin and keratan sulphate, and core proteins of the chondroitin sulphate proteoglycan, neurocan and phosphacan, were barely detected in the migratory pathway from the olfactory placode. By in situ hybridization histochemistry, N-syndecan mRNA was localized in virtually all of migrating neurons as well as in cells of the olfactory epithelium and the vomeronasal organ. N-syndecan immunoreactivity surrounded cells migrating along the vomeronasal nerves that were immunoreactive for neural cell adhesion molecules, NCAM, L1 and TAG-1. Considering that NCAM is implicated in the migratory process of LHRH neurons and specifically binds to heparan sulphate, it is likely that a heterophilic interaction between NCAM and N-syndecan participates in the neuronal migration from the rat olfactory placode.

Calretinin immunoreactivity in the prenatally developing olfactory systems of the tree shrew Tupaia belangeri

The prenatal patterns of calretinin immunoreactivity were studied in the olfactory systems of Tupaia belangeri. We investigated the peripheral and primary central parts of the vomeronasal system and of the main olfactory system from the 19th to the 43rd (last) day of gestation and compared the findings with the known calretinin immunoreactivity patterns in adult T. belangeri and the published data on other mammals. The onset of calretinin immunoreactivity was noted in the main olfactory system on the 23rd day of gestation and, in the vomeronasal system, on the 25th day of gestation: single precursors of receptor cells with calretinin immunoreactive perikarya and processes were observed in both epithelia. Their neuronal identity was proven by olfactory marker protein immunoreactivity. On the 42nd day of gestation, almost all receptor cells and nerve fibers, many interneurons and projecting cells were calretinin immunoreactive in the main olfactory and in the vomeronasal systems. In contrast to the intensive calretinin labeling previously observed in virtually all vomeronasal epithelial cells of adult T. belangeri, among developing receptor cells a population of intensively labeled, basally located perikarya was distinguishable from a population of less intensively stained, more apically located ones. In the main olfactory epithelium of fetal T. belangeri, calretinin immunoreactive receptor cells occurred in the middle layers. Whereas in the vomeronasal sensory epithelium differently reacting layers of receptor cells might represent the two known subfamilies of receptor cells, in the main olfactory epithelium the differing calretinin expression in the layers of the epithelium, most probably, did not reflect known subfamilies of odour receptor cells. Transiently, ectopic calretinin immunoreactive receptor cells were observed in the future non-sensory epithelium of the vomeronasal organ.

Ephrin-As as receptors in topographic projections

The Eph family of receptor tyrosine kinases and their 'ligands', the ephrins, have been implicated in a large number of developmental processes, such as boundary formation, cell migration, axon guidance and vasculogenesis. A characteristic of the EphB subclass is that both EphBs and transmembrane-anchored ephrin-Bs function as receptors and as ligands, a phenomenon commonly described as 'bi-directional signalling'. Here we review recent data indicating that EphA receptors and glycosylphosphatidylinositol (GPI)-anchored ephrin-As can also mediate bi-directional signalling. Moreover, characterization of the expression of ephrin-As on axons of the retinotectal and vomeronasal projections suggests that the EphA subfamily is involved in both repulsive and attractive guidance mechanisms during establishment of neuronal connections.

Genetic labelling of specific axonal pathways in the mouse central nervous system

We report on transgenic mouse lines in which several sensory systems in the brain are specifically visualized genetically. We employed GAP-lacZ as an axon-targeted reporter protein that was constructed by fusing the membrane-anchoring domain of the GAP-43 protein to lacZ. The reporter gene was introduced into the genome under the control of a promoter element of Brn3b transcription factor to establish transgenic mouse lines. The individual lines thus generated afforded clear images of specific axonal pathways of the visual, vomeronasal, pontocerebellar, and auditory systems. The reporter protein labelled the entire axonal process as well as the cell body of developing and mature neurons on staining with X-gal. We show that these lines facilitate the developmental and anatomical study of these neural systems. This strategy should be applicable to a variety of neural systems by using various specific promoter elements.

[A comparative morphometrical study on development of anterior septonasal accessory organs in the cleft palate mice]

OBJECTIVE

The aim of this study was to investigate whether cleft palate is associated with the mal-development of the vomer, Jacobson's organ and para-septal cartilage.

METHODS

Sixty craniomaxillary complexes, which came from 30 pairs of trisomy 18 with cleft palate (Ts + CP) and euploid mouse fetuses were precisely orientated in the coronal plane, and serially sectioned at 7 microns thickness. With the aid of computer imaging analysis system the vomer in coronal and sagittal direction, Jacobson's organ and para-septal cartilage in the boundary plane of the primary and secondary palate were measured and compared quantitatively.

RESULTS

The hard tissues (including the vomer and para-septal cartilage) of the anterior nasal septum in Ts + CP foetuses show serious developmental hypoplasia, however, the Jacobson's organ had no significant different between the cleft and non-cleft samples.

CONCLUSION

Development hypoplasia of these hard tissues may be more closely related with the autosomal trisomic condition more than the cleft palate.

Reappraisal of the vomeronasal system of catarrhine primates: ontogeny, morphology, functionality, and persisting questions

The vomeronasal organ (VNO) is a chemosensory organ that functions in sociosexual communication in many vertebrates. In strepsirhine primates and New World monkeys, the bilateral VNOs are traditionally understood to exist as a well-developed chemosensory epithelial unit. In contrast, the VNOs of catarrhine primates are thought to be absent or exist only as reduced epithelial tubes of uncertain function. However, the VNO of New World monkeys shows substantial variation in the extent of sensory epithelium. Recent findings that the chimpanzee (Pan troglodytes) possesses a VNO similar to humans suggest the variability of the VNO among haplorhine primates may be more extensive than previously thought, and perhaps more at par with that observed in chiropterans. The atypical histologic structure and location of the human/chimpanzee VNO suggest accessory glandular secretion and transport functions. Other catarrhine primates (e.g., Macaca spp.), may truly be characterized by VNO absence. Unique aspects of facial growth and development in catarrhine primates may influence the position or even presence of the VNO in adults. These recent findings demonstrate that previous investigations on some catarrhine primates may have missed the VNO and underestimated the extent of variability. As an understanding of this variation increases, our view of VNO functionality and associated terminology is changing. Further investigations are needed to consider phylogenetic implications of VNO variability and the association of craniofacial form and VNO anatomic position in primates.

Immunohistochemical localization of calbindin D-28k in the migratory pathway from the rat olfactory placode

The spatiotemporal localization of calbindin D-28k (Calb), a calcium-binding protein, was examined immunohistochemically in the developing rat olfactory system with special reference to cell migration from the olfactory placode. Calb immunoreactivity was first detected at embryonic day 12 (E12) in a few cells just outside the olfactory epithelium, and at E13, Calb-immunoreactive cells were found scattered in the laminin-rich mesenchyme. By E14, Calb-immunoreactive cells had increased in number and were seen along the entire migratory route between the vomeronasal organ, a derivative of the medial olfactory pit, and the ventromedial surface of the telencephalic vesicle. Calb neurones were not seen in the olfactory epithelium, a derivative of the lateral olfactory pit. Although the distribution pattern of Calb-immunoreactive cells was similar to that of luteinizing hormone releasing hormone (LHRH)-producing neurones, which are known to originate in the vomeronasal organ and migrate into the forebrain, Calb and LHRH immunoreactivities were contained in separate neuronal populations. Calb-immunoreactive cells were localized along the vomeronasal nerves, identified by labelling the vomeronasal organ with the lipophilic dye, DiI, and strongly immunoreactive for neural cell adhesion molecule (NCAM). These data strongly suggest that, in addition to LHRH neurones, the rat vomeronasal organ generates Calb-immunoreactive neurones which migrate along the vomeronasal nerves to enter the forebrain. The final fate and functional importance of these cells remains to be determined.

Human vomeronasal organ function: a critical review of best and worst cases

The human vomeronasal organ (VNO) has been the subject of some interest in the scientific literature and of considerable speculation in the popular science literature. A function for the human VNO has been both dismissed with ridicule and averred with conviction. This question of VNO function has been needlessly tied to the separate question of whether there is any place for pheromone communication among humans, a topic that is itself bogged down in conflicting definitions. This review is an attempt to weigh the evidence for and against human VNO function, to deconvolve that question from the question of pheromone communication and finally to provide a working definition of 'pheromone'. Further experimental work is required to resolve the conflicting evidence for and against human VNO function but chemical communication does appear to occur among humans. However, several examples reported in the literature do not meet the proposed definition for communication by pheromones: 'chemical substances released by one member of a species as communication with another member, to their mutual benefit'.

Immunohistochemical studies on the differential maturation of three types of olfactory organs in the rats

Differential maturation of three types of olfactory organs, the olfactory epithelium (OE), the vomeronasal organ (VNO) and the septal olfactory organ of Masera (MO), was examined immunohistochemically in embryonic and newborn rats by the use of antiprotein gene product 9.5 (PGP 9.5) serum. These olfactory organs were derived in common from the olfactory placode as neuroepithelia. In the OE, PGP 9.5-immunopositive olfactory cells first appeared at 13 days of gestation. The OE maturated completely, and showed the same cytological features as in the adult at 20 days of gestation. The MO first appeared as a dense mass of PGP 9.5-immunopositive sensory cells on the most ventrocaudal part of the nasal septum at 15 days of gestation and was evidently isolated from the OE by the decrease of immunopositive cells in the intercalated epithelium between the OE and the MO at 20 days of gestation. However, even at 7 days after birth, the MO did not complete its development and contained sensory cells aggregating in the mass. The VNO was separated from the nasal cavity at 13 days of gestation as a tubular structure of a neuroepithelium including PGP 9.5-immunopositive sensory cells. These cells gradually increased in number in the sensory epithelium of the VNO and extended their dendritic processes to the free surface at 7 days after birth. These findings clarified the differential maturation of these olfactory organs. That is, the OE completes its development before birth, while the MO and VNO after birth.

Vomeronasal epithelial cells of human fetuses contain immunoreactivity for G proteins, Go(alpha) and Gi(alpha 2)

Two G protein subfamilies, Go(alpha) and Gi(alpha 2), were identified and localized immunohistochemically in the vomeronasal organ (VNO) of 5-month-old human fetuses. Immunoreactivity for Go(alpha) and Gi(alpha 2) was present in a subset of vomeronasal epithelial cells. Prominent immunoreactivity was observed in apical processes and their apical terminals facing onto the vomeronasal lumen. Nerve fibers associated with the VNO exhibited intense immunoreactivity for Go(alpha) and weak immunoreactivity for Gi(alpha 2). Since Go(alpha) and Gi(alpha 2) are characteristically expressed and coupled with putative pheromone receptors in rodent vomeronasal receptor neurons, the present results suggest the possibility that vomeronasal epithelial cells containing Go(alpha) and Gi(alpha 2) in human fetuses are chemosensory neurons.

A role for the EphA family in the topographic targeting of vomeronasal axons

We have investigated the role of the Eph family of receptor tyrosine kinases and their ligands in the establishment of the vomeronasal projection in the mouse. Our data show intriguing differential expression patterns of ephrin-A5 on vomeronasal axons and of EphA6 in the accessory olfactory bulb (AOB), such that axons with high ligand concentration project onto regions of the AOB with high receptor concentration and vice versa. These data suggest a mechanism for development of this projection that is the opposite of the repellent interaction between Eph receptors and ligands observed in other systems. In support of this idea, when given the choice of whether to grow on lanes containing EphA-F(c)/laminin or F(c)/laminin protein (in the stripe assay), vomeronasal axons prefer to grow on EphA-F(c)/laminin. Analysis of ephrin-A5 mutant mice revealed a disturbance of the topographic targeting of vomeronasal axons to the AOB. In summary, these data, which are derived from in vitro and in vivo experiments, indicate an important role of the EphA family in setting up the vomeronasal projection.

Expression of glycoproteins in the vomeronasal organ reveals a novel spatiotemporal pattern of sensory neurone maturation

The main olfactory and the accessory olfactory systems are both anatomically and functionally distinct chemosensory systems. The primary sensory neurones of the accessory olfactory system are sequestered in the vomeronasal organ (VNO), where they express pheromone receptors, which are unrelated to the odorant receptors expressed in the principal nasal cavity. We have identified a 240 kDa glycoprotein (VNO(240)) that is selectively expressed by sensory neurones in the VNO but not in the main olfactory neuroepithelium of mouse. VNO(240) is first expressed at embryonic day 20.5 by a small subpopulation of sensory neurones residing within the central region of the crescent-shaped VNO. Although VNO(240) was detected in neuronal perikarya at this age, it was not observed in the axons in the accessory olfactory bulb until postnatal day 3.5. This delayed appearance in the accessory olfactory bulb suggests that VNO(240) is involved in the functional maturation of VNO neurones rather than in axon growth and targeting to the bulb. During the first 2 postnatal weeks, the population of neurones expressing VNO(240) spread peripherally, and by adulthood all primary sensory neurones in the VNO appeared to be expressing this molecule. Similar patterns of expression were also observed for NOC-1, a previously characterized glycoform of the neural cell adhesion molecule NCAM. To date, differential expression of VNO-specific molecules has only been reported along the rostrocaudal axis or at different apical-basal levels in the neuroepithelium. This is the first demonstration of a centroperipheral wave of expression of molecules in the VNO. These results indicate that mechanisms controlling the molecular differentiation of VNO neurones must involve spatial cues organised, not only about orthogonal axes, but also about a centroperipheral axis. Moreover, expression about this centroperipheral axis also involves a temporal component because the subpopulation of neurones expressing VNO(240) and NOC-1 increases during postnatal maturation.