Neurogenesis and migration of receptor neurons in the vomeronasal sensory epithelium in the opossum, Monodelphis domestica

Morphological Analysis for Neuron-Like Cells in the Vomeronasal Organ of Human Fetuses at the Middle of Gestation

The vomeronasal organ (VNO) of 5-month-old fetuses was examined immunohistochemically by the use of an antiserum to protein gene product 9.5 (PGP). The purpose was to identify if the human fetal VNO is lined by neuroepithelium. The PGP antiserum labeled abundant cells within the vomeronasal epithelium (VE), nerve fiber bundles in its lamina propria, and cells associated with these bundles. PGP-immunoreactive (ir) vomeronasal epithelial cells were classified into three subtypes. Type I cells, about 44% of the total cells observed, did not have any processes and tended to be located in the basal layer of the VE. Type II cells, about 37% had a single apical process that projected toward the lumen, ending at the epithelial surface. Type III cells sent a prominent process mainly toward the basement membrane, and occupied about 19% of the total cells observed. In the lamina propria, a considerable number of PGP-ir cells was observed. Some of them were present in nerve fiber bundles and contained processes parallel to the bundles. In addition, PGP-ir nerve fiber bundles and cells associated with them were even present in the portion of the nasal septal mucosa that was very close to the brain. The present results strongly suggested that the VE in human fetuses at mid-gestation is a neuroepithelium and that the VE may produce migrating cells toward the brain.

A lifetime of neurogenesis in the olfactory system

Neurogenesis continues well beyond embryonic and early postnatal ages in three areas of the nervous system. The subgranular zone supplies new neurons to the dentate gyrus of the hippocampus. The subventricular zone supplies new interneurons to the olfactory bulb, and the olfactory neuroepithelia generate new excitatory sensory neurons that send their axons to the olfactory bulb. The latter two areas are of particular interest as they contribute new neurons to both ends of a first-level circuit governing olfactory perception. The vomeronasal organ and the main olfactory epithelium comprise the primary peripheral olfactory epithelia. These anatomically distinct areas share common features, as each exhibits extensive neurogenesis well beyond the juvenile phase of development. Here we will discuss the effect of age on the structural and functional significance of neurogenesis in the vomeronasal and olfactory epithelia, from juvenile to advanced adult ages, in several common model systems. We will next discuss how age affects the regenerative capacity of these neural stem cells in response to injury. Finally, we will consider the integration of newborn neurons into an existing circuit as it is modified by the age of the animal.

How neurogenesis finds its place in a hardwired sensory system

So far most studies on adult neurogenesis aimed to unravel mechanisms and molecules regulating the integration of newly generated neurons in the mature brain parenchyma. The exceedingly abundant amount of results that followed, rather than being beneficial in the perspective of brain repair, provided a clear evidence that adult neurogenesis constitutes a necessary feature to the correct functioning of the hosting brain regions. In particular, the rodent olfactory system represents a privileged model to study how neuronal plasticity and neurogenesis interact with sensory functions. Until recently, the vomeronasal system (VNS) has been commonly described as being specialized in the detection of innate chemosignals. Accordingly, its circuitry has been considered necessarily stable, if not hard-wired, in order to allow stereotyped behavioral responses. However, both first and second order projections of the rodent VNS continuously change their synaptic connectivity due to ongoing postnatal and adult neurogenesis. How the functional integrity of a neuronal circuit is maintained while newborn neurons are continuously added—or lost—is a fundamental question for both basic and applied neuroscience. The VNS is proposed as an alternative model to answer such question. Hereby the underlying motivations will be reviewed.

Goα expression in the vomeronasal organ and olfactory bulb of the tammar wallaby

The vomeronasal organ (VNO) detects pheromones via 2 large families of receptors: vomeronasal receptor 1, associated with the protein Giα2, and vomeronasal receptor 2, associated with Goα. We investigated the distribution of Goα in the developing and adult VNO and adult olfactory bulb of a marsupial, the tammar wallaby. Some cells expressed Goα as early as day 5 postpartum, but by day 30, Goα expressing cells were distributed throughout the receptor epithelium of the VNO. In the adult tammar, Goα appeared to be expressed in sensory neurons whose nuclei were mostly basally located in the vomeronasal receptor epithelium. Goα expressing vomeronasal receptor cells led to all areas of the accessory olfactory bulb (AOB). The lack of regionally restricted projection of the vomeronasal receptor cell type 2 in the tammar was similar to the uniform type, with the crucial difference that the uniform type only shows expression of Giα2 and no expression of Goα. The observed Goα staining pattern suggests that the tammar may have a third accessory olfactory type that could be intermediate to the segregated and uniform types already described.

Neurogenesis in subclasses of vomeronasal sensory neurons in adult mice

The vomeronasal sensory epithelium contains two distinct populations of vomeronasal sensory neurons. Apical neurons express G(i) (2) (α) -linked V1R vomeronasal receptors and project to the anterior portion of the accessory olfactory bulb, while basal neurons express G(o) (α) -linked V2R receptors and project to the posterior portion. Sensory neurons expressing V1R and V2R vomeronasal receptors are sensitive to different stimuli. Neurons in the vomeronasal system undergo continuous cell turnover during adulthood. To analyze over time neurogenesis of the different sensory cell populations, adult mice were injected with bromodeoxyuridine (BrdU) and sacrificed at postinjection days 1, 3, 5, 7, and 11. Newborn vomeronasal neurons were revealed by antibodies against BrdU while subclasses of vomeronasal neurons were identified using antibodies against G(o) (α) or G(i) (2) (α) proteins. To ascertain whether G proteins are early expressed during neurogenesis, multiple labeling experiments using PSA-NCAM and doublecortin were performed. Distribution of BrdU-labeled cells was analyzed in angular segments from the margin of the sensory epithelium. No sexual differences were found. Within survival groups, BrdU-G(o) (α) labeled cells were found more marginally when compared with BrdU-G(i) (2) (α) labeled cells. The number of BrdU-positive cells decreased from day 1 to day 3 to remain constant afterwards. The relative proportions of BrdU-G(i) (2) (α) and BrdU-G(o) (α) labeled cells remained similar and constant from postinjection day 1 onwards. This rate was also comparable with BrdU-positive cells starting day 3. These results indicate an early, constant, and similar rate of neurogenesis in the two major subclasses of vomeronasal neurons, which suggests that both cell populations maturate independently.

Fate of marginal neuroblasts in the vomeronasal epithelium of adult mice

Chemical stimuli are sensed through the olfactory and vomeronasal epithelia, and the sensory cells of both systems undergo neuronal turnover during adulthood. In the vomeronasal epithelium, stem cells adjacent to the basal lamina divide and migrate to replace two classes of sensory neurons: apical neurons that express G(i2alpha)-linked V1R vomeronasal receptors and project to the anterior accessory olfactory bulb, and basal neurons that express G(oalpha)-linked V2R receptors and project to the posterior accessory olfactory bulb. Most of the dividing cells are present in the margins of the epithelium and only migrate locally. Previous studies have suggested that these marginal cells may participate in growth, sensory cell replacement or become apoptotic before maturation; however, the exact fate of these cells have remained unclear. In this work we investigated the fate of these marginal cells by analyzing markers of neurogenesis (bromodeoxyuridine incorporation), apoptosis (caspase-3), and neuronal maturation (olfactory marker protein and Neurotrace Nissl stain). Our data reveal a pool of dividing cells in the epithelial margins that predominantly give rise to mature neurons and only rarely undergo apoptosis. Newly generated cells are several times more numerous than apoptotic cells. These marginal neuroblasts could therefore constitute a net neural addition zone during adulthood.

Neurogenesis, migration, and apoptosis in the vomeronasal epithelium of adult mice

The location of neurogenesis and the direction of migration of neurons in the adult mouse vomeronasal organ is controversial. Cell division occurs at the center, and particularly, at the edges of the epithelium. Newly generated cells at the center of the epithelium participate in neurogenesis, however, it is unknown to what extent dividing cells at the edges participate in growth, become apoptotic or mature into neurons. Premitotic cells were labeled with bromodeoxyuridine (BrdU) in adult mice and animals allowed to survive for different postinjection periods. The terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end-labeling (TUNEL) method was used to show the distribution of apoptotic cells. The vertical and horizontal position of BrdU-labeled cells was analyzed as a function of postinjection survival time. Vertical and horizontal migration of BrdU-labeled cells were detected. Cells in the central portions of the epithelium migrated vertically to become neurons as demonstrated by co-expression of olfactory marker protein. Cells at the edges migrated horizontally very slowly (less than 10% of the distance from the edge to the center of the epithelium per month), thus indicating that these cells participate in cell renewal exclusively in marginal regions. Neural turnover in the mouse vomeronasal epithelium, therefore appears to occur through a process of vertical migration. Data on the distribution of apoptotic cells indicate that a number of dividing cells throughout the epithelium, but particularly at the edges, die before becoming functional neurons. Accordingly, most dividing cells at the edges probably constitute a reservoir of stem cells dying before differentiation.

Calbindin D28k, parvalbumin, and calretinin immunoreactivity in the main and accessory olfactory bulbs of the gray short-tailed opossum,Monodelphis domestica

The vertebrate main and accessory olfactory bulbs (MOB and AOB) are the first synaptic sites in the olfactory pathways. The MOB is a cortical structure phylogenetically well conserved in its laminar structure and overall synaptic organization, while the AOB has significant species variation in size. In order to better understand signal processing in the two olfactory systems and the species differences, immunocytochemical staining and analysis were done of the neuronal expression patterns of the calcium-binding proteins calbindin D28k (CB), parvalbumin (PV), and calretinin (CR) in the MOB and AOB in a marsupial species, the gray short-tailed opossum, Monodelphis domestica. In the MOB, antibody to CB labeled periglomerular cells, superficial short axon cells / Van Gehuchten cells; antibody to PV labeled Van Gehuchten cells; and antibody to CR immunostained periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. In the AOB, CB immunoreactivity was detected in periglomerular cells and a subpopulation of granule cells; antibody to PV labeled the superficial short axon cells / Van Gehuchten cells and granule cells; and antibody to CR labeled a small number of periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. These results showed that the patterns of CB, PV, and CR expression differ in the opossum main and accessory olfactory bulbs and differ from that in other animal species. These varying patterns of neuronal immunostaining may be related to the different functions of the main and accessory olfactory bulbs and to the differing signal processing features.

Sexual dimorphism in the central nervous system of marsupials

It is now evident that gonadal steroids, acting within a limited critical period during fetal or neonatal life, bring about sexual differentiation of both the reproductive tract and the central nervous system (CNS) in eutherians. This results in structural dimorphism in several regions of the brain and spinal cord and the programming of future patterns of adult reproductive behavior. At birth the CNS of marsupials is very underdeveloped and debate continues as to the importance of hormones in its sexual differentiation. Nevertheless, some sexually dimorphic regions have been identified, including the lateral septal nucleus in the hypothalamus and the spinal nucleus of the bulbocavernosus and dorsolateral nucleus in the spinal cord, but interestingly not the cremasteric nucleus, which is dimorphic in eutherians. To date, no apparent sex differences in estrogen and androgen receptor-immunoreactive structures have been detected in the marsupial brain; however, higher levels of aromatase activity during early development in male opossums have been reported. Sex differences have been identified in the localization of cholecystokinin-immunoreactive structures in the marsupial brain indicating that the expression of this neuropeptide is differentially regulated in each sex. A sex difference also exists in the density of arginine vasopressin-immunoreactive fibers. Arguments continue as to whether sexually dimorphic behavior in marsupials, as in eutherians, is largely predetermined by hormones acting on the CNS early in development or if it is entirely dependent on the adult steroid hormonal environment.

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.

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.

Proliferation and migration of receptor neurons in the vomeronasal organ of the adult mouse

Cell proliferation and differentiation in the vomeronasal organ of the adult mouse was studied by bromodeoxyuridine (BrdU) immunohistochemistry coupled to immunostaining for specific markers of the differentiation, such as carnosine, B50-GAP43 (growth-associated protein) and stathmin. The present study shows that three populations of proliferating elements are present in the vomeronasal sensory epithelium that are placed, respectively, in the supporting cell layer, at the boundaries between the sensory epithelium (S-VNO) and the non-sensory (NS-VNO) and in the basal region of the S-VNO. The number of dividing cells at the boundaries of the S-VNO is by far prevailing. Few proliferating cells located adjacent to the basal membrane are, however, present 1 day after BrdU inoculations. Seven days after BrdU treatment immunopositive nuclei were detected in more central regions of the VNO and at longer survival times they were also positive to carnosine, a marker of fully differentiated neurons. In conclusion, the present results suggest that at least two populations of VNO neuronal precursors are responsible for cell replacement throughout life.

Neurogenesis in the vomeronasal epithelium of adult rats: evidence for different mechanisms for growth and neuronal turnover

The pattern of cell migration during neuronal turnover in the vomeronasal sensory epithelium (VN-SE) is controversial. In mice, proliferating cells were detected at the edges and were described as migrating to the center of the VN-SE. In rats, in addition to proliferating cells at the margins of the epithelium, dividing cells are also present along the entire basal lamina of the VN-SE. In marsupials, dividing cells have also been observed in the margins and in the center of the VN-SE, the latter of which migrate vertically and become neurons. To investigate whether the process of neuronal turnover in placental mammals consists of horizontal and/or vertical migration, and whether or not this process is common to mammals, adult rats were injected with bromodeoxyuridine (BrdU) and allowed to survive for different periods of time. The distribution of BrdU-labeled cells in the horizontal and vertical dimension of the VN-SE was analyzed as a function of time. Both horizontal and vertical migrations of BrdU-labeled cells were detected. Since cells in the center of the VN-SE migrate vertically, and, as demonstrated by coexpression of markers of neuronal maturity and BrdU, become mature one day after undergoing mitosis, it is very likely that these cells participate in neuronal turnover. Conversely, because cells in the margins of the VN-SE stop migrating horizontally on day 14 before they have reached the center of the VN-SE, and since the VN-SE continues to grow during adulthood, it is likely that most of these latter cells constitute pools for growth.

Cell turnover in the vomeronasal epithelium: evidence for differential migration and maturation of subclasses of vomeronasal neurons in the adult opossum

Previous investigations of cell turnover in the mammalian vomeronasal sensory epithelium (VN-SE) raised two issues. First, if, in addition to the already demonstrated vertical migration, horizontal migration from the edges of the VN-SE participates in neuronal replacement. Second, whether or not migration and maturation is differential in upper and lower populations of vomeronasal neurons, since these two cell populations are chemically, physiologically, functionally, and perhaps evolutionarily different. By injecting bromodeoxyuridine (BrdU) into adult opossum (Monodelphis domestica) and permitting different survival times, the pattern of distribution of BrdU-labeled cells was analyzed. No evidence of horizontal migration in neuronal replacement was found. To investigate vertical migration and maturation of subclasses of vomeronasal neurons, double immunohistochemistry of BrdU and markers of the lower (G(oalpha) protein) and upper [G(i2alpha) protein and olfactory marker protein (OMP)] cell populations were performed. Three days after administration of BrdU, some mature neurons were observed in both lower and upper layers of the VN-SE, as demonstrated by coexpression of BrdU with G(oalpha) protein and OMP, respectively. The data on vertical distribution, however, indicate that most of the daughter cells enter the G(oalpha)-protein-expressing zone of the VN-SE by day 5, whereas most daughter cells do not reach the G(i2alpha)-protein-expressing zone until day 7, suggesting that these two populations mature at slightly different rates. These results are the first evidence of differential neurogenesis of subclasses of vomeronasal neurons.

Characterisation of glycoconjugate sugar residues in the vomeronasal organ of the armadillo Chaetophractus villosus (Mammalia, Xenarthra)

Conventional carbohydrate histochemistry and the binding patterns of 21 lectins were analysed to characterise the glycoconjugate content in the components of the vomeronasal organ of the armadillo Chaetophractus villosus. The mucomicrovillous complex of the sensory epithelium bound most of the lectins studied. No reaction was observed with Con A, PSA, S-Con A and SBA, and the sustentacular cells were-stained with UEA-I, DSL, LEL, STL and Con A. The vomeronasal receptor neurons were labelled with S-WGA, WGA, PNA, UEA-I, STL, Con A, S-Con A, ECL and RCA120. The basal cell layer reacted with S-WGA, WGA, LCA, UEA-I, DSL, LEL, STL, Con A, JAC and VVA. The nonsensory epithelium exhibited a differential staining in relation to the different components. The mucociliary complex stained with ECL, DBA, JAC, RCA120, STL, LCA, PHA-E, PHA-L, LEL, BSL-I and VVA. However, SJA and UEA-I stained the mucus complex lining a subpopulation of columnar cells. The cytoplasm and cell membranes of columnar cells was labelled with DBA, DSL and LCA. The apical region of these cells exhibited moderate reactivity with LEL and SJA. None of the lectins bound specifically to secretory granules of the nonsecretory cells. Basal cells of the nonsensory epithelium were labelled with DSL, LEL, LCA, BSL-I and STL. The vomeronasal glands showed a positive reaction with WGA, DSL, LEL, LCA, DBA, PNA, RCA120 and SBA. Subpopulations of acinar cells were observed with ECL, S-WGA, Con A, S-Con A and DBA. PNA and RCA120 stained the cells lining the glandular ducts. In comparison with previous results obtained in the olfactory mucosa of the same group of armadillos, the carbohydrate composition of the vomeronasal organ sensory epithelium differed from the olfactory sensory epithelium. This is probably related to the different nature of molecules involved in the perireceptor processes.

Ultrastructural characterisation of the olfactory mucosa of the armadillo Dasypus hybridus (Dasypodidae, Xenarthra)

The ultrastructure of the olfactory mucosa of the armadillo Dasypus hybridus was studied. A comparison with the olfactory mucosa of another armadillo (Chaetophractus villosus) was made. The olfactory mucosa of D. hybridus shows many features which are similar to those of other mammals. Interestingly, it differs from the olfactory mucosa of the armadillo C. villosus. A suggestion is made that these differences may be due to differences in the digging habits of these species. In Dasypus, the supporting cells (SCs) showed dense vacuoles, multivesicular bodies and lysosome-like bodies probably related with the endocytotic system. The SCs show a dense network of SER presumably associated with xenobiotic mechanisms. The olfactory receptor neurons exhibit lysosome-like bodies and multivesicular bodies in their perikarya. These organelles suggest the presence of an endocytotic system. Duct cells of Bowman's glands exhibit secretory activities. Bowman's glands are compound-branched tubulo-acinar mixed glands with merocrine secretory mechanisms.

The vomeronasal organ of the South American armadillo Chaetophractus villosus (Xenarthra, Mammalia): anatomy, histology and ultrastructure

The vomeronasal organ (VNO) is a chemoreceptive structure that has not been extensively studied in the Xenarthran order. Tissue samples from the VNO of the armadillo Chaetophractus villosus were prepared for light and electron microscopy. The VNO is located in the anterior part of the base of the nasal septum. It is tubular in shape, approximately 18 mm in length and opens in the rostral region of the nasal cavity and with a blind caudal end. Its lumen is lined by sensory (SE) and nonsensory (NSE) epithelium. The SE shows sensory, supporting and basal cells whereas the NSE contains ciliated and nonciliated secretory cells and basal cells. At the ultrastructural level, the sensory cells appear as bipolar neurons with conspicuous microvilli on their free surface. The supporting cells of the SE contain numerous membrane-bound vesicles in their apical regions. A peculiar feature not found in other mammals, is the presence of concentric whorls of RER cisterns frequently observed in their basal expansions. Infiltrating plasma cells can be detected in the SE basal region close to the dorsal junctional area. This region also exhibits an unusual type of basal cell, probably responsible for the generation of new vomeronasal receptor neurons. The ciliated NSE cells exhibit numerous ovoids or irregularly shaped membranous protrusions projecting from the plasma membrane of the cilia. As far as we know, this is the first study reporting the presence of this feature in ciliated NSE cells. The nonciliated cells are characterised by scarce large secretory granules and apical microvilli. The vomeronasal glands are compound-branched tubuloacinar glands with serous acinar cells. Four types of secretory granules are present. The ducts of these glands reach the lumen in the dorsolateral region between the NSE and SE. Hypolemmal nerve terminals were observed contacting secretory cells. Fenestrated and nonfenestrated capillaries constitute the vascular supply to these glands. Plasma cells, intimately associated with acinar cells, were frequently observed.

Proliferation and apoptosis in the mouse vomeronasal organ during ontogeny

Differential cell proliferation and apoptosis play a key role in organ morphogenesis. We have analyzed these two processes in the development of murine vomeronasal organ (VNO), an olfactory structure involved in the detection of pheromones. Using the TUNEL (TdT-mediated dUTP nick end labelling) method we demonstrate that dying cells are relatively more abundant in non sensory vomeronasal organ (NS-VNO) rather than in sensory epithelium (S-VNO), particularly in early stages of development. During ontogeny cell proliferation, studied with bromodeoxyuridine (BrdU) labelling, shows a broad pattern of localization, since proliferating cells are distributed throughout the VNO and not confined between NS-VNO and S-VNO. Quantification of BrdU-labelled cells indicates that proliferation is rather stable in both components.