Anatomical and immunohistological demonstration of the primary neural connections of the vomeronasal organ in the dog

Macro- and microdissection methods together with conventional histology and lectin immunohistochemistry have been used to identify the course of the vomeronasal nerves and their site of termination (accessory olfactory bulb; AOB) in the dog. The AOB in this species is small and variable in size, situated on the medial surface of the main olfactory bulb, and has an anatomical structure unlike that described for other mammals. The vomeronasal nerves and their terminal glomeruli in the AOB are easily identifiable by selective immunohistochemical staining using Ulex europeus agglutinin I. © 1992 Wiley-Liss, Inc.     

Axonal growth of newly formed vomeronasal receptor neurons after nerve transection

Chemosensory neurons in the vomeronasal epithelium (vomeronasal neurons) regenerate following experimentally induced degeneration. Transection of the vomeronasal nerves leads to retrograde degeneration of vomeronasal neurons followed by replacement of the cell population. The projection of the axons of regenerated vomeronasal neurons was examined by horseradish peroxidase(HRP) histochemistry and electron microscopy. HRP-wheat germ agglutinin (WGA) was placed on the surface of the vomeronasal organ of the rat. Dense distribution of HRP-labeled fibers was observed in the vomeronasal nerve and glomerular layers in the accessory olfactory bulb (AOB) of the intact rat. At one week after transection, HRP-labeled fibers were not found in the AOB, and no labeled fibers could be observed on the medial surface of the olfactory bulb where the vomeronasal nerve traversed. Three weeks after transection, labeled fiber bundles were observed on the medial surface of the olfactory bulb in all animals. No labeled fibers were detected in the AOB. From 12 to 32 weeks after transection, projection of HRP-labeled fibers was identified in the AOB in 8 out of 26 rats (the incidence of projection was 30%). But the number of projection fibers on the operated side was much smaller than on the control side. Electron microscopy confirmed that the HRP-labeled terminals make synaptic contacts with neurons in the AOB.     

A descriptive and comparative lectin histochemical study of the vomeronasal system in pigs and sheep

The accessory olfactory bulb (AOB) is the primary target of the sensory epithelium of the vomeronasal organ (VNO), and thus constitutes a fundamental component of the accessory olfactory system, which is involved in responses to behaviour-related olfactory stimuli. In this study we investigated the characteristics of the AOB, VNO, vomeronasal nerves (VNNs) and caudal nasal nerve (CdNN) in pigs and sheep, species in which olfaction plays a key behavioural role both in the neonatal period and in adulthood. The patterns of staining of the AOB by the Bandeiraea simplicifolia and Lycopersicon esculentum lectins were the same in the 2 species, whereas the Ulex europeus and Dolichos biflorus lectins gave different patterns. In both species, lectin staining of the AOB was consistent with that of the VNNs, while the CdNN did not label any of the structures studied. The entire sensory epithelium of the pig was labelled by Ulex europeus and Lycopersicum esculentum lectins, and all 4 lectins used labelled the mucomicrovillar surface of the sensory epithelium in sheep.     

Subdivision of the accessory olfactory bulb in the Japanese common toad, Bufo japonicus, revealed by lectin histochemical analysis

Lectin binding patterns in the olfactory bulb of the Japanese common toad, Bufo japonicus, were examined using 21 types of lectin. Ten out of 21 lectins, WGA, s-WGA, LEL, STL, DBA, VVA, SJA, RCA-I, PNA, and PHA-L, stained the olfactory nerve, the glomeruli in the main olfactory bulb (MOB), the vomeronasal nerve, and the glomeruli in the accessory olfactory bulb (AOB). The binding patterns of LEL, STL, DBA, and PHA-L subdivided AOB glomeruli into rostral and caudal regions, where LEL, STL, and DBA stained the rostral region more intensely than the caudal region, and PHA-L had the opposite effect. Another lectin, BSL-I, stained both AOB glomeruli and the vomeronasal nerve, but not MOB glomeruli or the olfactory nerve. This is the first report of histological subdivision in the AOB of an amphibian, which suggests that the AOB development in Bufo may be unique.     

Male hamster copulatory responses to a high molecular weight fraction of vaginal discharge: effects of vomeronasal organ removal

The importance of the vomeronasal (accessory olfactory) system for the copulatory responses of male hamsters to a high molecular weight fraction (HMF) of vaginal discharge was assessed in animals that had their vomeronasal organs (VNO) removed. These organs were extirpated bilaterally using an oral approach through the palate so as to eliminate the peripheral afferents to the accessory olfactory bulb (AOB) with minimal or no damage to the main olfactory system. The selective peripheral deafferentation procedure was verified by applying horseradish peroxidase intranasally following intraperitoneal injections of epinephrine to facilitate the vomeronasal pumping mechanism that draws fluids into the VNO. Heavy, bilateral anterograde labeling was evident in the olfactory nerve afferents within the main olfactory bulb of males that had their VNO removed and of animals that received sham surgery. Sham-operated males also had heavy, bilateral labeling in the vomeronasal nerve afferents within the AOB, whereas no such labeling occurred among animals with bilateral removal of the VNO. In sham-operated animals, both the HMF and the unfractionated discharge significantly increased the incidence of intromission attempts toward anesthetized males (surrogate females) whose hindquarters were scented with these stimuli. The unfractionated discharge also produced a significant elevation of overt copulatory behavior in males with selective peripheral deafferentation of the vomeronasal system, whereas the HMF did not facilitate copulatory behavior in these animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Bandeiraea simplicifolia lectin I and Vicia villosa agglutinin bind specifically to the vomeronasal axons in the accessory olfactory bulb of the rat.

The binding of 21 lectins to the accessory olfactory bulb (AOB) of the rat was examined by histochemistry. Two lectins [Bandeiraea simplicifolia lectin I (BSL-I) and Vicia villosa agglutinin (VVA)] bound specifically to the vomeronasal (VN) axons in the AOB. Seven lectins (Datura stramonium lectin, Erythrina cristagalli lectin, Lycoperisicon esculentum lectin, Ricinus communis agglutinin I, soybean agglutinin, Solanum tuberosum lectin, and Ulex europaeus agglutinin) bound to both VN axons in AOB and olfactory axons in the main olfactory bulb. BSL-I and VVA are useful as the marker of VN axons. This selective binding of lectins indicates the presence of specific glycoconjugates on the surface of VN axons.     

Tyrosine hydroxylase-like immunoreactive neurons in the olfactory bulb of the snake,Elaphe quadrivirgata,with special reference to the colocalization of tyrosine hydroxylase- and GABA-like immunoreactivities

Summary The distribution and structural features of tyrosine hydroxylase-like immunoreactive (TH-LI) neurons were studied in the olfactory bulb of a snake, Elaphe quadrivirgata, by using pre-and post-embedding immunocytochemistry at the light microscopic level. In contrast to rodent olfactory bulbs previously reported, many TH-LI neurons were seen not only in the main olfactory bulb (MOB) but also in the accessory olfactory bulb (AOB). With regard to the TH-like immunoreactivity, there appeared no appreciable differences between MOB and AOB. As in mammalian MOB, the majority of TH-LI neurons were clustered in the periglomerular region and appeared to send their dendritic branches into glomeruli, which as a whole make an intense TH-LI band in the glomerular layer (GML). In the external plexiform/mitral cell layer (EPL/ML) of MOB and AOB as well as in the outer sublamina of the internal plexiform layer (OSL) of AOB, an appreciable number of TH-LI neurons were scattered, extending dendritic processes which appeared to make a loose meshwork. TH-LI neurons in EPL/ML (including OSL) appeared to consist of at least two morphologically different types. The first had a small perikaryon and one or two smooth dendrites which usually extended to GML and were frequently confirmed to enter into glomeruli. The second had a larger perikaryon and 2–3 dendrites which branched into several varicose processes extending in EPL/ML/OSL but appeared not to enter into glomeruli. The TH-like immunoreactivity was rarely seen in the internal plexiform layer and internal granule cell layer. The colocalization of GABA-like and TH-like immunoreactivities was further studied. Almost all TH-LI neurons in both EPL/ ML/OSL and GML contained GABA-like immunoreactivity irrespectively of the type of TH-LI cells.Abbreviations in Figures AOB accessory olfactory bulb - MOB main olfactory bulb - Hem hemisphere - ON olfactory nerve layer - VN vomeronasal nerve layer - GM glomerular layer - EP/M external plexiform layer/Mitral cell layer - IP internal plexiform layer - IG internal granular layer - OS outer sublamina of the IPL of AOB - MS middle sublamina of the IPL of AOB - IS inner sublamina of the IPL of AOB     

An electrophysiological study of the accessory olfactory bulb in the rabbit--I. Analysis of electrically evoked potential fields

Following electrical stimulation of the vomeronasal nerves, the primary olfactory nerves, the lateral olfactory tract and the corticomedial amygdala, we have made a study of evoked potentials in the rabbit accessory olfactory bulb. Vomeronasal nerve stimulation evoked a complex field potential consisting of a compound action potential followed by 4 negative waves (N1, N2, N3, N4). In contrast to the field potential elicited in the main olfactory bulb following primary olfactory nerve stimulation, there was either no evoked wave or only a weak positive component of the field in the accessory bulb. Amygdala stimulation caused a long latency, long duration negative-positive dipolar field potential in the accessory olfactory bulb. Both antidromic and orthodromic field potentials showed sign reversal when the electrode penetrated the bulb at a point corresponding to the lower border of the mitral cell band. Stimulation of the lateral olfactory tract elicited a weak, short-latency wave which did not show any sign reversal when the electrode was lowered into the accessory bulb. This wave was presumably due to fibres arising in the main bulb and projecting through the accessory bulb into the lateral olfactory tract. Electrical stimulation of the primary olfactory nerves did not induce any response in the accessory bulb neither did vomeronasal nerve stimulation evoke a response in the main olfactory bulb. The origin of these potential fields is discussed and it is concluded that the synaptic organization of the accessory olfactory bulb resembles that of the main olfactory bulb in lower vertebrates. There is no detectable communication between the two olfactory systems.     

Oestrogen infusions into the amygdala potentiate excitatory transmission from the accessory olfactory bulb to tuberoinfundibular arcuate neurones in the mouse.

We have previously shown that oestrogen increases the percentage of tuberoinfundibular (TI) arcuate neurones that respond to electrical stimulation of the accessory olfactory bulb (AOB). This study focuses on the amygdala as a possible site for the hormonal modulation of AOB input to TI arcuate neurones. Local infusions of 17 beta-oestradiol (30 pmol) into the amygdala of ovariectomized female mice significantly potentiated excitatory responses of TI arcuate neurones to AOB stimulation. This effect appeared rapidly (less than 10 min) after infusion. The inactive oestrogen isomer, 17 alpha-oestradiol, infused in the same manner, was without effect. These results suggest that oestrogen acts directly on amygdala neurones, thereby modulating olfactory information relayed along the vomeronasal pathway to TI arcuate neurones.     

Projections of two subclasses of vomeronasal nerve fibers to the accessory olfactory bulb in the rabbit

The organization of the projections of subclasses of vomeronasal nerve fibers to the accessory olfactory bulb was analysed using monoclonal antibodies generated against a homogenate of the rabbit olfactory bulb. Monoclonal antibody R2D5 labels all the somata of vomeronasal receptor cells in the vomeronasal organ as well as all their axons (vomeronasal nerve fibers). Another monoclonal antibody (R4B12), which has been shown to selectively bind and thus identify a subclass of olfactory nerve fibers, also labels a subclass of vomeronasal nerve fibers. The R4B12-positive subclass of vomeronasal nerve fibers project to the glomeruli in the rostrolateral part of the accessory olfactory bulb. The third monoclonal antibody (R5A10) recognizes a complementary subclass of vomeronasal nerve fibers projecting to the glomeruli in the caudomedial part of the accessory bulb. In contrast to the clearly segregated terminations in the accessory bulb, the two subclasses of vomeronasal nerve fibers are intermingled with each other in the vomeronasal nerve bundles. Retrograde labeling of vomeronasal receptor cell somata following injection of horseradish peroxidase within the rostrolateral (R4B12-positive) part of the accessory bulb indicates that vomeronasal receptor cells of this subtype are widely distributed in the vomeronasal sensory epithelium. These results demonstrate the heterogeneity of vomeronasal receptor cells and the specificity of projections arising from subclasses of vomeronasal nerve fibers to the accessory olfactory bulb.     

Effect of bilateral accessory olfactory bulb lesions on volatile urinary odor discrimination and investigation as well as mating behavior in male mice

Previous research raises the possibility that urinary volatiles from estrous female mice activate mitral cells in the accessory olfactory bulb (AOB) of male mice following detection via the main olfactory epithelium as opposed to the vomeronasal organ. We asked whether bilateral lesions of the AOB would disrupt the ability of male mice to discriminate between urinary volatiles from mice of different sexes or endocrine states, or affect their interest in investigating these odors when they were presented sequentially in home-cage habituation/dishabituation tests. Males with either partial or complete bilateral lesions of the AOB resembled sham-operated control males in their ability to discriminate between ovariectomized and estrous female urinary volatiles as well as between male and estrous female urinary volatiles. However, males with either complete or partial AOB lesions spent significantly less time than sham-operated control males investigating urinary volatiles from estrous females, especially during tests when the alternative stimulus presented was male urine. Placement of AOB lesions failed to disrupt males' mating performance. Our results suggest that the incentive value of opposite-sex (female) volatile urinary odors which are initially detected by the main olfactory system is enhanced when they are further processed by the male's AOB.     

Shared and differential traits in the accessory olfactory bulb of caviomorph rodents with particular reference to the semiaquatic capybara

The vomeronasal system is crucial for social and sexual communication in mammals. Two populations of vomeronasal sensory neurons, each expressing Gαi2 or Gαo proteins, send projections to glomeruli of the rostral or caudal accessory olfactory bulb, rAOB and cAOB, respectively. In rodents, the Gαi2‐ and Gαo‐expressing vomeronasal pathways have shown differential responses to small/volatile vs. large/non‐volatile semiochemicals, respectively. Moreover, early gene expression suggests predominant activation of rAOB and cAOB neurons in sexual vs. aggressive contexts, respectively. We recently described the AOB of Octodon degus, a semiarid‐inhabiting diurnal caviomorph. Their AOB has a cell indentation between subdomains and the rAOB is twice the size of the cAOB. Moreover, their AOB receives innervation from the lateral aspect, contrasting with the medial innervation of all other mammals examined to date. Aiming to relate AOB anatomy with lifestyle, we performed a morphometric study on the AOB of the capybara, a semiaquatic caviomorph whose lifestyle differs remarkably from that of O. degus. Capybaras mate in water and scent‐mark their surroundings with oily deposits, mostly for male–male communication. We found that, similar to O. degus, the AOB of capybaras shows a lateral innervation of the vomeronasal nerve, a cell indentation between subdomains and heterogeneous subdomains, but in contrast to O. degus the caudal portion is larger than the rostral one. We also observed that four other caviomorph species present a lateral AOB innervation and a cell indentation between AOB subdomains, suggesting that those traits could represent apomorphies of the group. We propose that although some AOB traits may be phylogenetically conserved in caviomorphs, ecological specializations may play an important role in shaping the AOB.     

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.     

Specialized neuroglial arrangement may explain the capacity of vomeronasal axons to reinnervate central neurons

The neurosensory cells of the primary olfactory and vomeronasal projections are in a state of continuous replacement throughout adult life. Since their axons form synaptic terminals with neurons in the olfactory and accessory olfactory bulbs, this system is an apparent exception to the rule that peripheral axons cannot grow into the central nervous system of adult mammals. Electron microscopy of sections (especially in a plane tangential to the surface of the accessory olfactory bulb) shows a unique glial arrangement. By virtue of their greater electron density and "secretory-type" organelle content (Golgi apparatus and dense-core vesicles) the glial cells of the superficial layers of the accessory olfactory bulb are distinguished both from the glia of the vomeronasal nerves and from the astrocytes of the deeper bulbar layers. The synapses between the vomeronasal axons and the postsynaptic elements are formed in glomeruli which are encapsulated by an inner layer of glial cytoplasm derived from the superficial glia, and an outer layer derived from the astrocytes. The principle of the organization is that the superficial glial processes are reflected off the axons before they reach the synaptic terminal zone. Conversely, for the postsynaptic elements, the astrocytic processes are reflected off the dendrites of the accessory olfactory bulb neurons before they enter the core of the glomeruli. In effect, the synapses are formed in a "no-man's-land" between the two glial cell types. This peculiar glial arrangement may be important for the unique regenerative capacity of this system.     

Coculture of the vomeronasal organ and olfactory bulb of the fetal rat

The vomeronasal organ and the olfactory bulb of the rat were cocultured from 15-day embryo siblings on collagen-coated membrane in Dulbecco's modified Eagle's medium containing fetal calf serum, horse serum, and antibiotics. At 4 days in vitro (DIV), vomeronasal axons forming two to three large fascicles were seen originating from the explants of the vomeronasal organ. Differential axonal growth was observed. Some fascicles made connections with the explants of the olfactory bulb. Twenty percent of the cocultures studied here showed the formation of connections. At 6–10 DIV many fascicles that did not connect with the olfactory bulb had degenerated, and large fascicles that were connected with the olfactory bulb survived for more than 10 DIV. The formation of connections between the vomeronasal organ and the olfactory bulb in coculture favors the survival of large nerve fascicles, but it could not be determined whether or not the presence of the olfactory bulb affects the initial orientation of the fibers and fascicles from the explants of the vomeronasal organ.     

A putative functional vomeronasal system in anuran tadpoles

We investigated the occurrence and anatomy of the vomeronasal system (VNS) in tadpoles of 13 different anuran species. All of the species possessed a morphologically fully developed VNS with a highly conserved anatomical organisation. We found that a bean-shaped vomeronasal organ (VNO) developed early in the tadpoles, during the final embryonic stages, and was located in the anteromedial nasal region. Histology revealed the presence of bipolar chemosensory neurones in the VNO that were immunoreactive for the Gαo protein. Tract-tracing experiments demonstrated that chemosensory neurones from the VNO reach specific areas in the brain, where a discernible accessory olfactory bulb (AOB) could be observed. The AOB was located in the ventrolateral side of the anterior telencephalon, somewhat caudal to the main olfactory bulb. Synaptophysin-like immunodetection revealed that synaptic contacts between VNO and AOB are established during early larval stages. Moreover, using lectin staining, we identified glomerular structures in the AOB in most of the species that we examined. According to our findings, a significant maturation in the VNS is achieved in anuran larvae. Recent published evidence strongly suggests that the VNS appeared early in vertebrate evolution and was already present in the aquatic last common ancestor of lungfish and tetrapods. In this context, tadpoles may be a good model in which to investigate the anatomical, biochemical and functional aspects of the VNS in an aquatic environment.     

Electrophysiological studies of the tongue and accessory olfactory bulb in garter snakes

Tongue flicking in snakes transports environmental chemicals to the vomeronasal (Jacobson's) organ (VNO) which acts as a chemoreceptor. This paper provides the first electrophysiological evidence that the vomeronasal/accessory-olfactory system is activated following tongue flick. In acute experiments with partially anesthetized snakes, single units recorded in the accessory olfactory bulb (AOB) failed to show changes in firing rate following tongue flicks but changes in AOB activity did occur when cotton swabs soaked in prey extract were pressed to the roof of the mouth (where the VNO ducts open). In chronic experiments, multiunit recordings from electrodes implanted in the AOB of freely moving animals did reveal changes in neural activity following tongue flicks. EMG recordings from the hyoglossus (tongue retractor) muscle (and in one animal from the genioglossus [tongue extender] as well) were used to indicate precisely the time of tongue flicking. The techniques used in these experiments are detailed.     

A morphological study of the vomeronasal organ and the accessory olfactory bulb in the Korean roe deer, Capreolus pygargus

The vomeronasal organ (VNO) and accessory olfactory bulb (AOB) of the Korean roe deer (Capreolus pygargus) were studied histologically to evaluate their morphological characteristics. Grossly, the VNO, encased by cartilage, has a paired tubular structure with a caudal blind end and a rostral connection through incisive ducts on the hard palate. In the VNO, the vomeronasal sensory epithelium (VSE) consists of galectin-3-positive supporting cells, protein gene product (PGP) 9.5-positive receptor cells, and basal cells. The vomeronasal respiratory epithelium (VRE) consists of a pseudostratified epithelium. The AOB strata included a vomeronasal nerve layer (VNL), a glomerular layer (GL), a mitral/tufted cell layer, and a granular cell layer. All lectins used in this study, including Bandeiraea simplicifolia agglutinin isolectin B4 (BSI-B4), soybean agglutinin (SBA), Ulex europaeus agglutinin I (UEA-I), and Triticum vulgaris wheat germ agglutinin (WGA), labeled the VSE with varying intensity. In the AOB, both the VNL and the GL reacted with BSI-B4, SBA, and WGA with varying intensity, but not with UEA-I. This is the first morphological study of the VNO and AOB of the Korean roe deer, which are similar to those of goats.     

An electrophysiological study of the accessory olfactory bulb in the rabbit--II. Input-output relations as assessed from analysis of intra- and extracellular unit recordings

The input-output relations of the rabbit accessory olfactory bulb were studied by intra- and extracellular single unit recordings following electrical stimulation of the vomeronasal nerves, the lateral olfactory tract and the corticomedial amygdala. Cellular activity of accessory bulb mitral cells evoked by stimulation of the vomeronasal nerves consisted of a brief excitation with a latency of 16 ms. This initial response was followed by a period of reduced firing probability which was due to an inhibitory postsynaptic potential. In many cases this secondary response was followed by a second excitatory postsynaptic potential on which action potentials were generated at higher stimulus intensities. Deeper cells in the granule cell layer responded with a long latency, long duration, excitation, often consisting of bursts of 2-3 spikes. The majority of mitral cells were antidromically invaded by amygdala stimulation. The latencies of the antidromic spikes showed a wide range of variation (12-80 ms). Due to this great variation in antidromic latency the inhibitory postsynaptic potential following the antidromic action potential was rather modest but prolonged in duration. In many cases the onset of the inhibitory postsynaptic potential preceded the antidromic response. The majority of cells did not respond to lateral olfactory tract stimulation. Only 10% of the tested cells were invaded antidromically by stimulation at this site. These neurons were also driven antidromically by amygdala stimulation. We conclude that, although the physiological characteristics of mitral cells of the main and accessory olfactory bulb are very similar, there are important differences. The efferent fibres of the accessory bulb conduct at very slow and variable rates and project directly to the corticomedial amygdala.     

A Golgi study on the accessory olfactory bulb in the snake, Elaphe quadrivirgata

The intrinsic organization of the accessory olfactory bulb (AOB) in the snake was studied using the rapid Golgi method. A distinct laminar organization was observed in the snake AOB. Beginning with the most superficial surface, the following layers were distinguished: the layer of the vomeronasal fibers, the olfactory glomeruli, the mitral cells, the deep fiber plexus, the granule cells and the ependymal cells. While the general organizational pattern of the snake AOB resembles that of the main olfactory bulb (MOB) and the AOB reported in various vertebrate species, the present study shows that: (1) the external and internal plexiform layers cannot be identified as independent layers and are considered to be included in the mitral cell layer; (2) the afferent and efferent paths, which are disseminated in the granule cell layer in the mammalian MOB, accumulate external to the granule cell layer to form the layer of the deep fiber plexus: and (3) as a result of accumulation of the afferent and efferent paths in the layer of the deep fiber plexus, the granule cell layer is very fiber-sparse. These structural patterns are quite similar to those of the snake MOB.