Interaction of the transplanted olfactory placode with the optic stalk and the diencephalon in Xenopus laevis embryos

Transplantation of Ears Provides Insights into Inner Ear Afferent Pathfinding Properties

Numerous tissue transplantations have demonstrated that otocysts can develop into normal ears in any location in all vertebrates tested thus far, though the pattern of innervation of these transplanted ears has largely been understudied. Here, expanding on previous findings that transplanted ears demonstrate capability of local brainstem innervation and can also be innervated themselves by efferents, we show that inner ear afferents grow toward the spinal cord mostly along existing afferent and efferent fibers and preferentially enter the dorsal spinal cord. Once in the dorsal funiculus of the spinal cord, they can grow toward the hindbrain and can diverge into vestibular nuclei. Inner ear afferents can also project along lateral line afferents. Likewise, lateral line afferents can navigate along inner ear afferents to reach hair cells in the ear. In addition, transplanted ears near the heart show growth of inner ear afferents along epibranchial placode-derived vagus afferents. Our data indicate that inner ear afferents can navigate in foreign locations, likely devoid of any local ear-specific guidance cues, along existing nerves, possibly using the nerve-associated Schwann cells as substrate to grow along. However, within the spinal cord and hindbrain, inner ear afferents can navigate to vestibular targets, likely using gradients of diffusible factors that define the dorso-ventral axis to guide them. Finally, afferents of transplanted ears functionally connect to native hindbrain vestibular circuitry, indicated by eliciting a startle behavior response, and providing excitatory input to specific sets of extraocular motoneurons.

Sensory afferent segregation in three-eared frogs resemble the dominance columns observed in three-eyed frogs

The formation of proper sensory afferent connections during development is essential for brain function. Activity-based competition is believed to drive ocular dominance columns (ODC) in mammals and in experimentally-generated three-eyed frogs. ODC formation is thus a compromise of activity differences between two eyes and similar molecular cues. To gauge the generality of graphical map formation in the brain, we investigated the inner ear projection, known for its well-defined and early segregation of afferents from vestibular and auditory endorgans. In analogy to three eyed-frogs, we generated three-eared frogs to assess to what extent vestibular afferents from two adjacent ears could segregate. Donor ears were transplanted either in the native orientation or rotated by 90 degrees. These manipulations should result in either similar or different induced activity between both ears, respectively. Three-eared frogs with normal orientation showed normal swimming whereas those with a rotated third ear showed aberrant behaviors. Projection studies revealed that only afferents from the rotated ears segregated from those from the native ear within the vestibular nucleus, resembling the ocular dominance columns formed in three-eyed frogs. Vestibular segregation suggests that mechanisms comparable to those operating in the ODC formation of the visual system may act on vestibular projection refinements.

Olfactory epithelial transplantation: possible mechanism for restoration of smell

PURPOSE OF REVIEW

To discuss the unique properties of the olfactory epithelium and the potential use of olfactory epithelial grafts to restore olfactory function.

RECENT FINDINGS

Sensory neurons in the olfactory epithelium undergo continuous regeneration, grow new axons, and reestablish connections with the olfactory bulb throughout life. When transplanted into different regions of the brain, olfactory epithelial graft cells retain their morphological and regenerative properties. Olfactory cells within the grafts grow axons that enter into the surrounding brain tissue. Recent studies have shown that the olfactory epithelium can be grafted directly to the olfactory bulb.

SUMMARY

The olfactory epithelium has a remarkable capacity to continuously generate new sensory neurons and survives grafting into different regions of the brain. A review of the literature and the future use of olfactory grafts as a potential method to restore olfactory function is discussed.

Brain regeneration in anuran amphibians

Urodele amphibians are highly regenerative animals. After partial removal of the brain in urodeles, ependymal cells around the wound surface proliferate, differentiate into neurons and glias and finally regenerate the lost tissue. In contrast to urodeles, this type of brain regeneration is restricted only to the larval stages in anuran amphibians (frogs). In adult frogs, whereas ependymal cells proliferate in response to brain injury, they cannot migrate and close the wound surface, resulting in the failure of regeneration. Therefore frogs, in particular Xenopus, provide us with at least two modes to study brain regeneration. One is to study normal regeneration by using regenerative larvae. In this type of study, the requirement of reconnection between a regenerating brain and sensory neurons was demonstrated. Functional restoration of a regenerated telencephalon was also easily evaluated because Xenopus shows simple responses to the stimulus of a food odor. The other mode is to compare regenerative larvae and non-regenerative adults. By using this mode, it is suggested that there are regeneration-competent cells even in the non-regenerative adult brain, and that immobility of those cells might cause the failure of regeneration. Here we review studies that have led to these conclusions.

Functional regeneration of the olfactory bulb requires reconnection to the olfactory nerve in Xenopus larvae

Larvae of the South African clawed frog (Xenopus laevis) can regenerate the telencephalon, which consists of the olfactory bulb and the cerebrum, after it has been partially removed. Some authors have argued that the telencephalon, once removed, must be reconnected to the olfactory nerve in order to regenerate. However, considerable regeneration has been observed before reconnection. Therefore, we have conducted several experiments to learn whether or not reconnection is a prerequisite for regeneration. We found that the olfactory bulb did not regenerate without reconnection, while the cerebrum regenerated by itself. On the other hand, when the brain was reconnected by the olfactory nerve, both the cerebrum and the olfactory bulb regenerated. Morphological and histological investigation showed that the regenerated telencephalon was identical to the intact one in morphology, types and distributions of cells, and connections between neurons. Froglets with a regenerated telencephalon also recovered olfaction, the primary function of the frog telencephalon. These results suggest that the Xenopus larva requires reconnection of the regenerating brain to the olfactory nerve in order to regenerate the olfactory bulb, and thus the regenerated brain functions, in order to process olfactory information.

Vertebrate cranial placodes I. Embryonic induction

Cranial placodes are focal regions of thickened ectoderm in the head of vertebrate embryos that give rise to a wide variety of cell types, including elements of the paired sense organs and neurons in cranial sensory ganglia. They are essential for the formation of much of the cranial sensory nervous system. Although relatively neglected today, interest in placodes has recently been reawakened with the isolation of molecular markers for different stages in their development. This has enabled a more finely tuned approach to the understanding of placode induction and development and in some cases has resulted in the isolation of inducing molecules for particular placodes. Both morphological and molecular data support the existence of a preplacodal domain within the cranial neural plate border region. Nonetheless, multiple tissues and molecules (where known) are involved in placode induction, and each individual placode is induced at different times by a different combination of these tissues, consistent with their diverse fates. Spatiotemporal changes in competence are also important in placode induction. Here, we have tried to provide a comprehensive review that synthesises the highlights of a century of classical experimental research, together with more modern evidence for the tissues and molecules involved in the induction of each placode.

Lineage specification of olfactory neural precursor cells depends on continuous cell interactions

We transplanted, as a single cell suspension, cells dissociated from the mature and immature olfactory epithelium of rats or TgR(ROSA26)26Sor mice expressing constitutively the LacZ gene into the developing brain (cerebellum, striatum, inferior colliculus, lateral ventricles) of E15 rat fetuses. Grafted cells or their descendants were still present in the central nervous system more than a month after transplantation. Transplanted cells either integrated as isolated cells or, during the first day after transplantation, reaggregated into clusters. Scattered cells, despite their placodal origin, differentiated into neuron or glial cells with a central phenotype. This was demonstrated by anatomical methods and selective amplification of cDNA encoding for neuronal specific transcripts (microtubule-associated protein 2 and middle-molecular-mass neurofilament protein) expressed by the engrafted cells. Cells in large clusters generated an epithelium containing mature olfactory neurons. Some of them were immunoreactive for the olfactory marker protein. Our findings show that cells dissociated from the developing and adult olfactory organs when transplanted into the rat fetal brain can either completely change their fate and differentiate according to their final position or generate an olfactory epithelium if they reaggregate into large clusters.

Influence of receptor axons on the formation of olfactory glomeruli in a hemimetabolous insect, the cockroach Periplaneta americana

The embryonic development of the hemimetabolous insect Periplaneta americana requires approximately 31 days. Deafferentation experiments were used to investigate the role of ingrowing receptor axons during embryogenesis, specifically their influence 1) on the subdivision of the antennal lobe neuropil into glomeruli, 2) on the morphology and number of glial cells, and 3) on the arborization pattern of central neurons. The flagellum of one antenna was removed from embryos at different developmental stages starting with day 10. Subsequently, they were raised in culture until a total age of 26 days. At day 10, the deutocerebrum has received only a very small number (ca. 0.4%) of antennal receptor axons; deafferentation at this stage allowed us to deprive the deutocerebrum of approximately 99% of its normal antennal input. Deafferentation has marked effects on the organization of the antennal lobe neuropil. The deafferented lobe is reduced in volume compared to the control side; the characteristic glomeruli are missing. During normal development glomeruli are formed between day 19 and 22, first in dorsal and then in ventral antennal lobe regions. By removing the antenna before day 20, their formation is disturbed in all parts of the antennal lobe. If deafferentation is performed after stage 20, glomeruli persist in dorsal regions, but are missing in ventral regions. On day 24 or later, glomeruli in both dorsal and ventral regions are unaffected by deafferentation. Glial cells continue to extend fine processes into the neuropil in the absence of ingrowing receptor axons. The number of glial cells is reduced compared to control lobes. Multiglomerular local interneurons and other gamma-amino butyric acid-immunoreactive neurons, as well as projection neurons, fail to develop glomerular arborization patterns in antennal lobes deprived of sensory axons.

Multiple factors shape development of olfactory glomeruli: insights from an insect model system

The antennal system of the moth Manduca sexta is a useful model for studies of the development of olfactory glomeruli, the complex synaptic structures that typically underlie the initial processing of olfactory input in vertebrates and invertebrates. In this review, we summarize cellular events in the construction of glomeruli in Manduca and highlight experiments that reveal factors that influence glomerulus development. By methodically manipulating each of various cell types, both neuronal and glial, that contribute to glomerular architecture, we have found that: olfactory receptor axons lay a template for developing glomeruli, stabilization of the template by glial cells is necessary to permit subsequent steps in development of the glomeruli, and the hormone that regulates adult development causes production of adequate numbers of glial cells. Neither electrical activity nor the presence of a serotonin-containing neuron that persists throughout development is required for a glomerular pattern to develop; these factors might, however, influence the synaptic organization of individual glomeruli.

An ultrastructural study of glomeruli associated with vomeronasal organs transplanted into the rat CNS

Rat neonate vomeronasal organs were transplanted into the parietal cortex of littermates to examine their survival and the behavior of axon growth into the surrounding host brain parenchyma. After survival times of 10-100 days the brains were processed for ultrastructural examination. The transplanted vomeronasal organs (VNO) formed several vesicles lined with a sensory epithelium. From these sensory epithelia, VNO neurons leave the epithelium and enter the host brain. Transplant neurons grew axons that fasciculated into bundles surrounded by sheath cell processes and formed one or more fiber plexuses containing distinct globose or spherical-shaped glomerular-like structures. The glomeruli consisted of nerve terminals between which existed asymmetric synaptic contacts. Rarely did we observe clear reciprocal synapses. The glomeruli also contained terminals that showed signs of degeneration, such as increased density of the terminals, clumping of mitochondria and multivesicular bodies. The glomeruli were not partitioned or subdivided by glial septa; however, glial profiles were interspersed among the sensory terminals. Transplant glomeruli also lacked periglomerular cells and had no definitive glial envelope. These results suggest that glomerular formation is not dependent on dendrite contribution of second order neurons or glial support, but rather on a complementary population of receptor neurons.

A molecular approach to the pathophysiology of the X chromosome-linked Kallmann's syndrome

The human KAL gene is responsible for the X chromosome-linked Kallmann's syndrome, which consists of an association between hypogonadotropic hypogonadism and anosmia (or hyposmia). Additional symptoms are occasionally observed. The olfactory defect is associated with hypoplasia of the olfactory bulbs and tracts. The hypogonadism may be due to a defect in the embryonic migratory process of GnRH-synthesizing neurones from the olfactory pits up to the brain. The human and chicken KAL genes have been isolated. From the amino acid sequences deduced, it has been postulated that the KAL protein is an extracellular matrix component, with putative antiprotease activity and adhesion function. Various point mutations and, in a few cases, deletions of KAL have been detected in patients. By in situ hybridization, KAL expression has been studied during embryonic development in the chick. From embryonic day 2 (ED2) to ED8, the KAL gene is expressed in various endodermal, mesodermal and ectodermal derivatives, whereas the expression from ED8 is almost entirely restricted to definite neuronal populations in the central nervous system, most of which still express the gene after hatching. According to such a spatiotemporal pattern of expression, we suggest that the KAL gene is involved both in morphogenetic events and in late neuronal differentiation and/or neuronal trophicity. With respect to the olfactory system, the KAL gene is expressed in the mitral cells of the olfactory bulbs from ED8 onwards. In contrast, no expression of the KAL gene is detected at any stage in either the embryonic olfactory epithelium or the surrounding nasal mesenchyme. Therefore, assuming that similar conditions are found in the human embryo, we suggest that the olfactory anomaly in X-linked Kallmann's syndrome results from a central target cell defect. Current hypotheses regarding the pathophysiology of the GnRH deficiency are also discussed. In situ hybridization experiments in the human embryo, as well as characterization of the KAL protein, are in progress.

Transplantation of postnatal vomeronasal organ in the CNS of newborn rats

The present study was conducted to examine the survival and development of intracerebral transplanted neonatal rat vomeronasal organs (VNs). Complete neonatal (P5-P10) VNs were transplanted into the parietal cortex region of littermates and examined at 10-100 days by light microscopy. The VN survived and was organized into a series of vesicles lined by respiratory and/or sensory epithelia. Sensory neurons grew long axons that fasciculated and invaded the surrounding brain parenchyma. The newly developed axons did not prefer a specific brain region. The axons developed a complex fiber plexus either at the interface between transplant and host tissue or deep within the host brain parenchyma. Vomeronasal axons consistently formed glomerular-like structures within the fiber plexus. Our results suggest that glomerular formation is not dependent on specific target of length of axon development, but rather on a set of complementary axons that display mutual recognition.

Cell migration from the transplanted olfactory placode in Xenopus

The eye vesicle of Xenopus borealis has been replaced with the transplanted olfactory primordium from Xenopus laevis in an attempt to determine whether cells from the transplant could migrate along the regrowing olfactory nerve and become incorporated into the CNS of the host. The use of X. laevis and X. borealis pairs allowed us to distinguish the cells of the host from those of the donor at the cellular level by means of the characteristic fluorescent nuclear spots (Q bands) of X. borealis. Transplantation was performed on pairs of animals at stages 23/24. The olfactory anlage was readily incorporated into the host, often fusing with the host homolateral organ and inhibiting the regrowth of the eye vesicle. An olfactory nerve developed from the transplanted organ. In the majority of cases, the nerve reached the diencephalon at the level of entrance of the optic nerve. Along the nerve originating from the transplanted organ we observed a stream of cells with the characteristics of the donor. These cells penetrated the host's CNS and became incorporated into it. The nature of these cells has not been ascertained by specific neuronal markers. However, on the basis of their morphology and disposition, the hypothesis suggested is that some of the migrating cells are neurons.

Eye primordium transplantation in Xenopus embryo

A part of the eye primordium, the presumptive retinal anlage, was transplanted from stage-23/24 Xenopus borealis to replace the removed olfactory anlage of Xenopus laevis. Cells of the two species can be distinguished under fluorescence microscopy, and we used the resulting chimeras to determine whether the transplanted eye primordium would inhibit the regeneration of the olfactory anlage, whether it would connect with its usual target, the diencephalon, and whether migration of cells would occur from the transplant to the host CNS or from the host CNS to the transplant. In all cases, the olfactory anlage regenerated promptly, and normal olfactory bulbs developed. Omission of the eye stalk in the transplant resulted in failure of an optic nerve to develop from the developing retina. A cellular bridge containing the optic axons connected the transplanted retina to the diencephalon. Cells from the transplant migrated freely through the cellular bridge to several CNS regions. Their morphology, topographic arrangement, number, and relations with other host elements are consistent with the hypothesis that these cells belong to both glia and neuron types.

Microscopic structure of the olfactory organ of the clearnose skate, Raja eglanteria

The olfactory organ of juvenile clearnose skates (Raja eglanteria) was studied with the light and electron microscopes. The organ is ovoid in shape, and its free surface is complicated by the presence of some 20 lamellae. Each lamella has a folded surface lined by a typical neurosensory olfactory epithelium. Bipolar olfactory receptor neurons, ciliated sustentacular cells, and basal cells are the pre-eminent cellular components of the epithelium. Two types of receptor neurons, both bearing microvilli but not cilia, were identified. The type 1 neuron is similar to that previously described in other fishes. The type 2 neuron has a characteristic morphology justifying a separate description. Its dendritic knob is larger than that of type 1, and its microvilli, which are shorter and thicker, are straight and regularly arranged. Tight bundles of filaments provide a skeleton to each microvillus, and these filament bundles reach more than 5 microns down into the dendrite. Type 2 receptor neurons have a well-developed Golgi complex and sparse rough endoplasmic reticulum (rER), whereas type 1 receptor neurons have a less well-developed Golgi complex and a conspicuous system of rER lamellae. The mucous layer on the epithelial surface is provided by the secretion of goblet cells that are situated mostly in the peripheral regions of each lamella. Secretory granules in the sustentacular cells and glands in the lamina propria were not observed.

In vitro analyses of neurite outgrowth indicate a potential role for tenascin-like molecules in the development of insect olfactory glomeruli

Tenascin-like material is associated with glial cells that form borders around developing glomerular units in the olfactory (antennal) lobe of the moth Manduca sexta and is present at critical stages of glomerulus formation (Krull et al., 1994, J. Neurobiol. 25:515-534). Tenascin-like immunoreactivity declines in the mature lobe, coincident with a wave of synapse formation within the glomeruli and glomerulus stabilization. Tenascin-like molecules associated with neuropilar glia are in the correct position to influence the branching patterns of growing neurites by constraining them to glomeruli. In this study, we examine the growth of cultured moth antennal-lobe neurons in response to mouse CNS tenascin. Uniform tenascin provides a poor substrate for cell-body attachment and neurite outgrowth. Neuronal cell bodies provided with a striped substratum consisting of tenascin and concanavalin-A (con-A)/laminin attach preferentially to con-A/laminin lanes. Most neurons restrict their branching to con-A/laminin lanes both at early and later times in culture but others send processes across multiple tenascin and con-/laminin lanes in an apparently indiscriminate manner. Tenascin can inhibit the neuritic outgrowth of most antennal-lobe neurons, and this raises the possibility that the tenascin-like molecules associated with neuropilar glia in vivo act to constrain growing neurites to glomeruli. Thus, glial cells, acting in concert with olfactory axons, might act to promote glomerular patterns of branching by antennal-lobe neurons.

Morphological and quantitative evaluation of olfactory bulb development in Xenopus after olfactory placode transplantation

We found previously that the number of olfactory axons is correlated with the number of mitral/tufted cells (output neurons of the olfactory bulb) during normal larval development. To examine the significance of this quantitative relationship, we evaluated the effects of transplanting an extra olfactory placode on the development of the larval olfactory bulb. We found that the transplanted tissue retained the normal, pseudostratified, columnar appearance and had the same cell types as normal olfactory epithelium, and the olfactory bulbs had the same laminar organization as control bulbs. With gross examination of the olfactory bulb, the side innervated by the transplant appeared slightly larger than the contralateral side in animals analyzed at a young larval stage (stage 50) and in 2 of the 9 animals examined at late larval stages (57/58). Tissue sections and area measurements, however, revealed that the volume of the olfactory bulbs in animals with a transplant was not significantly different from control values. Our quantitative analysis also showed that in stage-50 animals with a transplant, the total number of olfactory axons (in nerves from the transplanted and host olfactory organs) appeared to be greater than in control animals, but not to a statistically significant level. The number of mitral/tufted cells was not different from controls. In animals examined at stage 57/58, there was no difference from controls in either the total number of olfactory axons, total number of mitral/tufted cells, or convergence ratio of olfactory axons onto mitral/tufted cells. Thus, in the late-stage larvae, the quantitative relationship between olfactory axons and mitral/tufted cells was not altered by the experimental manipulation. These results suggest that the olfactory bulb can regulate the number of afferent fibers.

Expression of the KAL gene in multiple neuronal sites during chicken development

The human KAL gene is responsible for the X chromosome-linked Kallmann syndrome. A partial cDNA sequence from the chicken KAL homologue was determined and used to study expression of the KAL gene, by in situ hybridization, during chicken development, from day 6 of incubation. The KAL gene is mainly expressed in neurons of the central nervous system during the second half of embryonic life. High levels of transcript were detected in mitral neurons of the olfactory bulbs, in striatal neurons, in Purkinje cells of the cerebellum, in retinal neurons, and in isolated neurons of the brainstem and spinal cord. No expression was observed in glial cells. A low level of expression was observed in some mesenchymal derivatives. In the adult, expression is maintained or increased in several neuronal populations, especially in optic tectum and striatum. A possible role for the KAL protein in synaptogenesis at these stages is discussed. These results in the chicken embryo help to elucidate the mechanisms of anosmia and gonadotropin-releasing hormone deficiency, which define Kallmann syndrome. In addition, most of the occasional symptoms described in Kallmann syndrome patients, such as cerebellar ataxia, abnormal ocular movements, abnormal spatial visual attention, mirror movements, and renal aplasia, could be ascribed to malfunction of areas that, in the chicken, express the KAL gene.

Cellular organization and growth-related plasticity of the crayfish olfactory midbrain

Little knowledge is available concerning the detailed anatomy of the crusctacean central olfactory pathway. We are using radiolabeling, Golgi and biocytin/neurobiotin tracer methodologies, at the correlated light and electron microscopical levels, to study the olfactory midbrain of the freshwater crayfish. We have found that primary afferent fibers from the antennular olfactory receptor cells branch extensively throughout the length of the glomerular columns within the olfactory lobes in the midbrain. Globuli cells of the lateral cell clusters ramify as dendritic arborizations within both the olfactory and accessory lobes; their axons project out the olfactory-globular tracts to the lateral protocerebrum, often branching to both sides. Developmental plasticity involving the connections made by afferent fibers within the olfactory lobes may permit detailed examination of organizational changes within the midbrain as the animal grows and adds new afferent input from the periphery.

Molecular basis of the X-chromosome-linked Kallmann's syndrome

Kallmann's syndrome combines hypogonadotropic hypogonadism and anosmia. The most frequent form of the disease is linked to the X chromosome and has been proposed to be due to a defect in the embryonic migration of GnRH neurons and olfactory axons from the nose to the brain. A candidate gene for the X-linked form of the disease has been isolated by positional cloning. Mutations in the open reading frame have been identified in several patients, providing convincing evidence that this gene is the actual gene, KAL, responsible for the X-linked Kallmann's syndrome. Correlations between molecular and clinical data extend the role of the KAL gene to other neuronal pathways and kidney organogenesis. The deduced amino acid sequence led us to postulate that the KAL protein is an extracellular matrix component with possible antiprotease and adhesion functions. Such functions are known to be involved in neuronal migration, axonal guidance and targeting, and also in synaptogenesis. Further experiments will enable the elucidation of the role of the KAL protein.

Transient pattern of exuberant projections of olfactory axons during development in the rat

The purpose of our study was twofold: (1) to trace the development of the olfactory axons from early embryonic stages until the mature pattern of connectivity and (2) to determine whether a transient penetration of them exists beyond the olfactory glomeruli. Two techniques were employed: DiI applied in the olfactory epithelium after aldehyde fixation, and olfactory marker protein (OMP) immunostaining. At E13 and E14 olfactory axons were observed spreading over the telencephalic vesicle and entering deeply into the prospective olfactory bulb, extending near the ventricular zone. Growth cones were seen at the end of these axons. At E15, the bundles of olfactory axons form a network, in which axons, growth cones and cells were seen. Some of these axons entered the olfactory bulb. Using OMP immunostaining olfactory axons were observed along the external plexiform layer, the mitral cell layer and in the granular layer from E19 to P6. At P9 some OMP immunoreactive axons were observed in the external plexiform layer. No OMP immunostained axons could be observed outside the glomeruli at P10. Our conclusions are that a transient immature pattern of early invasion over the telencephalic vesicle and of the olfactory bulb by olfactory axons occurs in the olfactory system. By the second postnatal week the glomerular layer reaches its mature configuration, and no olfactory fibers are seen outside the glomerular layer.

The influence of the olfactory placode on the development of the telencephalon in Xenopus laevis

Removal of the sensory plate in Xenopus laevis embryos was performed to study the influence of the olfactory anlage on the development of the forebrain. Embryos, which at stage 22-23 underwent removal of the olfactory anlagen, were killed from stage 47 to 60. In 79% of the animals, two olfactory organs reformed and gave origin to two olfactory nerves which contacted the forebrain. In this instance, the telencephalic hemispheres developed normally. In 14% of the animals, one olfactory organ reformed which contacted the brain by means of one olfactory nerve. This resulted in the development of a unique, reduced in size, cone-shaped telencephalic lobe. In the remaining animals, only a rudiment of the olfactory organ, unconnected with the brain, was present; in these cases, the telencephalon did not develop. Similar results were observed in embryos where olfactory anlagen removal was coupled with damage to, or partial removal of, the prosencephalic vesicle. In animals where lesion of the forebrain was performed without placodal removal, normal development of the forebrain was observed. The developmental relationship observed between the olfactory organ and the forebrain suggests an active role of the nose on the development of the brain.

Immunochemical markers for the frog olfactory neuroepithelium

Monoclonal antibodies (mAbs) were generated that react with the major cell types in the olfactory neuroepithelium of the frog, Rana catesbeiana. This pseudostratified epithelium consists of apical supporting cells, a middle layer of olfactory receptor neurons and a heterogeneous population of basal cells consisting of basal cells proper and globose basal cells. Both olfactory receptor neurons and globose basal cells were labelled by mAb 13-OE, which recognized the neural cell adhesion molecule NCAM. The identity of these NCAM positive cells was established by analysing regenerating olfactory epithelium and by a double-antibody labelling immunofluorescence technique. The olfactory nerve was lesioned, which induced the death of olfactory receptor neurons and the subsequent proliferation of basal cells. When the regenerating olfactory epithelium was analysed prior to the reconstitution of mature olfactory neurons, mAb 13-OE reacted specifically with globose basal cells and not the basal cells proper. Simultaneous labelling of normal olfactory epithelium with mAb 13-OE and polyclonal anti-keratin antibodies, the latter of which labels supporting cells and basal cells proper, revealed no double-labelled cells. These results further confirmed that NCAM was expressed by both globose basal cells and receptor neurons but not by other cell types within the epithelium. Additional cell types in the olfactory epithelium reacted with other new mAbs: 4-OE, 5-OE, 7-OE and 9-OE. Supporting cells were stained by mAb 4-OE. Olfactory receptor neurons and the entire population of basal cells were immunoreactive with mAb 7-OE. The cilia and knobs of receptor neurons were strongly immunoreactive with mAb 5-OE whereas mAb 9-OE selectively stained olfactory knobs and not the cilia on these chemosensory cells. These studies are a first step towards experimental approaches designed to elucidate the mechanisms underlying the unique proliferative properties of the olfactory neuroepithelium in frog.

The migration of luteinizing hormone-releasing hormone (LHRH) neurons from the medial olfactory placode into the medial basal forebrain

Over the years, investigators have noticed, in a wide variety of species of vertebrates, large numbers of cells migrating from the olfactory placode to the forebrain. These cells were considered to be Schwann cells or ganglion cells of the terminalis nerve. Recently, immunocytochemical localization studies have shown that many of these migrating cells contain luteinizing hormone-releasing hormone (LHRH), a brain peptide that regulates reproductive functions by evoking the release of luteinizing hormone and follicle-stimulating hormone from the anterior pituitary gland. The origin of LHRH cells in the epithelium of the medial olfactory placode, their migration across the nasal septum and into the forebrain, with branches of the terminalis nerve, also a derivative of the medial part of the olfactory placode, has led to some interesting speculations, from evolutionary and physiological perspectives, about the origin of these cells and the role of the terminalis nerve in their migration.

Requirement for olfactory axons in the induction and stabilization of olfactory glomeruli in an insect

The role of antennal sensory axons in the induction and stabilization of olfactory glomeruli has been explored in the moth Manduca sexta. First, we asked the question: how many axons are necessary to induce glomerulus formation within the first-order olfactory neuropil of the brain? Axons from as few as 10 of the normal 70-80 repeating antennal segments were sufficient to induce glomeruli. However, there was a dose dependence in the number of glomeruli that developed in partially innervated lobes. When only 11 segments of the antenna were allowed to provide innervation to the lobe, only 37 of the normal 59 +/- 2 glomeruli developed; over 20 segments were necessary to induce the normal number of glomeruli. In a second set of experiments, we asked: for how long must antennal axons be present to stabilize newly formed glomeruli? We found that antennal axons must be intact for at least 2 to 4 stages (roughly equivalent to 2 to 4 days) for glomeruli to be stable even if the axons are subsequently severed. This finding, taken in the light of other recent findings in our laboratory, suggests that the formation of synapses may be a crucial element in the stabilization of glomerular structure. All together, the results of the present study indicate that induction and stabilization of glomeruli are separable events with different underlying cellular bases.

Glial cells form boundaries for developing insect olfactory glomeruli

Like many other first-order olfactory centers, the olfactory lobe of the moth brain is organized into histologically conspicuous synaptic glomeruli delimited by glial cells. These glomeruli in the moth offer an advantageous system in which to study the parcellation of central neuropils during development. In this article, we review the evidence that glial cells play an essential role in the induction of glomeruli by olfactory sensory axons, and we present new evidence that leads us to propose that a major role for glia in the development of glomeruli is to define for ingrowing dendrites of second-order olfactory neurons edges that lead to the formation of these cells' characteristic morphology.

Precocity and plasticity: odor deprivation and brain development in the precocial mouse Acomys cahirinus

Altering the early olfactory environment of animals can have dramatic consequences on the development of brain regions which subserve olfaction. The present study indicates that early odor deprivation has a more severe effect on a species which is born relatively mature than it does on related species which are not. The results call into question prevailing notions about the developmental continuity between animals born in divergent ontogenetic states.

Single olfactory organ associated with prosencephalic malformation and cyclopia in a Xenopus laevis tadpole

A cyclops Xenopus laevis tadpole with a single olfactory organ is described. At a stage comparable to 48, the telencephalon was severely atrophic and only the region where the olfactory fibres terminated appeared to have the cytoarchitecture of the olfactory bulb. In this animal the central nervous system (CNS) appeared normally developed only posterior to the preoptic area. The hypothesis of a diencephalic origin of the region where the olfactory fibres terminated is discussed in the light of our previous results of olfactory placode transplantation. By analogy between this case and other malformations (cyclopia, holoprosencephaly) in higher vertebrates and humans, the need is emphasized for a more precise anatomical description of the olfactory input in related malformations.

Neuronal changes in the forebrain of mice following penetration by regenerating olfactory axons

Following total, unilateral bulbectomy in neonatal mice, the olfactory sensory axons regrow from a reconstituted population of sensory neurons, cross the lamina cribrosa, and invade the spared forebrain that has leaned forward toward the anteroventral wall of the cranial cavity. The sensory axons invade several regions of the spared forebrain, at times penetrating deeply into the brain parenchyma. These axons terminate in characteristic globose structures resembling the glomeruli of the olfactory bulb. However, they can be distinguished from the latter by the absence of periglomerular cells. These ectopic glomerular structures are formed by the commingling of the olfactory axon terminals and the dendrites of brain neurons that lie in their proximity. Previously we have established that synaptic contacts occur between the sensory axon terminals and the dendrites of the brain neurons. Our present study describes large neurons, resembling mitral cells, that expand their dendrites into the intracerebral glomeruli. These neurons are recognized by virtue of their relatively large diameter, their selective stainability with silver methods, and the unorthodox arrangement of their dendrites in comparison with the neurons of the region. Their appearance is contingent upon the presence of ectopic glomeruli. The possibility is discussed that the large argyrophilic neurons may be derived from developing neuronal elements of the brain.