Masako Fujii

Long-distance fiber outgrowth from the heterotopically transplanted olfactory bulb in the rat

An embryonic olfactory bulb was heterotopically inserted and allowed to mature in young adult rat brains. The projection of the transplanted olfactory bulb to the host brain was examined by injections of peroxidase-labeled wheatgerm agglutinin into the host olfactory bulb (and anterior olfactory nucleus). Neurite elongation to the host olfactory area occurred most frequently from the transplant which had been inserted into the anterior horn of the lateral ventricle and fused medially with the lateral septum in host brains with no detectable damage of host olfactory connections. Transplants in the septum, olfactory tubercle, nucleus of the horizontal limb of the diagonal band, or anterior piriform cortex also showed the projection to the host olfactory area. These results indicate that the transplanted olfactory bulb projection neurons have potent abilities to detect the target and project to it even if there is a considerable distance (2-5 mm).

Heterotopically transplanted embryonic olfactory bulb projection neurons form selective and appropriate axonal projections over considerable distances in adult host brains

Embryonic olfactory primordia were transplanted into the region of the septum, the adjacent lateral ventricle (LV) and olfactory tubercle (Tu) in adult host rats. After a minimum of 7 weeks, wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) were injected into the host anterior olfactory nucleus (AO). The injection retrogradely labeled the neural cell bodies in the large neuron area (the mitral and tufted cells) of the olfactory bulb (OB) transplant. In addition, the anterogradely labeled fibers projected from the host AO to the transplant. These results indicated that the transplanted mitral and tufted neurons were able to grow axons selectively to an appropriate host terminal region (the AO) and receive fibers from the AO, even when the transplant itself was in an inappropriate host site, at a considerable distance from the host AO.

Non-specific characteristics of intracerebral fiber elongation from the olfactory bulb transplanted into the young adult host neocortex or hippocampal formation, demonstrated immunohistochemically by the mouse Thy-1 allelic system

The olfactory bulb of an embryo (BALB/c strain mouse) was transplanted into the neocortex or hippocampal formation in a young female adult host (AKR strain mouse) and matured for at least 1 month. Projection fibers from the transplant were demonstrated immunohistochemically using the allelic form of the Thy-1 system between both mice. Fibers from the olfactory bulb transplanted into the frontal cortex showed non-specific elongation in two directions where normally there are no targets of the olfactory bulb fibers: one was toward the cortical surface among radially oriented host fiber systems and another on the corpus callosum running rostrocaudally together with the hosts long association bundle. Transplants placed in the hippocampal dentate and hilar regions emit fibers mainly towards the dentate molecular layer and these fibers spread medially and laterally within the molecular layer. The results suggest that projection neurons (mitral and tufted cells) in the transplanted olfactory bulb have the potential ability of non-target-directing axon elongation into the host nerve tissue which usually prevents the penetration of newly growing fibers.

Transplant-to-Host Neuron Migration and Neurite Projection from Homotopically Transplanted Olfactory Bulb as Demonstrated by Mouse Allelic Thy-1 Form

Transplant-to-host neuron migration and neurite projection were demonstrated using the mouse allelic Thy-1 system, namely, BALB/c (Thy-1.2) embryonic olfactory bulb (OB) as the graft and 5- to 6-week-old AKR (Thy-1.1) OB as the host. From OB transplants inserted into the host OB, small neurons were often extensively moved mainly in the internal granular layer and showed almost the same morphology as the normal granule neurons. Some large neurons also migrated. Furthermore, inside OB the transplants sent axons mainly into the internal granular layer and dendrites into the external plexiform layer. Outside OB the axons arrived at the anterior olfactory nucleus, primary olfactory cortex, olfactory tubercle, and cortical nucleus of the amygdaloid complex. These fibers appeared to terminate in normal target areas. These findings show that the olfactory system at 5-6 weeks of age still has the capacity to integrate newly migrated neurons and to receive newly growing fibers from the transplant.

DIFFERENTIATION OF CHEMICALLY DEFINED NEURONAL POPULATIONS IN THE TRANSPLANTED OLFACTORY BULB WITHOUT OLFACTORY RECEPTOR INNERVATION

Olfactory bulbs (OBs) from embryonic day 15 and 17 and postnatal day 1 mice were transplanted into the lateral ventricle of juvenile host mice without bulbectomy, and fine structural and chemical features of neurons and glia in the OB transplants were investigated immunocytochemically and electron microscopically. In the OB transplants there were neither clearly defined glomeruli nor layers, nor olfactory marker protein immunoreactive elements. However, chemically defined neuronal populations resembling those in the normal OBs such as those immunoreactive for gamma-aminobutyric acid (GABA), tyrosine hydroxylase and Ca(2+)-binding proteins (calbindin-D28K, calretinin, parvalbumin) were observed. Electron microscopically, dendrodendritic and somatodendritic reciprocal synapses, that is, synapses characteristic of the OB, were occasionally observed in the OB transplants. These results indicated that at least some embryonic or newborn mouse OB neurons and/or precursor cells could exhibit chemical properties and form typical synaptic contacts observed in normal OB, even when they received no inputs from olfactory receptor cells.

Axons from the olfactory bulb transplanted into the cerebellum form synapses with dendrites in the granular layer, as demonstrated by mouse allelic form of Thy-1 and electron microscopy

Transplant-to-host axon projection and synapse formation from the olfactory bulb (OB) transplant to the host cerebellum were studied using the mouse allelic form of Thy-1 (AKR strain of Thy-1.1 was used as host and BALB/c strain of Thy-1.2 as graft). Thy-1.2 immunohistochemical and ultrastructural examination showed that numerous OB axons elongate into the cerebellar granular layer and form asymmetrical synapses there with dendrites of host origin (perhaps of the host granule neuron). Factors which support this mismatched synapse formation are discussed.

Cases of two patients whose food aversions disappeared following severe traumatic brain injury.

Following traumatic brain injury (TBI) the complete disappearance of food aversions was observed in the cases of two patients. In one of the cases, a young female, this change in her food aversion was manifested several months after the TBI, from the time when she was able to eat normally. The other patient, a young man, exhibited the disappearance of his food aversion immediately after recovery from his unconscious state following TBI. These results indicate that the disappearance of food aversions was a consequence of TBI.

AXONS FROM THE OLFACTORY BULB TRANSPLANTED INTO THE HIPPOCAMPAL FORMATION SHOW AXON PREFERENCES AND FORM TRANSPLANT-TO-HOST SYNAPSES SIMILAR TO THOSE OF NORMAL AFFERENTS

Using a mouse Thy-1 allelic system (an AKR strain of Thy-1.1 was used as the host and a BALB/c strain of Thy-1.2 as the graft), characteristics of transplant-to-host axon projection and synapse formation of the olfactory bulb (OB) were studied in the hippocampal formation by immunohistochemical and ultrastructural examinations. Thy-1.2-positive axon projections were most prominent in the dentate molecular layer (DM), although there was a labeled-axon-poor layer in the innermost part of the DM. Some of the axons in the DM entered the hippocampus proper, forming two clear borders between the two axon-rich outer layers and the inner axon-poor central layer. These results suggested that axons from the transplanted OB extended into the hippocampal formation, showing axon preferences. In the ultramicroscopic observation of the DM, three different synaptic patterns could be distinguished among the transplant-to-host synapses. These three synaptic patterns are similar to those of the synapses formed by perforant path fiber terminals in the DM, indicating that the transplanted OB forms fiber connections with the dendrites of the dentate granule neurons similar to those of normal afferents.

Migration of small and large cells from the grafts of embryonic olfactory bulbs, transplanted into the anterior wall of the lateral ventricle

Embryonic olfactory bulbs (OBs) were transplanted into the anterior wall of the lateral ventricle (ALV) of juvenile and adult host mice. Neurons originated from the transplanted OBs were detected immunohistochemically by using mouse Thy-1 allelic system. Within the OBs of both host groups, a substantial number of Thy-1.2-positive small presumed granule cells were observed. Furthermore, Thy-1.2 labeled large cells, although far smaller in number, were encountered in some cortical areas, including the anterior olfactory nucleus of the juvenile host group, but not within the host OBs. These results indicated that both small and large cell precursors had the capacity to migrate from the host ALV, but only the small cell precursors could migrate tangentially into host OBs.