Olfactory bulb transplantation into the olfactory bulb of neonatal rats: a WGA-HRP study

Implications of olfactory lamina propria transplantation on hyperreflexia and myelinated fiber regeneration in rats with complete spinal cord transection

Transplantation with olfactory ensheathing cells (OECs) has been adopted after several models of spinal cord injury (SCI) with the purpose of creating a favorable environment for the re-growth of injured axons. However, a consensus on the efficacy of this cellular transplantation has yet to be reached. In order to explore alternative parameters that could demonstrate the possible restorative properties of such grafts, the present study investigated the effects of olfactory lamina propria (OLP) transplantation on hyperreflexia and myelinated fiber regeneration in adult rats with complete spinal cord transection. The efficacy of OLP (graft containing OECs) and respiratory lamina propria (RLP, graft without OECs) was tested at different post-injury times (acutely, 2- and 4-week delayed), to establish the optimum period for transplantation. In the therapeutic windows used, OLP and RLP grafts produced no considerable improvements in withdrawal reflex responses or on the low-frequency dependent depression of H-reflex. Both lamina propria grafts produced comparable results for the myelinated fiber density and for the estimated total number of myelinated fibers at the lesion site, indicating that the delayed transplantation approach does not seem to limit the regenerative effects. However, animals transplanted with OLP 2 or 4 weeks after injury exhibit smaller myelin sheath thickness and myelinated fiber area and diameter at the lesion site compared to their respective RLP groups. Despite the ongoing clinical use of OECs, it is important to emphasize the need for more experimental studies to clarify the exact nature of the repair capacity of these grafts in the treatment of SCI.

Olfactory ensheathing cell transplantation following spinal cord injury: Hype or hope?

Olfactory ensheathing cells (OECs) are unique glia found only in the olfactory system that retain exceptional plasticity, and support olfactory neurogenesis and the re-targeting across the PNS:CNS boundary in the olfactory system. Because they are also relatively accessible in an adult rodent or human, OECs have become a prime candidate for cell-mediated repair following a variety of CNS lesions. A number of different labs across the world have applied OECs prepared in many different ways in several different acute and chronic models of rodent SCI, some of which have suggested surprising degrees of functional recovery. OECs can stimulate tissue sparing and neuroprotection, enhance outgrowth of both intact and lesioned axons (to different degrees), activate angiogenesis, change the response status of endogenous glia after lesion and remyelinate axons after a range of demyelinating insults. Their ability to stimulate regeneration in specific tracts is, however, limited. Despite this, the ongoing clinical use of cell preparations containing OECs has proceeded as a therapeutic approach for human spinal cord injury (SCI). Here, we review the current status of OEC research in SCI, and focus on potential mechanisms for OECs in the SCI repair response that may help to explain the biological reasons underlying the wide variation of results obtained in this promising, yet contentious, field.

An olfactory bulbectomy model to study plasticity of the nervous system in rat: a review focused on published data from the Slovak Republic

This review surveys the main data published at the Institute of Neurobiology, Slovak Republic, pertaining to both morphological and functional observations in the olfactory bulbectomy model. The review utilized this experimental approach in homo- and heterotopic transplantations, in experiments for studying neurodegenaration/regeneration and phenomena of adult neurogenesis. The results, discussed in context of relevant literary data, indicate that the model of olfactory bulbectomy can be utilized to investigate problems not only related to the olfactory system but it can also address the problems of neuronal development, neuronal plasticity and interaction concerning the nervous system as a whole.

Lamina propria and olfactory bulb ensheathing cells exhibit differential integration and migration and promote differential axon sprouting in the lesioned spinal cord

Olfactory bulb-derived (central) ensheathing cell (OB OEC) transplants have shown significant promise in rat models of spinal cord injury, prompting the use of lamina propria-derived (peripheral) olfactory ensheathing cells (LP OECs) in both experimental and clinical trials. Although derived from a common embryonic precursor, both sources of OECs reside in different nervous system compartments postnatally, and their ability to promote regeneration and efficacy after transplantation may differ depending on both their source and mode of transplantation. Here, we have purified green fluorescent protein-expressing LP and OB OECs, assayed their biological differences in vitro, and transplanted them acutely either directly into or rostral and caudal to a dorsolateral funiculus crush. LP and OB OECs exhibit multiple morphological and antigenic similarities in vitro, and, after transplantation, they both attenuate lesion and cavity formation and promote angiogenesis, endogenous Schwann cell infiltration, and axonal sprouting. However, an increased mitotic rate and migratory ability of LP OECs in vitro was reflected in vivo by their superior ability to migrate within the spinal cord, reduce cavity formation and lesion size, and differentially stimulate outgrowth of axonal subpopulations compared with OB OECs. An undesired behavior (autotomy) was also significantly enhanced by LP OEC, over OB OEC, transplantation. These results suggest that LP and OB OECs exhibit intrinsic biological differences that, after transplantation into the lesioned CNS, result in differences in postlesion spinal cord neuropathology and anatomical and behavioral regeneration outcomes that also vary depending on direct versus rostrocaudal transplantation.

Developmental localization of GAP-43 and olfactory marker protein in rat olfactory bulb transplants

In an effort to identify and understand the laminar disorganization that occurs in the transplanted (TX) rat olfactory bulb (OB), we examined the development of fiber systems within these TX OBs. One antibody for olfactory marker protein (OMP) was used to identify axons of mature olfactory receptor neurons (ONs) and a second antibody, for a growth-associated protein (GAP-43), provided a marker for all extending or immature fibers. Donor OBs were taken from fetuses on embryonic days 14 or 15 (sperm-positive day is zero) and TX directly into the cavity produced by removal of an OB in 1-day-old hosts of the same strain. After survival times of 1 and 2 weeks and at maturity, adjacent 8 microns paraffin sections from the TX material were examined for OMP and GAP-43 reactivity. Fiber bundles, reactive for OMP, were found within the TX by 1 week post-TX, indicating rapid re-innervation of the donor OB by ONs. The appearance of OMP reactivity gradually shifted from tightly packed, well-defined fiber bundles at 1 week post-TX to a diffuse reticulated pattern of individual fibers emerging from bundles at maturity. The OMP-reactive fiber bundles of the TX OB also contained GAP-43-reactive fibers, but GAP-43 reactivity also extended to other (OMP-negative) bundles and fields. Reactivity for GAP-43 in the TX OB was nearly ubiquitous at 2 weeks post-TX but, as development progressed (in both the TX and normal OB), such reactivity gradually decreased. Thus, while maturation in sensory afferent fiber systems in the TX OB may be delayed, it eventually follows a pattern similar to that in the normal OB, suggesting that factors other than the timing of fiber extension may be responsible for the laminar disorganization of the TX OB.

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.

Olfactory bulb transplants establish afferent and efferent connections with host forebrain in rat

We are using wheat germ agglutinin-conjugated horseradish peroxidase (WHRP) to study reconnectivity in the transplanted (TX) olfactory bulb (OB) in Sprague-Dawley rats. Tritium-labeled OBs from fetal rat donors of Embryonic Days 14-15 were immediately grafted into neonatal rats in the site from which the host OB had been removed. Following survival times of 7 weeks and longer, WHRP solution was injected into the TX OB, and subjects were perfused after 24 h. The WHRP transport is seen in fibers from the TX OB into layer I of the host olfactory peduncle (OP) and olfactory cortex (OC) and in cell bodies in layers II and III of the OP and OC, the lateral hypothalamus, and the contralateral anterior olfactory nucleus (AON). These findings reaffirm that the axons from a TX OB make connections with some appropriate areas of the host brain and also indicate that axons from cells in the target areas of the host brain, including contralateral AON, reinnervate the TX OB.

Host primary olfactory axons make synaptic contacts in a transplanted olfactory bulb

Previous light microscopic studies have shown that host olfactory neurons are able to grow into a transplanted fetal olfactory bulb, and behavioral studies have shown that animals with transplanted olfactory bulbs recover functional olfactory abilities. We examined the olfactory bulb transplant at the ultrastructural level to determine whether synaptic contacts are reestablished between host olfactory neurons and donor olfactory bulb. Mature rats that, as neonates, had received embryonic olfactory bulb transplants following olfactory bulb removal were studied. An antibody specific for olfactory marker protein was used to identify the primary olfactory neurons; it was bound by a gold-conjugated secondary antibody for visualization. To preserve the antigenicity of the olfactory marker protein for immunolabeling, Lowicryl K4M hydrophilic resin was used. Synaptic contacts were unmistakable between labeled axons of host olfactory neurons and unlabeled processes within glomerulus-like areas of the transplanted olfactory bulb. The surrounding neuropil contained other elements similar to those found in normal tissue, including synaptic contacts between unlabeled profiles. We clearly show that the transplanted olfactory bulb exhibits sufficient plasticity to form an array of normal synaptic contacts, including the contacts from host primary olfactory neurons.

The development of glial fibrillary acidic protein-positive cells and the appearance of laminin-like immunoreactivity in fetal olfactory bulb transplants

Embryonic rat olfactory bulbs were transplanted into the site vacated by aspiration of an olfactory bulb from a neonatal rat. This paper presents our findings related to the development of glial fibrillary acidic protein (GFAP) positive (+ve) glial cells and the appearance of laminin-like immunoreactivity in these transplants. The GFAP+ve glial cells formed perivascular end-feet on the invading vasculature and formed a glia limitans along the pial surface of the transplant. This reconstituted glia limitans was continuous with that of the host brain, there being no glia limitans at the donor-host interface. Thus, the donor tissue was well-integrated with that of the host brain.

Recovery of olfactory behavior. II. Neonatal olfactory bulb transplants enhance the rate of behavioral recovery

Previous experiments in this laboratory have shown that transplants of a fetal olfactory bulb into a neonatal rat are viable and that they establish connections with the olfactory peduncle and olfactory cortex. The focus of this experiment was to investigate the anatomical correlates of any behavioral recovery seen in rats that had one olfactory bulb removed along with an immediate transplant of a fetal olfactory bulb. Anatomical details, such as transplant organization and olfactory nerve repenetration patterns were analyzed using a variety of histological and immunohistochemical techniques. The rats in this experiment showed behavioral recovery of olfactory ability. The recovery rates observed in these animals were compared to two other groups of rats that this laboratory has shown to be behaviorally competent: normal rats and rats with neonatal ablations of the olfactory bulb but no transplant. Although the animals with transplants did not recover to completely normal levels of olfactory ability, they did start behavioral testing in a more behaviorally competent condition than rats with simple neonatal lesions. Anatomical analysis revealed that the transplanted olfactory bulb was heavily penetrated by incoming olfactory nerve fibers but olfactory nerve penetration was not limited to the transplanted olfactory bulb. The extra-bulbar host regions that were penetrated included the orbital frontal cortex and three olfaction-related areas; olfactory cortex, olfactory peduncle and the subependymal cell layer. The olfactory nerve penetration patterns observed beyond the transplant were essentially the same as those observed in rats with only neonatal lesions of the olfactory bulb. Thus, multiple pathways may have contributed to the recovery observed in the rats with olfactory bulb transplants.

Olfactory ensheathing glia and platelet-derived growth factor B-chain reactivity in the transplanted rat olfactory bulb

Using a monoclonal antibody against the B-chain of platelet-derived growth factor as a marker, we have examined the behavior of olfactory ensheathing glia in the normal and transplanted rat olfactory bulb. In the normal postnatal olfactory bulb, these glia are found to ensheath the bundles of incoming primary olfactory nerve fibers as well as those in the olfactory nerve layer. Olfactory marker protein antibody was used to identify the olfactory nerve proper. Within the transplant, the same glia: (1) ensheath bundles of both primary olfactory and non-primary olfactory axons, (2) ensheath axonal bundles deep within the donor tissue, and (3) eventually permit radiation of individual axons from bundles to surrounding neuropil. We believe that ensheathing glia (being rich in growth-related factors and extracellular matrix molecules) may be useful in providing trophic support and guidance for the reconstruction of developmentally or traumatically damaged neuronal pathways not directly related to the olfactory system. The evidence presented here indicates that ensheathing glia are capable of existing in deep brain areas and ensheathing other than primary olfactory axons. The special molecular characteristics of these glia along with the morphological findings presented here provide a foundation for further studies of these unique glia and their potential utility in the restoration of damaged neural pathways.