Study of the fifth-type cell in the olfactory epithelium

The olfactory epithelium has been shown to contain fifth-type cells and microvillar cells and olfactory receptor cells along with supporting and basal cells. However, the morphological differences among them are unknown, especially between the former two, and most of their functions are still unclear. In this study, the fifth-type cell and the microvillar cell in the olfactory epithelium of mouse were micromorphologically investigated by SEM and TEM. With the aim of elucidation their functions, the morphological change in these cells after resection of olfactory bulb were studied. In addition, the changes in the olfactory epihelium after HRP injections into the postganglionic fiber for the trigeminal nerve and the olfactory bulb were examined. The TEM observation revealed that the fifth-type cell has finger-like microvilli, which are strong and straight. The microvilli were characterized by a specific core structure consisting of microfilament bundles. On the other band, the microvilli of the microvillar cell were meandering and had no core structure, indicating that the two cells are clearly different types. By SEM observation, neither the fifth-type cell nor the microvillar cell was found in the normal olfactory epithelium. When the olfactory cillial mat disappeared as a result of recection of the olfactory bulb, the fifth-type cell became observable. On the 300th-day after the resection, it was found that the olfactory receptor cells had disappeared and the microvilli of the supporting cells were shortened. Thus, the surface structure of the microvillar cell became clearly observable. Then, the microvilli were found to be shorter than those of the fifth-type cell and were distributed radially. Neither the fifth-type cell nor the microvillar cell was not affected by resection of the olfactory bulb. Most of the olfactory vesicles were HRP positive after the HRP injection into the olfactory bulb; however, HRP was not detected in either the fifth-type cells or the microvillar cells. After the HRP injection into the trigeminal postganglionic fiber, it was detectable in part of the nerves in the lamina propria mucosa and the epithelal basement, but not in the fifth-type cell or microvillar cell. These results suggest that the fifth-type cell is a mechanoreceptor for a system other than the olfactory one and the microvillar cell is a kind of supporting cell in an early stage or final stage near death.     

Cell surface-associated extracellular distribution of a neural proteoglycan, 6B4 proteoglycan/phosphacan, in the olfactory epithelium, olfactory nerve, and cells migrating along the olfactory nerve in chick embryos

The immunocytochemical and immuno-electron microscopic distribution of a neural proteoglycan (PG) was investigated with a monoclonal antibody, MAb 6B4, in the olfactory epithelium, the olfactory nerve, and the cells originating the epithelium and migrating along the olfactory nerve toward the forebrain in chick embryos. The PG recognized by MAb 6B4, that is 6B4 PG, in the brain of early postnatal rats, is identical to phosphacan. In chick embryos, immunoreactivity to 6B4 PG appeared on embryonic day (ED) 3-3.5 in a thin layer beneath the olfactory epithelium. It disappeared immediately, then becoming apparent in the bundles of the olfactory nerve. The immunoreactivity in the nerve bundles gradually increased during ED 5-11. On the other hand, cell surface-associated extracellular localization of the immunoreactivity was seen in the olfactory epithelium on ED 6 and afterwards. Immunofluorescent double-labeling of 6B4 PG and gonadotropin-releasing hormone (GnRH) revealed that the cell bodies of both GnRH-containing cells and other cells migrating along the olfactory nerve were surrounded by a rim immunoreactive to the PG. Under an electron microscope, the surfaces of the cell bodies and of the neurites in the nerve bundles were surrounded by deposits immunoreactive to 6B4 PG. These results indicate that 6B4 PG in chick embryos is one type of cell surface-associated extracellular matrix molecule, and that 6B4 PG covered the surfaces of migrating cells and of elongating olfactory nerve. The cell surface-associated extracellular localization of 6B4 PG found in the nasal region, taken together with the binding properties of this PG with cell adhesion molecules shown in rat brains, suggested that 6B4 PG played a role in guiding the migration of cells along the olfactory nerve in chick embryos.     

Development and further characterization of a small subclass of rat olfactory receptor neurons that shows immunoreactivity for the HSP70 heat shock protein

We previously described a rat olfactory receptor neuron (ORN) subpopulation [the 2A4(+) ORNs] that shows uniquely strong reactivity with antibodies to the 70-kD heat shock protein (HSP70) family of molecular chaperones (Carr et al. [1994] J. Comp. Neurol. 348:150-160). The 2A4(+)ORNs are dispersed through zones II-IV of the olfactory epithelium (OE), and their axons project to only two or three glomeruli that are located consistently in each olfactory bulb (OB). To date, the 2A4(+)ORN subpopulation is the only cell population to show such distinct HSP70 immunoreactivity as well as the most discrete ORN subpopulation to be so labeled. The present report shows that 2A4(+)ORN neurons first appear between postnatal days 7 (P7) and P10. Initially, low cell numbers rise to a density of 0.1 2A4(+)ORNs/mm OE length by P14, plateau at 0.9 2A4(+)ORNs/mm by P49, then fall to adult values of 0.4 cells/mm. Autoradiographic birthdating indicates that almost all of these early appearing 2A4(+)ORNs are generated postnatally, in contrast to the prenatal generation of all ORN subpopulations characterized to date by their expression of olfactory receptor protein mRNAs. A developmentally related increase in the mean depth of 2A4(+)ORNs within the OE also occurs. In the OB, initial 2A4(+)axonal projections are to only two or three glomeruli, as in adults. Slight but significant rostral shifts in (+)glomerular location occur with development. The 2A4(+)ORN immunoreactivity was found to be due to expression of HSP70, the dominant stress-inducible member of the HSP70 family, rather than constitutively expressed HSC70. In addition, despite their presence in rat OE, no 2A4(+)ORNs were found in mice, gerbils, guinea pigs, or hamsters.     

Study of the function of nerve growth factor in the olfactory tract of the mouse

The aim of this study was to examine the function of nerve growth factor (BGF) in the olfactory tract of mice. Using the mice which had received unilateral olfactory bulbectomy and in which antibodies to NGF had been continuously infused with into the contralateral olfactory blub, three kinds of analysis were performed: histological analysis of the olfactory epithelium by HE staining, immunohistochemical analysis of the olfactory epithelium using polyclonal antibodies to trk which forms the NGF receptor, and olfactory-mediated behavioral analysis with cycloheximide. These animals had been sacrificed at day 1, 3, 7, 14, 21 or 28. Several findings were obtained as a result of the above analysis. Degeneration of the olfactory epithelium and trk expression by the olfactory cells were observed on day 7, and the olfactory epithelium was incompletely regenerated on day 28. However, trk expression by the olfactory cell was still recognized and the olfactory function was not restored by day 28. These examinations suggest that NGF produced in the olfactory bulb was transported retrogradely to olfactory cells through the olfactory nerves, and was associated with sustaining the existence of those cells and with regenerating the olfactory tract after injury.     

Association of ecto-5'-nucleotidase with specific cell types in the adult and developing rat olfactory organ

A unique feature of the olfactory epithelium is its ability to give rise to new sensory neurons throughout life and also following injury. Cells at the basal side of the epithelium serve as neurogenic progenitor cells. The enzyme ecto-5'-nucleotidase is expressed at the surface of developing nerve cells and is regarded as a marker of neural development. To study the expression pattern of the enzyme, we analyzed its distribution in the adult and developing rat olfactory organ. Labeling is restricted to specific cell types and varies between the epithelia investigated. At the basal side of the olfactory epithelium, activity of 5'-nucleotidase is associated specifically with the dark/horizontal basal cells. Neither the light/globose basal cells, which are the immediate precursors of the sensory receptor cells, nor subsets of potentially immature olfactory receptor cells are labeled. On the other hand, microvillar cells dispersed at the lumenal side of the epithelium contain 5'-nucleotidase activity. The enzyme is also present at the inner lining of the ducts of Bowman's glands as they traverse the epithelium. Within the respiratory epithelium, activity of 5'-nucleotidase is associated with basal cells as well as with the epithelial surface. During development, 5'-nucleotidase is initially limited to the respiratory epithelium, including its basal cells. Dark/horizontal basal cells of the olfactory epithelium, which are positive for 5'-nucleotidase, first appear at the border of the respiratory epithelium, suggesting that they might originate from immigrating basal cells of the respiratory epithelium. Within the vomeronasal organ, labeling is largely restricted to the receptor-free epithelium. Although the functional role of 5'-nucleotidase in the olfactory system needs to be further defined, the distribution of the enzyme can be used successfully as a marker for defined cell types.     

Amelioration of virulent Babesia bovis infection in calves by administration of the nitric oxide synthase inhibitor aminoguanidine

The expression of synaptosomal-associated protein (SNAP-25), neural growth-associated protein (GAP-43) and neural cell adhesion molecule (NCAM) were studied in mouse olfactory cells and axons for 2 weeks following unilateral bulbectomy. The olfactory cells and axons in the control olfactory epithelium were positive for SNAP-25 but levels decreased in the atrophic olfactory epithelium 3 days after bulbectomy. There was no expression of SNAP-25 in the olfactory epithelium on the bulbectomy side 7 days after bulbectomy, indicating that this protein may be a good marker for the degeneration of olfactory cells. The expression of NCAM was still found in the atrophic olfactory epithelium at 7 days after bulbectomy, while the expression of NCAM in the olfactory epithelium of the bulbectomy side was stronger than that on the control side at 14 days after bulbectomy. The expression of GAP-43 in the olfactory axonal bundles of the bulbectomy side at 3 and 4 days after bulbectomy was stronger than that on the control side. These results suggest that upregulation of NCAM may be related to the regeneration of the olfactory cells, with upregulation of GAP-43 probably playing a role in axonal regeneration after bulbectomy.     

Olfactory receptor proteins. Expression, characterization and partial purification

A rat olfactory epithelium cDNA library was screened for olfactory receptor clones. One of the positively hybridizing cDNA clones was sequenced and found to encode a new member of the olfactory receptor superfamily. This cDNA, termed olp4, was used as a model of olfactory receptor for expression, both in vitro and in vivo. Expression of olp4, as well as of another previously cloned olfactory receptor (F5), was monitored by immunoprecipitation was a monoclonal antibody directed against a Flag peptide epitope tag, inserted at the N-terminus of the open reading frame, and a specific polyclonal antibody against a C-terminal peptide of olp4. Translation in vitro, followed by immunoprecipitation, showed a major olp4-specific band of 27-29 kDa. The olp4 and F5 polypeptides were found to be inserted into microsomal membranes as expected for integral membrane proteins. Expression in vivo of Flag-olp4 in Sf9 insect cells, using the baculovirus expression system, showed a specific polypeptide of the same size as the in vitro species, with an additional band of 34 kDa, which is most likely a glycosylated form. Fluorescence cytometry and immunohistochemical assays demonstrated the localization of the Flag-olp4 product on the cell surface of the infected host Sf9 cells, with the N-terminus and C-terminus in the proper orientation. Affinity chromatography was used for the partial purification of the olp4 polypeptide from infected Sf9 cells. The identification and purification of this expressed olfactory receptor polypeptide could open the way for further characterization and functional studies of the olfactory receptor superfamily members.     

Mash1 activates a cascade of bHLH regulators in olfactory neuron progenitors

The lineage of olfactory neurons has been relatively well characterized at the cellular level, but the genes that regulate the proliferation and differentiation of their progenitors are currently unknown. In this study, we report the isolation of a novel murine gene, Math4C/neurogenin1, which is distantly related to the Drosophila proneural gene atonal. We show that Math4C/neurogenin1 and the basic helix-loop-helix gene Mash1 are expressed in the olfactory epithelium by different dividing progenitor populations, while another basic helix-loop-helix gene, NeuroD, is expressed at the onset of neuronal differentiation. These expression patterns suggest that each gene marks a distinct stage of olfactory neuron progenitor development, in the following sequence: Mash1>Math4C/neurogenin1>NeuroD. We have previously reported that inactivation of Mash1 function leads to a severe reduction in the number of olfactory neurons. We show here that most cells in the olfactory epithelium of Mash1 mutant embryos fail to express Math4C/neurogenin1 or NeuroD. Strikingly, a subset of progenitor cells in a ventrocaudal domain of Mash1 mutant olfactory epithelium still express Math4C/neurogenin1 and NeuroD and differentiate into neurons. Cells in this domain also express Math4A/neurogenin2, another member of the Math4/neurogenin gene family, and not Mash1. Our results demonstrate that Mash1 is required at an early stage in the olfactory neuron lineage to initiate a differentiation program involving Math4C/neurogenin1 and NeuroD. Another gene activates a similar program in a separate population of olfactory neuron progenitors.     

Proliferation in the rat olfactory epithelium: age-dependent changes

Vertebrate olfactory sensory neurons are replaced continuously throughout life. We studied the effect of age on proliferation in olfactory epithelium in postnatal rats ranging in age from birth (P1) until P333. Using BrdU to label dividing cells, we determined the proliferation density of basal cells, i.e., the number of labeled nuclei/unit length (240 microm) of olfactory epithelium in coronal sections from six different anterior-posterior levels from each animal. A total length of >1 m of olfactory epithelium was counted in each age group. We observed a dramatic decrease of proliferation density from P1 through P333. At P1, proliferation density is 151 cells/mm; it decreases to approximately half at P21 (70 cells/mm), and half again at P40 (37 cells/mm). At P333 the proliferation density was only 8/mm, approximately 5% of that seen at P1. The changes were clearly related to age and not to body weight, because the values were essentially identical for males and females of the same age but of different body weight. Proliferating cells appear in patches that, after P40, become more separated from one another and contain fewer cells. In 6- and 11-month-old rats, 30 and 45% of all units contained no labeled cells. We confirmed the data of others that the olfactory surface area continuously increases with age; we showed that there is a reciprocal relationship between proliferation density and surface area. The proliferating cells provide neurons to sustain growth as well as to replace dying cells.     

Effect of sulfamethazine on sexual precocity and neuropeptide Y neurons within the tuberoinfundibular region of the chick brain

A hitherto ignored microvillous cell type, distinct from microvillous supporting cells and other microvillous cell types, was encountered in olfactory and respiratory epithelia of nasal turbinates of rat fetuses, near the transition between these two epithelia. The apex of the cell resembles the apices of vestibular hair cells. The cell has a cone-shaped bundle of microvilli, resembling the complex bundle of hair-cell stereocilia, accompanied by a cilium. Therefore we called this cell type the nasal hair cell. Cilium and microvilli seemed adhered. Cell numbers were very low, up to about 5 per turbinate. The cell's appearance is precocious compared to that of olfactory receptor and supporting cells. Also, while the apices of olfactory receptor and supporting cells and of ciliated respiratory cells underwent major morphological maturation during the developmental period from embryonic day 16 to day 21, the apical structures of the nasal hair cell only changed marginally from embryonic day 16, when they were first seen, through to at least embryonic day 21. The cell's location and precociously mature appearance suggests that it plays a special role in the development of nasal epithelia.     

FGF2 promotes neuronal differentiation in explant cultures of adult and embryonic mouse olfactory epithelium

Neurogenesis in the adult olfactory epithelium is highly regulated in vivo. Little is known of the molecular signals which control this process, although contact with the olfactory bulb or with astrocytes has been implicated. Explants of mouse olfactory epithelium were grown in the presence or absence of several peptide growth factors. Basic fibroblast growth factor (FGF2) stimulated differentiation of sensory neurons in adult and embryonic olfactory epithelium. Other growth factors tested were ineffective. FGF2-stimulated neurons were born in vitro and expressed neurofilament, neural cell adhesion molecule, and beta-tubulin. The cells also expressed olfactory marker protein, a marker for mature olfactory sensory neurons in vivo. These bipolar neurons did not express glial fibrillary acidic protein or low-affinity nerve growth factor receptor. These results indicate that neither astrocytes nor olfactory bulb are necessary for differentiation of olfactory sensory neurons in vitro.     

Evidence for a role of the chemorepellent semaphorin III and its receptor neuropilin-1 in the regeneration of primary olfactory axons

To explore a role for chemorepulsive axon guidance mechanisms in the regeneration of primary olfactory axons, we examined the expression of the chemorepellent semaphorin III (sema III), its receptor neuropilin-1, and collapsin response mediator protein-2 (CRMP-2) during regeneration of the olfactory system. In the intact olfactory system, neuropilin-1 and CRMP-2 mRNA expression define a distinct population of olfactory receptor neurons, corresponding to immature (B-50/GAP-43-positive) and a subset of mature (olfactory marker protein-positive) neurons located in the lower half of the olfactory epithelium. Sema III mRNA is expressed in pial sheet cells and in second-order olfactory neurons that are the target cells of neuropilin-1-positive primary olfactory axons. These data suggest that in the intact olfactory bulb sema III creates a molecular barrier, which helps restrict ingrowing olfactory axons to the nerve and glomerular layers of the bulb. Both axotomy of the primary olfactory nerve and bulbectomy induce the formation of new olfactory receptor neurons expressing neuropilin-1 and CRMP-2 mRNA. After axotomy, sema III mRNA is transiently induced in cells at the site of the lesion. These cells align regenerating bundles of olfactory axons. In contrast to the transient appearance of sema III-positive cells at the lesion site after axotomy, sema III-positive cells increase progressively after bulbectomy, apparently preventing regenerating neuropilin-1-positive nerve bundles from growing deeper into the lesion area. The presence of sema III in scar tissue and the concomitant expression of its receptor neuropilin-1 on regenerating olfactory axons suggests that semaphorin-mediated chemorepulsive signal transduction may contribute to the regenerative failure of these axons after bulbectomy.     

Crypt hyperplasia related to increased lymphocyte activation in the rectal mucosa of children with ulcerative colitis

The presence of cells positive for interleukin-2 receptors (CD25) and the proportion of Ki-67 positive dividing cells in the crypts were studied with monoclonal antibodies using sensitive immunohistochemical techniques in the rectal biopsy specimens taken from 13 children with inflammatory bowel disease (4 Crohn's disease and 9 ulcerative colitis) and 10 controls. In all specimens, but one, from patients CD25+ cells were found in the surface and crypt epithelium, while in none of controls were seen such cells. The mean percentage of dividing crypt cells was significantly higher in ulcerative colitis (36.0%) and in Crohn's disease (34.6%) than in controls (18.4) (p < 0.001 in both comparisons). In ulcerative colitis, a correlation between CD25/CD3 ratios in the surface and crypt epithelium and the percentage of dividing crypt cells was found. We conclude that activated lymphocytes have a role in the crypt hyperplasia of ulcerative colitis.     

Morphology and cytology of the nasal cavity and vomeronasal organ in juvenile and adult blind mole rats (Spalax ehrenbergi)

The blind mole rat (Spalax ehrenbergi) is a fossorial solitary rodent which exhibits extensive intraspecific aggression and uses scent markings to deter contraspecific invaders. Mole rats of different ages were captured near Tel Aviv, Israel, and sacrificed by an overdose of Xylazine hydrochloride. Olfactory epithelium sites from the nasal cavity (NC) and the vomeronasal organ (VNO) were dissected and fixed for light and electron microscopy. The mole rat's olfactory epithelium of the NC consists of several cell types, of which two types are supporting cells that comprise both microvilli and cilia but differ in staining and the presence of rough endoplasmic reticulum. The third type has no cilia. Secretory goblet cells were frequent among supporting cells of adults alone. Two types of receptor cells protrude into the NC with olfactory knobs at their apical region; one type has up to 177.6 +/- 9.4 cilia per knob plus microvilli, while the other type has only microvilli. The third type of sensory cell has no knob and contains microvilli only. The basal epithelium layer consists of short-bodied cells with round nuclei. The VNO of the mole rat is situated beneath the nasal septum, consisting of supporting, sensory, and basal cell types, with many cilia at the apical portion. At its anterior part, the VNO is connected to the NC by narrow canals. The abundance of cilia and microvilli in the mole rat olfactory cells provides the first anatomical evidence for their olfactory acuity. Such acuity is important in mole rats, compensating for their loss of vision and enabling them to detect and avoid rivals prior to potential aggressive encounters as well as to select food plants during foraging.     

Response of olfactory Schwann cells to intranasal zinc sulfate irrigation

The response of olfactory Schwann cells was assessed at 2, 4, and 7 days following intranasal zinc sulfate irrigation in 1-month-old mice. Ultrastructural and immunohistochemical observations showed dramatic differences between experimental and control mice which had been washed with saline intranasally. Two days after zinc sulfate treatment, many olfactory nerve bundles contained patchy areas of axonal degeneration, while the cell bodies of the olfactory Schwann cells appeared to have increased in electron density and to have shifted peripherally. Some of the cell bodies protruded from the surface of the axon fascicle, suggesting that the olfactory Schwann cells were in the initial process of migrating away. On the fourth day when most of the olfactory axons had degenerated, some olfactory Schwann cells were aligned immediately beneath the basal lamina of the olfactory epithelium. These cells were immunopositive for the S-100 protein and possessed an expanded perinuclear space. Many olfactory Schwann cells were present in the region beneath the cribriform plate, while some appeared to have passed through the gaps between the bony plates to reach the olfactory bulb. Hence, the results showed that many olfactory Schwann cells migrated towards the olfactory bulb following loss of axonal contact. Furthermore, on the seventh day following zinc sulfate treatment, some olfactory Schwann cells in the vicinity of the olfactory bulb appeared phagocytic, as indicated by their extension of processes around fragments of cell debris and the presence of lysosome-like organelles in the perikaryon. The control mice which had been intranasally irrigated with saline did not demonstrate massive olfactory axonal degeneration, and the morphology of the nasal cavity region was similar to that of normal mice.     

Cloning and localization of two multigene receptor families in goldfish olfactory epithelium

Goldfish reproduction is coordinated by pheromones that are released by ovulating females and detected by males. Two highly potent pheromones, a dihydroxyprogesterone and a prostaglandin, previously have been identified, and their effects on goldfish behavior have been studied in depth. We have cloned goldfish olfactory epithelium cDNAs belonging to two multigene G-protein coupled receptor families as a step toward elucidating the molecular basis of pheromone recognition. One gene family (GFA) consists of homologs of putative odorant receptors (approximately 320 residues) found in the olfactory epithelium of other fish and mammals. The other family (GFB) consists of homologs of putative pheromone receptors found in the vomeronasal organ (VNO) of mammals and also in the nose of pufferfish. GFB receptors (approximately 840 residues) are akin to the V2R family of VNO receptors, which possess a large extracellular N-terminal domain and are homologs of calcium-sensing and metabotropic glutamate receptors. In situ hybridization showed that the two families of goldfish receptors are differentially expressed in the olfactory epithelium. GFB mRNA is abundant in rather compact cells whose nuclei are near the apical surface. In contrast, GFA mRNA is found in elongated cells whose nuclei are positioned deeper in the epithelium. Our findings support the hypothesis that the separate olfactory organ and VNO of terrestrial vertebrates arose in evolution by the segregation of distinct classes of neurons that were differentially positioned in the olfactory epithelium of a precursor aquatic vertebrate.     

Formulation and evaluation of albuterol metered dose inhalers containing tetrafluoroethane (P134a), a non-CFC propellant

The objectives of this study were to determine: (1) the frequency and distribution of carbonic anhydrase (CA) activity in the bullfrog nasal cavities, and (2) whether inhibition of nasal CA affects the olfactory receptor response to CO2 or other odorants. It was found, using Hansson's staining technique, that some olfactory receptor neurons exhibited CA activity and that these CA-positive receptors were distributed throughout the nasal cavity with peak densities in the dorsal and ventral sensory epithelial regions. To test for the role of CA in olfactory transduction, electro-olfacto-grams (EOGs) were recorded from the surface of the ventral sensory epithelium in response to 2-s pulses of 5% CO2 and amyl acetate before and after topical CA inhibition with acetazolamide (10(-3) mol.l-1). In 52 bullfrogs, 1222 sites on the ventral epithelium were tested resulting in 23 locations that exhibited a response to 5% CO2. Inhibition of CA caused an immediate 65% reduction in the EOG response to CO2 while the response to amyl acetate was not affected. These results, along with the histochemical localization of CA in some olfactory receptor neurons, indicate that CA plays a role in the detection of CO2 in frog olfactory neurons and that only a small population of olfactory receptor neurons are CO2 sensitive.     

Macrophage and microglia-like cells in the avian inner ear

Recent studies suggest that macrophages may influence early stages of the process of hair cell regeneration in lateral line neuromasts; numbers of macrophages were observed to increase prior to increases in hair cell progenitor proliferation, and macrophages have the potential to secrete mitogenic growth factors. We examined whether increases in the number of leukocytes present in the in vivo avian inner ear precede the proliferation of hair cell precursors following aminoglycoside insult. Bromodeoxyuridine (BrdU) immunohistochemistry was used to identify proliferating cells in chicken auditory and vestibular sensory receptor epithelia. LT40, an antibody to the avian homologue of common leukocyte antigen CD45, was used to label leukocytes within the receptor epithelia. Macrophages and, surprisingly, microglia-like cells are present in normal auditory and vestibular sensory epithelia. After hair cell loss caused by treatment with aminoglycosides, numbers of macrophage and microglia-like cells increase in the sensory epithelium. The increase in macrophage and microglia-like cell numbers precedes a significant increase in sensory epithelial cell proliferation. The results suggest that macrophage and microglia-like cells may play a role in releasing early signals for cell cycle progression in damaged inner ear sensory epithelium.