SEARCH

SEARCH BY CITATION

Keywords:

  • morphology;
  • morphometry;
  • postnatal development;
  • olfactory mucosa;
  • rabbit

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED

Rabbits are born blind and deaf and receive unusually limited maternal care. Consequently, their suckling young heavily rely on the olfactory cue for nipple attachment. However, the postnatal morphofunctional adaptations of olfactory mucosa (OM) are not fully elucidated. To clarify on the extent and the pattern of refinement of the OM following birth in the rabbit, morphologic and morphometric analysis of the mucosa were done at neonatal (0–1 days), suckling (2 weeks), weanling (4 weeks), and adult (6–8 months) stages of postnatal development. In all the age groups, the basic components of the OM were present. However, proliferative activity of cells of the mucosal epithelium decreased with increasing age as revealed by Ki-67 immunostaining. Diameters of axon bundles, packing densities of olfactory cells, and cilia numbers per olfactory cell knob increased progressively with age being 5.5, 2.1, and 2.6 times, respectively, in the adult as compared with the neonate. Volume fraction values for the bundles increased by 5.3% from birth to suckling age and by 7.4% from weaning to adulthood and the bundle cores were infiltrated with blood capillaries in all ages except in the adult where such vessels were lacking. The pattern of cilia projection from olfactory cell knobs also showed age-related variations, that is, arose as a tuft from the tips of the knobs in neonates and sucklings and in a radial pattern from the knob bases in weanlings and adults. These morphological changes may be attributed to the high olfactory functional demand associated with postnatal development in the rabbit. Anat Rec, 2012. © 2012 Wiley Periodicals, Inc.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED

One of the first challenges facing any newborn mammal is finding a nipple, attaching to it, and suckling (Raihani et al., 2009). Later on depending on species-specific factors, the young reach autonomy from the mother in both social and alimentary terms (Hudson and Altbäcker, 1994). Born functionally blind and deaf, rabbit pups need, as in almost all mammalian neonates, to locate the teats and suckle (Coureaud et al., 2008). Among mammals, rabbits are unusual in that they give their young ones minimal assistance during suckling and, in addition, only nurse them for a few minutes once each day (Hudson and Distel, 1983, 1984; Coureaud et al., 2008; Hudson et al., 2008). Therefore, successful suckling in the rabbit depends on the detection of specific odors on the mother's ventrum, the so-called nipple-search pheromone (Hudson et al., 2008), which has been attributed to the presence of the substance 2-methylbut-2-enal in the doe's milk (Schaal et al., 2003). Supposedly for this reason, newborn rabbits are difficult to raise with the bottle, and when rendered anosmic, are completely unable to suckle from their mother (Hudson and Distel, 1984; Distel and Hudson, 1985; Hudson and Distel, 1987).

The olfactory mucosa (OM), which mainly lines the posterior roof of vertebrate nasal cavity, detects and discriminates between a myriad of odors (Buck and Axel, 1991; Liberles and Buck, 2006). The mucosa comprises an atypical epithelium having basal, supporting, and olfactory cells, and a lamina propria that accommodates Bowman's glands, bundles of olfactory nerve axons, and blood vessels (Mendoza, 1993; Menco and Morrisson, 2003; Kavoi et al., 2010). The olfactory cell is a bipolar neuron with a dendrite that terminates at the surface of the olfactory epithelium (OE) in form of a cilia-bearing knob, a cell body, and a basally directed axon (Menco and Morrison, 2003; Nomura et al., 2004). Axons of the olfactory cells run close to each other forming small intraepithelial fascicles which, on piercing the basal lamina, become entrapped by processes of olfactory ensheathing cells (Williams, 1995; Field et al., 2003; Nomura et al., 2004). On reaching the lamina propria, the fascicles progressively group together to form large axon bundles, which eventually pierce the cribriform plate of the ethmoid bone to reach the olfactory bulb glomeruli where connections are established with central relay cells (Meisami, 1989; Williams, 1995; Herrera et al., 2005; Levine and Marcillo, 2008). Through vertically oriented ducts, the Bowman's glands deliver secretions to the surface of the OE that form the environment for dissolving odorants and allowing their diffusion to the sensory receptor sites (Getchell and Getchell, 1992; Williams, 1995). The supporting cells, which constitute a major population of OE-resident cells, act as glia to the epithelium (Vogalis et al., 2005) and serve other functions such as K+ transport and maintenance of water/salt balance in the neuroepithelium mucus layer (Menco et al., 1998; Rochelle et al., 2000; Vogalis et al., 2005). Previous reports indicate that the basal cells, which are situated deep within the OE, continually divide to replace the olfactory neurons (Graziadei and Monti Graziadei, 1978; Calof et al., 1998). However, it is not known whether the capacity of the basal cells changes with age.

Reports on the basic structure of the adult rabbit olfactory system date as far back as 1940s (Allison and Warwick, 1949; Le Gros Clark, 1951, 1956; Allison, 1953; Mulvaney and Heist, 1970; Yamamoto, 1976; Mori et al., 1985; Onoda and Fujita, 1988; Harkema et al., 2006). However, regarding postnatal development in this species, relatively little is known particularly regarding how the OM structurally adapts to the unusually high olfactory functional demands. Modification in structure of the nasal olfactory tissue during juvenile development would be essential as a preparative measure for a self-dependent adult life in regard to olfactory functional needs (Schluessel et al., 2010; Kavoi et al., 2010). This is particularly critical in the rabbit where the young are abruptly weaned at about day 26 in preparation for the next litter and have to typically make the transition to independent feeding without the direct behavioral assistance of their mother (Hudson and Altbäcker, 1994; Hudson et al., 1996). More importantly, the magnitude and pattern of OM structural refinement vary among species depending on olfaction reliance (Kavoi et al., 2010). Using histological, immunohistochemical, ultrastructural, and morphometric methods, this study analyzed the structural changes of the various OM components in newborn, suckling, weanling, and adult rabbits in an attempt to ascertain whether such changes may provide some implications on the functional status of the postnatally developing rabbit OM.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED

Experimental Animals

For this study, a total of 40 male New Zealand White rabbits were used. These animals were from a breeding stock kept in the Department of Veterinary Anatomy and Physiology Animal Facility under housing conditions of 12-hr light/dark cycle, temperature 23°C ± 2°C and relative humidity 55% ± 15%. The animals were fed on rabbit pellets (Unga Feeds, Nairobi) and provided with tap water ad libitum. The rabbits were assigned to four age groups of 10 animals each comprising newborns (neonates), sucklings, weanlings, and adults (respective ages of 0–1 days, 2 weeks, 4 weeks, and 6–8 months). All protocols for the experimentation of these animals were approved by the Animal Care and Use Committee of the University of Nairobi.

Tissue Harvesting and Fixation

The animals were euthanized with intraperitoneal injection of pentobarbital sodium (200 mg/kg b.wt.) and perfusion fixation of the OM was done intracardially via the left ventricle, with 10% formaldehyde for histology and immunohistochemistry (N = 5 animals per group), and with 2.5% phosphate buffered glutaraldehyde (pH 7.4) for electron microscopy (N = 5 animals per group). Harvesting and sampling of the OM were performed as published earlier (Kavoi et al., 2010). In brief, ethmoid conchae were harvested following sagittal sectioning of the animal skulls and removal of the nasal septum. The conchae were then transected perpendicular to their long axes into posterior, middle, and anterior portions. By systematic random sampling, tissues for microscopy were selected from subsegments obtained from each of the above levels.

Processing for Light and Electron Microscopy

Decalcification of the olfactory tissues was done using 5% ethylenediaminetetraacetic acid (EDTA) (Alers et al., 1999) after which the tissues were dehydrated in increasing concentrations of ethanol (70, 80, 95 and twice in 100%) and infiltrated and embedded in paraffin. The embedded tissues were sectioned on the transverse plane at 5 μm using a rotary microtome (Leitz Wetzlar, Germany) and stained using H&E and Masson's trichrome.

Tissues for transmission electron microscopy (TEM) were postfixed in 1% osmium tetroxide in 0.1 M sodium cacodylate buffer and contrasted in 0.5% uranyl acetate in 0.05 M maleate buffer. Dehydration of the tissues was done in graded series of ethanol (70%, 80%, 95%, and twice in 100%) after which ethanol in the tissues was gradually replaced with propylene oxide before infiltrating and embedding the tissues in epoxy resin. Semithin and ultrathin sections were cut from the resin blocks using a Sorvall® ultramicrotome. The semithin sections were picked on glass slides and stained with 0.5% toluidine blue for light microscopy. The ultrathin sections were mounted on 200-mesh carbon-coated copper grids, stained with lead citrate, and observed with a Hitachi H 7100 TEM. For scanning electron microscopy (SEM), some sections selected after dehydration were critical-point dried, mounted on aluminum stubs, sputter coated with gold–palladium complex, and viewed on a Leo 1530 or Jeol 330 SEM.

Immunohistochemical Detection of Ki-67 in the OM

Sections of 3–4 μm thickness were cut from representative paraffin blocks of the OM. Following deparaffinization and rehydration, tissue sections were boiled for 15 min at 600 W in 10 mM citrate buffer (pH 6.0) in a microwave oven followed by cooling at room temperature. After washing the tissue sections in Tris-buffered saline (TBS) (50 mM Tris base, 150 mM NaCl, and 0.002% Triton X-100, pH 7.6), the tissues were quenched for endogenous peroxidases by incubating the slides in 3% H2O2 in methanol for 30 min (Rodriguez-Burford et al., 2002; Brenner et al., 2003). The sections were then incubated for 60 min at room temperature with primary antibody mouse monoclonal anti-human Ki-67 antigen clone MIB-1 (Immunotech S.A., Marseille, France) (1:25 and 1:50, dilutions in TBS). Subsequently, the sections were washed in 0.17 M NaCl and incubated with biotinylated anti-mouse IgG (1:500, in antibody diluent, DAKO) for 45 min at room temperature after which they were washed in the NaCl and further incubated with avidin–biotin complex solution (Vectastain) for 45 min at room temperature. After a second wash in the NaCl, immunostaining was visualized by the use of diamino-benzidine (DAB) (mix 7.5 μL H2O2 to 1 mL of 1% DAB) stained for 10 min followed by counterstaining with Meyer's hematoxylin. Assessment was performed on the entire epithelium following distinct nuclear staining of the dividing cells.

Morphometric Analysis

Quantitative data were analyzed at light microscopy and SEM levels. For each age group, tissue samples were obtained and randomly selected from 10 animals (five for light microscopy and five for SEM). From each tissue block, 10–15 histological and 8–10 SEM micrographs were prepared. For each of these methods, quantitative parameters were analyzed on 30–35 test fields generated from randomly selected micrographs. At light microscopy, thicknesses of the olfactory epithelia, diameters of axon bundles, volume fraction for the bundles, Bowman's glands and blood vessels, and Ki-67 labeling index of OE cells were determined. At SEM, packing densities of olfactory cells and cilia numbers per olfactory cell knob were also estimated.

Epithelial thickness was measured from the basal lamina to the apical surface on images projected to a computer monitor from a light microscope using a digital ruler. To determine the cross-sectional diameters of the axon bundles, mean linear intercept lengths were used (Karlsson and Gokhale, 1997). For this purpose, a set of test lines were overlaid with a uniform random position on a micrograph. The set of lines intercepting the bundle profiles generated linear intercepts within the profiles whose mean total length ( equation imageTL) and number ( equation imageTN) were used to calculate the mean diameter of the bundle profiles ( equation imageDi) as follows:

  • equation image

According to Weibel (1979), the volume fraction (also called volume density) of a tissue component can be estimated by point counting using an overlay of coherent test system of points. For this study, estimation of volume fraction for the axon bundles, Bowman's glands, and blood vessels was performed as earlier described for avian pectineal components (Kiama et al., 2001). Thus, a transparent test grid bearing a square lattice of points was overlaid with random position on histological fields of the lamina propria projected on a computer screen and the total number of test points hitting the components of interest (bundles, glands, or vessels) and those falling on the projected field of the propria were counted. The volume fraction for the component of interest Vv(c) was then worked out as the ratio of the total number of points falling on the component of interest (ΣNPc) to the total sum of points falling on the entire field of the propria (ΣNPp) and expressed as a percentage as follows:

  • equation image

To quantify the Ki-67-labeled cells in the OE, the direct manual counting technique previously applied in measuring the mitotic rates in the endometrium of primates was used (Brenner et al., 2003). To this end, Ki-67-positive cells were counted on randomly chosen areas of a section (of the basal region of the OE) on a light microscope at a magnification of 400× with a mechanical tabulator and with nonoverlapping fields being selected with the help of an ocular grid. In each area of the section, negative (unstained) cells were also counted. Four trained laboratory technicians counted the same sections and the counts made by each observer was summed and averaged. Where KiT denotes the total number of cells counted (positive and negative for Ki-67) and Ki+ stands for the number of Ki-67-positive cells counted on the same area, the Ki-67 labeling index, also called proliferative index (PI), was calculated and expressed as a percentage using the formula

  • equation image

At SEM level, estimation of packing densities of the olfactory cells was done by counting, on the epithelial surface, the number of knobs projected by the olfactory cells per millimeter square area (Apfelbach et al., 1991), and with the application of the forbidden line rule (Gundersen, 1977). Using the method of Menco (1978, 1980) who proposed that a quarter (25%) of the cilia remain obscured behind visible structures during observation, the total number of cilia per olfactory cell knob was worked out from the number of cilia observed and counted on each knob. Thus, where NCk is the total number of cilia per olfactory cell knob and NCc is the number of cilia counted on each knob, then NCc = 3/4 NCk and therefore

  • equation image

Student t distribution was used to analyze for quantitative changes in structure of the OM. In all cases, statistical significance was set at P < 0.05 and mean values were presented together with their standard deviations (SD).

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED

Morphology

In all the age groups, the structure of the OM was basically similar with all the components including Bowman's glands, axon bundles, and blood vessels being present (Fig. 1). These components were fully formed except in the neonates where developing Bowman's glands and blood vessels were observed in the region of the lamina propria subjacent to the mucosal epithelium (Fig. 1A). The height of the epithelium was also observed to increase progressively with age (Fig. 1A–D).

thumbnail image

Figure 1. Histological sections of OM of newborn (A), suckling (B), weanling (C), and adult (D) rabbits. All mucosal components including Bowman's glands (g), axon bundles (b), and blood vessels (v) are present in all the age groups. In the neonate, developing Bowman's glands and blood vessels (arrows) are observed in the region of the lamina propria subjacent to the mucosal epithelium. Notice also the age-related increase in the height of the epithelium. H&E, Bar = 50 μm in A–D.

Download figure to PowerPoint

Age-related variations were noted with regard to the structural forms (types) of the Bowman's glands whereby in the neonates, sucklings, and weanlings, the glands were of the acinar type (Fig. 2A–C), whereas in the adults, the glands were predominantly tubular (Fig. 2D).

thumbnail image

Figure 2. Light micrographs of Bowman's glands of newborn (A), suckling (B), weanling (C), and adult (D) rabbits. Notice the change in the form (type) of the Bowman's glands (*) from being acinar (alveolar) in the neonate, suckling, and weanling to being predominantly tubular in the adult. H&E stain, Bar = 50 μm in A–D.

Download figure to PowerPoint

In all age groups, axon bundles were surrounded by a fibroblastic sheath while individual fascicles within the bundles were encircled by sheet-like processes of olfactory ensheathing cells, whose nuclei appeared superimposed on cross-sections of the bundles (Fig. 3). In the neonate, suckling, and weanling animals, cores of the axon bundles contained blood capillaries, which were lacking from the bundle cores of the adult animals (Fig. 3).

thumbnail image

Figure 3. Cross-sections of axon bundles taken from suckling (A) and adult (B) rabbits to demonstrate features of the bundles profiles. In these age groups and also in the neonates and sucklings, the bundles are surrounded by a fibroblastic sheath (arrows) and their cores are infiltrated with blood capillaries (asterisks) except in the adults where such vessels are absent. Note also the ensheathment of individual bundle fascicles by processes of olfactory ensheathing cells, whose nuclei (arrow heads) appear superimposed on sections of the bundles. Masson's trichrome stain, Bar = 20 μm in A and B.

Download figure to PowerPoint

On sections across axon bundles, relative sizes of the constituent fascicles were observed to increase with age (Fig. 4). These fascicles were separated by distinct gaps, which became smaller and smaller as the animals progressed from neonates to sucklings, weanlings, and adults (Fig. 4).

thumbnail image

Figure 4. Semithin sections across axon bundles demonstrating the structural features of the bundle fascicles in newborn (A), suckling (B), weanling (C), and adult (D) rabbits. With progressing age, relative sizes of the fascicles (*) increase while the gaps between the fascicles (arrow heads) become narrower and narrower. Toluidine blue stain, Bar = 20 μm in A–D.

Download figure to PowerPoint

Ultrastructurally, cross-sectional profiles of the bundle fascicles in all the age groups demonstrated the presence of the unmyelinated axons of the olfactory nerves, which contained numerous neurofibrils (Fig. 5A–D). In the neonates and to a small extend the sucklings, extensive processes of olfactory ensheathing cells were seen to entrap and compact newly forming axons (Fig. 5A, B). In the weanlings and adults, the axons were relatively more closely packed (well compacted) and were without such entrapments (Fig. 5C, D).

thumbnail image

Figure 5. Ultrathin sections across bundle fascicules of newborn (A), suckling (B), weanling (C), and adult (D) rabbits. Notice the unmyelinated axons (*) of olfactory nerves, which are packed with numerous neurofibrils (arrow heads) in all age groups. In the newborn and suckling animals, extensive sheaths of olfactory ensheathing cell processes (arrows) entrap and compact groups of newly forming axons. Bar = 0.1 μm in A–D.

Download figure to PowerPoint

On the surface of the OE of newborn and suckling animals, olfactory cell cilia projected from the tips of the dendric knobs to run parallel in form of a bundle (Fig. 6A, B), whereas in the weanlings and adults, the cilia emerged from around the bases of the knobs in a radial pattern (Fig. 6C, D). In the weanlings and adults, supporting cells projected their apical parts on the surface of the OE in contrast to the situation in the neonates and sucklings where the apices of these cells were not discernible from the epithelial surface (Fig. 6A, B). Also observed on the olfactory epithelial surface were tunnel-like openings, which were more predominant in the adult, weanling, and suckling animals (Fig. 6B–D).

thumbnail image

Figure 6. Scanning electron micrographs showing the surface features of the olfactory epithelia of newborn (A), suckling (B), weanling (C), and adult (D) rabbits. In the newborn and suckling animals, the cilia (arrows) project as a bundle from the tips of the olfactory cell knobs (k) in contrast to the weanlings and adults where the cilia radiate from around the bases of the knobs. Projecting apices of supporting cells (s) are present on the epithelial surfaces of adults and weanlings but are absent in neonates and sucklings. Notice also the predominance of tunnel-like openings (arrow heads) on the epithelial surfaces of adult, weanling, and suckling animals. Bar = 2 μm in A–D.

Download figure to PowerPoint

As defined by Ki-67 immunostaining, proliferative activity of the OE cells appeared to decrease with age. Thus, the Ki-67-positive cells were more prevalent in the neonates followed by sucklings and weanlings and least in the adults (Fig. 7). In all cases, the Ki-67 reactivity was confined to a population of cells localized in the basal region of the OE (Fig. 7).

thumbnail image

Figure 7. Ki-67 immunostaining of the olfactory epithelia of newborn (A), suckling (B), weanling (C), and adult (D) rabbits. The Ki-67 reactivity is restricted to the basal part of the epithelium just above the basement membrane (Bl) and the proliferative activity, as evidenced by the prevalence of the Ki-67-positive cells (arrows), appears to decrease with advancing age. Bar = 30 μm in A–D.

Download figure to PowerPoint

Morphometry

Morphometric values related to the thickness of the OE, cross-sectional diameters of axon bundles, packing densities of olfactory cells, and cilia counts per olfactory cell knob are shown in Table 1. Relative to the value at birth (55.5 ± 2.7 μm), the height of the OE increased by 1.3, 1.5, and 1.7-fold in the sucklings, weanlings, and adults, respectively (Table 1). The mean diameter of the axon bundles, which was 27.4 ± 4.3 μm at birth, increased 2.0, 2.3, and 5.5 times at suckling, weaning, and adult ages, respectively (Table 1). The packing density (mm−2 × 103) of the olfactory cells was 42.3 ± 3.5 at birth, a value which increased by 1.3, 1.8, and 2.1-fold at suckling, weanling, and adult ages, respectively (Table 1). At birth, the number of cilia per olfactory cell knob was estimated at 9 ± 4 and this value increased 1.4, 1.9, and 2.6 times at suckling, weanling, and adult stages, respectively (Table 1). The Ki-67 labeling index (%) was estimated in the newborn as 31.2 ± 6.7, a value that decreased to 28.8 ± 6.0 in the suckling animals, 18.6 ± 5.5 in the weanlings and 11.3 ± 4.6 in the adults (Table 1).

Table 1. Mean values (±SD given in parentheses) for olfactory epithelial height (μm), axon bundle diameter (μm), olfactory cell densities (mm2 × 103), cilia counts per olfactory cell knob, PI (%) of olfactory epithelial cells in the newborn, suckling, weanling, and adult rabbits, and the growth ratio of each of the parameter relative to the value at birth
ParameterNewbornSucklingWeanlingAdult
  1. Growth ratio = mean of a given age group/mean of newborn (Meisami et al., 1990).

Epithelial height55.5 (2.7)69.8 (5.3)82.4 (4.8)92.9 (2.3)
 Growth ratio 1.31.51.7
Bundle diameter27.4 (4.3)54.6 (8.3)63.4 (6.1)149.5 (9.4)
 Growth ratio 2.02.35.5
Olfactory cell density42.3 (3.5)56.2 (4.3)74.6 (5.1)87.6 (4.8)
 Growth ratio 1.31.82.1
Cilia number9 (4)13 (3)17 (3)23 (4)
 Growth ratio 1.41.92.6
PI31.2 (6.7)28.8 (6.0)18.6 (5.5)11.3 (4.6)

Figure 8 shows the volume fraction (%) for axon bundles, Bowman's glands, and blood vessels. In all age groups, volume fraction values for the glands remained higher compared to those for the bundles and vessels. The volume fraction for the glands, which was 32.1% ± 3.4% in neonates, 31.3% ± 2.9% in sucklings, 40.4% ± 3.2% in weanlings, and 44.3% ± 3.7% in adults, was significantly different only between the suckling and weanling ages (Fig. 7). For the axon bundles, the volume fraction values increased with progressing age, with differences in these values being significantly different between newborn and suckling ages (9.3% ± 2.1% to 14.6% ± 2.5%) and between weaning and adulthood (16.7% ± 2.8% to 24.1% ± 3.3%) (Fig. 7). Volume densities for the blood vessels did not show any significant differences between the various ages (13.0% ± 1.5% in neonates, 11.5% ± 1.4% in sucklings, 13.7% ± 1.6% in weanlings, and 12.5% ± 1.5% in adults) (Fig. 7).

thumbnail image

Figure 8. Mean volume fraction (%) for Bowman's glands, axonal bundles, and blood vessels in newborn, suckling, weanling, and adult rabbits. Volume fraction values for the glands are generally higher than for the other structures and the increase in the volume fraction for the glands is age-related, being significantly different only between suckling and weanling ages. Volume fraction for the bundles increases with increasing age, with significant differences occurring between birth and suckling age and also between weanling and adult ages. No significant differences are noted with regard to volume fraction values for the blood vessels between the various ages, P < 0.05.

Download figure to PowerPoint

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED

The basic structure of the adult rabbit olfactory system has extensively been studied (Allison and Warwick, 1949; Le Gros Clark, 1951, 1956; Allison, 1953; Mulvaney and Heist, 1970; Yamamoto, 1976; Mori et al., 1985; Onoda and Fujita, 1988, Harkema et al., 2006) and investigations on olfactory function are also well documented (Shepherd, 1971; Freeman and Schneider, 1982; Hudson and Distel, 1983; Distel and Hudson, 1984; Chaput and Holley, 1985; Hudson and Distel, 1987; Imamura et al., 1992; Schaal et al., 2003; Hudson et al., 2008). In view of the unique nature of the rabbit OM with respect to functional challenges during postnatal life (Hudson and Distel, 1983, 1984; Hudson et al., 2008), it is surprising that the postnatal morphology of the OM has been so little studied. In a study by Meisami et al. (1990) in which the structure of the nasal olfactory tissue was compared in newborn and weanling rabbits, key mucosal components including axon bundles, Bowman's glands, and blood vessels were ignored. Moreover, histology was the only analytical method used. Hence, in this work, several other analytical techniques including scanning and TEM and Ki-67 immunostaining are used to investigate the morphological changes of the rabbit OM from birth through weaning, suckling, and adult stages.

Despite the relatively mature cytoarchitecture of the rabbit OE at the time of birth, results of this study show a progressive modification in the qualitative and quantitative structure of the OE from birth to adulthood. In a related study in rats (Meisami, 1989), a dramatic refinement involving an increase in the surface area of the olfactory receptor sheet and the number of primary sensory afferent units was observed in the first few postnatal weeks. In a study by Sakashita et al. (1995), epithelial thickness has been cited as a key developmental index of the OE. From the time of birth in the rabbit, the OE, whose height measures 55.5 μm, increases progressively to reach 92.9 μm at adult age. This is in contrast to the situation in the rat (Sakashita et al., 1995) where the height of the OE has been shown to increase from 94 to 98 μm between birth and postnatal day 10 after which it declines to 57 μm at adult age.

Barber and Boyde (1968) recognize the use of SEM as a key breakthrough in the analysis of surface structures of biological specimens. In this study, SEM is used in the estimation of packing densities of olfactory cells and in describing the structure of olfactory nerve endings. In newborn and weanling rabbits, Meisami et al. (1990) determined the densities of olfactory cells based on cell shape and staining characteristics with no regard to the state of their maturation. It is generally accepted that only those olfactory neurons whose apical dendrites terminate in a ciliated knob can be considered functional (Le Gros Clark, 1956; Graziadei, 1971; Monti Graziadei et al., 1980). Accordingly, the data by Meisami et al. (1990) in which the packing densities of the olfactory cells were determined on histological sections may be less reliable in relating aspects of mucosal morphology with olfactory function. In a study on the adult rabbit by Allison and Warwick (1949) using light microscopy, the number of olfactory cells was estimated at 120,000 mm−2 while our counts using SEM give a value of 87,600 mm−2. The discrepancy in these values may have resulted from errors in counting procedures and/or differences in the degree of shrinkage where, at SEM, the shrinkage is minimal as it is compensated by the obligatory metal coating (Menco, 1978).

On the surface of the rabbit OE, the number of cilia per olfactory cell knob increases with progressing age, reaching a mean of 24 cilia per knob in the adult. In earlier studies, mean values for the cilia number per knob were 18 in the dog, seven in the sheep (Kavoi et al., 2010), and 17 in the bovine (Menco, 1978). As the receptors for odor binding are mainly located on the cilia of olfactory neurons (Getchell, 1986; Buck and Axel, 1991; Kinnamon and Getchell, 1991; Menco et al., 1992, 1997; Lowe and Gold, 1993; Liberles and Buck, 2006), it can be argued that the number of cilia per olfactory cell knob is reminiscent of the olfactory functional capability of a particular species.

In the rabbit OE, the prevalence of Ki-67-positive cells decreases progressively with increasing age, with the labeling index being 11.3% in adults as compared to 31.2% in the adults, and Ki-67 reactivity is restricted to the population of cells positioned at the base of the epithelium. Mitotic activity of OE cells during postnatal development has been demonstrated in other species including guinea pigs (Nakamura et al., 1998; Higuchi et al., 2005), mice (Ohta and Ichimura, 2000), and rats (Weiler and Farbman, 1997, 1998) with a similar finding that the rate of cell proliferation decreases postnatally. In the rat OE (Weiler and Farbman, 1998), 5-bromo-2′-deoxyuridine (BrdU)-labeling index for the basal cell population was shown to decreased from a high of 30% at postnatal day 1 to a low of 5% at postnatal day 181. The age-related decrease in the number of labeled basal cells have been attributed to a number of factors including age-related decrease in the concentration of growth-associated factors (Weiler and Farbman, 1998), longer cell cycle time in older animals (Weiler and Farbman, 1997, 1998), and greater rate of expansion of the olfactory area in the younger than in the older animals (Weiler and Farbman, 1997). In the basal region of the OE, multipotent basal cell progenitors give rise to immature neuronal and supporting cells that migrate with the passage of time to take defined positions in the apical region of the epithelium (Graziadei and Graziadei, 1979; Williams, 1995; Schwob, 2002). This apical migration of maturing neuronal cells has actually made it possible to determine the neuronal age by position (Graziadei and Monti-Graziadei, 1979; Farbman and Margolis, 1980). In the current investigation, it is suggested that the early part of postnatal life in the rabbits is mainly devoted to basal cell replication, which slows down in the later stages when an adequate number of mature epithelial cells have been attained. This therefore offers a likely explanation to the finding of a relatively higher Ki-67 reactivity in the basal region of the OE of neonates and sucklings as compared to weanling and adult animals.

At neonatal stage, axon bundles in the rabbit appear as fully formed structures while Bowman's glands and blood vessels continued to develop in the area of the lamina propria subjacent to the mucosal epithelium. Similar variations in the level of development of these structures have been reported in prenatally developing animals where, for example, in the Syrian hamster (Taniguchi and Taniguchi, 2007), the primitive axonal bundles were found at the base of the OE at midgestation, whereas in the mouse (Cuschieri and Bannister, 1975), small out growths representing the future Bowman's glands appeared in the same location at late gestation. Developing vasculature has also been demonstrated in the olfactory neuroepithelium of prenatal humans (Sangari et al., 2000) and mice (Herken et al., 1989).

This study documents an age-related variation in the structural form of the Bowman's glands where the glands are of acinar type in the neonates, sucklings, and weanlings and are mainly tubular in the adults. In a study on the histogenesis of the OM in humans (Sangari et al., 1992), the Bowman's glands were shown to develop as buds of epithelial tissue in the lamina propria during fetal development. In the postnatal rabbits, the age-associated variation in the structural forms of the Bowman's glands may probably be linked to the anatomical transformation that take place as the glands develop from primitive to definitive forms. Between birth and adulthood, diameters of axon bundles increase over five times and the volume fraction for the bundles increase by over 2.5-fold in comparison to the Bowman's glands whose volume fraction increment is merely 1.3-fold. According to studies by Van Drongelen et al. (1978) and Meisami (1989), large-sized axon bundles are required for a functionally effective OM as the size of a bundle is directly related to the ratio of convergence between its axons and those of secondary neurons in the olfactory bulb. Moreover, olfactory receptor proteins located on the axonal processes are believed to act as molecule sensors for odorants as well as cell recognition molecules guiding the axons to their appropriate glomerulus in the olfactory bulb (Menco et al., 1994; Strotmann et al., 2004).

In the neonate, suckling, and weanling animals, blood vessels occur within the cores of the axonal bundles. In a recent study (Kavoi et al., 2010), similar vessels demonstrated in axon bundles of dogs were associated with the great thicknesses of the bundles and hence the great diffusion distances that oxygen and nutrients must cross to supply olfactory ensheathing cells located deep within the bundle cores. In the neonate, suckling, and weanling rabbits, the bundle sizes are not so large to limit oxygen diffusion and thus the presence of vasculature in the bundle cores in these age groups is likely to be a feature of development. Moreover, vascularization of developing olfactory mucosal structures such as the OE (Sangari et al., 1992) has been attributed to the increased metabolic demand of the replicating cells toward the completion of their maturation. During postnatal development in the rabbits, the relative sizes of the bundle fascicles increase while the gaps separating these fascicles narrowed with age. The increase in the fascicle sizes and the decrease in the interfascicular gaps with age may be associated with the age-related increase in olfactory neuronal densities, the increased number of axons arising from the newly forming neurons, and consequently the increased rate of packaging of the neuronal axons within individual fascicles (as evidenced by TEM in neonates and sucklings). Furthermore, the increase in the number of primary olfactory cells and hence the higher convergence upon the central relay neurons enhances the physiological capacities of the olfactory afferent pathway by increasing the opportunity for spatial summation and facilitation thereby resulting in improved olfactory sensitivity with development (Meisami, 1989).

At the time of birth in the rabbit, dendritic endings of the olfactory cells possess fully developed cilia. In mice (Cuschieri and Bannister, 1975) and Syrian hamsters (Taniguchi and Taniguchi, 2007), growth of cilia on dendritic terminals occurs 3–4 days before term. In prenatally developing rats, densities of odor binding proteins, which take the form of freeze-fracture intramembranous particles in the ciliary membrane, increase with development (Menco, 1987), whereas immunoreactivity for antibodies to the olfactory signal-transduction proteins parallel cilium development (Menco et al., 1994). These findings, coupled with the fact that the cilia are the principal sites for the initial events of olfactory transduction (Getchell, 1986; Buck and Axel, 1991; Kinnamon and Getchell, 1991; Lowe and Gold, 1993; Menco et al., 1997; Jenkins et al., 2009), imply that the full development of the cilia in the rabbit neonates is reflective of an early functional maturation of the OM in this species.

Regarding the projection of the cilia from the olfactory cell knobs, the change in pattern from parallel in the newborns and sucklings to radial in the weanling and adults is not clear. However, the radial pattern, which has also been demonstrated in dogs (Kavoi et al., 2010), horses (Kumar et al., 2000), and humans (Lenz, 1977), seems to be associated with higher cilia numbers when compared with the parallel pattern commonly reported in bovids (Menco, 1978; Kavoi et al., 2010). At the early stages of development of the OE, olfactory cells proliferate in the superficial layer of the epithelium and send extensions toward the free surface while on the contrary, proliferation of the supporting cells remain confined in the middle or basal layers of the epithelium (Taniguchi and Taniguchi, 2007). This may therefore explain why, in the neonatal and suckling stages, the supporting cells had not projected their apices on the surface of the OE. In a previous study by Barber and Boyde (1968), the tunnel-like openings seen on SEM micrographs of the rabbit OE were identified as the exit points for the Bowman's gland ducts on the surface of the OE.

Studies conducted in several mammals have provided a more dynamic view of the role of olfaction in the regulation of maternal care (Le'vy et al., 2004). In the sheep, for example, parturient ewes were shown to be more attracted to a model lamb smeared with amniotic fluid than to one without amniotic fluid (Vince et al., 1985), an indication that olfactory cues (provided by amniotic fluid) are necessary to ensure appropriate maternal behavior at parturition. Similar results were reported in rabbits (Melo and Gonzalez-Mariscal, 2003). In the rat, however, olfaction played no crucial role in the initiation of maternal behavior at parturition. In this species, following prepartum destruction of the OM by zinc sulfate application, normal onset of maternal behavior was found in primiparous parturient females (Benuck and Rowe, 1975; Jirik-Babb et al., 1984; Kolunie and Stern, 1995). Functional studies in the rabbit (Distel and Hudson, 1984) revealed that, between birth and postnatal day 5, the median time taken by the pups to attach to nipples decreases from 11.8 to 3.2 sec. This improvement suggests an age-related increase in the ability to react to odors and may reflect a likely contribution of the aforementioned modifications in olfactory mucosal structure to the enhancement of olfactory sensitivity with development.

In conclusion therefore, results of this study show that between birth and adulthood, the rabbit OM undergoes a progressive refinement in its qualitative and quantitative structure and that such modifications may be ascribed to the unusually high olfactory functional demands reported in postnatal rabbits. In contrast to earlier work on the postnatal morphology of the nasal olfactory tissue in the rabbit (Meisami et al., 1990), this study provides quantitative data on the proliferating rates of cells of OE and volume fractions of axon bundles, Bowman's glands, and blood vessels. This study also demonstrates, for the first time, the presence of vasculature within the cores of the axon bundles in early postnatal rabbits. In conformity with findings in other species, values for neuronal densities (Apfelbach et al., 1991; Weiler and Farbman, 1997, 1998), cross-sectional diameters of axon bundles, and cilia numbers per olfactory cell knob (Kavoi et al., 2010) increase postnatally. However, for corresponding postnatal ages, the values for these parameters show great interspecies differences. This observation suggests that the principles of development and morphology are similar across taxa but that the quantitative and temporal variations are maintained. Besides opening grounds for further investigations on a wider range of age groups and species, findings of this study may provide important basis for future work involving chemical, microbial, or physical perturbations of the OM.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED

The authors are grateful for the excellent technical assistance accorded by the following: (1) Aikaterini Anagnostopoulou of the Institute of Anatomy, University of Bern, (2) Shem Ochieng' of the International Centre for Insect Physiology and Ecology, Nairobi, (3) Peter Kiguru, George Kariuki, John Kiai, Irene Osoro, and Amos Mwasela of the Department of Veterinary Anatomy and Physiology, University of Nairobi, and (4) Jackson Gachoka of the Department of Veterinary Pathology, Microbiology, and Parasitology, University of Nairobi.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. LITERATURE CITED