Light and Scanning Electron Microscopic Study on the Structure of the Lingual Papillae of the Feathertail Glider (Acrobates pygmeus, Burramyidae, Marsupialia)
Article first published online: 10 OCT 2007
Copyright © 2007 Wiley-Liss, Inc.
The Anatomical Record
Volume 290, Issue 11, pages 1355–1365, November 2007
How to Cite
Jackowiak, H. and Godynicki, S. (2007), Light and Scanning Electron Microscopic Study on the Structure of the Lingual Papillae of the Feathertail Glider (Acrobates pygmeus, Burramyidae, Marsupialia). Anat Rec, 290: 1355–1365. doi: 10.1002/ar.20606
- Issue published online: 10 OCT 2007
- Article first published online: 10 OCT 2007
- Manuscript Accepted: 10 AUG 2007
- Manuscript Received: 17 OCT 2005
- lingual papillae;
- feathertail glider;
The structure of the tongue of the marsupial feathertail glider (Acrobates pygmeus) was observed under a light and scanning electron microscope. The elongated tongue with a sharpened apex is ca. 10 mm in length. Only the posterior half of the tongue is attached to the bottom of the oral cavity by the frenulum, which facilitates considerable mobility of the anterior free part of the tongue. On the dorsal surface of the tongue, three types of lingual papillae were distinguished, that is, mechanical filiform papillae and gustatory fungiform and vallate papillae. The arrangement, shape, and size of filiform papillae and the direction of their keratinized processes change depending on the part of the tongue, so that the surface of the apex and the body of the tongue resembles a brush adapted to effective holding of semiliquid food and collection of pollen. The fungiform papillae have a single taste bud and are uniformly scattered between filiform papillae only on the anterior half of the tongue. On the smooth root of the tongue, three oval vallate papillae are arranged in the form of a triangle, similarly as it is the case in other marsupials. The posterior biggest vallate papilla is oriented perpendicularly to the smaller anterior papillae. The results of the study on the feathertail glider show that the special arrangement of lingual papillae is strongly adapted to feeding behavior of this nectar-eating and frugivorous animal. Anat Rec, 290:1355–1365, 2007. © 2007 Wiley-Liss, Inc.
The arrangement and structure of gustatory lingual papillae, responsible for the reception of gustatory sensations, and mechanical lingual papillae, aiding the transfer of food, documented in vertebrates, constitute general traits typical of individual taxonomic units, for example, orders or families (Chamorro et al., 1986; Iwasaki et al., 1987; Iwasaki and Miyata, 1989; Azalli et al., 1991; Kobayashi and Wanichanon, 1992; Kobayashi et al., 1995; Kumar et al., 1998; Emura et al., 2000, 2002; Eerdunchaolu et al., 2001; Yoshimura et al., 2002; Jackowiak and Godynicki, 2004). On the other hand, an important factor affecting the structure of the lingual mucosa is the type of ingestion of food, the method of its grinding in the oral cavity, as well as the method of its passage to further segments of the alimentary tract. Thus, in comparative studies on morphological traits of the tongue conducted so far on mammals, the degree of adaptation of the animals to a given alimentary group, that is, carnivores, herbivores, or omnivores, is also investigated (Doran, 1975; Thome, 1999).
The object of the investigations in this study is the feathertail glider, an arboreal marsupial living in eastern Australia, belonging to Burramyids, which is described as the world's smallest gliding mammal (Flannery, 1994; Starck, 1995). Among marsupials, the structure of lingual papillae has been investigated so far in such species as the wombat, wallaby, kangaroo, koala, and opossum (Kubota et al., 1963; Beg and Qayyum, 1976; Krause and Cutts, 1982; Abe et al., 2001; Kobayashi et al., 2003). Due to the geographic isolation of Australia, marsupials exhibit a wide morphological diversification, depending on the ecological niche they occupy and their anatomical structure, including also the morphology of the alimentary tract, with features found in the representatives of orders in Eutheria. Results of previous microscopic observations showed in marsupials that some characteristics of the structure of the tongue and the mucosa covering it are closely correlated with their diet and similar to those found in large ruminants or carnivores (Kubota et al., 1963; Kobayashi et al., 2003). Most species belonging to Burramyids are omnivorous, but in the feathertail glider, living in dry sclerophyl forests and woodlands, pollen and nectar feeding is characteristic (Turner and Mckay, 1989).
In the present study, the aim was to describe the morphology of the tongue in the feathertail glider and examine the distribution and microstructure of lingual papillae on the dorsal surface of the tongue of the feathertail glider (Acrobates pygmeus) in view of the adaptation to its diet.
MATERIALS AND METHODS
The study was conducted on 7 tongues of adult feathertail gliders (Acrobates pygmeus), donated by the Zoological Garden in Poznan (Poland). Three dissected tongues were cleaned in saline and fixed by immersion in Bouin solution for observations under a light microscope. Fixed samples of the tongue were dehydrated in a graded series of ethanol (70–99.8%) and embedded in Paraplast. The 4-μm serial sections were stained with hematoxylin–eosin (HE) and Masson–Goldner trichrome, mounted with DPX, and examined under an Axioscope 2 plus microscope (ZEISS, Germany). The morphometric data were obtained using a KS 400 computer morphometry system (ZEISS, Germany).
Samples from four tongues for scanning electron microscopic (SEM) study were fixed in 10% neutral formalin. After dehydration in an ethanol series, tissues were processed with acetone, and subsequently dried at critical point using CO2 (Critical Point Dryer K850, EMITECH, England). The specimens were mounted on aluminium stubs covered with carbon tabs, sputtered with gold (Sputter Coater S 150B, EDWARDS, England), and observed under a ZEISS 435 VP (Germany) scanning electron microscope at the accelerating voltage of 15–20 kV.
The tongue of the feathertail glider has a markedly elongated lingual body and a short pointed apex (Figs. 1a, 2a). The tongue of the feathertail glider is approximately 9–11 mm long, while the width of the tongue is constant, amounting to ca. 2.3–2.4 mm. The posterior part of the root of the tongue, with the length of 2 mm, located under the pharyngeal–palatal arch, is narrowed to 0.7 mm (Fig. 4a). Macroscopically, the whole dorsal surface of the lingual mucosa, apart from the root of the tongue, is covered with papillae, giving it a rough appearance (Figs. 1a, 2a, 3a).
The ventral surface of the mucosa is smooth without papillae and covered by a multilayered nonkeratinized epithelium, 40–52 μm in height. No lingual lyssa is observed. The tongue is connected with the bottom of the oral cavity in the posterior part of the body of the tongue by a thin, folded, ca. 1.5–2 mm frenulum of the tongue.
Apex of the Tongue
On the apex of the tongue, mechanical filiform papillae and gustatory fungiform papillae are found. Filiform papillae on the sharpened lingual apex in the feathertail glider are elongated, thorn-like structures (Fig. 1a–c). They are distributed only in the area within approximately 1.2 mm from the tip of the tongue. The total height of filiform papillae is ca. 240 μm and the mean density ca. 158/mm2.
Each papilla has a short base and is split into three to four processes of identical, ca. 80–135 μm length (Fig. 1c,d). Processes protruding over the surface of the tongue are composed only of keratinized cells, forming scales with a flat surface. There are no microridges on the surface of horny cells.
Fungiform papillae on the apex of the tongue are evenly distributed between filiform papillae and have a markedly elongated basal part, so that the protruding apex of the papillae is distinctly visible between long processes of filiform papillae (Fig. 1b–d). The fungiform papillae are as much as 107–120 μm high, while their diameter ranges from 65 to 85 μm. The mean density of fungiform papillae is 20/mm2. The elongated connective tissue core of the fungiform papilla is covered by a thin layer of keratinized epithelium, ca. 23–27 μm in height (Fig. 1d). The keratin layer is 4–5 μm thick. On the dorsal surface of each fungiform papilla only a single taste bud is observed (Fig. 1d). Taste buds are situated very deep in the epithelium, so that small taste pores are very hard to localize between superficial keratinized epithelial cells observed on SEM.
Body of the Tongue
On the body of the tongue of the feathertail glider, two zones may be distinguished, differing in the arrangement and morphology of filiform papillae. The mean density of filiform papillae on the body of the tongue is 237/mm2. Figures 1a and 2a present filiform and fungiform papillae covering the dorsal surface of the anterior part of the body of the tongue, reaching approximately up to three-quarters the length of the tongue (Figs. 1a, 2b). Filiform papillae of this region of the tongue are higher than papillae located on the apex, as the total height of papillae is ca. 370 μm. From the base of papilla rise three to four flattened processes with pointed tips, which are tilted toward the back of the tongue (Fig. 2b,c,e). A single posterior process is dominant in size, while two or sometimes three lower anterior processes overlap it. The structure of the multilayered epithelium differs in the anterior and posterior processes. (Fig. 2d). Anterior processes are in fact thin laminae formed of a homogenous keratin layer.
The structure of the epithelium on the posterior process of filiform papillae differs on both sides. On the anterior surface of this process, the epithelium has a clearly visible cornified layer with cells containing keratin granules. On the posterior surface of processes, the keratin layer of the epithelium is a homogenous lamina (Fig. 2d). The surfaces of superficial cells on the anterior and posterior side of the processes are flat or occasionally small hollows are present (Fig. 2f). The dimensions of filiform papillae in the medial part of the tongue are as follows: the length of the posterior process ranges between 78 and 114 μm, while the length of anterior processes is 57–71 μm, respectively. The width of the base of filiform papillae is 30–47 μm. At the margins of the body of the tongue, anterior processes of filiform papillae are generally slightly wider and lower than in the papillae located in the medial part of the body of the tongue (Fig. 2c).
Both on the apex and the body of the tongue the mucosal epithelium covering the interpapillary areas is parakeratinized and its thickness ranges from 100 to 120 μm. On the surface of cells in this epithelium, which retain cellular nuclei, there is a labyrinth-like pattern of microridges.
Fungiform papillae on the body of the tongue are ca. 120–140 μm in height and their diameter is ca. 57–71 μm. The density of fungiform papillae is 9–11/mm2. The papillae are uniformly distributed between filiform papillae (Fig. 2b). On the dorsal surface of fungiform papillae also only one taste bud is found.
In the posterior part of the body of the tongue, in the area of approximately 2 mm in front of the root of the tongue, the arrangement and structure of filiform papillae change radically. Figure 3a presents two groups of filiform papillae, on the left and right side of the tongue, the bases of which are situated lateral and the apexes of the papillae tilted perpendicular to the median line of the tongue. Each filiform papilla is divided already at the short base into two to four narrow, keratinized processes, reaching as much as 166–291 μm in length (Fig. 3b). The mean density of these papillae is 130/mm2. The SEM of the surface of the keratinized processes shows horny scales coiling around the axis of the process and desquamate as horny tubules (Fig. 3c). No fungiform papillae are observed between these filiform papillae.
On both edges of the posterior part of the tongue, where filiform papillae adjoin the smooth lateral surface of the tongue, papillae resembling the fungiform type due to their shape and convex surface are found (Fig. 3b). On the cross-sections of the epithelium covering those papillae taste buds are not observed, which may indicate that they may be reduced, undivided filiform papillae.
Root of the Tongue
The mucosa of the root of the tongue in the feathertail glider is smooth and devoid of mechanical papillae (Fig. 4a). On the surface of the root of the tongue, three oval papillae are found together with openings of lingual papillae (Fig. 4a,e). Vallate papillae are arranged in the form of a triangle (Fig. 4a). Two smaller anterior vallate papillae are distributed symmetrically on the left and right side of the root of the tongue (Fig. 4a,b). The larger diameter of the body of the papillae is 94–100 μm, while the smaller is ca. 58 μm.
The third, largest vallate papilla is located centrally in the posterior part of the root of the tongue (Fig. 4a). The longer axis of this papilla is located perpendicularly to the anterior vallate papillae. The larger diameter of the body of this papilla is 320–340 μm, while the smaller is 200 μm. All the vallate papillae are surrounded by a continuous groove, and the pad of each papilla is flat or slightly protruding over the surface of the mucosa (Fig. 4a,b,d). The height of vallate papilla ranges from 280 to 340 μm. Taste buds are distributed in the epithelium of the body of papillae on the bottom of the groove (Fig. 4c). Single taste buds are present also near the dorsal surface of the body of posterior vallate papillae (Fig. 4d,e).
Vallate papillae, similarly to the root of the tongue, are covered by a parakeratinized epithelium (Fig. 4c,f). As is case of the epithelium in the interpapillary areas of the previous parts of the tongue, SEM observations reveal the presence of microridges on the surface of superficial cells (Fig. 4g). The thickness of the multilayered epithelium covering the surface of vallate papillae is ca. 60–63 μm and on the root of the tongue it is 70–93 μm.
To date, results of morphological studies of the tongue of marsupials indicate that there is no taxonomic relationship among marsupial species, and the primary factor is the kind of food. Thus, the tongues in individual species are characterized, for example, by a varied shape and relief of the dorsal surface of the mucosa and the muscular coat of the tongue. Hence, the described shapes of the tongue adopt from ovoid up to elongated with a rounded or sharp-ended tip, and on the surface of the tongues in the herbivorous species, such as the koala and gray kangaroo, there are various protrusions are observed on the lingual body, resembling lingual prominence in great ruminants (Kubota et al., 1963; Krause and Cutts, 1982; Kobayashi et al., 2003). The structural variation on the dorsal surface of the tongue in Marsupialia is found also in the distribution of lingual papillae, variation of types of papillae and their microstructure.
Our study on the feathertail glider revealed structural adaptations of the tongue, which are closely related with the feeding behavior of this nectar-eating and frugivorous species. A typical method of food uptake in these animals is licking of honeydew, eucalyptus pollen, fruit exudates and/or scraping fruit pulp, and feeding on arthropods such as lerps, soft-bodied termites, or white ants (Turner, 1984; Turner and Mckay, 1989; Goldingay and Kavanagh, 1995). Under keeping conditions of the Zoological Garden, artificial feed for feathertail gliders are mixed chopped fruits, honey, lactogen, baby cereal, moths, and fresh high nectar flowers, if available.
It needs to be noted that characteristic behavior in these animals is frequent self-grooming, mainly using claws, but during this procedure pollen gathered on the fur is mixed with saliva and the tongue transfers it to the mouth (Turner and Mckay, 1989). We have found that the macro- and microscopical features of the tongue contribute to the effective performance of these activities. In this respect, a long frenulum of the tongue needs to be mentioned, reaching to half of the tongue, thus increasing mobility and manipulation ability of the tongue.
The dorsal surface of the tongue in the glider is characterized by numerous, densely arranged mechanical papillae represented only by filiform papillae. In the feathertail glider, the variability of structure of filiform papillae is connected with the shape and length of keratinized processes of filiform papillae. Light microscopic and SEM micrographs of the epithelium covering processes of the filiform papillae show that it presents the commonly observed in mammals dual pattern of keratinization, where cells produce soft and hard keratin, determining the biomechanical qualities of filiform papillae such as hardness, but also a specific elasticity (Farbman, 1970).
A unique feature, which has not been reported so far in marsupials or any higher groups of mammals and which in our opinion results from the influence of the increased adhesability of semiliquid food, is the arrangement of filiform papillae on the whole surface of the tongue in three directions. The presence of filiform papillae directed perpendicularly to the median line of the tongue may canalize the food into the root of the tongue before the passage to the pharynx. A comparison of the types, distribution, and structure of mechanical papillae in the feathertail glider and other Marsupials shows great differences. In the previously investigated marsupials, filiform papillae usually resemble lingual papillae in carnivores. Both in the koala and in the opossum filiform papillae are composed of a large main posterior process and several slender long anterior processes (Kubota et al., 1963; Krause and Cutts, 1982; Kobayashi et al., 2003). In the abovementioned species, apart from filiform papillae on the tongue also conical papillae were observed, whereas in the koala, they are found in the posterior part of the tongue, as in carnivores, and in the opossum they form a specific patch in the anterior part of the lingual body. The processes in all of these papillae are oriented only posteriorly.
Gustatory papillae in the feathertail glider are represented by fungiform and vallate papillae. The arrangement of uniformly scattered fungiform papillae between filiform papillae in the feathertail glider is also found in other marsupial species (Sonntag, 1924; Kubota et al., 1963; Krause and Cutts, 1982; Kobayashi et al., 2003; Thome, 1999). However, a typical feature of this nocturnal species is their distribution limited only to the anterior half of the tongue, what may be connected with the need for enhanced perception of substances in the ingested food. It is important to note the greater density of fungiform papillae on the apex of the tongue. Additionally, in fungiform papillae, we observed only a single taste bud, which so far has been considered typical of certain rodents (Miler and Preslar, 1975; Grandi et al., 1994; Jackowiak and Godynicki, 2005).
The occurrence of three vallate papillae, differing in size and arranged in the form of a triangle, found in the feathertail glider, is a feature commonly observed in all marsupials (Kubota et al., 1963; Krause and Cutts, 1982; Abe et al., 2001; Kobayashi et al., 2003). However, if we compare the morphology of vallate papillae in marsupials it turns out that their shapes are species-specific. According to Kobayashi et al. (2003), in the koala, round or slit-like vallate papillae are found with a distinctly marked pad of papilla. In the opossum and kangaroo bodies of papillae, similarly as in the feathertail glider, are rounded (Krause and Cutts, 1982). Among mammals the arrangement of three vallate arranged in the form of a triangle is found only in bats (Azalli et al., 1991; Emura et al., 2002). A rare feature among mammals, found in the feathertail glider, is the occurrence of pores of taste buds not only in the bottom of the papillary groove, but also near the dorsal surface of the body of the posterior vallate papilla, which may also be an element facilitating tasting of food.
Despite that in Acrobates and other species of marsupials Sonntag (1925) describes the occurrence of the so-called lateral organ, which is analogous to the foliate papillae in other mammals, in our investigations on the posterolateral surface of the tongue, we did not observe either the presence of any structures that could point to the presence of foliate papillae or any slits, commonly found in marsupials, which are the places releasing the secretion of lingual glands.
In conclusion, observations on the tongue in the feathertail glider showed a unique pattern of the distribution of the lingual papillae, well-adapted to the efficient ingestion of liquid or comminuted food. Analogous studies on other species may show how frequent and typical this pattern is in other nectar-eating and frugivorous animals. The premise for this statement can be the occurrence of such an arrangement of filiform papillae in a frugivorous Egyptian Fruit Bat (Rousettus aegyptiacus), where the arrangement of elongated filiform papillae in the posterior part of the body of the tongue is similar as in the feathertail glider, while between filiform papillae scarce fungiform papillae are reported (Jackowiak et al., 2006).
Authors thank Dr. Paweł Botko and Dr. Ewa Trzęsowska from Zoological Garden in Poznan for supplying the specimens for the research and discussion about the biology of the feathertail glider. We also thank Mrs. Katarzyna Jackowiak for excellent technical assistance.
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