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Microscopy and imaging science: practical approaches to applied research and education (A. Méndez-Vilas, Ed.)
LM and SEM Studies on Tongue and Lingual Papillae in the Donkey
(Equus asinus)
Hanna Jackowiak1, Hassen Jerbi2, Kinga Skieresz-Szewczyk1 and Ewelina Prozorowska1
1
Department of Histology and Embryology, Poznan University of Life Sciences, Wojska Polskiego71C, PL-60-625
Poznań, Poland
2
Department of Anatomy, Veterinary School of Sidi Thabet, CP2020, Tunisia
The macroscopic, LM and SEM observations were carried out on 27 tongues of the donkey. The three age groups were
determined: 0 - 1.5 year, 2 - 5 year, 6 - 10 year. The tongues of the donkeys in all studied age groups are elongated and
consist of the spatula-like apex, narrow body, torus linguae and the root of tongue. It was found no sexual dimorphism of
the morphological features of the donkey's tongue. The length of the tongue in studied age groups increases from 20 cm to
26 cm and the width of the tongue on torus area increases from 4.8 cm to 6.0 cm. The special-specific features, in
comparison to the tongue of horses, is rare presence of the cartilage of dorsum of the tongue (cartilago dorsi linguae). The
mucosa of the tongue forms filiform, fungiform, foliate and vallate papillae. The filiform papillae as only one type of
mechanical papillae, cover the entire surface of the tongue up to the area of the vallate papillae. SEM studies reveal the
presence of the two types of filiform papillae, differing in density and size of processes on the anterior part of tongue and
on torus linguae. The fungiform papillae are scattered between the filiform papillae. Interesting observation in the donkey
tongue is age related reduction of the number of fungiform papillae positioned on lateral surface of donkey's tongue, from
85 to 60. In each age group foliate papillae are located on the lateral surface of the posterior part of torus linguae and
consist of 7-8 mucosal folds. Characteristic feature in donkey is variable number and shape of the vallate papillae. There
are predominantly three vallate papillae located in one row behind the torus linguae. We also observed 2 to 5 vallate
papillae in each age groups. The results are discussed in terms/aspect of feeding function, food manipulation in oral cavity
and grooming behavior.
Keywords: Functional Morphology; Tongue; Lingual papillae; Donkey; LM&SEM
1. Introduction
The donkey is a member of the Equidae family along with horses and mules. Donkeys are domesticated asses, and they
constitute their own species, Equus asinus. The genetic studies on donkey and wild ass from all over the world showed
that wild asses in Northeast Africa are the ancestors of modern donkey [1].
So far the studies on morphology of organs of the oral cavity in Equidae were focused mainly on horses [2-7]. The
donkey as a member of the Equidae family plays a significant economic importance in some areas of the world. Some
morphological data about tongue were given by Abd-Elmaeim et al. [3].
Therefore, we aimed to prepare detailed macroscopic description of donkey’s tongue and also scanning electron
microscopic (SEM) and light microscopy (LM) observations of the distribution and microstructure of mechanical and
gustatory lingual papillae on the surface of the tongue to look on species-specific features and functional adaptations.
Such description could be important not only for veterinary practice but also be used in the discussion of the function of
the lingual papillae according to the diet, mechanism of food intake (grazing and chewing) in herbivorous animals.
2. Material and methods
The macroscopic observations were carried out on 27 tongues (16 female tongues and 11 male tongues) of the donkey
collected from the slaughterhouse. The tongues after dissection were rinsed in saline and immersed in 4% neutralized
formalin. The tongues were documented with a digital camera and divided into three age groups. Group 1: 0-1.5 year,
group 2: 2-5 year and group 3: 6-10 year. Tissue samples from the apex, body and torus linguae were collected. For
light microscopy (LM) observation, samples were dehydrated in a series of ethanol (70% - 96%), and embedded in
Paraplast®. Paraplast blocks were cut 4.5 - 5 µm slides stained with the Masson-Goldner trichrome [8]. The slides were
examined using an Axioscope2plus light microscope (Zeiss, Germany). For scanning electron microscope (SEM)
analysis the samples were dehydrated in a series of ethanol (70 - 100%) and acetone (96% - abs.), and subsequently
dried at critical point using CO2 (CPD300, Leica, Germany). All specimens were mounted on aluminums stubs covered
with carbon tabs, sputtered with gold (Sputter Coater S150B, Edwards) and observed under the LEO 435 VP at an
accelerating voltage of 10-15 kV. Morphometric analysis were conducted by using a Multiscan system (ver. 10.2, CSS,
Warsaw, Poland).
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3. Results
The tongue of the donkey is elongated and consists of the spatula-shape apex, narrow body, torus linguae and the root
of the tongue (Fig. 1). The macroscopic observations reveal no sexual dimorphism of the morphological features of the
tongue in donkey. The morphometric data reveals that the length of the tongue increases in age groups from 20.0 cm to
26.0 cm. The width of the apex increases from 3.8 cm to 4.6 cm and the width of the torus linguae increases from 4.8 m
to 5.6 cm . (Graph 1). Four types of lingual papillae, i.e., fungiform papillae, filiform papillae, vallate papillae and
foliate papillae are observed on the dorsal surface of the tongue (Figs. 4, 6, 7, 10, 13). Among 20 examined tongues
only in 2 specimens cartilage of the dorsum of the tongue (cartilago dorsi linguae) is observed (Fig. 2).
Fig. 1. Dorsal view on the tongue in the donkey. A-apex, B-body, T-torus linguae, R-root of the tongue. Scale bar =1 cm.
Fig. 2. Cross section through the torus linguae. Arrow shows the cartilage of the dorsum of the tongue. Scale bar =1 cm.
Graph 1.
Length of the tongue and width of
the apex and torus linguae in the
donkey in the three age groups.
3.1. Filiform papillae
The filiform papillae are the only one type of the mechanical papillae present on the entire surface of the tongue, from
the lingual apex to the area of the vallate papillae. Two subtypes of filiform papillae differ in size of keratinized
processes and density. On the anterior part of tongue i.e. the apex and the body of the tongue, the filiform papillae are
sparsely arrange and their height is about 130.3 µm (Figs. 3, 4). On the torus of the tongue the densely distributed
filiform papillae create the dense brush and their height is 205.8 µm (Figs. 5, 6).
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Fig. 3. Cross section through the apex of the tongue of the donkey. Arrow shows the short keratinized processes of the filiform
papillae. Ep-epithelium, Lp-lamina propria. LM, scale bar =100 µm.
Fig. 4. Dorsal surface of the apex of the tongue of the donkey with keratinized processes of the filiform papillae (arrows). SEM, scale
bar =100 µm.
Fig. 5. Cross section through the torus linguae in donkey. Arrow shows elongated processes of the filiform papillae.
Ep-epithelium, Lp-lamina propria. LM, scale bar =50µm.
Fig. 6. Dorsal surface of the torus linguae of the donkey with densely distributed processes of the filiform papillae (arrow). SEM,
scale bar =50 µm.
3.2. Fungiform papillae
Fungiform papillae are evenly scattered between the filiform papillae on the entire dorsal surface of the apex and body
of the tongue and are also observed on the lateral surface of the anterior part of the tongue (Fig. 7). These papillae, on
the dorsal surface of the lingual apex, are slightly raised over the surface of mucosa, so that they are not well visible in
macroscopical observations (Figs. 8, 9). The papillae on the lateral surface of the tongue are visible and interestingly the
number of papillae decreases from 85 to 60 in studied age groups. Microscopic observations show that the fungiform
papillae are covered with multilayered keratinized epithelium with long connective tissue papillae. The horny layer of
epithelium is relative thin and is approximately 15.8 µm (Figs. 8). The average diameter of the fungiform papillae is
529.8 µm.
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Fig. 7. Lateral view on the lingual body (B) in the
donkey. Dashed line marks the surface with the
fungiform papillae. Scale bar =300 µm.
Fig.8. Cross section through the fungiform papillae.
Arrow points the keratinized layer of the epithelium.
Ep-epithelium, Lp-lamina propria. LM, scale bar =100
µm.
Fig. 9. Higher magnification of the surface of slightly
protruded fungiform papillae (Fu) on surface of the
body of the donkey's tongue. SEM, scale bar =100 µm.
3.3. Vallate papillae
The vallate papillae are present on the posterior part of the torus linguae. In each age groups the number of vallate
papillae vary between 2 and 5. Predominantly there are 3 vallate papillae located in one row, positioned perpendicular
to the medial line of the tongue (Fig. 10). Characteristic feature of these papillae is variable shape, from rounded
papillae, through the oval shape papillae to heart - shape papillae (Fig. 9). The diameter of the papillae varies between
1.7 mm and 3.5 mm. The LM and SEM observations show that the middle part of the vallate papillae i.e. the surface of
the body of these papillae is concave (Fig. 11, 12). The body of each papilla is surrounded by wide furrow and flat outer
wall. The height of multilayered, partly keratinized epithelium on the body of papillae, is about 502.6 µm. In the thinner
nonkeratinized multilayered epithelium of the lateral wall of the furrow, which height is about 102.4 µm, are present
numerous of the taste buds (Fig. 11). In the lamina propria below the furrows are located serous mucous glands (Fig.
11).
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Fig. 10. Macroscopic pictures of the different shape of the vallate papillae in the donkey's tongue (V). Scale bar =1 mm.
Fig. 11. Cross section through the vallate papillae. Arrowhead points the taste buds. Ep-epithelium, Lp-lamina propria, M-muscles,
Sg-serous glands. LM, scale bar =1mm.
Fig. 12. Higher magnification of the vallate papillae with the wide furrow of the papilla (arrowhead) and concave middle part of the
papillae (arrow). SEM, scale bar =1 mm.
3.4. Foliate papillae
Foliate papillae are located symmetrically on the lateral surface of the posterior part of the torus linguae near pallatoglossal arch (Fig. 13). Each papillae consist of about 7-8 leaves in each age groups of male and female donkey. Leaves
are separated by deep furrows (Fig. 14, 15). Height of the leaves is approximately 1.2 mm and their wide is about 1.2
mm. The foliate papillae are covered with nonkeratinized epithelium, which height is about 691.0 µm (Fig. 13). In the
epithelium of the lateral wall of the leaves are present taste buds (Fig. 14). In the lamina propria below the vallate
papillae numerous of the secretory units of the serous glands are present (Fig. 14).
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Fig. 13. Lateral view on the posterior part of the torus
linguae (T). Dashed line marks the surface with foliate
papillae. Scale bar =4 mm.
Fig. 14. Cross section through the foliate papillae (F).
Arrows show grooves. Arrowhead points the taste buds.
Ep-epithelium, Lp-lamina propria, Sg-serous glands. LM,
scale ba r=0,5 mm.
Fig. 15. Higher magnification of the foliate papillae (F).
Arrow shows groove. SEM, scale bar =0,5 mm.
4. Discussion
The macro - and microscopical studies on the tongue of donkey show the general similarity with a tongue in horses. In
the age groups studied in the donkey we observe the elongation of 6 cm of the tongue, whereas its width extends of 2
cm. This shows that donkey’s tongue is well constituted in postnatal period. The presence of spatula-like apex,
elongated torus and muscular eminence called torus linguae is characteristic for all Equidae, studied so far [2-7]. It is
important to mention that such eminence called intermolar prominence is typical for some Artiodactyls - ruminants,
Perissodactyls - equines, but also is described in rodents [5,6,10,11-16]. The torus linguae or intermolar prominence is
in contact with posterior keratinized part of hard palate and soft palate and also with teeth and play an important
function during transport of food, mastication and formation of food bolus in all herbivorous species.
The macro- and microscopic observations of the lingual papillae in donkey allow us to describe the distribution and
microstructure of the mechanical papillae represented by filiform papillae and also three kinds of gustatory papillae. To
describe a functional relationship between the feeding and the mucosal structure is important to know, that donkey is
characterise with small nutrient requirements, hence its vegetable diet is varied (grass, hay, twigs and bark). Donkey is
adapted to digest high fibre diets and utilize organic acids for energy [9].
The mechanical filiform papillae are visible on the surface of the lingual mucosa as single processes embedded
deeply in the high epithelium. These mechanical papillae in donkey shows the similarity with horse and clear difference
with ruminants, in which filiform papillae possess one main process and two lateral shorter processes [3-7,10,14-16].
According to the length of the well-keratinized processes of filiform papillae there are distinguished, similar to horse,
two subtypes of filiform papillae. The short filiform papillae in the anterior part of the tongue in donkey play an
important function in food intake. This papillae are responsible for the adhesion of the plants parts and water and also
for the hair grooming. On the apex and anterior body of the tongue between these filiform papillae, the low fungiform
papillae are dispersed. Our study shows that the gustatory papillae are partly covered with keratinized epithelium, what
suggests the protection of the epithelium against to the hard particles of the food. The explanation of the age related
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decrease of number of lateral fungiform papillae in postnatal period in donkey is probably an aging effect or
phenomenon connected with destruction of papillae in contact of food. Both hypotheses could be study deeply in future.
Observations of the long densely arranged filiform papillae, which form a dense comb on the surface of the torus,
bring to the conclusion that this area is specialized in adhesion of masticated food, which is prepared to swallowing.
The occurrence of numerous green food particles and hairs between these papillae on the macro - and microscopic
photos is a sign of strong adhesion of masticated food. The long filiform papillae on torus occur in horse and hippo
[5,6,16]. In hippo the papillae help in positioning of wet food. Contrary observations are delivered by studies of the
ruminants torus, that is covered with short, well-keratinized conical papillae, which help grinding the solid food
[10,14,15].
The present observations of the vallate papillae in donkey show diversity of the shape and size of vallate papillae.
Our study reveals that the number of these papillae varied between 2 and 5, with predominantly number of 3 papillae.
This results is contrary to the earlier studies on donkey’s tongue where the authors observe only 2 - 3 vallate papillae, so
that similar with horse [2,3]. As a comparison, the vallate papillae are absent in hippo and are distributed in rows along
of intermolar prominence in ruminants [10,14-16]. Interesting, from functional point of view, is the presence of very
thick multilayered epithelium of the concave surface of vallate papillae which has a special protective function. Such
protective layer of the epithelium both, on the surface of the vallate papillae, as well as previously mentioned fungiform
papillae, seems to be a species-specific feature of the donkey. The structure of third type of gustatory papillae i.e. foliate
papillae of the donkey is typical to the other mammals, however the species-specific feature in donkey is constant
number of foliae of papillae in male and female.
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