Scanning Electron Microscopical Study of the Lingual Epithelium of Green Iguana (Iguana iguana)

Department of Morphology, Biochemistry, Physiology and Animal Productions, Section of Morphology, University of Messina, Messina, Italy

Scanning Electron Microscopical Study of the Lingual Epithelium of Green Iguana (Iguana iguana)

F. Abbate*, G.Latella, G. Montalbano, M. C. Guerrera, M. B. LevantiandE. Ciriaco

Address of authors: Department of Morphology, Biochemistry, Physiology and Animal Productions, Section of Morphology, University of Messina – Polo Universitario Annunziata, 98168 Messina, Italy; *Corresponding author: Tel.: +390903503598;

fax: +390903503935; e-mail: abbatef@unime.it

With 2 figures Received July 2007; accepted for publication July 2007

Summary

During the last few years, green iguanas (Iguana iguana) have turned out to be one of the most popular pets. They are omnivorous. In their way of feeding, this crucial function is performed by capturing of the preys and mostly, this is car- ried out by the tongue. The role of the tongue is also fun- damental during the intra-oral transport and during the swallowing of food. This has been reported in several studies about chameleons, agamids and iguanids, nevertheless pub- lished data about the mechanisms of capturing and swal- lowing the prey, and the morphological descriptions about the tongue epithelium, are scarce. Therefore, the aim of this present study was to analyse the morphology of the lingual epithelium in green iguanas by scanning electron microscopy.

Three different areas were demonstrated on the tongue sur- face: the tongue tip, characterized by a smooth epithelium without papillae, a foretongue, completely covered by numerous closely packed cylindriform papillae, and a hind- tongue with conical-like papillae. Some taste buds were rec- ognized on the middle and the posterior parts of the tongue.

Different functional roles could be hypothesized for the three tongue areas: the tongue tip could have a role related to the movements of the prey immediately after the capturing, while the middle papillae and the hindtongue could have an important role concerning the swallowing phase.

Introduction

During the last few years, green iguanas (Iguana iguana)have turned out to be some of the most popular pets. In their natural habitat, especially during the early period of their life, they are omnivorous, so they eat both plants and insects. In captivity they eat plants, vegetables, fruits and meat. In their characteristic way of feeding, the crucial function is repre- sented by the capturing of preys and most of this role is carried out by the tongue. The role of the tongue is fundamental also during the intra-oral transport and for swallowing the food. This has been reported in several studies about the components of the iguana family as chameleons (Bels and Baltus, 1988; Bell, 1990; Wainwright et al., 1991;

Wainwright and Bennett, 1992), agamids (Schwenk and Bell, 1988; Schwenk and Throckmorton, 1989; Kraklau, 1991;

Herrel et al., 1995) and iguanids (Schwenk and Throckmor- ton, 1989; Bels, 1990; Bels and Goosse, 1990; Bels and

Delheusy, 1992). Data present in literature about the mech- anisms of capturing and swallowing the preys also consider the lingual prehension, described in iguania, as the primitive way of prey capture in squamata (Schwenk, 1988; Schwenk and Bell, 1988; Schwenk and Throckmorton, 1989). The lingual epithelium of reptiles has considerable differences as demonstrated by many macroscopical and light microscopical studies (Smith, 1986; Schwenk, 1988; Winokur, 1988; Delh- eusy et al., 1994) and also by electron microscopical studies that characterize the lingual epithelium of reptiles (Iwasaki and Miyata, 1985; Rabinowitz and Tandler, 1986; Iwasaki, 1990). Iguanians typically have broad fleshy tongues that are moved within, or protruded out of, the oral cavity during prey transport and capture respectively (Herrel et al., 2005).

Having a large fleshy tongue is critically important as the adhesive forces that keep the prey attached to the tongue are largely surface-dependent (Emerson and Diehl, 1980; Herrel et al., 2000; Schwenk, 2000).

Nevertheless, the morphological descriptions about the lingual epithelium of the iguanidae are scarce and no data are present about the morphology of the lingual epithelium of the green iguana. The aim of the present study was to analyse the three-dimensional features of the lingual epithelium of the green iguana by scanning electron microscopy to elucidate the relationships between the lingual morphology and the igua- nian-feeding mechanisms.

Materials and Methods

The heads of five iguanas were taken from animals immedi- ately after their natural death, thanks to the collaboration with some veterinarians. For the clear exposure of the oral cavity, the temporo-mandibular joints were disarticulated and cut and the samples were fixed in a phosphate-buffered solution 0.1m of 2.5% glutaraldehyde. After several rinsings in the same phosphate buffer, they were dehydrated in a graded alcohols series, critical-point dried in a Balzers CPD 030, sputter coated with 3 nm gold in a Balzers BAL-TEC SCD 050 and examined under a Cambridge Stereoscan 240 electron microscope (Zeiss ex Cambridge Instruments, Cambridge, UK) operating with an accelerating voltage of 20 kV. For ascertaining the morphological details better, some pieces were washed in 5%

neutral Extran(Merck, Damstadt, Germany) to remove the mucus.

2008 The Authors. Journal compilation2008 Blackwell Verlag www.blackwell-synergy.com

Anat. Histol. Embryol.37, 314–316 (2008) doi: 10.1111/j.1439-0264.2008.00847.x

ISSN 0340–2096

Results

The iguana tongue was somewhat triangular and elongated in the antero-posterior direction, and the apical part of it was bifurcated with the presence of a deep median sulcus (Fig. 1a). Scanning electron microscopy demonstrated the presence of three different areas that could be visualized on the tongue surface. The tongue tip was characterized by a smooth epithelium without papillae and notable for absence of taste buds (Fig.1b,d). Between the tongue tip and the foretongue, an evident and blunt borderline could be observed (Fig. 1c). The adjacent and posteriorly situated part changed and the foretongue was completely covered by numerous closely packed and cylindriform papillae that appeared flattened on their surface (Fig. 1e). In the hindton- gue, the presence of many conical-like papillae was visualized.

These papillae, organized in an inter-digitating pattern, were distinct and prominent, separated each other by very small furrows (Fig. 1f).

In all the above-detailed zones, the inter-papillary areas were very narrow. Some taste buds, which protrude on the surrounding epithelium on epithelial papillae, were recognized in the middle and the posterior part of the tongue. They were characterized by short-branched microvilli arising from the external part of their cells (Fig.2a,b).

Discussion

The tongue is often considered an innovative key in the evolution of a terrestrial lifestyle as it allows animals to transport food items through the oral cavity in air, a medium with low density and viscosity (Herrel et al., 2005). The three- dimensional observations of the present study show that there are differences within the dorsal lingual epithelium among the three different zones in the green iguana.

The presence of different areas appear to reflect the need for multiple functions of the tongue and therefore the different functional roles that could be hypothesized for the tongue areas are: the tongue tip could have a role as regards the movements of the prey immediately after the capturing, while the middle papillae and the hindtongue could have an important role concerning the swallowing phase. Each surface plays successive roles during food ingestion, intrabuccal transport, and swallowing (Delheusy et al., 1994; Iwasaki, 2002). The mucous inter-papillary spaces would serve to ensure the adherence between the tongue and the food, the smooth epithelium could facilitate movements of the prey toward the pharynx, and conical papillae of the hindtongue present a rough surface which could act on the prey during the swallowing phase (Herrel et al., 2005). This study can be correlated with those ones regarding the tongue protrusion

(a) (b)

(c) (d)

(e) (f)

Fig. 1. (a) Scanning electron micrograph of the lower part of the oral cavity showing the tongue (asterisk), the teeth (arrowheads) and the epiglottis (arrow) (10·).

(b) The bifurcated tongue tip with a deep median sulcus (arrow) (25·). (c) Particular of the blunt borderline between the tongue tip and the foretongue (220·).

(d) Smooth epithelium of the tongue tip without papillae (480·).

(e) Closely packed and cylindri- form papillae in the foretongue (140·). (f) Conical-like papillae in the hindtongue separated by very small furrows (120·).

Scanning Electron Microscopical Study of the Lingual Epithelium of Green Iguana 315

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and flicking in iguanian lizards (Herrel et al., 1998; Meyers and Nishikawa, 2000) and moreover, considering the increased presence of iguanas as pets in the houses of the civilized countries, our results could also make a useful contribution for clinical applications.

Acknowledgements

The authors thank Mr V. Sidoti for his technical assistance particularly working on the electron micrographs. This work was supported by a grant of the University of Messina (PRA 2005 – Prof. Abbate).

References

Bell, D. A., 1990: Kinematics of prey capture in the chameleon. Zool.

J. Anat.94,247–260.

Bels, V. L., 1990: Quantitative analysis of prey-capture kinematics in Anolis equestris(Reptilia: Iguanidae). Can. J. Zool.68,2192–2198.

Bels, V. L., and I. Baltus, 1988: The influence of food items on the feeding cycle in Anolis equestris (Reptilia: Iguanidae). Copeia,2, 479–481.

Bels, V. L., and V. Delheusy, 1992: Kinematics of prey capture in iguanid lizards: comparison betweenAnolis carolinensisandOplurus cuvieri(Oplurinae). Belg. J. Zool.122,223–234.

Bels, V. L., and V. Goosse, 1990: Comparative kinematic analysis of prey capture in Anolis carolinensis (Iguania) and Lacerta viridis (Scleroglossa). J. Exp. Zool.255,120–124.

Delheusy, V., G. Toubeau, and V. L. Bels, 1994: Tongue structure and function inOplurus cuvieri(Reptilia: Iguanidae). Anat. Rec. 238, 263–276.

Emerson, S. B., and D. Diehl, 1980: Toe pad morphology and mech- anisms of sticking in frogs. Biol. J. Linn. Soc.13,199–216.

Herrel, A., J. Cleuren, and F. De Vree, 1995: Prey capture in the lizard Agama stellio. J. Morph.224,313–329.

Herrel, A., J. P. Timmermans, and F. De Vree, 1998: Tongue flicking in agamid lizards: morphology, kinematics and muscle activity patterns. Anat. Rec.252,102–116.

Herrel, A., J. J. Meyers, P. Aerts, and K. C. Nishikawa, 2000: The mechanics of prey prehension in chameleons. J. Exp. Biol. 203, 3255–3263.

Herrel, A., N. Canbek, U. Ozelmas, M. Uyanoglu, and M. Karakaia, 2005: Comparative functional analysis of the hyolingual anatomy in

lacertid lizards. Anat. Rec. A. Discov. Mol. Cell. Evol. Biol.284, 561–573.

Iwasaki, S., 1990: Fine structure of the lingual epithelium of the lizard Gekko japonicus(Lacertilia, Gekkonidae). Am. J. Anat.187,12–20.

Iwasaki, S., 2002: Evolution of the structure and function of the ver- tebrate tongue. J. Anat.201,1–13.

Iwasaki, S., and K. Miyata, 1985: Scanning electron microscopy of the lingual dorsal surface of the japanese lizard, Takydromus tachy- dromoides. Okajimas Folia Anat. Jpn.62,15–26.

Kraklau, D. M., 1991: Kinematics of prey capture and chewing in the lizardAgama agama(Squamata: Agamidae). J. Morph.210,195–

212.

Meyers, J. J., and K. C. Nishikawa, 2000: Comparative study of tongue protrusion in three iguanian lizards:Sceloporus undulatus, Pseudotrapelus sinaitusandChamaeleo jacksonii. J. Exp. Biol.203, 2833–2849.

Rabinowitz, T., and B. Tandler, 1986: Papillary morphology of the tongue of the American chameleon:Anolis carolinensis. Anat. Rec.

216,483–489.

Schwenk, K., 1988: Comparative morphology of the lepidosaur tongue and its relevance to squamate phylogeny. In: Phylogenetic Rela- tionships of the Lizard Families: Essays Commemorating Charles L. Camp (R. Estes and G. Pregill ed.). Stanford: Stanford Univer- sity Press, pp. 569–598.

Schwenk, K., 2000: Feeding in lepidosaurs. In: Feeding: Form, Function and Evolution in Tetrapod Vertebrates (K. Schwenk, ed.).

San Diego, CA: Academic Press, pp. 175–291.

Schwenk, K., and D. A. Bell, 1988: A cryptic intermediate in the evolution of chameleon tongue projection. Experientia44,697–700.

Schwenk, K., and G. Throckmorton, 1989: Functional and evolu- tionary morphology of lingual feeding in squamate reptiles: phy- logenetics and kinematics. J. Zool., Lond.219,153–175.

Smith, K. K., 1986: Morphology and function of the tongue and hyoid apparatus inVaranus(Varanidae, Lacertilia). J. Zool., Lond.187, 261–287.

Wainwright, P. C., and A. F. Bennett, 1992: The mechanism of tongue projection. I. Electromyographic tests of functional hypotheses. J.

Exp. Biol.168,1–21.

Wainwright, P. C., D. M. Kraklau, and A. F. Bennett, 1991: Kine- matics of tongue projection inChamaeleo oustaleti. J. Exp. Biol.

159,109–133.

Winokur, R. M., 1988: The buccopharyngeal mucosa of the turtles (testudines). J. Morphol.196,33–52.

(a) (b)

Fig. 2. (a) Scanning electron micrograph of the posterior part of the tongue with the presence of taste buds (arrows) (800·). (b) The posterior part at higher magnifica- tion showing a taste bud (2000·).

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