Lingual morphology of domesticated Asian small-clawed otters in Yogyakarta, Indonesia

Abstract

This study aimed to observe the lingual morphology of the domesticated Asian small-clawed otter, Aonyx cinereus (A. cinereus), from Yogyakarta, Indonesia. Six domesticated A. cinereus adults were obtained from a local otter breeder in Yogyakarta, with no regard to sex. The animals were acclimated to the laboratory for one day, following this, the animals underwent macroscopy identification and scanning electron microscopy (SEM) and light microscopy (LM) analysis. Macroscopically, the tongue of domesticated A. cinereus is divided into three parts: the apex, corpus, and radix. The apex is the shortest part and can move freely. A median groove is bent along the corpus. Typically, the radix contains circumvallate papillae and the epiglottic valleculae. The SEM and LM observations revealed that the lingual morphology of A. cinereus consisted of two types of papillae: mechanical papillae (horny filiform, leaf-like filiform, bifid filiform, trifid filiform, elongated leaf-like filiform, triangular filiform and conical papillae) and gustatory papillae (fungiform and circumvallate papillae). The lingual glands consisted of Weber's glands and von Ebner's glands secreting acid and neutral mucins. Collagen fibres are found in the lamina propria and muscular layer. In conclusion, the papillae of the Asian short-clawed otter have the same structure as those of other Mustelidae family members.

Keywords: Aonyx cinereus; electron and light microscopy; lingual glands; lingual papillae.

Figure 1. Gross macroscopy of the tongue of Aonyx cinereus

(A) Macroscopic depiction of the tongue of Aonyx cinereus: (a) apex; (c) corpus; (r) radix; (e) epiglottis; (v) vallecula; (mg) median groove. (B) Macroscopic dorsal view of the apex of the tongue of Aonyx cinereus. (C) Macroscopic dorsal view of the corpus of the tongue of Aonyx cinereus: (ac) anterior corpus; (pc) posterior corpus. (D) Macroscopic dorsal view of the radix of the tongue of Aonyx cinereus (black arrow) circumvallate papilla

Figure 2. The schematic illustration of the papillae distribution on the tongue of Aonyx cinereus

Figure 3. Scanning electron microscopy (SEM) images of papillae on the apex region of the tongue of Aonyx cinereus

(A) Leaf-like filiform papillae that occupy most parts of the apex: (lf) leaf-like filiform papilla. (B) Horny filiform papilla that occupies a small portion on the tip of the tongue: (hf) horny filiform papilla. (C) Fungiform papilla between the filiform papillae: (fu) fungiform papilla

Figure 4. Scanning electron microscopy (SEM) images of papilla on the corpus region of the tongue of Aonyx cinereus

(A) Bifid filiform papilla that occupies anterior corpus alongside trifid filiform papilla; (bf) bifid filiform papilla. (B) Trifid filiform papilla that occupies anterior corpus alongside bifid filiform papilla; (tf) trifid filiform papilla. (C) Cornflower filiform papilla that occupies posterior section on the anterior corpus; (cf) cornflower filiform. (D) Elongated leaf-like filiform papilla occupying posterior corpus; (elf) elongated leaf-like filiform. (E) Triangular filiform papilla occupying the latero-posterior section on posterior corpus; (trf) triangular filiform papilla. (F) Fungiform papilla that scatters between filiform papilla on the corpus of the tongue; (fu) fungiform papilla

Figure 5. Scanning electron microscopy (SEM) images of papillae on the radix region of the tongue of Aonyx cinereus

(A) half section of the radix regio; (cv) circumvallate papilla; (sc) short conical papilla; (fu) fungiform papilla. (B) higher magnification of the circumvallate papilla. (C) short conical papilla occupying the median section of the radix; (sc) short conical papilla. (D) long conical papilla occupying the lateral section of the radix; (lc) long conical papilla

Figure 6. Photomicrograph of the Aonyx cinereus tongue apex region, with haematoxylin and eosin staining

(A) Apex region histologically divided into three layers: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae; and (tmss) textus muscularis striatus syncytialis. The tip of the apex is dominated by horny filiform (hf) papillae. (B) Higher magnification of horny filiform (hf) with thick keratinisation (*) which are elongated and form a concave in the dorsal end. (C) Transition part from the anterior tip showing horny filiform (hf) and leaf-like filiform (lf) with elliptical epithelial layer covered by thick keratinisation forming lateral processes. Fungiform (fu) papilla is located among the filiform papillae. (D) Higher magnification of a dome-shaped fungiform (fu) papilla, characterised with several taste buds (arrow) in the inner epithelial layer and covered by a thin keratin layer

Figure 7. Photomicrograph of the Asian small-clawed otter’s tongue corpus region, with haematoxylin and eosin (H&E) staining

(A) Anterior corpus histologically divided by three layers: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae; and (tmss) textus muscularis striatus syncytialis. The surface of the mucous layers forming papillae named cornflower filiform (cf), bifid filiform (bf), and trifid filiform (tf). (B) Higher magnification of bifid filiform papilla (bf) with elongated lamina propria branched into two. Thick keratin layer (*) forming pointed lateral tips. (C) Higher magnification of trifid filiform papilla (tf) with three branches of elongated lamina propria, covered by a thick keratin layer (*). Cornflower papillae (cf) is covered by a bud-like keratin layer (*). (D) Posterior corpus histologically divided by three layers: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae; and (tmss) textus muscularis striatus syncytialis. There are elongated leaf-like filiform (elf) papillae in the medial and triangular filiform (tf) papillae in the lateral part. (E) Higher magnification of triangular filiform papilla (tf) forming a pointed shape in the upper one-third part, covered by thick keratinisation (*). (F) Higher magnification of elongated leaf-like filiform (elf) papilla that has a lean elongated shape with thick keratinisation (*). (G) Fungiform papillae (fu) located among the filiform papillae. (H) Higher magnification of fungiform papillae (fu), equipped with several taste buds (arrow)

Figure 8. Photomicrograph of the Asian small-clawed otter’s tongue radix region, with haematoxylin and eosin (H&E) staining

(A) Three layers of radix: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae; and (tmss) textus muscularis striatus syncytialis. Short conical papillae (sc) is located in the medial part of radix with von Ebner’s gland (eg) underneath. (B) Higher magnification of short conical papillae (sc). (C) Higher magnification of long conical papillae (lc) that are located in the lateral part of radix. (D) Circumvallate papillae (vp) in the anterior radix, surrounded by a groove and showing von Ebner’s gland (eg) underneath. (E) Higher magnification of circumvallate papillae (vp) that have round shape with the surface lamina propria mucosae layer forming processes. (F) Higher magnification of the ventral part of circumvallate papillae (vp), which are equipped by taste buds (arrow) in the ventrolateral part and lingual gland’s duct (d) in the base of the groove. (G) Fungiform papillae (fu) in the posterior radix with taste buds in the dorsal and lateral surface. (H) Another form of fungiform papillae (fu) and circumvallate papillae (vp) in the posterior radix. Both have numerous taste buds in the epithelial layer (arrow). Underneath the circumvallate papillae, Weber’s gland (wg) is equipped with a duct (d). (I) Higher magnification of the ventral part of circumvallate papillae (vp) showing taste buds (arrow) in the ventrolateral surface

Figure 9. Location of the lingual glands in the radix region of Asian small-clawed otter’s tongue with haematoxylin and eosin (H&E) staining

(A) Von Ebner’s gland found in the anterior part of the radix, located in upper layer of textus muscularis striatus syncytialis (tmss) under the circumvallate papillae: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae. (B) Von Ebner’s gland (eg) in the medial of the middle part of radix. (C) Weber’s gland (wg) in the lateral of the middle part of radix. (D) Weber’s gland in both medial and lateral of posterior part of the radix. (E) Higher magnification of von Ebner’s gland (eg), showing the structure of the serous gland. (F) Higher magnification of Weber’s gland (wg), showing the structure of seromucous gland that is dominated by mucous cells and a small number of serous cells (arrow)

Figure 10. Photomicrograph of the apex and corpus region of Asian small-clawed otter’s tongue with alcian blue (AB) and periodic acid-Schiff (PAS) staining, showing no positive reaction to the lingual gland’s secretion

(A) Apex region, AB stain: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae; and (tmss) textus muscularis striatus syncytialis. (B) Corpus region, AB stain. (C) Apex region, PAS stain: (lem) lamina epithelialis mucosae; (lpm) lamina propria mucosae; and (tmss) textus muscularis striatus syncytialis.(D) Corpus region, PAS stain

Figure 11. Photomicrograph of the radix region of Asian small-clawed otter’s tongue with alcian blue (AB) and periodic acid-Schiff (PAS) staining

(A) Medial part of radix shows von Ebner gland (eg) with a weak positive (+) reaction under a gustatory papillae, AB stain. (B) Von Ebner’s gland (eg) with a moderate positive (++) reaction, PAS stain. (C) Lateral part of radix shows Weber’s glands (wg) with moderate positive (++) reaction which located in the textus muscularis striatus syncytialis, AB stain. (D) Weber’s gland with a strong positive (+++) reaction, PAS stain

Figure 12. Photomicrograph of Asian small-clawed otter’s tongue with Masson’s Trichrome staining

(A) Apex region of the tongue, showing various shapes of epithelial tissue in lamina epithelialis mucosae (lem), dense collagen fibres in lamina propria mucosae (lpm), and looser collagen fibres in textus muscularis striatus syncytialis (tmss). Corpus (B) and radix (C) part shows the same as apex. (D) Higher magnification of mucous layer of the lingual apex including the lamina epithelialis mucosae (lem) and lamina propria mucosae (lpm). (E) Mucous layer of the lingual corpus. (F) Mucous layer of the lingual radix. (G) Higher magnification of textus muscularis striatus syncytialis (tmss) layer in the lingual apex, showing loose collagen fibres (stained in blue) between the longitudinal and transverse muscle fibres (stained in red). (H) Muscular layer of the lingual corpus. (I) Muscular layer of the lingual radix