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J. Mamm. Soc. Japan 15( 1) : 1 -13
November 1990
Surface Structure and Keratinization of the Mucosal
Epithelium of the Domestic Cat Tongue
Shin-ichi IWASAKI
Department of Anatomy, School of Dentistry at Niigata,
The Nippon Dental University, Niigata 951, Japan
(Accepted 23 December 1989)
Abstract. The tip of the cat tongue was examined by transmission and scanning elec
tron microscopy. Many filiform papillae were densely distributed over its dorsal sur
face. At low magnification, all of the filiform papillae were seen to be slightly in
clined toward the back. Each filiform papilla had a baso-frontal circular slope and
some conical twigs were arranged in a semicircle from the back of the slope. Histo
logically, the keratinized layer was thickest in the epithelium on the posterior side of
filiform papillae. The surface of this area contained abundant pits and hollows. A
cornified layer was also observed on the anterior side of filiform papillae, but it was
not as thick as on the posterior side of filiform papillae. The cell surface of this
area contained faint microridges. The interpapillary epithelium showed little evidence
of keratinization, but well-developed microridges were visible on its surface. From
these observations, it appears that microridges are lost from the epithelial surface in
conjunction with the advance of keratinization.
Key words : Cat ; Tongue ; Filiform papillae ; Epithelium ; Keratinization.
Ultrastructural studies by Farbman (1966, 1970) revealed that the dorsal
epithelium of the rat tongue consists of the epithelium on the anterior side of the
filiform papillae, the epithelium on the posterior side of the filiform papillae, and
the interpapillary epithelium. Reports by Cane & Spearman (1969) and by Hume
& Potten (1976) suggested a more complex composition for the epithelium of
each filiform papilla in the mouse tongue. Farbman (1966, 1970) also demon-
strated that the fine-structural features of each area of the lingual dorsal epithe-
lium were closely related to the pattern of keratinization. However, little fur-
ther information is available about the lingual epithelium of mammals other than
rodents (Boshell et al., 1980, 1982; Iwasaki & Miyata, 1985; Steflik et al.,
1983).
Boshell et al. (1982) have already reported the histology of the dorsal lingual
epithelium of cats. The main purpose of the present study is to enhance our
knowledge of the fine-structural characteristics of the three regions of the dorsal
lingual epithelium of cats by transmission electron microscopy.
Materials and Methods
Tongues from three male and three female adult cats were used. Animals
were sacrificed by an overdose of pentobarbital anesthetic. Tongues were re-
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S. Iwasaki
moved and prefixed in Karnovsky solution. After being cut into small blocks and
rinsed with 0.1 M cacodylate buffer, samples from the tip of each tongue for
transmission electron microscopy were postfixed in a phosphate-buffered solution
of 1% osmium tetroxide at 4 •Ž for 1.5 hrs. This procedure was followed by
dehydration, embedding in epoxy resin, ultrathin sectioning, and uranyl acetate-
lead citrate double staining. The stained specimens were observed under a
transmission electron microscope (JEOL JEM-1200EX).
Some of the materials embedded in epoxy resin were stained with toluidine
blue, and were examined under the light microscope.
Samples from the tip of each tongue for scanning electron microscopy were
postfixed in a phosphate-buffered solution of 1% osmium tetroxide at 37 •Ž for 2
hrs after washing in 0.1 M cacodylate buffer. Then they were treated with 8 N
hydrochloric acid at 60 °C for 30 min in order to remove the mucus from the
surface of the tissue. This procedure was followed by dehydration, critical-point
drying and gold-ion sputtering. The specimens were observed under a scanning
electron microscope (Hitachi S-800).
Results
As shown by Boshell et al. (1982), there were short filiform papillae with
several conical twigs at the tip of the cat tongue, and there were large filiform
papillae, each with a single, sharp sinuous process, in the midportion of the dor-
sum of the tongue. The present results deal primarily with the short filiform
papillae with several conical twigs (Fig. 1).
Under the light microscope, the filiform papillary epithelium was seen to
consist of an anterior side and a posterior side. The interpapillary epithelium
was restricted to the narrow area between filiform papillae. The posterior side
of the filiform papillae was strongly and densely stained by toluidine blue. This
staining pattern is a characteristic feature of "hard keratinization". By contrast,
in the epithelium on the anterior side of the filiform papillae, the surface layer
was also strongly stained by toluidine blue, but not so densely. This staining
pattern is a characteristic feature of •gsoft keratinization•h. The surface area of
the interpapillary epithelium was not stained so effectively by toluidine blue (Fig.
2).
Under the transmission electron microscope, the cells in the germinal layer
of the interpapillary epithelium (Fig. 3) were seen to be oval in shape. The
nuclei were located in the central area of these cells. In the cytoplasm of the
cells of the germinal layer of the interpapillary epithelium, many free ribosomes
and a few tonofibrils and mitochondria were seen. Cytoplasmic processes were
widely distributed around almost the whole perimeter of each cell (Fig. 4).
In the deep intermediate layer (Fig. 3), the cells became flatter in the hori-
zontal plane. An oval nucleus was located in the central area of each cell.
Abundant free ribosomes and some tonofibrils were observed in the cytoplasm.
The cell membrane formed a lot of fine spines (Fig. 5a).
In the shallow intermediate layer (Fig. 3), few nuclei could be observed.
Mucosal Epithelium of Cat Tongue
3
Fig. 1. Scanning electron micrograph of the surface of the tip of the cat tongue. Arrows, long and
broad, central twigs of the filiform papillae ; arrowheads, small and thin, lateral twigs of the filiform
papillae ; asterisk, a fungiform papilla.
Fig. 2. Light micrograph of a sagittal section of the dorsal lingual epithelium of a cat. LP, lamina
propria ; A, the epithelium on the anterior side of a filiform papilla ; P, the epithelium on the pos-
terior side of a filiform papilla ; I, the interpapillary epithelium. Epoxy-resin embedding, toluidine
blue staining.
Fig. 3. Diagram showing the localization of cells in each cell area of the epithelium. A, the epithe-
lium on the anterior side of a filiform papilla ; P, the epithelium on the posterior side of a filiform
papilla ; I, the interpapillary epithelium ; LP, lamina propria ; GL, germinal layer ; DIL, deep in-
termediate layer ; SIL, shallow intermediate layer ; SL, surface layer ; SPL, spinous layer ; GrL,
granular layer ; UKL, unkeratinized surface layer ; SKL, soft-keratinized layer ; HKL, hard-
keratinized layer.
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S. Iwasaki
Fig. 4. Transmission electron micrograph of the germinal layer of the interpapillary epithelium.
N, nucleus ; Tf, tonofibrils ; R, free ribosomes ; arrowheads, desmosomes.
The cytoplasm of these cells was filled with filamentous structures which con-
sisted of tonofibrils and tonofilaments. The cell membrane also formed a lot of
fine spines (Fig. 5h) -
Approaching the surface layer (Fig. 3), the cells became significantly flat-
tened. The major part of the cytoplasm was filled with tonofibrills and tonofila-
ments. Marginal bands were recognizable, attached to the plasma membrane.
The spines of each cell were very fine (Fig. 12a). Neighboring cells were linked
together by desmosomes from the germinal layer to the surface layer of the
interpapillary epithelium.
Under the scanning electron microscope, we observed densely distributed,
fine microridges, which were coincident with the fine processes of cell mem-
branes seen under the transmission electron microscope, on the free surface of
the interpapillary epithelium. Elevated intercellular borders were easily recogniz-
able (Fig. 12b).
On the anterior side of filiform papillae, the outline of the cells of the ger-
minal layer (Fig. 3) was almost the same as that of the cells in the interpapillary
epithelium. A large nucleus was located in the central area of each cell. Many
free ribosomes, tonofibrils and a few mitochondria were distributed in the cyto-
plasm.
In the deep intermediate layer (Fig. 3), the cells became flattened. A
large, oval nucleus was located in the central area of each cell. Some nuclei
were condensed. In the cytoplasm, tonofibrils and free ribosomes were abun-
dant, and small keratohyalin granules were scattered around. Each cell appeared
to have a rugged surface with spines (Fig. 6a).
Mucosal Epithelium of Cat Tongue
5
Fig. 5. Transmission electron micrograph of the deep intermediate (a) and the shallow intermediate
(b) layers of the interpapillary epithelium of the lingual dorsum. N, nucleus ; Tf, tonofibrils ; R,
free ribosomes ; arrowheads, desmosomes.
Fig. 6. Transmission electron micrograph of the deep intermediate (a) and the shallow intermediate
(b) layers of the epithelium on the anterior side of a filiform papilla. N, nucleus ; M, mitochondria ;
Tf, tonofibrils ; K, keratohyalin granules ; Kc, keratinocyte.
6 S. Iwasaki
In the shallow intermediate layer (Fig. 3), the cells became significantly flat-
tened. The nuclei could hardly be seen. Keratohyalin granules of various sizes
were abundantly scattered in the cytoplasm. Tonofibrils and free ribosomes
were also observed (Fig. 6b). A few keratinocytes were located between these
cells with keratohyalin granules.
At higher magnification, some of the small keratohyalin granules, in particu-
lar in their peripheral regions, were seen to be composed of distinct, small parti-
cles of defined size, which were probably ribosomes. Among the keratohyalin
granules, small round bodies with moderate electron-density were recognizable.
Their diameters ranged between about 0.2 and 0.25 um (Fig. 8). By contrast,
the major fraction of the large keratohyalin granules seemed to be composed of
homogeneous structures. The tonofibrils were abjacent to or attached to large
keratohyalin granules (Fig. 9).
The surface layer (Fig. 3) was composed of keratinocytes. They were sig-
nificantly flattened. A very few condensed nuclei were observed. The cyto-
plasm was filled with densely packed, fibrous structures. These cells also had
many fine protrusions. A marginal band was recognizable in the cell membrane
(Fig. 13a). Adjacent cells were linked by desmosomes from the germinal layer
to the surface layer of the epithelium on the anterior side of filiform papillae.
Under the scanning electron microscope, the surface of the epithelium on
the anterior side of the papillae was seen to be covered with densely distributed,
fine microridges, which were coincident with the fine cellular protrusions on the
free-surface side under the transmission electron microscope. Elevated inter-
cellular borders were clearly recognizable (Fig. 13b).
Fig. 7. Transmission electron micrograph of the deep intermediate (a) and the shallow intermediate
(b) layers of the epithelium on the posterior side of a filiform papilla. N, nucleus ; Tf, tonofibrils ;
arrowhead, desmosome.
Mucosal Epithelium of Cat Tongue 7
Fig. 8. Higher magnification of small, round keratohyalin granules (K). Tf, tonofibrils ; arrows,
round bodies of 0.2 to 0.25 p m in diameter with moderate electron density ; arrowheads, ribo-
somes.
Fig. 9. Higher magnification of a large keratohyalin granule (K). Tf, tonofibrils ; R, ribosomes.
8
S. Iwasaki
Fig. 10. Transmission electron micrograph of the critical area between the shallow intermediate
layer and the keratinized surface layer (Kc) of the epithelium on the posterior side of a filiform
papilla. R, free ribosomes ; Kf, keratin fibers.
Fig. 11. Higher magnification of the keratin fibers (Kf) in the critical area between the shallow in-
termediate layer and the keratinized surface layer of the epithelium on the posterior side of a fili-
form papilla. R, free ribosomes; Kf, keratin fibers; arrow, the longitudinal section of the
filamentous structures ; arrowhead, the cross section of the filamentous structures of about 10 nm
in diameter.
Mucosal Epithelium of Cat Tongue
9
Fig. 12. Transmission electron micrograph of the surface layer of the interpapillary epithelium of
the lingual dorsum (a), and scanning electron micrograph of its surface (b). Tf, tonofibrils; T,
tonofilaments; Mr, microridges; arrowhead, desmosome; arrow, the elevated intercellular border.
Fig. 13. Transmission electron micrograph of the surface layer of the epithelium on the anterior
side of a filiform papilla (a), and scanning electron micrograph of its surface (b). N, nucleus ; Kc,
keratinocytes; Mr, microridges; arrowhead, desmosome; arrow, the elevated intercellular border.
10
S. Iwasaki
In the epithelium on the posterior side of filiform papillae, the outline of the
cells of the germinal layer (Fig. 3) was almost the same as that of cells on the
anterior side of filiform papillae. A large nucleus was located in the central area
of each cell. Free ribosomes, tonofibrils, and mitochondria were distributed in
the cytoplasm.
In the deep intermediate layer (Fig. 3), the cells appeared to be relatively
spherical. A large, oval nucleus was located in the central area of each cell.
Tonofibrils gradually increased in number from the basal side to the surface side.
Free ribosomes were also abundant between tonofibrils. A few mitochondria
were observed (Fig. 7a).
In the shallow intermediate layer (Fig. 3), the outline of cells gradually be-
came flattened along the longitudinal axis of each filiform papilla. The nuclei
were also elongated in the same direction as the cells themselves ; they became
condensed and then disappeared. Large numbers of tonofibrils occupied the
cytoplasm and formed thick, electron-dense bundles. Many free ribosomes were
dispersed in the spaces between these bundles. Cell membranes were smooth
(Fig. 7b).
Approaching the keratinized surface layer, the number of free ribosomes
gradually decreased. By contrast, keratin fibers, which seemed to be derived
from tonofibrils and ribosomal material, were significantly increased in number.
Marginal bands were recognizable, being attached to the plasma membrane (Fig.
10). Each keratin fiber was composed of numerous filamentous structures of
about 10 um in diameter (Fig. 11).
In the keratinized surface layer of the epithelium on the posterior side of
Fig. 14. Transmission electron micrograph of the surface layer of the epithelium on the posterior
side of a filiform papilla (a), and scanning electron micrograph of its surface (b). Kc, keratinocytes ;
arrowhead, desmosome ; small arrows, small pits ; large arrow, the elevated intercellular border.
Mucosal Epithelium of Cat Tongue
11
filiform papillae (Fig. 3), each cell became significantly flattened. Nuclei and free
ribosomes were completely absent. The cytoplasm was occupied by keratin fi-
bers. Marginal bands were recognizable, attached to the plasma membrane.
The cell membranes had very fine pits on the free-surface side (Fig. 14a). Des-
mosomes were recognizable between the neighboring cells from the germinal
layer to the keratinized surface layer.
Under the scanning electron microscope, the surface of the epithelium on
the posterior side of filiform papillae revealed many pits and hollows. The ele-
vated intercellular borders were also distinct in this area (Fig. 14b).
Discussion
As shown first by Boshell et al. (1982), the present study demonstrates
that the three different areas of the lingual epithelium of the cat have a tendency
toward different degrees of keratinization. Furthermore, the same epithelial
areas in other mammals do not always show the same tendencies with respect to
the degree of keratinization. In the dorsal lingual epithelium of the cat, there is
a continuum in the tendency toward keratinization, extending from the absence of
keratinization to •gparakeratinization•h, and from •gparakeratinization•h to "orthokera-
tinization" Thus, an exact categorization in terms of only three types of kera-
tinization is impossible.
The interpapillary epithelium of cats does not contain any keratohyalin gran-
ules, while that of rats (Farbman, 1966, 1970) and mice (flume & Potten, 1976)
does contain such granules. The frequency of distribution of keratohyalin gran-
ules may be coincident with the tendency toward keratinization of the surface
layer in this epithelial area. Scanning electron microscopy of the free cell sur-
face on the interpapillary epithelium of the cat revealed that microridges were
more distinct than in rats (Iwasaki et al., 1987a) and mice (Iwasaki et al.,
1987b), but were similar to those in musk-shrews (Iwasaki et al., 1987a).
However, the present study also showed evidence of a relationship between the
degree of keratinization and the development of microridges. The microridges
on the epithelial surface gradually became indistinct in parallel with the progres-
sion of keratinization. These results suggest that the distribution of microridges
may also be a reflection of the tendency toward keratinization of the epithelium.
There are two types of keratohyalin granule, and both have different pat-
terns of formation. One type originates from the complex of tonofibrils and
keratohyalin substances (Fukuyama & Epstein, 1967). The other type origiantes
predominantly from ribosomes alone (Suzuki et al., 1973). The former is
observed in large areas of the skin of mammals (Brolly, 1959, 1960). The latter
is observed in the skin of newborn rats (Fukuyama & Epstein, 1967; Matoltsy
& Matoltsy, 1970), in the footpads of mice (Suzuki et al., 1973) and in the
epithelium on the anterior side of the filiform papillae and the interpapillary
epithelium of the tongues of rats, mice and musk-shrews (Farbman, 1966, 1970;
Cane & Spearman, 1969; Hume & Potten, 1976; Iwasaki & Miyata, 1985).
Recent biochemical analyses of keratin fibers in normal human skin have re-
12 S. Iwasaki
vealed that these fibers consist of peptides of 50K, 56.5K, 55K and 63-67K
daltons (Bowden & Cunliffe, 1981, Woodcock-Mitchell et al., 1982). However,
no detailed biochemical analysis has been reported of keratohyalin granules,
which appear morphologically to consist of coagulated ribosomes in some areas of
skin or epithelium. Therefore, it seems very important to resolve by biochemi-
cal techniques whether this type of keratohyalin granule contains substances that
originate from ribosomes.
Small round bodies with moderate electron density scattered between kera-
tohyalin granules seem to be different from the membrane-coating granules be-
cause they have neither the limiting membrane nor a lamellar structure.
Although their functional role and biochemical nature remain to be determined, it
is possible that they are cores, or matrices of ribosome-coagulated keratohyaline
granules.
The keratinized layer is thickest in the epithelium on the posterior side of
filiform papillae in all mammals so far examined. However, in every case, kera-
tohyalin granules of ribosomal origin were absent, or present at only very low
levels, in the epithelium of this area. These observations suggest that keratohy-
alin granules of ribosomal orgin are not important for •ghard keratinization•h.
Cells in this area are found to contain a great many tonofibrils which are more
important for •ghard keratinization•h than are keratohyalin granules of ribosomal
origin. The keratohyalin granules of ribosomal origin may rather play a role in
the inhibition of keratinization. That is, the coagulation of free ribosomes may
inhibit the active formation of tonofibrils. If this process does not occur, so
many tonofibrils are made as to ensure that •ghard keratinization•h results.
As shown in filiform papillae of some other animals (Farbman, 1970; Iwasaki
& Miyata, 1985, 1989), the keratinized layer of the epithelium of the cat filiform
papillae is thicker on the posterior side than on the anterior side. Studies by
scanning electron microscopy, by contrast, have shown that the filiform papillae
of various mammals are usually inclined toward the pharynx (Farbman, 1966;
Greenbaum & Phillips, 1974; Iwasaki & Sakata, 1985; Iwasaki et al., 1987a,b ;
Kutuzov & Sicher, 1951, 1953; Steflik et al., 1983). These two facts indicate
that the filiform papillae have the ability to incline toward the pharynx, but not in
the opposite direction. These features are appropriate for the transport of food
through the mouth to the pharynx, and for lapping food. Furthermore, cats fre-
quently use their tongues for brushing their body hair. For this purpose, the
filiform papillar structure serves to promote the tongue's function as a hairbrush.
Acknowledgments
A part of the present study was supported by a Grant-in-Aid from the Kaza-
to Foundation. We wish to thank Mr. K. Ishizuka, Department of Oral Physiolo-
gy, School of Dentistry at Niigata, The Nippon Dental University, for kindly
donating his valuable material.
Mucosal Epithelium of Cat Tongue
13
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