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THE ANATOMICAL RECORD 248:176–188 (1997)
Ultrastructure of the Parotid Salivary Glands in Seven Species
of Fruit Bats in the Genus Artibeus
BERNARD TANDLER,1* TOSHIKAZU NAGATO,2 AND CARLETON J. PHILLIPS3
1Department of Oral Anatomy II, Kyushu Dental College, Kitakyushu 803, Japan
22nd Department of Oral Anatomy, Fukuoka Dental College, Fukuoka 814, Japan
3Department of Biological Sciences, Illinois State University, Normal, Illinois
ABSTRACT
Background: In previous studies, we determined that the
submandibular glands of five species of Neotropical fruit bats in the genus
Artibeus had seromucous granules in their demilune cells with substructures
that varied interspecifically in accordance with systematic relationships.
Moreover, the striated ducts in these frugivores exhibited structural modifications that apparently are related to the consumption of a diet rich in potassium, but deficient in sodium. We now turn our attention to the parotid gland
in a large number of species in this genus to determine if it follows the same
structural pattern as does the submandibular gland.
Methods: Members of seven different species of Artibeus were livetrapped in various Neotropical locations. The parotid glands were extirpated from euthanized bats, fixed in the field, and prepared for electron
microscopic examination by conventional means.
Results: The parotid glands in all seven species were virtually identical in
morphology. The acinar cells (determined to be seromucous on the basis of
ultrastructural criteria) contain large numbers of what appear to be vacuoles,
but which are a type of secretory granule. These granules have an electronlucent matrix and may contain one or several circular membranous profiles
arranged either concentrically or in a random array. These granules appear to
form by progressive dilatation of the termini of Golgi saccules, with the
nascent granules finally severing their connection with the Golgi apparatus.
Many of the internal membranous profiles are formed simply by invaginations
of the limiting membrane of the granule; others may result from indentation of
the limiting membrane by protrusions from adjacent granules; the source of
multiple internal membranes in certain granules is unclear. The exocytosis of
these granules results in the acinar and intercalated duct lumina being filled
with an abundance of membranous material. Such extruded membranes are
present in some striated ducts, but not in others, suggesting that they are
degraded during passage through the duct system. The striated ducts are of
conventional appearance, lacking the frondose processes that are prominent
in the submandibular glands of Artibeus.
Conclusions: The parotid gland in Artibeus shows none of the interspecific ultrastructural variability that characterizes the submandibular
gland in bats of this genus. The seromucous acinar cells secrete granules
that release phospholipids as well as glycoconjugates into the saliva.
Based on the lack of frondose processes with their sodium-transporting
portasomes, the striated ducts of the parotid gland are less concerned
with electrolyte homeostasis than are those in the submandibular gland.
Anat. Rec. 248:176–188, 1997. r 1997 Wiley-Liss, Inc.
Key words: parotid gland; salivary glands; seromucous cells; Golgi apparatus; secretion; striated ducts; chiropterans; fruit bats
Salivary glands have had a major biological role in
the evolution and adaptive radiation of mammals. In
part, this is because these organs can produce products
of immediate survival value, can be responsive to both
r 1997 WILEY-LISS, INC.
*Correspondence to: Dr. Bernard Tandler, Department of Oral
Anatomy II, Kyushu Dental College, 2-6-1 Manazuru, Kokurakitaku,
Kitakyushu 803, Japan.
Received 4 December 1996; accepted 23 December 1996.
177
PAROTID GLAND IN SEVEN SPECIES OF FRUIT BATS
external and internal environments, and can exhibit
remarkable plasticity in terms of gene regulation and
secretory processes (Phillips, 1996; Phillips and Tandler, 1996).
In the present article, we describe the ultrastructure
of the parotid salivary glands in seven species of
Neotropical fruit bats in the genus Artibeus. In 1956,
Wimsatt reported that the acinar cells in the Artibeus
jamaicencis parotid gland are unusual at the light
microscopic and histochemical levels (Wimsatt, 1956).
It was later shown that the ultrastructure of these cells
in A. phaeotis is also unusual in that the secretory
granules look like vacuoles rather than typical acinar
cell products (Phillips et al., 1977). Although the acinar
secretory cells in the Artibeus parotid gland superficially resemble mucous cells, they actually are seromucous cells according to the criteria set forth by Young
and van Lennep (1978) and by Tandler and Phillips
(1993). Moreover, our previous comparative analyses of
the bat family Phyllostomatidae have shown that, from
an evolutionary perspective, the unusual acinar cells in
Artibeus most likely evolved from a more typical parotid serous cell (Phillips et al., 1987, 1993). In the
present comparative study of a number of Artibeus
species, we sought to clarify the ultrastructure not only
of the secretory cells, but of the remainder of the gland
as well.
Neotropical fruit bats of the genus Artibeus are
geographically widespread, occurring from western
Mexico through Central America and as far south as
northernmost Argentina. They also have reached the
Antilles and one species, A. jamaicencis, occurs on
nearly every island (Phillips et al., 1989). For the
present investigation, we selected seven species in this
genus because we sought to determine whether or not
there are interspecific differences in their parotid salivary glands, as there are in the submandibular glands
(Tandler et al., 1986). The possibility of interspecific
differences is important because the species selected
represent two significantly different size categories
(small, represented by A. cinereus, and large, represented by A. lituratus) with presumably different diets
and different metabolic strategies.
In addition, there has been some controversy about
the selected species. Owen (1991) has argued that the
species used in the present study represent three
genera (Dermanura cinereus, Koopmania concolor, and
Artibeus). However, recent molecular genetic analysis
supports placement of all of the species in question in a
common genus (Pumo et al., 1996). The case for inclusion of the species in a single genus would be strengthened if their parotid salivary glands were identical or
similar. We previously have shown the systematic value
of salivary gland ultrastructure (e.g., Tandler et al.,
1986) and one would anticipate that A. cinereus and A.
concolor would substantially differ from one another
and from the other five species if they truly represent
different genera.
MATERIALS AND METHODS
Parotid glands were obtained from seven species of
fruit bats in the genus Artibeus. The bats were captured
at night in mist nets as they were feeding or using
fly-ways leading to and from fruit trees. Identification
of some of the South American species in this genus can
be difficult; in the present investigation, we used the
criteria of Lim and Wilson (1993) for species identification. Voucher specimens used in this research were
deposited in the research collection at the Carnegie
Museum of Natural History, Pittsburgh.
The species examined, sample sizes, geographic origins, and mode of initial fixation of parotid gland
specimens are listed below:
A. cinereus (n 5 7); Suriname; triple aldehyde-DMSO
A. jamaicensis (n 5 4); Jamaica and Trinidad; Karnovsky-DMSO
A. lituratus (n 5 3); Suriname; triple aldehyde-DMSO
A. planirostris (n 5 3); French Guiana; KarnovskyDMSO
A. concolor (n 5 1); Suriname; triple aldehyde-DMSO
A. amplus (n 5 1); Suriname; triple aldehyde-DMSO
A. obscurus (n 5 1); French Guiana; Karnovsky-DMSO
After capture, bats were euthanized in the field with
T-61 euthanasia solution and the parotid glands quickly
removed and fixed by immersion in either triple aldehyde-DMSO (Phillips, 1985) or in half-strength Karnovsky’s (1965) fluid in 2.5% DMSO, a mixture that we
formulated for field fixation of salivary glands.
After transport to the laboratory, aldehyde-fixed tissues were thoroughly washed in buffered sucrose, then
postfixed in phosphate-buffered 2% osmium tetroxide
(Millonig, 1961a). After rinsing in distilled water, the
specimens were soaked overnight in acidified 0.25%
uranyl acetate (Tandler, 1990). Another rinse in distilled water was followed by dehydration in ascending
concentrations of ethanol, passage through propylene
oxide, and embedment in Epon:Maraglas (Tandler and
Walter, 1977). Thin sections were stained with acidified
methanolic uranyl acetate (Tandler, 1990), followed by
staining with lead tartrate (Millonig, 1961b) or lead
citrate (Venable and Coggeshall, 1965), and examined
in a JEOL (Tokyo, Japan) 1200EX or Siemens (Knoxville, TN) Elmiskop 1A or 101 electron microscope.
Semithin sections were stained with methylene blueazure II (Richardson et al., 1960) and examined in an
Olympus Vanox (Tokyo, Japan).
RESULTS
Histologically, the parotid glands of Artibeus in all
seven species match Wimsatt’s (1955, 1956) light microscopic descriptions based on paraffin sections; they
consist of secretory endpieces and ducts in the usual
arrangement (Tandler, 1993a), although the intercalated ducts are quite short. In semithin sections stained
with methylene blue-azure II, the acinar cells appear to
be filled with unstained vacuoles. The ducts are unremarkable.
At the ultrastructural level, the parotid gland acini in
all seven species are strikingly similar. At low magnification, the secretory cells [seromucoid according to
histochemical tests (DiSanto 1960); seromucous according to ultrastructural criteria (Young and van Lennep,
1978; Tandler and Phillips, 1993)], echoing their light
microscopical appearance, contain large numbers of
putative vacuoles (Fig. 1). At higher magnification (Fig.
2), it becomes apparent that these structures are not
vacuoles, but represent secretory granules of a peculiar
type. In many granules, the matrix is totally ‘‘empty’’
178
B. TANDLER ET AL.
Fig. 1. Artibeus jamaicensis. Survey electron micrograph of a parotid gland. The acinar cells appear to
be filled with vacuoles. The arrow indicates a distal intercalated duct that is shading into a striated duct.
31,500.
(Fig. 3), but in some—not all—of the acinar cells in A.
cinereus, the matrix contains a sparse, weblike array of
faint threads, and an eccentrically placed, dense spherule may be present. An almost ubiquitous substructural
feature of the lucent matrix in all seven species is
membranes, most often of a circular configuration.
Although these internal membranes are of the same
thickness (,7 nm) as are the limiting membranes of the
granules, the two membrane systems (limiting and
internal) probably are of different composition, since
the internal membranes occasionally become contaminated by particles during staining with lead tartrate,
whereas the limiting membranes of the same granules
never do (staining with lead citrate, however, frequently leads to deposition of particles on both types of
membranes). Usually, the internal membranes are
disposed as a single large circle, but may have several
satellite circles, either inside or outside themselves.
Some granules contain a number of more or less
concentric circular membranes; this configuration is
especially prominent in A. planirostris, where such a
panoply may consist of well above a dozen (Fig. 4).
Certain of the internal membranes may be the result
of the invagination of a granule by a pseudopod-like
extension of a neighbor (Figs. 5 and 6), but such a
relationship should result in a pair of tightly apposed
membranes (Fig. 7), a membranous profile rarely encountered in the Artibeus granules. At its simplest, the
internal membranes appear to arise by invagination of
the limiting membrane (Fig. 8), with the invagination
eventually pinching off to form an independent circular
membranous inclusion.
Although in most of the species of Artibeus that we
examined, the Golgi apparatus appears to consist of a
number of separate dictyosomes, these seemingly independent structures in salivary gland secretory cells
probably are all interconnected (Yamashina, 1995). In
A. jamaicensis, the Golgi apparatuses appear to be in
the form of small, independent dictyosomes. In fully
charged seromucous cells in all seven species of Artibeus, the Golgi apparatuses are relatively inconspicuous. The Golgi saccules exhibit a degree of dilatation,
with this expansion being most evident at the saccular
terminations (Fig. 9).
Although, in general, the seromucous cells have
relatively few basal folds, a few acinar cells have an
abundance of such folds and lateral folds as well (Fig.
10). Contiguous seromucous cells often form microvilluslined intercellular canaliculi (Fig. 11). That secretory
granules engage in exocytosis into these canaliculi is
PAROTID GLAND IN SEVEN SPECIES OF FRUIT BATS
179
Fig. 2. A. planirostris. Survey electron micrograph of a seromucous acinus. Echoing the previous figure,
the cells appear to contain numerous vacuoles. 34,900.
shown by the abundance of V-shaped invaginations
along their limiting membranes (Fig. 12). Exocytotic
granules occasionally are observed whose limiting membrane has fused with the plasma membrane and whose
internal membranes are herniated into the acinar or
intercellular canalicular lumen (Fig. 13). That the
internal membranes are liberated during this process is
shown by their abundance not only in the lumina of the
acini (Fig. 14) and intercellular canaliculi, but in those
of the ducts as well (see Fig. 16).
The acinar cells exhibit all of the structural hallmarks of protein-secreting cells; as they become charged
with secretory granules, the intervening cytoplasm is
correspondingly reduced. In cells containing an abundance of seromucous granules, the mitochondria appear to have been displaced peripherally—a layer of
these organelles appears to separate the surface folds
from the rest of the cell.
The intercalated ducts consist of simple cuboidal
epithelium surrounding a lumen of relatively small
caliber (Fig. 15) that often contains a plethora of
circular membrane profiles. Each duct cell has a large,
deeply notched nucleus and small aggregates of rough
endoplasmic reticulum. The most noteworthy feature of
these cells is the presence of a modicum of uniformly
dense, serous-type secretory granules. These granules
vary between 0.5–0.9 µm in diameter among the seven
species of Artibeus.
Lumina of proximal striated ducts contain circular
membrane profiles (Fig. 16), whereas more distal ducts
are devoid of such material (Fig. 17). Otherwise, the
striated ducts are of typical appearance (Tandler, 1993c),
with long, slender, basally situated mitochondria alternating with folded plasma membranes (Fig. 18). Mitochondria in the supranuclear cytoplasm tend to be
ovate or elliptical. Short, flattened vesicles are fairly
abundant in the region subjacent to the apical membrane, especially in A. lituratus, but the duct cells lack
secretory granules altogether (Fig. 19). These cells bear
variable numbers of short microvilli.
Excretory ducts closely resemble striated ducts (Fig.
20), but are readily distinguishable from the latter by
virtue of their mantle of fibrous connective tissue. The
walls of these ducts harbor an occasional dark cell,
180
B. TANDLER ET AL.
Fig. 3. A. planirostris. A seromucous granule that is totally devoid of
substructure, appearing to be ‘‘empty.’’ 345,000.
Fig. 4. A. planirostris. A seromucous granule that contains a multiplicity of more or less concentric membranes as well as some circular
which is interlocked with the usual lighter cells (Fig.
20). It could not be determined whether these cells are
fixation artefacts or whether they are a normal component of parotid excretory duct epithelium in a variety of
Artibeus species.
DISCUSSION
The seromucous granules present in the parotid
glands of the seven species of Artibeus that we examined are of a type rarely seen previously. Seromucous
granules studied in other mammals tend to have a
well-defined substructure, e.g., rat and mouse (Ciofi
Luzzatto et al., 1968; Martinez-Hernandez et al., 1972),
cat (König and Kühnel, 1986), mink (Tandler, 1991),
ferret (Jacob and Poddar, 1987), and Siberian weasel
(Matsunaga, 1992). In contrast, the matrix of the
Artibeus granules is completely lucent.
At first, the ultrastructural appearance of the parotid
gland secretory granules of Artibeus seems to be a
result of inadequate fixation. When osmium tetroxide
was the mainstay of fixation for electron microscopy,
secretory granules in a variety of salivary glands
appeared extracted and empty (see, for example, Reinecke, 1967). With the advent of aldehyde fixatives,
these same granules were seen to have a dense content
(for comparison, see figure 40 in the compendium by
Young and van Lennep, 1978). The obvious improvement in preservation of parotid secretory granule contents afforded by aldehyde fixation was documented by
Robinovitch et al. (1966). The initial fixative that we
used in our comparative ultrastructural studies, namely,
triple aldehyde-DMSO (Kalt and Tandler, 1971, as
modified by Phillips, 1985), seems to be a near universal fixative for salivary gland granules, preserving
profiles inserted between the concentric membranes. Note that some
of the internal membranes are contaminated by small, dense lead
particles. 327,600.
delicate details of granule substructure that permit
differentiation of closely related species of bats (Tandler, 1993b; Tandler and Phillips, 1993). Moreover, in the
case of the Artibeus glands, the same fixation protocol
was applied simultaneously to the submandibular
glands of the selfsame animals that furnished the
parotid glands examined in the present study. The
seromucous granules in the principal submandibular
glands of four Artibeus species previously described
(Phillips et al., 1977; Tandler et al., 1986) exhibited
characteristic structural patterning for each individual
species; the same holds true for the three additional
species examined in the course of the present study
(Tandler, unpublished observations). In other words,
where a substructure existed in Artibeus submandibular gland seromucous granules, our fixation protocol
revealed its presence. Furthermore, using the same
protocol, we have not encountered granules of the
Artibeus parotid gland type in the parotid glands of
more than 230 other species of bats that we have fixed
in the same manner. For these reasons, we believe that
the ‘‘empty’’ appearance of the Artibeus parotid seromucous granules is representative of their true structure.
The granules that most closely resemble those in the
parotid glands of the fruit bats occur in standard pigs
(Ferrandi, 1969; Boshell and Wilborn, 1978; Boshell,
1981). Boshell and Wilborn (1978) identify these granules as those of special serous cells, a cellular designation that we have argued elsewhere is based on inadequate fixation (Tandler and Phillips, 1987). Moreover,
histochemical studies of the pig parotid gland have
shown that the gland is seromucous in nature (Kamiya,
1977). Similar granules also are present in the parotid
gland of Gottingen strain minipigs (Ginsbach and Küh-
PAROTID GLAND IN SEVEN SPECIES OF FRUIT BATS
Figs. 5–9.
181
182
B. TANDLER ET AL.
nel, 1978; Lotz et al., 1990). Although the authors of the
latter reports designate them as a mucous granules or
as special serous granules, respectively, Ginsbach and
Kühnel (1978) also mention that the minipig parotid
gland secretes a ‘‘. . . watery, serous product . . .’’ leaving little doubt that the parotid secretory granules in
this porcine variant also are seromucous in nature. The
aforementioned studies on pig parotid glands were
based on specimens initially fixed in glutaraldehyde,
yet the granules maintained their lucency. Although it
is true that certain secretory granules are better preserved by initial fixation in osmium tetroxide than in
aldehyde mixtures (Tandler and MacCallum, 1974),
this is not the case in the pig where, even after fixation
solely in osmium, Yamada (1977) found the porcine
parotid granules to have a lucent matrix. Finally, using
morphological criteria (Young and van Lennep, 1978;
Tandler and Phillips, 1993), we consider that their
ultrastructural appearance indelibly marks pig parotid
granules as seromucous.
In addition to their lucency, another feature shared
by seromucous granules in both Artibeus species and
pigs is the presence of circular membranes in their
matrices; these lack the tightly furled configuration of
myelin figures (Revel et al., 1958). Although some of
these membranes conceivably could result from the
invagination of one granule by a process from another,
some seromucous granules in the fruit bats have too
many internal, circular membrane profiles (as exemplified by Fig. 4) to have originated in this fashion.
Granules connected by a process usually have only a
single membrane between the two moieties rather than
two appressed membranes. Such a configuration is
more in keeping with a fusion process (Tandler and
Poulsen, 1976; Neutra and Schaeffer, 1977; Vidić, 1977;
Tanaka, 1982; Shimono et al., 1984, Odajima and
Nakane, 1984) than with internalization of limiting
membranes of granules. We recently described mucous
droplets in the accessory submandibular glands of
long-winged bats (Miniopterus schreibersi and M. magnator) that were delimited by multiple membranes
(Tandler et al., 1994); it was posited that these supernumerary membranes originated by the simple expedient
Fig. 5.A. lituratus. These paired granules illustrate one possible way
that some of the internal membranes originate. A protrusion from one
granule penetrates a neighbor, invaginating its limiting membrane. A
section passing through the plane indicated by the line would give rise
to a granule with the configuration shown in the next micrograph.
340,000.
Fig. 6.A. lituratus. A granule with two concentric internal membranes, conceivably resulting from a transversely-sectioned pseudopod within an invagination. 340,000.
Fig. 7.A. lituratus. In these coupled granules, the surface protusion
of the granule on the left has only a single membrane separating its
contents from that of its consort. This configuration may be a prelude
to granule fusion. 3121,000.
Fig. 8.A. lituratus. This granule shows an early stage in the
acquisition of internal membranes. The limiting membrane has
formed a small invagination without the assistance of a neighboring
granule. 321,000.
Fig. 9.A. lituratus. A Golgi apparatus in an acinar cell fully charged
with seromucous granules. The expanded portions of the saccules
eventually may become separated from the Golgi membranes to form
nascent secretory granules. 337,500.
of Golgi saccules wrapping themselves around the
nascent droplets. A similar mechanism cannot be operative in the Artibeus granules, because the membranes
in question are suspended in their matrices, not applied
to their surfaces. Figure 8 shows what probably is an
early stage in the development of the internal membranes, with a surface membrane invaginating into the
granule matrix to yield what in transverse section
would appear to be a circular membrane profile. That
granules conjoined by a pseudopod-like process are not
simply produced by chemical fixation was shown by
Dylewski and Keenan (1987), who used rapid freezing
of rat mammary gland; ‘‘ball and socket’’ connections
between granules were abundant. In order for the panoply
of internal membranes that characterize some of the Artibeus parotid granules to have formed by this mechanism,
the invagination process would have to be repeated many
times. As the surface membranes become internal ones,
they must undergo a physical or chemical change that
leads to their becoming contaminated by lead stain when
their precursors remain resistant to such an artifact.
The origin of the lucent seromucous granules could
not be unequivocally ascertained because we encountered virtually no secretory cells in any of our specimens that were at the beginning of their secretory
cycle. It is in such cells that both the RER and Golgi
apparatuses would be expected to be at their most
prominent. The relatively small and inactive (on a
morphological basis) Golgi apparatuses in cells filled
with seromucous granules only hint at their role in
producing these structures. At this point, the termini of
the Golgi saccules are somewhat dilated and have a
lucent content.
Two possible Golgi-related pathways in the elaboration of the lucent granules present themselves. The
simpler one is based on gradual enlargement of the
termini of the Golgi saccules until the expansions
attain the dimensions of mature granules, followed by
scission of these amplified parts from the saccules to
give direct rise to full-blown granules. A second possibility involves the release of relatively small granules
from the termini of the Golgi saccules, and their
progressive enlargement by accretion of progranules, a
process that occurs in certain exocrine cells (Tooze et
al., 1991; Lew et al., 1994). Regardless of which program the Golgi apparatus follows, the point at which
the seromucous granules initially acquire their complement of internal membranes is undetermined. However, our observations suggest that this accession is a
dynamic, ongoing process. The second of the two scenarios that we posited requires a massive reduction in
surface area of the maturing granules (Lew et al.,
1994); perhaps the internalization of membranes is an
integral part of this process in seromucous granules of
the Artibeus parotid type.
The question arises as to what glycoproteins are
present in the ‘‘empty’’-appearing granules of Artibeus.
Of three different species of Artibeus tested by Junqueira et al. (1973) (A. concolor, A. jamaicensis, and A.
lituratus), only A. jamaicensis had more than a trace of
a-amylase, but the activity was trifling compared to
that of bats such as the velvety free-tailed bat, Molossus
ater, or the spear-nosed bat, Phyllostomus hastatus. A
very low level of protease activity was evident in A.
jamaicensis; the other two species had none. By histo-
PAROTID GLAND IN SEVEN SPECIES OF FRUIT BATS
Fig. 10. A. lituratus. The base of two seromucous acinar cells
showing its array of basal folds. 317,600.
Fig. 11. A. jamaicensis. A longitudinally sectioned intercellular
canaliculus showing its close approach to the cell base. The canaliculus is lined with microvilli. 314,500.
Fig. 12. A. lituratus. An intercellular canaliculus with connected
membrane profiles (*) suggestive of a recent exocytotic event. 336,200.
183
Fig. 13. A. lituratus. A seromucous granule in the process of exocytosis, with an internal membrane herniating into an intercellular
canaliculus. 356,200.
Fig. 14. A. jamaicensis. An acinar lumen replete with numerous
membranous vesicles and vacuoles representing exocytosed seromucous granule internal membranes. 311,100.
184
B. TANDLER ET AL.
Fig. 15. A. jamaicensis. An intercalated duct. The cells contain a few
secretory granules that have a maximum diameter of ,0.9 µm. 37,000.
Fig. 16. A. lituratus. A longitudinally sectioned striated duct. Note
the abundance of membranous detritus in the duct lumen. This
material undoubtedly represents the internal membranes of the
seromucous granules that were exocytosed into the acinar lumen and
that so far have survived the exigencies posed by passage through the
duct system. 33,000.
Fig. 17. A. jamaicensis. A transversely sectioned striated duct whose
lumen is devoid of membrane profiles. This sequence of membrane
clearance from the lumina occurred in all seven species of Artibeus
that were examined. 35,200.
Fig. 18. A. jamaicensis. A portion of the striated duct wall showing
the extremely long mitochondria alternating with folded plasma
membranes. 38,400.
PAROTID GLAND IN SEVEN SPECIES OF FRUIT BATS
Fig. 19.A. lituratus. Numerous round and elongated vesicles are
present in the supranuclear cytoplasm of a striated duct cell. 358,000.
Fig. 20.A. jamaicensis. A portion of the wall of an excretory duct. The
cells still retain a degree of basal striations. A dark cell is present at
185
the right in the ductular epithelium. It is not clear whether or not such
cells are fixation artefacts, or represent a distinct population of duct
cells. 39,500.
186
B. TANDLER ET AL.
chemistry, the parotid acinar cells in A. jamaicensis
were negative to antiserum against lysozyme (see
figure 25 in Phillips et al., 1993).
The expanding population of seromucous granules
within acinar cells appears to ‘‘drive’’ the mitochondria
toward the cell base where they form a layer that
separates the basal folds from the rest of the cytoplasm.
Unlike the situation in the mouse submandibular
(Sharawy et al., 1978) and rat parotid (Sampson and
Montalvo, 1983) glands, these organelles are not directly associated with the basal plasma membrane, and
no energetic inferences should be drawn based on the
propinquity of the two structures.
The internal membranes of the mature seromucous
granules are delivered into the acinar lumina during
exocytosis of the seromucous granules, ultimately filling these passageways. Although pieces of plasma
membrane are avulsed from salivary gland mucous
cells during exocytosis (Tandler and Poulsen, 1976),
such shed plasmalemmal fragments cannot by themselves account for the plethora of membranes present in
the lumina of the fruit bat parotid glands. The circular
membranes (which must be spherical in three dimensions) continue unaltered into the duct system. Whether
some membranes survive transit through the main
excretory duct to the mouth or while en route are
degraded to their constituent molecules was not determined, although our observations suggest that breakdown of the membranes occurs in the striated duct. In
either case, it is plain that a great deal of phospholipid
is present in the final saliva entering the mouth. In the
case of the two Miniopterus species mentioned earlier,
we postulated that salivary lipids may be used to
counter chemical defenses erected by insects that are
their normal prey (Tandler et al., 1994). Phospholipidrich saliva might have a different role in species of
Artibeus. In these frugivorous bats, the gastric glands
have great numbers of acid-producing parietal cells and
relatively few surface mucous cells or mucous neck cells
(Phillips et al., 1984). The parietal cells in Artibeus [and
in related phyllostomid fruit bats (Studholme et al.,
1986)] ultrastructurally appear to be highly active and
breakdown and passage of fruit is a remarkably rapid
process. Studier et al. (1983) postulated that saliva may
have a role in protection of the gastric mucosa, and
Studholme et al. (1986) noted the possibility that
surface-active phospholipids might be involved. Our
documentation of salivary phospholipid in the form of
membrane-like materials in the parotid initial saliva of
Artibeus is consistent with hypotheses regarding mechanisms for protecting the stomach lining.
The major difference between the striated ducts of
the Artibeus parotid gland and those of the submandibular gland in the same animals is that the ducts in the
former lack the array of leaflike microvilli (frondose
processes) that characterize not only the latter, but
striated ducts in the submandibular gland of most
frugivorous bats (Tandler et al., 1989; Tandler, 1993c).
Frondose processes have repeating units (portasomes)
on the inner aspect of their limiting membranes that
presumably function in sodium transport, i.e., Na1
conservation (Harvey et al., 1981), a necessity in fruit
eaters, since their diet, although rich in potassium, is
deficient in sodium. Whatever their ultimate function
proves to be, the striated ducts in Artibeus parotid
glands probably play a lesser role in electrolyte homeostasis than do those in their companion submandibular
glands.
Elsewhere, we have argued that salivary gland plasticity has been an extremely important factor in adaptive radiation in mammals (Phillips, 1996), and we also
have shown that salivary glands in bats are greatly
diversified, often at the species level, in obvious relationship to diet and other variables (Phillips and Tandler,
1987; Tandler et al., 1990; Phillips et al., 1993). By
contrast, we have documented in the present comparative analysis an example in which seven different
species of fruit bats of the genus Artibeus exhibit a
‘‘conserved’’ parotid gland, at least as far as secretory
cell ultrastructure is concerned. An eighth species, A.
phaeotis, can be added to the list on the basis of an
earlier study (Phillips et al., 1987). This conservation is
interesting because although in ultrastructural terms
the parotid acinar cells are essentially the same in all
eight studied species of Artibeus, this cell type differs
from homologous cells in related genera of Neotropical
fruit bats (Phillips et al., 1987; Tandler et al., 1986).
Although there are some similarities among salivary
glands in various genera of Neotropical fruit bats, the
unusual secretory granule ultrastructure described in
the present report thus is not a ‘‘signature’’ for frugivory
in general. Instead, the granules described herein are a
‘‘generic’’ characteristic for just Artibeus and their
conservation across the broad spectrum of the surveyed
species of this genus implies that the parotid gland
carries out a fundamentally important function in the
lives of these related species. Salivary gland conservatism is not a general phenomenon in the genus, because the
seromucous cells in the submandibular glands in Artibeus
exhibit interspecific differences (Tandler et al., 1986).
Generally speaking, the parotid gland and, more
specifically, the parotid acinar cells appear to have two
main roles in mammals. The obvious one is production
and secretion of a variety of enzymes such as amylase
and proteinases. However, as explained previously, this
does not appear to be the case in Artibeus. At this point,
it seems that phospholipids and some unknown but
possibly nonenzymatic glycoprotein(s), perhaps prolinerich proteins, are the primary constituents of the
watery parotid saliva. Another role of parotid acinar
cells is production and release of HCO3, a function that
has been extensively investigated in rats (e.g., Turner,
1993). Interestingly, the saliva of Artibeus is significantly more acidic and has a much lower buffering
capacity than does the saliva of Carollia (a frugivore) or
any of the tested insectivorous species (Dumont, 1996).
The implication of these observations is that the parotid acinar cells in Artibeus do not buffer saliva to any
meaningful extent and, to the contrary, might be different because they are exempt from this function. This
would be consistent with the importance of high acid
production by the gastric mucosa in Artibeus (Phillips
et al., 1984).
Patterns of diversity at the cellular level reflect
evolutionary adaptation (Phillips, 1996). Patterns of
conservation of sets of unique features are suggestive of
‘‘key innovations’’ (compare Nitecki, 1984) at the root or
base of a group of species. In the case of the parotid
gland in Artibeus, additional investigation is required
in order to fully explain the biological significance of the
PAROTID GLAND IN SEVEN SPECIES OF FRUIT BATS
unusual parotid secretory granules in these particular
bats.
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