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The amphibian cutaneous glands^: C/^
some aspects of their structure and adaptive role
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R. Brizzi', G. Delfino', S. Jantra', B.B. Alvarez^ D.M. Sever^
1. Dip. di Biologia Animale e Genetica, Universita di Firenze,
Via Romana 17, 50125 Firenze Italia
2. Dep. de Biologia, Facultad de Ciencias Exactas y Naturales y Agrimensura,
Universidad Nacional del Nordeste, 9 de Julio 1449, 3400 Corrientes, Argentina
3. Dep. of Biology, Saint Mary's College, Notre Dame, I N 46556, U S A
Abstract
This paper reviews the main characteristics of the cutaneous glands in two amphibian orders,
Anura and Urodela, and provides indications on the most significative morphological and functional patterns. On the basis of structural and ultrastructural data, the skin glands can be distinguished in four types: serous, mucous, mixed and lipid in nature. In addition, a prevalently topographical criterion, related to their diffuse or localized distribution on the body skin, permits
to recognize ordinary and specialized cutaneous glands. These latter are essentially involved in
defensive strategies or in fiinctions related to social communication and/or reproduction.
Introduction
The cutaneous apparatus plays a crucial role in the homeostasis o f the Amphibia.
Basically it exchanges substances as water, respiratory gas and salts, acting as interface
between the organism and the external environment. I n addition, a lot o f specific functional roles may be referred to the different skin components, each exhibiting unicellular, multicellular or extracellular organization (see Duellman & Trueb, 1986; F o x ,
1994). Among the cutaneous multicellular structures, noteworthy is the rich supply of
exocrine glands, characterized by very different morphology and functions.
In this paper are summarized the main morphological traits o f the integumental glands
and their ordinary or specialized roles. For the study we used skin strips o f Anura and
Urodela, and observed gland morphology, cytology and histochemistry using light
( L M ) and transmission ( T E M ) electron microscopes. I n the lack o f personal observations, data on the third major group o f amphibians, the caecilians, are not included in
this repertory.
Results and Discussion
Ordinary cutaneous glands and their function
With the term "ordinary" glands we define multicellular secretory structures widely
scattered in the integument o f the various body regions. On the basis o f their structure,
the amphibian glands are usually distinguished in four types: granular glands (also
defined "serous" or "poison glands") (figs A - B ) , mucous glands (figs B , F ) , mixed
43
Herpetologia Candiana
glands (fig. C ) and lipid (or w a x ) glands (fig. D ) . Although the gland structural planes
are homogeneous throughout the class, some differences exist between different taxa.
In the anuran skin the secretory units o f serous and lipid glands are syncytial, whereas
discrete secretory cells form the mucous glands. I n this order mixed glands are rare. A n
epithelial organization characterizes, on the contrary, the three gland types o f the >
urodeles (serous, mucous and mixed). I n these amphibia lipid glands have not been, at
the moment, specifically described.
Usually the serous glands release a product in form o f granules, that appear strongly
eosinophilic at L M but very different in their structural and ultrastructural caracteristics according to the species. I n this gland type, the term "serous" defines the nature o f
its fluid product, including bioactive peptides and amynes. The term "poison" glands
indicates their possible antipredatory role. The toxic nature o f serous secretions in
many salamanders and frogs has led to assume that they are "poison glands" in all the
amphibians, although this function seems to lack in some lineages (Neuwirth et al,
1979). According to other authors (Bachmayer et al., 1967; Barberio et al,
1987;
Zasloff et al, 1988), the granular glands are involved in the production o f substances
aging as antimicrobical film over the body surface. However, the ancestral function o f
granular glands, probably involved in some forms o f defense, regulative functions,
storage, or all, is still an open question.
The product o f mucous glands is basophilic and usually P A S positive. The secretory
cells (mucocytes) are orderly arranged around an obvious lumen (figs B , F ) . The
mucous glands are involved in general functions as saline and gas exchange, as w e l l as
in regulation o f water loss during terrestrial phases or in friction reduction during
swimming.
M i x e d glands consist o f a large granular portion and a smaller mucigenous acinus (fig.
C ) . These are well distinguishable also on the basis o f their different histochemical
stain, typical o f the two gland types. Mixed glands are a stable type in urodeles
(Delfino et al., 1986), where they possibly represent an adaptation for the synergic production o f both mucous and serous products. I n anurans, on the contrary, these glands
seem to correspond to a transitory stage in granular gland development (Duellman &
Trueb, 1986) or restorafion (Delfino, 1980).
Currently, lipid glands (fig. D ) have been reported only in some hilids (Blaylock et al,
1976; C e i , 1980, Delfino et al,
1998), but we sfill consider them "ordinary glands"
since in this family they are widely distributed over the skin. A t L M w a x glands show
a relatively large lumen containing discrete secretory bodies which exhibit positive
response in trials for lipids. Observed under T E M the w a x glands reveal their syncytial nature and secretory aggregates consisting o f heterogeneous material. Their secretion serves as a cutaneous lubrificant in the water, or reduces dehydration during the
terrestrial life phases.
^
44
Brizzi et al.
Fig.A: Physalaemus biligonigerous. Ordinary serous gland observed under T E M . Bar = 5 |im; Fig.B:
Triturus carnifex. Ordinary serous and mucous glands observed under L M . Bar = 200nm; Fig.C:
Hydromantes genei. Note, in the mixed gland, the small mucous acinus inside the large serous portion
( L M ) . Bar = 50 |im; Fig.D: Phyllomedusa hypocondrialis. Patterns of lipids glands ( L M ) . Bar = 50 ^m;
Fig.E: Bufo bufo. As a rule, the antipredatory parotoid glands are very large serous glands inserted in the
dermis. (LM).(compare with Fig.F). 500 )xm; Fig.F: Salamandra lanzai. Also in this species the parotoid
glands exhibit serous characteristics and large dimensions ( L M ) (compare with Fig.E). Bar = 200 nm;
Fig.G: Plethodon glutinosus. Male mental glands are elongated, mucous glands specialized in production
of courtship chemosignals.(LM) (compare with Fig.H). Bar = 200)im; Fig.H: Hydromantes italicus.
Cloacal vent glands are male glands specialized in pheromone production. Note their mucous structure and
compare with Fig.G ( L M ) . Bar = 50 urn. Labels: dd = dense dermis, e = epidermis, gn = gland neck, Ig =
lipid gland, ma = mucous acinus, mec = myoepithelial cell, meg = mental gland, mg = mucous gland, pg
= parotoid gland, sd = spongy dermis, sg = serous gland, vg = vent gland.
Herpetologia Candiana
Main gland portions
A l l the gland types are simple alveolar and intradermal. Independently o f their nature,
they exhibit the same structural pattern based upon integration o f four parts.
Proceeding according to the functional way o f secretory release, a gland consists of: 1)
myoepithelial sheath; surrounding the gland acinus and involved in gland discharge, 2 )
secretory unit; the most characteristic portion o f a gland providing to its typical function, 3) intercalary tract, or neck; a trunco-conical, subepidermal region between gland
body and duct. It consists o f imbricated adenoblasts and myoblasts which serve as a
regenerative matrix after gland holocrine discharge, 4 ) duct; an intradermal channel for
the secretory product, bordered by epidermal cells. The myoephitelial sheath o f serous
and lipid glands possesses direct innervation, whereas, as a rule, this lacks in mucous
glands. I n this case the release o f the neurotransmitter from nerve terminals occurs,
very likely, in the stromal environment and diffuses more slowly towards the gland.
Specialized cutaneous glands
With the term "specialized" glands we define secretory organs localized in limited skin
regions and involved in peculiar roles related to the species biology. I n this connection,
although several different gland specializations have been reported in both Anura (see
Barthalmus, 1994) and Urodela (see Houck & Sever, 1994), all o f which are referable
to two main strategies: antipredatory defence or social communication and reproduction. When specialized glands occur, the most prominent differences with the ordinary
ones concern the secretory units. A s a rule these are large and/or elongated and closely packed together, forming, in some cases, obvious outgrovv1:hs (plicae, swellings,
patchs, a.s.o).
On the basis o f cytological traits and/or secretory products, most specialized glands o f
the amphibians pertain to the serous type. This is the rule for the antipredatory units o f
both anurans and urodeles. Serous are also the parotoid glands producing defensive
toxins and occurring in some anurans (pelobatids and bufonids; fig.E) as well as in several families o f salamanders, including ambystomatids, plethodontids and salamandrids (fig. F ) .
Other anurans and salamanders possess localized patches o f granular glands that perform the same defensive function as the parotoid ones. Among anurans, these are, for
example, the leg glands occurring in the skin o f the zeugopodium o f Bombina variegata (Delfino et al., 1982) and the inguinal glands observed in the dorsal pelvic region
of the leptodactylid Physalaemus
biligonigerous
(Delfino et al,
1999). Similar
antipredatory glands occur also in the caudal region o f some plethodontids (genus
Hydromantes;
B r i z z i et al,
1994) and salamandrids (genus Triturus; B r i z z i et
al,
2000). The combination of toxin biosynthesis and effective neuromuscular release sys46
Brizzi et al.
tem makes these glands a remarkable example o f a defensive device. From such perspective, one can imagine this mechanism being favored over evolutionary time at the
expense o f would-be predators. The flill array o f these specializations is yet to be
known, and future studies on different species w i l l permit an interpretation o f their evot
lutionary history.
However, other specializations o f cutaneous glands seem to derive from the mucous
type. This is typical o f peculiar glands, frequently sexually dimorphic, that in some
salamanders and frogs are related to courtship strategies and produce pheromones. I n
general, chemical signals that affect species identification and/or mate attraction are
more important for salamanders than for anurans. I n fact, chemical cues may be the
salamander counterpart o f auditory vocalization in frogs and toads. A list o f urodelan
skin glands producing chemosignals is reported below:
Caudal glands in the midventral tail base, posterior to the cloaca, in Amides
lugubris
(Staub & Paladin, 1997) and Eurycea (Sever, 1989). Genial glands in the salamandrid
Notophthalmus (Houck & Sever, 1994). Mental glands in the salamandrid Taricha
(Smith, 1941) and in some plethodontids as Plethodon (Houck & Sever, 1994; fig. G )
and Hydromantes ( B r i z z i et al, 1994). Nasolabial glands, especially prominent growth
into cirri in the plethodontid Eurycea (Sever, 1975). The fiinction o f other localized
glands as the intermaxillary ones (described by Wiedersheim, 1876), and the orbital
(lacrimal) glands (Noble, 1927) is still unclear, but in all probability they are involved
in social or reproductive communication.
Specialized glands, possibly related to breeding activities, have been occasionally
reported also in anurans. The ventral skin o f the sternal area in male Microhyla carolinensis and M. olivacea possesses glands o f the mucous type that appear to be under
testosterone control (Conaway & Metter, 1967). These glands exhibit slight morphological modifications o f mucocytes but significant changes in their secretory products.
Due to their role, these glands have been labeled as breeding glands.
Among the specialized exocrine glands o f ectodermal origin are also to include male
and female cloacal glands occurring in most urodeles and involved in reproduction (see
Sever & B r i z z i , 1998; B r i z z i et al, 1999). The chemical composition o f these gland
secretions seems to be rather complex. Most male cloacal glands produce primarily
glycosaminoglycans, whereas other ones release lipids and proteins. Thus, the products
of the glands that make spermatophores are "blended", containing carbohydrates,
lipids, and/or proteins. Finally, it is noteworthy that the cloacal vent glands occurring
in many salamanders (fig. H ) and involved in pheromone production exhibit a mucous
structural and istochemical characterization. This report seems to confirm the mucous
nature o f the cutaneous glands specialized in the release o f social or reproductive
chemosignals.
47
Herpetologia Candiana
Conclusive remarks
A comparative analysis o f ordinary and specialized cutaneous glands in the two most
representative orders of living amphibians (Anura and Urodela) indicates some general evolutionary trends: selective pressures due to environmental and social constrains
gave arise to the rich supply of ordinary and specialized glands observed in the skin of
the various body regions. Thus, the typical gland specializations of the different species
strongly reflect their survival and reproductive strategies. Urodeles display a larger
variety o f specialized glands, whereas in anurans selection see mingly acted shaping
the biosynthetic performance of ordinary glands, capable o f producing several bioactive molecules. A s a rule, specialized glands involved in chemical defense pertain to
the serous line and are chiefly localized in body regions which undergo attack from
predators during flight reactions. Glands specialized for reproductive strategies or
chemical communication appear particularly diffuse among salamanders, although
they also occur in some anurans. The pheromone producing glands share morphological characteristics in both amphibian orders and seem to pertain to the mucous gland
line.
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