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Transcript of Ultrastructure of the skin melanophores and iridophores in
www.elsevier.com/locate/micron
Micron 38 (2007) 81–84
Short communication
Ultrastructure of the skin melanophores and iridophores in
paddlefish, Polyodon spathula
Otilia Zarnescu
Bucharest University, Faculty of Biology, Department of Animal Biology,
Splaiul Independentei 91–95, 76201 Bucharest, Romania
Received 6 January 2006; received in revised form 27 March 2006; accepted 27 March 2006
Abstract
The ultrastructure of melanophores and iridophores of Polyodon spathula has been examined by transmission electron microscopy. In the skin,
two types of chromatophores, melanophores and iridophores were founded. Melanophores were localized both in epidermis and dermis. Epidermal
melanophores were present on the dorsal region of the trunk, sides, outer surface of the operculum and rostrum. Iridophores were founded in the
dermis from ventral skin. The cytoplasm of iridophores is filled with reflecting platelets with variable orientation. The length of the long axis of the
platelets varies from 1 to 2.10 mm.
# 2006 Published by Elsevier Ltd.
Keywords: Chromatophores; Iridophores; Melanophores; Melanosome; Paddlefish; Polyodon spathula; Reflecting platelets; Skin; Ultrastructure
Skin coloration in fishes depends on chromatophores or
pigment cells. Chromatophores were classified into light-
absorbing, as melanophores, erythrophores, xanthophores and
cyanophores and light-reflecting which include leucophores
and iridophores (Fujii, 1993).
North American paddlefish (Polyodon spathula, Walbaum)
belongs to the order Acipenseriformes, which consists of
families Acipenseridae, with 25 sturgeon species and Poly-
odontidae. Paddlefish is one of the two living species of
Polyodontidae, the other being the Chinese paddlefish
Psephurus gladius (Bremis et al., 1997).
Most of the work on the structure of the paddlefish skin has
been restricted to light microscopy (Mester and Zarnescu,
2000; Weisel, 1975), analysis of ampullary organs (Wilkens
et al., 2002; Wilkens, 2004) and the only transmission electron
microscopy investigations have been performed just on the
epidermis (Zarnescu and Mester, 2003).
The aim of the present work was to investigate at the electron
microscopic level the chromatophores of paddlefish skin. To the
best our knowledge no ultrastructural studies of Acipenser-
iformes chromatophores have been published.
E-mail address: [email protected].
0968-4328/$ – see front matter # 2006 Published by Elsevier Ltd.
doi:10.1016/j.micron.2006.03.015
1. Material and methods
1.1. Animals
The paddlefish were brought to Romania from the USA as
larvae and reared in captivity at Nucet Station. Juvenile
Polyodon spathula measuring 90 cm in total length were used
in this study.
1.2. Light microscopy
Skin samples were taken from various sites of the body
(middorsal and lateral regions of the trunk, belly, operculum
and rostrum) for fixation in Bouin-Hollande solution,
dehydrated in ethanol, cleared in xylene and embedded in
paraffin. The 7 mm thick sections were stained with hematox-
ylin–eosin–alcian blue (pH 2.5).
1.3. Electron microscopy
Small skin fragments were fixed in 2.5% glutaraldehide,
buffered at pH 7.4 in 0.1 M sodium cacodylate, then post-fixed
in 1% OsO4, in the same cacodylate buffer, dehydrated and
embedded in Epon 812. Ultrathin sections were stained with
uranyl acetate and lead citrate, and then studied with a Phillips
206S electron microscope operating at 80 kV.
O. Zarnescu / Micron 38 (2007) 81–8482
Fig. 2. Electron micrograph of epidermal melanophore located between epithe-
lial cells (E). Bar = 2 mm.
2. Results and discussion
The skin of Polyodon spathula as in most teleosts is
composed of two main layers: the epidermis, that in contrast
with other fish species has epithelial cells only, and underlying
dermis. The dermis contains two strata: laxum and compac-
tum. The thickness of stratum laxum situated at the base of
epidermis is variable in different parts of the body and contains
collagen fibres, nerves, capillaries, fibroblasts and pigment
cells. Stratum compactum is more developed than stratum
laxum of the dermis and is formed by densely compressed
bundles of collagen fibres that run mostly parallel to the skin
surface.
Electron microscopy observations show that in the skin of
paddlefish there are two types of chromatophores, melano-
phores and iridophores.
Melanophores localized both in epidermis and dermis were
present (Fig. 1A) on the dorsal and lateral regions of the trunk,
outer surface of the operculum and rostrum and lacking
(Fig. 1B) on throat, belly and inner surface of the operculum.
The shape of epidermal melanophores was irregular with many
cytoplasmic processes that are insinuated between epithelial
cells (Fig. 2). While epidermal pigment cells may be of variable
shape, dermal one are more or less flattened. In light
microscopy the dermal melanophores are abundant in the
stratum laxum and in the lower part of the stratum compactum.
Few melanophores can be seen between collagen bundles of the
Fig. 1. Histological cross section of the paddlefish dark skin with melanophores (a
laxum; SC, stratum compactum. Bar = 50 mm.
stratum compactum of the dermis (Fig. 1A). Ultrastructural
studies indicated that many melanophores were present near
blood vessels in the stratum laxum (Fig. 3). Dermal
melanophores have a centrally or excentrically placed nucleus
that can be round, elongated or indented. Throughout the
cytoplasm there are melanosomes. The shape of the melano-
rrows) on the middorsal trunk (A) and white skin on the belly (B). SL, stratum
O. Zarnescu / Micron 38 (2007) 81–84 83
Fig. 3. Electron micrograph of melanophores (arrows) in the stratum laxum of
the dermis. Bar = 3 mm.Fig. 5. Transmission electron micrograph of a dermal iridophore which con-
tains many reflecting platelets (RP) with different orientation. N, nucleus.
Bar = 0.5 mm.
somes is rounded or oval, with uniformly electron-dense
pigment. Some melanosomes show an electron-lucent core
(Fig. 4). Under the membrane of some melanosomes there is a
thin space filled with a granular material. Probably, this type of
melanosome is not fully differentiated. Besides melanosomes
in the cytoplasm of these cells could also be observed
mitochondria, lipid droplets, endoplasmic reticulum and Golgi
apparatus.
Iridophores were observed in the dermis from belly. These
cells are variable as shape, mostly dendritic and elongated
Fig. 4. A group of melanosomes (M) in the cytoplasm of dermal melanophore.
While some melanosomes contain an electron-lucent core (*) others show a
granular material at its periphery (arrow). Bar = 0.2 mm.
(Fig. 5). Iridophore contains a nucleus located near its central
region. The nuclei of these cells are elongated (Fig. 5) or with
deep invagination (Fig. 6). The cytoplasm of iridophores is
filled with hexagonal platelets with variable orientation (Fig. 6).
Because the contents of platelets are removed by fixation they
appeared as empty spaces. Each platelet is surrounded by a
membrane. The length of the long axis of the platelets varies
from 1 to 2.10 mm. The cytoplasm also contains mitochondria,
endoplasmic reticulum, ribosomes and Golgi apparatus.
Fig. 6. Vertical sections through a cytoplasmic process of an iridophore. The
reflecting platelets (RP) are numerous in the dendritic process of the iridophore.
Moreover, this cell has a nucleus with deep invagination. N, nucleus.
Bar = 1 mm.
O. Zarnescu / Micron 38 (2007) 81–8484
Skin of the paddlefish is dark on the back and sides and
white or cream on the throat, belly and inner side of the
operculum. This pattern of skin coloration is reflected by the
chromatophores described at the ultrastructural level, mela-
nophores and iridophores.
The melanophores are the most widely distributed group of
chromatophores which absorb light rays of a wide range of
wavelengths. They are mostly found in the dermis of the fish
skin, but especially in the larger species they may also occur in
the epidermis. In a few species of fishes melanophores appear in
the epidermis, generally at some later stage of development
(Meyer-Rochow, 2001).
Polyodon spathula iridophores are responsible for white
appearance of the skin from belly, throat and inner side of the
operculum. In terms of motility, two different populations –
static and motile iridophores – can be recognized in fishes
(Meyer-Rochow, 2001). Static iridophores were described in
some teleost species such as Atlantic salmon, Salmo salar
(Harris and Hunt, 1973), sand eel, Ammodytes personatus
(Kamishima, 1978), and blue wrasse, Achoerodus viridis
(Kawaguti, 1965). Motile iridophores were founded in neon
tetra, Paracheirodon innesi (Lythgoe and Shand, 1982), blue
damselfish, Chrysiptera cygnea (Oshima et al., 1985) and
freshwater goby, Odontobatis obscura (Matsuno and Iga,
1989). Further analysis needs to be done to understand motility
of iridophores in paddlefish. Investigations of the cytoskeletal
filaments would provide clues to elucidate physiological
characteristics of Acipenseriformes chromatophores.
Acknowledgements
The author thanks Dr. Lotus Mester, Professor of Vertebrate
Zoology, for her encouragement to start studies on the
paddlefish and Dan Vizitiu for kindly providing the fish.
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