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: otilia@bio.bio.unibuc.ro.

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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