Th e aquatic environment poses less restrictions on mechanical and barrier performances in the integument of both marine and freshwater vertebrates, mainly fi sh and some amphibians, in comparison to the skin of terrestrial vertebrates (Whitear 1977; Zaccone et al. 2001; Alibardi 2006). As a result, apart from specifi c locations of the body in a few species of fi shes and amphibians, the epidermis of aquatic vertebrates resembles the relatively poorly keratinised multi-layered epithelia found in the mucoses lining the respiratory or alimentary canals of terrestrial vertebrates (Whitear 1986a,b). In particular a corneal layer is missing over the general epidermis of fi shes and perennibranchiate amphibians. Th is includes also sarcopterygian fi sh, both crossopterygians (Latimeria, saltwater) and dipnoans (freshwater). Th is chapter is mainly concerned with the skin, especially the epidermis of the Australian lungfi sh, Neoceratodus forsteri, which is compared where studies are available, with the epidermis of other species of dipnoans (Kitzan and Sweeny 1968; Imaki and Chavin 1975a,b, 1984). In addition to a previously published study (Alibardi and Joss 2003), N. forsteri skin derived from two extra larval stages are described and the keratins have been partially characterised. Keywords: lungfi sh, skin, epidermis, ultrastructure, keratins
In the skin of a 1-2 months old hatchling lungfi sh, the epidermis comprises a fl at periderm covering a fl at to cuboidal layer of basal epidermis. Th e dermis beneath is composed of loose mesenchyme which is richly vascularized. At more advanced larval stages (e.g. 2-6 months old), the dermis is still composed of loose connective tissue (Fig. 1 A) and the epidermis has 2 or 3 layers of cells (Fig. 1 A). Th e number of epidermal layers increases to 4-5 in juveniles about 2 years old and 16 cm in length, and 5-7 layers in older individuals with lengths from 21 to 26 cm (Alibardi and Joss 2003; see Figs. 1, 2). Th e skin of early larvae is diff erentiated into an intermediate epidermal layer above the germinal layer, external layers formed by polygonal cells and a cuticle (Fig. 1 A). Th e latter is resolved under the electron microscope as the apical part of the cytoplasm of keratinocytes where most of the mucus granules are localized and discharge their content on the epidermal surface to form the cuticle (Fig. 1 C). Two main cell types are present, cells accumulating keratin fi laments (keratinocytes) and mucous cells with diff ering degrees of granule accumulation. Th e apical cells contain keratin fi laments that generally do not form dense tonofi laments as they do in amphibian and amniote keratinocytes. Th e microvilli that project from the outer surface of the apical cells are probably sections of micro-ridges (of unknown three-dimensional pattern) similar to the epidermis in actinopterigian fi sh (Bereither-Hahn et al. 1979; Whitear, 1986a). Hatchlings have a loose dermis with the uppermost layer in contact with the epidermis being formed from a layer of dermal melanosomes (Fig. 1 A). Sparse melanocytes are present in the epidermis and a few intracellular melanosomes can be seen in some keratinocytes. In some areas of the trunk epidermis in a later larval stage N. forsteri (about 2-4 months post-hatching) epidermal cells are organized around an empty space that may be a big mucous cell remnant or a true space where mucus is secreted to reach the surface (Fig. 1 B). Th is collection of cells around a lumen is clearer in some areas than in others, e.g. right on Fig. 1 B, where it appears that an epidermal cell has evaginated into a glandular-like or sensory structure. Th ese apparently multicellular, alveolar glands, resemble the forming mucous glands of metamorphosing amphibians (Delfi no et al. 1982, 1985). Th ese ‘glands’ were only observed in larval epidermis and not in adult skin. No studies to date have found glands in adult epidermis, probably due to the very heavy scales formed in the upper dermal layers and therefore it is likely that any amphibian-like glands in larval epidermis are lost at an early stage of development. In the 5 months old larva, the epidermis is 20-40 m thick and rich in mucous cells at diff erent stages of secretion (Fig. 2 A, B) or type of mucus. Some mucous cells are even seen extruded into the cuticular space, probably as degenerating
cells (see also ultrastructure). In both the tail and trunk there are some glandularlike organs, which in general appearance resemble a taste bud. In fact, their pale cytoplasm indicates that no secretory material is present (Fig. 2 C). As a consequence these ampullar-organs are interpreted as sensory organs in this specimen rather than glands as in the younger specimen (Fig. 2 C). Th eir true nature remains to be fully clarifi ed. Finally, the dermis shows a thin, denser layer and a deeper, loose layer. Th e dermal bone (scales) in a 5-month larva are already deposited beneath the thin dense layer, and form a series of pointed spines (Fig. 2 D).