Cutaneous Lesions in Cetaceans: An Indicator of Ecosystem Status?
New Approaches to the Study of Marine Mammals
New Approaches to the Study of Marine Mammals 124 skin of cetaceans, the occurrence of skin lesions among these mammals, the microbes that seem to be the causative agents, as well as contributing factors such as anthropogenic activities. Cetacean skin The Barrier: First line of defence The first study on the anatomy of whale skin was conducted early in the 20 th century by Japha  . He noted the unusual thickness of the epidermis when compared to other mammals (15 to 20 times thicker as in
... es thicker as in humans). Later, Parry  distinguished between the layers of epidermis, dermis and hypodermis. He differentiated between the epidermal layers and recognised the stratum corneum and the stratum germinativum (Figure 1) , consisting of the more superficial prickle cells and the deep cylindrical cells. The hypodermis was found to be thick and fatty, merging into the dermis that consists of white fibres. The dermis stretches into the epidermis by means of 'dermal papillary ridges', and therefore interdigitate with the epidermal papillae ( Figure 1 )  . Parry also studied the vascular system and conductivity of blubber, which affect temperature regulation in these mammals. A few decades later, Sokolov  summarized cetacean skin anatomy by stating that the skin of these animals is relatively smooth, and with a ratio of 0.3 -1.5 % in relation to body length, may reach greater 'absolute thickness' compared to other mammals. He noted that although the stratum corneum is very thick in cetaceans, layers of strata granulosum and lucidum are absent, with the upper layers of the epidermis not fully cornified. Sokolov concluded that sebaceous and sweat glands, as well as pelage, are absent in the skin of cetaceans. Using bottlenose dolphins (Tursiops truncatus) as model, it was found that the epidermal layer of cetaceans has a large capacity for cell population and a long turn over time, accompanied by rapid sloughing  . These characteristics account for the unusual thickness, as well as the smooth surface of the skin, thereby enhancing the barrier properties and ability to limit attachment by microbes [6, 7] . The stratum corneum of cetaceans ( Figure 1 ) is often referred to as the parakeratotic layer and is composed of moderately flattened cells, characterized by retained elongated nuclei and prominent organelles (including mitochondria), representing a form of parakeratosis [7, 8] . The latter process was attributed to a type of cornification, associated with evolutionary hair follicle loss. The phospholipid-rich cornified layer presumably also aids in waterproofing the skin of these mammals  . In addition to these general features of cetacean skin, several unique ultra-structural characteristics were reported for the stratum corneum of the southern right whale (Eubalaena australis)  . These include lipid droplets occurring in close association with the nucleus, as well as an abundance of intra-nuclear inclusions similar to small fragments of cytoplasmic keratin. Keratins are scleroproteins responsible for mechanical support in epithelial cells  . These macromolecules are mechanically hard, chemically unreactive, insoluble, fibrous, and very tough as a result of the numerous disulfide cross linkages  . In terrestrial mammals, keratins are produced by the so-called keratinocytes, found in the stratum basale or