Comparative evidence on the topography, climate and chemical nature of the mammalian skin surface ecosystem is reviewed. Potential and established surface defence systems have been identified and their significance and interactions discussed in an attempt to provide a basis for further multidisciplinary study of the protective role of the skin surface, a function which is more extensive than hitherto recognised.

I would like to recall that in 1957 Selby noted in thin sections of mammalian epidermis (Selby 1957) granules smaller than the keratohyalin granules. In subsequent studies these granules, called membrane-coating granules (MCG), were shown to migrate toward the plasma membrane and discharge their lammelar content into the intercellular spaces (Matoltsy and Parakkal 1965). The lamellae which spread over the surface of granular cells have been noted to reassemble into bileaflets and to form broad sheets in the intercellular spaces of the horny layer (Matoltsy 1976) as illustrated in Fig. 1. Since histochemical studies have indicated the presence of lipids in MCG-s (Breathnach and Wyllie 1966; Hashimoto 1971), and tracer studies (Elias and Friend 1975; Squier and Hopps 1976) have revealed that ferratin, lanthanum, ruthenium red and horseradish peroxidase do not pass through the horny layer it is reasonable to postulate that lipids derived from MCG-s form an important part of the defence system of the mammalian epidermis and control the flow of hydrophilic substances across the stratum corneum.

The wide variety of metazoan and microbial species frequenting the mammalian skin surface is described. The damage which they cause to the superficial barrier mechanisms and the host factors which modify their activity are examined. Predatory and inhibitory reactions between the skin inhabitants are discussed in relation to their ability to inhibit the activities of pathogenic organisms.

The cutaneous reactions induced in the skin of human volunteers by protein A from Staphylococcus aureus have been studied. This cell wall protein induces no response when applied in solution to intact skin. In contrast a vigorous inflammatory reaction is seen if it is injected intradermally or applied to skin from which the stratum corneum has been partially removed. This study demonstrates that the intact epidermis is an important protection against this microbial product.

The effect of artificially induced climatic changes or arthropod bites on the development of experimental dermatophilosis lesions was examined. The lesions were produced following application of Dermatophilus congolensis zoospores to the skin of goats.

When zoospores were applied to skin which had been treated with methanol, the resulting lesion healed rapidly. This observation contrasts with the disease under natural conditions, where lesions become chronic. There was no increase in the time for lesions to heal if the goats were sprayed with water to simulate rainfall. However, if a delayed hypersensitivity reaction to chlorodinitrobenzene was induced at the site of zoospore application (to simulate the host's response to an arthropod bite), the resulting lesion was morphologically similar to natural lesions and failed to heal so long as applications of the chemical were repeated.

It is suggested that arthropods play a primary role in producing chronic lesions by inducing an immune reaction at the site of infection with D. congolensis.

Birds have a layer of plumage over most of the body which provides thermal insulation. The vane-shaped feathers are more efficient than fur in decreasing air movement between the skin microclimate and the ambient environment. Around the feathers is a layer of air, the depth of which for optimum insulation is determined by their angles to the skin surface maintained by the follicular musculature. The mean avian core temperature is 41°C which is higher than in mammals, but skin surface temperature is several degrees lower. Over the exposed skin the microclimate is more variable, but the skin is warmed by the dermal blood vessels and so the feet do not freeze even at sub-zero temperatures. Transpiration occurs through the skin and helps cool the body. Some water vapour is absorbed by the feather keratin, which prevents the feathers from becoming brittle in dry ambient air. Maintenance of the feather vane arrangement requires a more rigid keratin than in hairs. Birds have only one skin gland, the secretion of which contains wax and lipids, but the epidermis is also strongly lipogenic. The sebaceous secretion probably waterproofs the skin, prevents the feathers from becoming too dry, and is anti-microbial. The avian skin surface provides habitats for many commensal micro-organisms and a few bacterial, fungal and arthropod parasites. Pathogenic microorganisms can penetrate the epidermal barrier in wounds or indirectly through the bites of animal parasites. Once in the dermis micro-organisms are combated by immune responses.

Ectoparasites of poultry in the United Kingdom are reviewed. Mention is made of poultry ectoparasites occurring in other parts of the world and of some ectoparasites of British birds. In south west Scotland ectoparasite infestations appear to have decreased following the intensification of the poultry industry. Occasional outbreaks of Dermanyssus gallinae, the red mite and Ceratophyllus gallinae, the hen flea have occurred in modern poultry units. Cases of louse infestation with Menacanthus stramineus have also been observed from farmyard chickens and bantams. The damage caused by ectoparasites and their role in disease transmission is discussed.

Skin diseases of poultry caused by bacteria, fungi and viruses are reviewed. Skin conditions not induced by trauma are extremely rare in south west Scotland. Traumatic induced skin diseases still occur in modern poultry units and are associated with fighting or the use of poor materials in the structure of the house. Cases of staphylococcal wound infection and staphylococcal arthritis have been observed in poultry units where poor quality wire netting or hard spiky litter have been used as floor materials. Mention is also made of two pigeon diseases, pigeon pox and canker.

The skin surface of teleost fish dififers from that of higher vertebrates in that it is a living vital substrate at the margin between the water and the tissues. Its surface is continually sloughing into the water, to be replaced from the mucoid and formed elements of the epidermis. The surface, or cuticle, is altered by cyclical physiological changes in the animal, and also by pathological conditions. These, and the nature of the external medium, affect the range and the ecology of the biota of the surface. The secretions on the skin surface may also have a protective capacity. Fish epidermis is delicate and easily traumatized and there is great mobility of the Malpighian cells which allows rapid sealing of any breaches.

The present contribution shows the extent and the limitations of our knowledge of the structure, functions and biota of the epidermis of just one family of teleost fish, the Salmonidae. Even within this group it is clear that the epidermis can respond, with remarkable rapidity in some cases, to changes in the environment and to internal stimuli, presumably mediated via the fish's endocrine system. Against this background of constantly changing structure it is believed that the emphasis of the multiple functions of the epidermis must also be changing. Thus, future studies on the skin offish must take into account the possibility of such variations and relate the biology of the skin and its inhabitants to broader aspects of the biology of the fish.

The epidermis of the sea horse Hippocampus kuda is characterised by flame cone cells, each of which protrudes 20-40 μm above the surface and is covered by a prominent mucous cap. Unlike normal surface cells, the mucoid caps can support epiphytic microbial growth.

Histochemically the mucous cap is a neutral mucopolysaccharide-protein complex possessing 1,2 glycol groups and SH-groups; acid mucopolysaccharides are absent. The acid mucopolysaccharide glycocalyx of unmodified surface cells is absent from the mucous cap surface.

Ultrastructurally two types of vesicle can be distinguished in flame cells. Type I is oval (0·3 × 0·6 um) with contents of medium electron density and occurs principally in mature flame cells. Type II, seen only in developing cells, is spherical (0·4 μm) and contains rod-like subunits characteristic of the cap mucous. Secretion is mediated by channels formed by smooth endoplasmic reticulum. Unmodified surface cells also secrete their vesicular contents, which resemble the Type I vesicles of flame cells and are similar to the goblet mucous vesicles of other fish, through similar channels.

Flame cell mucous caps, by virtue of their histochemistry, possible provide a suitable substratum for the adhesion and growth of epiphytes which in turn may afford protection against cnidarian nematocysts.

The Reptilia and Amphibia are two distinct groups of vertebrates. However, they show a number of similarities, such as ectothermy and ecdysis, and are therefore compared and contrasted in this paper.

The physical features of the skin of reptiles and amphibians vary considerably; in the case of the amphibians the skin is usually relatively thin and may play a part in respiration while that of the reptile is often thick and impervious due to heavy keratinisation. Little is known of the ‘climate’ of the integument and there are relatively few data on its chemistry. Various pathogens may gain entry to the skin but usually this necessitates breaching the epithelium or taking advantage of existing damage.

Regular moulting can prevent colonisation of the skin surface by epibionts, although some invading organisms are sufficiently mobile to overcome this protective device. Bacteria and sessile protozoans settle on the surface of the claws of Xenopus, where the keratinised cells are not shed as regularly as on the skin. Alternatively, parasites may inhabit the orifices, which also are not affected by moulting.

The skin of arthropods is also the exoskeleton and as this gives them mechanical protection, they have comparatively few ectoparasites. Those which are present are removed during the periodic moulting of the skin. However, some organisms, particularly mites, have become adapted to living on the cuticle of arthropods.

Photographs were displayed of the mouthparts of some important arthropod parasites of British livestock as viewed with a scanning electron microscope. Ectoparasites included in the demonstration were Damalinia ovis, the body louse of sheep; Haematopinus eurysternus, the short-nosed cattle louse; Ceratophyllus gallinae, the hen flea; Melophagus ovinus, the sheep ked and Ixodes ricinus, the sheep tick. Photographs of livestock infested with these parasites were also displayed.

The biological properties of the surface of helminth parasites are examined under three headings, morphology, digestive-absorptive function and defence. The concept of the host-parasite interface is explored.

Platyhelminth parasites are covered by a syncytial tegument with sunken parenchymal cell bodies. The surface is either microvillous or deeply folded, presenting a large surface area relative to volume. The acanthocephalan surface is structurally unique, possessing a branching pore-canal system. The majority of nematodes are surrounded by a thick cuticle composed of distinct layers; exceptions include the entomophilic nematodes.

The absorptive properties of the surface of helminth parasites are discussed and the published data on transtegumental transport of nutrient molecules examined. Most nematodes are impermeable to small molecules, but all other helminths utilise surface transport mechanisms. Cestodes and acanthocephalans lack a gut and the tegument becomes the major site of nutrient uptake.

Surface-bound enzymes have been described in several helminths and indicate the possible extent of surface digestion by parasites. Binding of host digestive enzymes to the helminth surface may also serve a protective as well as nutritional function.

Surface defence mechanisms are described whereby physiological interactions between host and parasite at the helminth surface effect evasion of the host's defences. The essence of these mechanisms is surface disguise.

Sera from patients suffering from the autoimmune skin diseases pemphigus vulgaris and bullous pemphigoid were used to demonstrate the presence of intercellular substance (ICS) or bullous pemphigoid antigen (BPA) on the surface of the schistosomula of Schistosoma mansoni which had penetrated mouse and human cadaver skin, and mouse skin percutaneously. Both ICS and BPA were absent from mechanically transformed schistosomula or those formed in the peritoneal cavity of mice. Schistosomula which penetrated slowly through mouse or human skin in vitro, acquired more ICS or BPA than those which penetrated rapidly.

During percutaneous infections of mice, schistosomula recovered from skin after 2h and 24h had acquired large quantities of these materials whereas those which were recovered from skin after 10min had no detectable ICS or BPA. These materials must be shed during subsequent migration since schistosomula from lungs and liver, and 7-week old adults do not possess them.

Histological examination of both mouse and human skin revealed that the schistosomula remained in the epidermis for varying lengths of time. Schistosomula which remained there for more than 2h became vacuolated, whereas schistosomula which were present in the dermis at 2h appeared undamaged.

On cercarial penetration of human skin, the granular layer of the epidermis became disrupted or condensed. The implications of these findings are discussed.

The mechanisms of interaction are discussed in general terms; this is followed by a review of interaction which may occur in tinea paedis.

The production of penicillin-like and other antibiotics by dermatophyte fungi in lesions of athletes foot may result in the selection of an antibiotic-resistant bacterial flora which, perhaps by production of antibiotic or other secondary metabolites, can reduce the viability of the fungi. The effect on the host is to bring about a change from a fungal skin lesion which is dry and scaly to a bacterial lesion which is macerated and malodorous.

Human skin propionibacteria have been shown to produce, in vitro, a wide range of inhibitory substances including bacteriocins, bacteriolytic enzymes, bacteriophages and low molecular weight metabolites (e.g. ammonia, propionic acid). Some or all of these substances may play a role in the ecology of cutaneous populations of micro-organisms and may be involved in maintaining the balance of the normal flora. There is no evidence to suggest that they deter or exclude other transient bacteria including pathogens. Similarly, evidence in support of the in vivo production of growth inhibitors by propionibacteria with activity against other skin micro-organisms is lacking (except for bacteriophage and propionic acid) so that their potential role in controlling cutaneous populations must be interpreted with caution.