Introduction
The aquaculture industries with high densities of fishes are susceptible to diseases caused by many pathogens. Fish possess numerous distinct and complex defense mechanisms to protect themselves from these pathogenic infections amongst which fish skin mucus acts as the first line of physical defense against pathogens (Wang et al., 2011).
The fish skin mucus provides a stable physical or chemical barrier against the invading pathogens. In fish, the epithelial surfaces are covered by a slimy, slippery layer called the mucus.
invade 침입하다
It can also be defined as a viscous colloid containing the antibacterial enzymes, proteins and water, etc known as mucins. It serves as an important component of innate immune mechanism in two ways.
Firstly, by producing continuously and being sloughed off regularly, it prevents the adherence of pathogens and stable colonization of potential infectious microbes and invasion of parasites (Arasu et al., 2013).
slough off ~을 버리다, 벗어나다
adherence 부착
Secondly, it contains a number of factors of innate immunity like proteins and enzymes such as lysozyme, immunoglobulin, complement proteins, lectins, C-reactive protein (CRP), proteolytic enzymes, transferring, alkaline phosphatise (ALP) and various other antibacterial proteins and peptides, etc (Arockiaraj et al., 2012, 2014; Arasu et al., 2013).
The aquatic environment is rich in pathogenic organisms and the aquatic animals including fish are obliviously prone to the invasion of these pathogens (Dash et al., 2008).
Therefore, the skin mucus in fish plays a significant role as it provides the first line of defence and is continuous with the linings of all body openings covering the fins also. The mucus has a wide range of functions including disease resistance, protection, as well as respiration, ionic and osmotic regulation, reproduction, excretion, communication, feeding and nest building (Ingram, 1980).
lining 내벽
respiration 호흡
osmotic regulation 삼투조절
reproduction 재생
excretion 배설
communication 의사소통
nest building 둥지만들기 행동
This review focuses on the immune mechanism of skin mucosa and their different components along with their potential role in innate immunity. Further, the antimicrobial role of the skin mucus has been discussed to provide a better understanding on the antimicrobial properties of skin mucus which could be useful in development of antimicrobial agents for therapeutic applications.
mucosa 점막
therapeutic 치료상의
application 적용
Components of fish epidermal mucus
The skin mucosa of fish is an essential barrier and serves as a protection against the surrounding environment with biotic and abiotic factors. The mucosa consists of a cellular and a humoral part. The cellular part consists of the mucous membrane and its underlying connective tissue and humoral part consists of the extracellular molecules present in the skin mucus (Salinas et al., 2011).
humoral 체액의
underly 아래로
connective 연결하는
The skin mucosa of fish has different components such as proteins, carbohydrates, lipids, metabolites (Zaccone et al., 2001). Many important proteins and enzymes such as proteases, antimicrobial peptides (AMP), lectins, lysozyme, immunoglobulin, complement proteins, CRP, transferrins, ALP and various other antibacterial proteins and peptides have been characterized in fish mucus which plays a significant role in innate immunity of fishes (Shoemaker et al., 2005; Swain et al., 2007).
protease 단백질 분해효소
The mucins present in mucus are high molecular weight, glycoproteins that impart viscoelastic and rheological properties to the mucus. Neutral glycoproteins may be found in the fish mucus but are often made acidic by sialic acid (a carboxylated monosaccharide) or sulfated monosaccharides. Mucins typically possess repetitive regions rich in threonine, serine and proline (Rose and Voynow, 2006).
viscoelastic 점성과 탄성을 지닌
high molecular weight 고분자량
impart 전하다, 주다
rheological property 리올로지 특성
The fish mucus also contains few carbohydrate components. Although their functions are not well defined, some protective roles have been suggested (Esteban, 2012). The skin mucus of fish is reported to contain different saturated fatty acids (SFA), monosaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA).
The SFA in the mucus are palmitic acid and stearic acids. Monosaturated fatty acid is the oleic acid. Polyunsaturated fatty acids are linoleic, alpha linolenic and moroctic acid. These fatty acids are believed to play an important role of defense against pathogens (Balasubramanian and Gunasekaran, 2015).
The fish skin mucus also reported to contain few metabolites having antibacterial properties such as azelaic acid, N-acetylneuraminic acid and Nacetylglucosamine, and hydroxyisocaproic acid (Ekman et al., 2015). Mucus composition varies amongst different fish species.
metabolite 대사산물
properties (물질의) 성질
The mucus cells and the composition of the mucus are influenced by various endogenous (e.g. sex, developmental stage) and exogenous factors (e.g. stress, hyperosmolarity, pH and infections) (Ellis, 2001). In some occasions, especially when fish specimens are frightened or injured a high amount of proteins are present on mucus. The epidermis of such fish secretes a gel-like material which adheres to the skin even when they swim at varying speeds and for several days (Esteban, 2012).
There are three mucus secreting cells in the epidermis of fish namely goblet cells, sacciform cells and club cells. Mucus secreting globlet cells are abundant on all fish epidermal surfaces and particularly on gill surfaces. These cells produce mucus granules which burst to release their contents. Goblet cells contain sialylated, sulfated or neutral glycoproteins (Shephard, 1993).
Other secretory cells have also been identified whose secretions mix with the goblet cell secretions to give rise to mucus. These include sacciform cells and acidophilic granular cells (orserous goblet cells) wherein the latter produce basic proteins rather than glycoproteins (Zaccone et al., 2001). Sacciform cells may be analogous to granular glands of amphibians which produce crinotoxic and repellent substances but secretions of these cells also have a protective and regulatory role. The club cell secretes have larger proteinaceous and lesser carbohydrate components (Fasulo et al., 1993; Zaccone et al., 2001).