Research advances in the structure, function, and regulation of the gill barrier in teleost fish(진행중)
1. Introduction
The fish gill is an interesting tissue due to it being a highly characteristic feature of fishes and multifunctional in nature. The gills are important respiratory organs in fish. In addition to the main function of respiratory gas exchange, the gill also functions in ion and water transfer, filter-feeding, ammonia nitrogen excretion, and osmoregulation (Evans and Nunez, 2015; Koppang et al., 2015).
To realize these complex physiological functions, gills have evolved complex morphological structures. These morphological structures include the gill arch, gill petals, gill filaments, gill operculum, gill flakes, and gill rakes, and these structures change accordingly with alterations in water flow, temperature, ion concentration, and salinity in the aquatic environment (Rita and Abhik, 2014).
The relationships among the structural characteristics, multi-physiological functions, and the adaptability to environmental changes of gills have attracted extensive attention (Chen et al., 2012). Due to the diversity of fish species and the complexity of their living environment, research on the structure and functions of gills is still a long-term, complex, and arduous task. The structure of gills is very sensitive to environmental changes (Evans and Nunez, 2015). Exploring the alterations and corresponding physiological and functional adjustments of gill structures resulting from environmental changes has become an important field of research (Papi et al., 2016). This article introduces the basic structure and corresponding functions of the gills of teleosts. In addition, the effects of feed and environmental factors on gill health and the regulation channels of gill health are also reviewed.
2. Basic structure of gills
2.1. Morphological structure of gills
The teleost fish has four pairs of holobranchs located on both sides of the skull arranged symmetrically in a comb shape, and each holobranch is arched (Ou et al., 2013). Each holobranch is composed of a gill arch and two gill petals. The gill arch is the main skeleton of the gills, and its cross section is approximately semicircular. The gill petals are arranged by many parallel gill filaments. One end of the gill filaments is attached to the convex surface of the gill arch, and the other end is free. A row of lamellar saccular gill flakes stretches out on both sides of each gill filament. The lamellae are evenly distributed, and water can flow in the interspaces of the lamellae. Serrated gill rakes are attached to the concave surface of the gill arch. Each gill rake is triangular cone-shaped, and the spacing between adjacent gill rakes is from 0.15 to 0.18 mm (Zhao et al., 2018b). The gill arch is located between the gill filament and the gill rake; the bone in the gill arch is shaped in a circular arc; the gill filament is a long strip with the end an expanded blind sac, and the spacing between adjacent gill filaments is about 0.10–0.13 mm (Zhao et al., 2018a). The gill operculum protects the gill cavity externally. Chondrichthyes are generally divided into fish with plate gills and the Holocephali. The gill plate class has plate-like gills with gill slits on both sides of the head and no gill operculum. The Holocephali have membranous gills. These marine species are less numerous than Chondrichthyes with plate gills (Wegner, 2015).
2.2. Fish gill ultrastructure
The surface of gill filaments is covered by a large number of pavement cells, chloride cells, and mucous cells. Pavement cells account for 90%, while chloride cells and mucous cells together account for only 10% (Xiang et al., 2018). Holes with a diameter ranging from 1.03 to 2.15 μm appear on the surface of the gill filament, gill rake, and gill arch (Ou and He, 2012). There are concave pits between adjacent pavement cells on the surface of the gill. The pit on the surface of the gill rake is deeper than those of the gill filament and gill arch. The striated morphology of the flat epithelium on the surface of the gill filaments has three main types: circular micro-ridged, star point rod-shaped, and microvilli (Xiang et al., 2018). The surface of the gill lamella is covered by a large number of pavement cells, and there are visible microvilli on the surface of the pavement cells that have large amounts of mucus attached to the surface of the gill lamella (Zhan et al., 2016). The surface of the gill arch is relatively smooth, covered by a large number of star point rod-shaped pavement cells. The surface of the gill rakes is largely covered by two different types of pavement cells: circular micro-ridged and star point rod-shaped, both of which exhibit a fingerprint appearance (Abumandour, 2019). The gill epithelium is without many mitochondria but contains numerous cytoplasmic vesicles or a distinct Golgi apparatus (Lucu, 2021).
2.3. Circulatory system
When the ventricle contracts, the blood is pressed into the abdominal artery, through the gill artery, the gill filament artery, the gill small artery, and finally into the capillaries of the gill for gas exchange. When the water flows through the mouth and gill filament layer, the dissolved oxygen in the water permeates into the capillaries of the gill filaments, while the carbon dioxide in the blood vessels diffuses outward and is discharged into the water. Blood moves through the afferent filament arteries of the primary lamellae into the blood spaces of the secondary lamellae, where CO2 is released into the water and O2 is taken up (Ren et al., 2018). The arterial blood passes through the area of gas exchange in the gills and is rich in oxygen. Arterial blood flows through the branchial artery and finally to the dorsal artery, and is then shunted to various parts of the fish body (Ou et al., 2014).
2.4. Neural tissue architecture of the gill
The branchial nerves of gills are derived from the VIIth (facial nerve), IXth (glossopharyngeal), and Xth (vagal) cranial nerves, each of which is divided into motor and sensory branches. The branchial arches are normally innervated by the vagus nerve. The nervous system shares perfusion control in part within the gills by acting on specialized vascular territories within the gills (Mu et al., 2022).