Vascular anatomy of the fish gill.

The fish gill is the most physiologically diversified vertebrate organ, and its vasculature the most intricate. Application of vascular corrosion techniques that couple high-fidelity resins, such as methyl methacrylate, with scanning electron microscopy yields three-dimensional replicas of the microcirculation that have fostered a better appreciate gill perfusion pathways. This is the focus of the present review. Three vascular networks can be identified within the gill filament. The arterioarterial (respiratory) pathway consists of the lamellae and afferent and efferent segments of the branchial and filamental arteries and lamellar arterioles. The body of the filament contains two post-lamellar pathways: the interlamellar and nutrient. The interlamellar system is an extensive ladder-like network of thin-walled, highly distensible vessels that traverses the filament between, and parallel to, the lamellae and continues around the afferent and efferent borders of the filament. Interlamellar vessels are supplied by short, narrow-bore feeder vessels from the medial wall of the efferent filamental artery. A myriad of narrow-bore, tortuous arterioles arise from the basal efferent filamental artery and efferent branchial artery and anastomose to form the nutrient circulation of the arch and filament. In the filament body, nutrient capillaries and interlamellar vessels are often closely associated, and the former may ultimately drain into the latter. Many of the anatomical characteristics of interlamellar vessels are strikingly similar to those of mammalian lymphatic capillaries, with the exception that interlamellar vessels are directly fed by arteriovenous-like anastomoses. It is likely that gill interlamellar and mammalian lymphatics are physiologically, if not embryologically, equivalent.