Die Vaskularisation der Kiemen von Myxine glutinosa L. (Cyclostomata)

The relationship between the secondary vascular system and the lymphatic vascular system in fish

New technologies have resulted in a better understanding of blood and lymphatic vascular heterogeneity at the cellular and molecular levels. However, we still need to learn more about the heterogeneity of the cardiovascular and lymphatic systems among different species at the anatomical and functional levels. Even the deceptively simple question of the functions of fish lymphatic vessels has yet to be conclusively answered. The most common interpretation assumes a similar dual setup of the vasculature in zebrafish and mammals: a cardiovascular circulatory system, and a lymphatic vascular system (LVS), in which the unidirectional flow is derived from surplus interstitial fluid and returned into the cardiovascular system. A competing interpretation questions the identity of the lymphatic vessels in fish as at least some of them receive their flow from arteries via specialised anastomoses, neither requiring an interstitial source for the lymphatic flow nor stipulating unidirectionality. In this alternative view, the 'fish lymphatics' are a specialised subcompartment of the cardiovascular system, called the secondary vascular system (SVS). Many of the contradictions found in the literature appear to stem from the fact that the SVS develops in part or completely from an embryonic LVS by transdifferentiation. Future research needs to establish the extent of embryonic transdifferentiation of lymphatics into SVS blood vessels. Similarly, more insight is needed into the molecular regulation of vascular development in fish. Most fish possess more than the five vascular endothelial growth factor (VEGF) genes and three VEGF receptor genes that we know from mice or humans, and the relative tolerance of fish to whole-genome and gene duplications could underlie the evolutionary diversification of the vasculature. This review discusses the key elements of the fish lymphatics versus the SVS and attempts to draw a picture coherent with the existing data, including phylogenetic knowledge.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

The branchial circulation and the gill epithelia in the Atlantic hagfish, Myxine glutinosa L

The vessels of the branchial circulation of the Atlantic hagfish, Myxine glutinosa, and their relationship with the gill epithelia have been studied by light and electron microscopy. The inner surface of the pouch wall (containing the interconnected radial arteries) and the afferent and efferent unbranched portions (cavernous tissues) of the radially oriented gill folds are covered by a multilayered epithelium. The lamellar portion that is characterized by pillar cells is lined by a thin bilayered epithelium. The thin-walled sinusoid system is part of an arterio-venous circulation. This is demonstrated by the presence of arterio-venous anastomoses and by the connection to the peribranchial sinus. The sinusoid system has a close spatial relationship to the multilayered epithelium. The multilayered epithelium consists of pavement cells, cells of the medial layer and basal cells. Granulated cells are often found in the basal half of the epithelium. The pavement cells are characterized by large vesicles in close apposition to the apical plasma membrane. Ionocytes, which display a cytoplasmic tubular system that is continuous with the intercellular space, a high number of mitochondria, and small apical vesicles, are present. The occurence of the ionocytes in the afferent multilayered epithelium as well as the bilayered lamellar epithelium, the morphology of the ionocyte, and the absence of accessory cells is reminiscent of the freshwater teleost gill, and in part the elasmobranch gill, and is discussed in relation to osmo- and ion regulation.

Gill ultrastructure of the Pacific hagfish Eptatretus stouti

At the gross anatomical level, hagfish gills show unusual features not seen in any other fish gills. Our study was undertaken to determine if peculiarities also characterize the microscopic anatomy and ultrastructure of hagfish gills. To the contrary, branchial respiratory lamellae of Pacific hagfish were found to resemble the lamellae of lampreys, elasmobranchs, and teleosts, often down to the finest subcellular details. As in other fish, hagfish lamellae are lined by epithelium containing pavement cells with organelles indicative of a secretory function, basal cells showing undifferentiated cell features, and branchial ionocytes. The ionocytes are identical to chloride cells of teleosts in cytostructure, distribution, and abundance. There are pillar and marginal capillaries in hagfish gill lamellae. Pillar cells contain bundles of 5-nm microfilaments, and they associate with collagen columns as in other fish. Hagfish pillar cells do exhibit odd features, however: They cluster (groups of up to nine were seen), and their extracellular collagen columns are rarer than in other fish gills (averaging only two columns per three pillar cells). Other special features of hagfish gills are the following: lipid droplets and smooth endoplasmic reticulum are well developed in all cell types; pavement cells secrete a lipomucous product (stains with periodic acid-Schiff, Alcian blue, and Sudan black B); and goblet cells are absent. The presence of "chloride cells" in hagfish is puzzling, as hagfish body fluids are iso-osmotic to seawater and there is no need to osmoregulate for sodium chloride; the ionocytes contain carbonic anhydrase, suggesting a function in acid/base regulation.

Assemblies of linear arrays of particles in the apical plasma membrane of mitochondria-rich cells in the gill epithelium of the Atlantic hagfish (Myxine glutinosa)

In the gill epithelium of the Atlantic hagfish Myxine glutinosa a mitochondria-rich cell type is described which ultrastructurally resembles the chloride cells in other fish species. Freeze-fracture replicas reveal different supramolecular structures in the luminal plasma membrane and in the intracellular amplification (tubular system) of the basolateral plasma membrane of these mitochondria-rich cells. In the luminal plasma membrane ordered assemblies of particles and fibril-like elements are regularly found. On the P face, the assemblies are composed of up to 20 linear arrays of particles which are preferentially located in the microvillar membrane on which they show a helicoidal orientation. The arrays are formed by globular (diameter 8-9 nm) and rod-shaped (length 16-20 nm) particles, which occasionally are so tightly end-to-end attached that they generate fibril-like structures. The distances between adjacent arrays within an assembly measure 10-15 nm. On the E face complementary patterns of linear grooves are present. These assemblies have not previously been demonstrated in the freeze-fractured plasma membrane of branchial chloride cells. In the membrane of the tubular system, emerging from the basolateral plasma membrane of the mitochondria-rich cells, infrequently repeating rows of 7-8 nm particles are present in addition to randomly distributed globular particles (diameter 7-8 nm). Complementary patterns of grooves are present on the E face. These intramembranous structures resemble the repetitive elements which have been previously described in the tubular system of branchial chloride cells in euryhaline teleosts and the subunits of which are considered to be the sites of ion pumps.