Differential occurrence of immune cells in the primary and secondary vascular systems in rainbow trout, Oncorhynchus mykiss (Walbaum).

The vascular system in fish is divided into two components, the blood circulation and a secondary circulatory system, which is a vessel system referred to by earlier authors as ‘lymphatics’. Also, Wardle (1971) used the latter term for his description of the secondary system. However, true lymphatics are defined solely anatomically (and not functionally) as a system of vessels without direct vascular connection to the arterial part of the blood circulation. Based on newer studies pointing to deviations from this picture in fish, the term secondary vascular system (SVS) was proposed for these special vessels, and accordingly, the normal blood circulation system was termed the primary vascular system (PVS) (Vogel 1985a, b). The SVS was described in some detail in rayfinned fishes (Actinopterygii) by Vogel & Claeviez (1981), Vogel (1985a,b), Steffensen & Vogel (1986) and Steffensen & Lomholt (1992) confirming that this system described thus far lacked the characteristic anatomy of lymphatics normally seen in other vertebrates, especially mammals. The latter authors showed that the SVS was in communication with the systemic arteries via a large number of anastomoses of capillary dimensions. The SVS forms capillary beds situated in the outer surfaces of the fish, such as the skin, gills, mouth and pharynx (Vogel 1985a,b), but appears to be absent in regions of the mesenteric and renal tissues, where the lymphatic system is normally found in mammals. In addition, red blood cells may, under certain circumstances, be present in the SVS, which was also shown by Wardle (1971) when describing lymphatic-like vessel systems in the plaice. Although this author interpreted this as an artefact (due to puncturing of vessels or abnormal blood pressure), the latter study does comply with the notion of the existence of a communication between PVS and SVS. These clear deviations from the structure in higher vertebrates advocate for a reassessment of whether the SVS has functions related to lymphatic functions in higher vertebrates – including immune-related elements – or not (Vogel 1985a,b; Steffensen & Lomholt 1992; Skov & Bennett 2004). As a first elucidation of this question, we have investigated whether the composition of immunologically central elements (various leucocytes) in SVS of rainbow trout, Oncorhynchus mykiss (Walbaum), differs from that of PVS. Rainbow trout (total body weight 300–400 g), were collected from the fish farm ‘Hvilested Dambrug’, Kolding, Denmark, and brought to the laboratory where they were kept in 1500-L tanks and provided with continuously aerated recirculated fresh water (10 C). Fish were acclimatized to laboratory conditions for 3 weeks, and remained unfed for at least 1 week before sampling to minimize physiological changes induced by differences in feed uptake. A total of five fish Correspondence K Buchmann, Section of Biomedicine, Faculty of Health and Medical Sciences, University of Copenhagen, Stigbøjlen 7, DK-1870 Frederiksberg C, Denmark (e-mail: [email protected])