Abstract：
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.
摘要：
新技术使人们在细胞和分子水平上更好地了解了血液和淋巴管的异质性。然而,我们仍然需要更多地了解不同物种在解剖和功能水平上的心血管和淋巴系统的异质性。即使是关于鱼淋巴管功能的看似简单的问题，也尚未得到最终的回答。最常见的解释假设斑马鱼和哺乳动物的脉管系统具有类似的双重设置：心血管循环系统，和淋巴管系统（LVS），其中单向流来自多余的间质液并返回到心血管系统。一种相互竞争的解释质疑鱼中淋巴管的身份，因为至少其中一些通过专门的吻合从动脉接受血流，既不需要淋巴流的间质来源，也不规定单向性。在这个替代视图中，“鱼淋巴管”是心血管系统的一个专门的亚室，称为次级血管系统（SVS）。文献中发现的许多矛盾似乎源于SVS通过转分化部分或完全从胚胎LVS发育的事实。未来的研究需要确定淋巴管胚胎转分化为SVS血管的程度。同样，需要对鱼类血管发育的分子调控有更多的了解。大多数鱼拥有超过五个血管内皮生长因子（VEGF）基因和三个VEGF受体基因，我们从小鼠或人类知道，鱼类对全基因组和基因重复的相对耐受性可能是脉管系统进化多样化的基础。这篇综述讨论了鱼类淋巴管与SVS的关键要素，并试图绘制与现有数据一致的图片，包括系统发育知识。
