This newly proposed model, diagrammed in Fig. 10, is supported by previous findings of tight junctions on arterial capillaries and a loose association between venous capillaries, as well as by a differential strength in light microscopy staining of actin filaments located on the cytoplasmic side of cell�Ccell adhesions. There is some physiological evidence suggesting the impermeability of the arterial capillaries in the rete mirabile. Wagner et al. showed in a perfusion experiment that tannic acid did not permeate arterial capillary walls but did permeate the venous capillaries of the eel rete mirabile. Analyses of capillary permeability of isolated eel rete mirabile using radiolabeled water, urea, glucose, insulin, and albumin demonstrated that water is exchanged more rapidly than would be expected, in terms of its molecular size. These results suggest that a significant barrier is present in arterial capillaries, and carriers mediate solute-specific transcellular permeation by the arterial capillaries. The rete mirabile has been a major focus of Columbianadin countercurrent system research and will become a useful model for further study of the O2 transport heterogeneity of the capillary endothelium. A similar system operates in the eyes of certain fish to support the metabolic needs of the retinal cells. High partial pressure of oxygen is maintained there by the Root effect and the countercurrent capillary network called the choroid rete mirabile, which has been known to exist in the fish for almost two centuries and to be evolved independently in several groups of teleosts. The present study may also shed light on the mechanism how the choroid rete acts to maintain a relatively high pressure of oxygen. The a/b hydrolase fold was Cetylpyridinium chloride monohydrate identified in 1992, by comparing five hydrolytic enzymes with widely different catalytic function. Since then, more than 50 members belonging to this family have been identified and characterized by structure determination. The a/b hydrolase fold involves a variety of enzymes including esterases, lipases, epoxide hydrolases, dehalogenases, proteases, and peroxidases, making it one of the most versatile protein families known. The conversed feature of the a/b hydrolase fold has been described as a mostly parallel, eight-stranded b sheet surrounded on both sides by a helices. The Rv0045c gene encodes a polypeptide chain of 298 amino acids with a putative hydrolase activity.