Qualitative and quantitative studies of the capillary structure in the rete mirabile of the eel, Anguilla vulgaris L.

The capillaries of the counter-current capillary organ, rete mirabile, have been studied by light microscopy and electron microscopy. The vasculature of this rete was found to have a cross-sectional area of about 5.25 mm2, and it consisted of 34,000 efferent (=arterial) and 22,000 afferent (=venous) capillaries. The total surface area was the same for the two types of capillaries. The capillary endothelial cells showed numerous pinocytotic vesicles, scattered giant vacuoles and cytoplasmic extrusions (microvilli) at the luminal surface. The majority of the intercellular junctions appeared to have patent gaps with a width of 110-120 A. The arterial capillaries appeared in many ways to be morphologically similar to skeletal muscle capillaries of mammals, whereas the fenestrated venous capillaries resembled those in the intestinal mucosa of mammals. Measurements of the ionic composition of the rete tissue indicated that the endothelial cells contained much less K+ than other cells, the rete containing approximately equimolar amounts of K+ and Na+. The functional significance of the structural and chemical observations are discussed.