How a highly complex three-dimensional network of blood vessels regresses: the gill blood vascular system of tadpoles of Xenopus during metamorphosis. A SEM study on microvascular corrosion casts

The morphology and vasculature of filter plates and filter rows of tadpoles of Xenopus laevis Daudin were studied from prometamorphosis (stage 56) to metamorphic climax (stage 62) by scanning electron microscopy (SEM) of (i) critical point-dried specimens and (ii) microvascular corrosion casts. The number and frequencies of the highest branching orders of venous filter row vessels, which reach their maximum (6) in stage 58, as well as vessel diameters, interbranching distances, and branching angles were measured. Branching orders continuously decrease until stage 62, so that only two are left. Vessel (luminal) diameters increased from 16.9 +/- 0.65 microm (SEM) (stage 56) to 19.3 +/- 0.61 microm (stage 58) (branching order 0) and then decreased to 16.7 +/- 0.91 microm (stage 62). Interbranching distances were largest in stage 59 (57.5 +/- 2.07 microm (branching order 0). Branching angles were highest in stage 62 (91.4 degrees +/- 8.82 degrees; branching order 2) and lowest in stage 56 (76.0 degrees +/- 4.64 degrees; branching order 4). From the morphology of cast vessels we conclude that vascular regression is organized by progressive "fusion" of opposite vessels in the highest branching orders ("miniaturization") until only a single capillary loop connects the terminal arteriole and the main filter fold venule. With obliteration of the single capillary loop, the regression of the gill vasculature is terminated. It is noted that the "fusion" of gill vessels is reminiscent of "reversed" intussusceptive microvascular growth.