Spatial growth and pattern formation in the small intestine microvascular bed from larval to adult Xenopus laevis: a scanning electron microscope study of microvascular corrosion casts.

The microvascular anatomy of the small intestine of metamorphosing tadpoles of the South African Clawed Toad, Xenopus laevis (Daudin) is studied from developmental stages 55 to 65 and in adults by scanning electron microscopy (SEM) of vascular corrosion casts (VCCs) and light microscopy. Up to stage 62, VCCs reveal a dense two-dimensional vascular network ensheating the intestinal tube, whose proximal portion forms a clockwise spiralling outer and its distal portion an anti-clockwise spiralling inner coil. Vessels of the intestinal network impose flat and run circularly to slightly obliquely. Locally, dense capillary plexus with small "holes" indicating ongoing intussusceptive microvascular growth (IMG) and vessel maturation, are present. The typhlosole, an invagination along the proximal portion of the small intestine, reveals a dense capillary bed with locally ongoing IMG. VCCs of stages 62/63 for the first time reveal a three-dimensional vascular bed with longitudinal intestinal folds of varying size and heights greatly enlarging the luminal exchange area of the intestinal tube. From stage 65 onwards, longitudinal intestinal folds undulate and, though smaller in size and less mature as indicated in VCCs by the presence of wider, sinus-like vessels with small "holes" interposed between, closely resemble the intestinal folds present in the small intestine of adult Xenopus. Our data suggest that maturation of the vascular pattern in the small intestine of X. laevis tadpoles takes place successively after stages 62-63, and growth during this period is preferentially by intussusception.