Ultrastructural analysis of teratoma tissue. (A) TEM on a representative Fgfr-1^+/− teratoma micrograph shows elongated, flattened endothelial cells with cytoplasmic infoldings (◀) and occasional fenestrations (∗). Endothelial basement membrane and perivascular smooth muscle cell coat was scant, but fibroblasts producing collagen (←) were identified in the perivascular area. (B) A representative Fgfr-1^−/− micrograph shows a vessel with endothelium resting on an endothelial basement membrane and surrounded by smooth muscle cells invested by basement membrane (◀). ← indicates abundant transport vesicles in protruding endothelial cell cytoplasm. (C) Higher magnification of vessel in panel B, showing increase in endothelial cell surface by basal invaginations covered by basement membrane and luminal protrusions. Tight junctions (∗) indicate the presence of 3 endothelial cells (◀) in this micrograph. Perpendicularly cut collagen is seen outside the smooth muscle cell layer. (D) Fgfr-1^−/− micrograph shows several adjacent, protruding endothelial cells (←). (E) Confocal analysis of immunofluorescent CD31 staining (red) shows similar image as in panel D with protruding endothelial cell nuclei (←) in Fgfr-1^−/− teratoma. Nuclei were visualized by Hoechst 33342 staining (blue). RBCs indicates red blood cells; L, vascular lumen; EC, endothelial cell; N, endothelial cell nucleus; FB, fibroblast; T, thrombocyte; Co, collagen; and SMC, smooth muscle cell. Original magnifications ×12 500 (A,B,D) and ×20 000 (C). Bars represent 500 nm (A-D) and 30 μm (E).