EC heterogeneity. (A) Electron micrograph showing the junction between two capillary ECs in a guinea pig pancreas (micrographs reprinted with permission from R.F. Bolender, The Journal of Cell Biology, 1974, vol. 61, p. 269). (B) Electron micrograph demonstrating the diversity of ECs from two types of capillaries: (1) vesicular invaginations (arrow) on both luminal and abluminal plasma membrane of a muscle capillary EC; (2) fenestrated capillary from the lamina propria of the colon with thin diaphragms (arrow) covering the plasma membrane pores (micrographs reprinted with permission from E. Weihe, Textbook of Histology, (ed 12), 1994, p. 391, courtesy of Chapman and Hall).