Anatomy and cell morphology. The rabbit collecting tubule originates near the surface of the kidney, descends through the cortex within the medullary rays and traverses the outer medulla as an unbranched structure. The cortical collecting tubule (CCT) receives the contents of an average of 6 nephrons [1]. The fusion of these nephrons generally occurs proximal to the origins of the collecting tubule. Thus, the collecting tubule is unbranched from the superficial portion of the cortex until it reaches the inner medulla where the collecting tubules begin to fuse to form larger and larger ducts eventually exiting through the papillary tip. Figure 1 schematically represents the collecting tubule as it courses through the cortex and medulla. The fact that the cortical and outer medullary collecting tubule (OMCT) in the rabbit are unbranched facilitates their examination using the technique of in vitro perfusion of isolated nephron segments.