Ultrastructural observations on the pars descendens of the proximal tubule in the kidney of the male rat

The pars descendens (pars recta) of the proximal tubule in the male rat kidney, consisting of the terminal part of the second proximal segment (P2) and of the third proximal segment (P3), was studied with the electron microscope. A technique of tissue orientation and trimming was used which permitted precise topographic definition of the tubules studied in the electron microscope. The terminal descending part of the P2 showed some minor differences from the convoluted part of this segment, and ultrastructure also changed along the course of the P3. In the beginning of the latter segment numerous, shallow interdigitations were observed between adjacent cells; along the course of the segment they decreased in number or disappeared. In the initial part of the P3 mitochondria were more abundant than in the terminal portion of the segment and at least as numerous as in the straight part of the P2. Also, the dense, acid phosphatase-positive cytoplasmic bodies decreased somewhat in size along the course of the P3. The smooth surfaced endoplasmic reticulum reached a higher development in the P3 than anywhere else in the proximal tubules.