The cells lining the proximal convoluted tubule have numerous microvilli and mitochondria. these structures are critical for the function of these cells, which is to

Microvilli which line down the luminal surface of those epithelial cells represent the resorptive function of the proximal tubule.
The high quantity of mitochondria inside the epithelial cells of the proximal convoluted tubule is needed in order to supply the energy for the active transport of sodium ions (to enter the cell from the luminal side).

Microvilli is the answer

The answer would be sand and gravel