Freeze-fracture differences between plasma membranes of descending and ascending branches of the rat Henle's thin loop.

Freeze-fracture of Henle's thin loop of the rat shows that the plasma membrane of epithelial cells in the descending branch has far more particles (approximately 4000 per sq. mum.) than the plasma membrane in the ascending branch (approximately 1000 per sq. mum). On the contrary, the tight junctions between epithelial cells in the two branches are equally developed. The large number of particles in the descending branch may be an important factor in the water permeability of this tubular segment.