Freeze-fracture of adrenal chromaffin granules

Freeze-fracture replicas of bovine adrenal chromaffin granules show convex and concave fracture faces. The concave fracture faces are studded with 9 nm diameter particles (on average 950/μm 2 ) while the convex fracture faces are almost devoid of particles (approx 150/μm 2 ). The asymmetry in particle distribution for the membranes of these intracellular organelles is consistent with that observed for plasma membranes. The significance of these results is discussed with reference to the exocytotic release of catecholamines from the adrenal medulla.