A morphometric study of coated pit formation on the subplasmalemmal surface of murine peritoneal macrophages after adhesion to glass.

Murine peritoneal macrophages were settled onto treated and untreated glass at 4 degrees C. The cells were warmed to 37 degrees C and at different time intervals, after being briefly in buffer containing 1 mM ZnCl2, they were subjected to a stream of buffer regulated by a peristaltic pump. By controlling the time of extraction with the buffer stream, it was possible to remove all attached cytoskeletal filaments and relatively clean preparations of the subplasmalemmal surface could be obtained. Platinum replicas of the remaining ventral subplasmalemmal surfaces were prepared and examined by transmission electron microscopy and the extent of clathrin pit formation was assessed morphometrically. It was found that clathrin-coated pits and vesicles formed on untreated or positively charged glass surfaces after only 2 min at 37 degrees C, reaching a peak in 2 hr, when approximately 5% of the available surface was occupied by these structures. The proportion of plasmalemma occupied by coated pits was higher on the ventral membranes of resident macrophages settled onto glass surfaces treated with BSA/anti-BSA (greater than 10% of the total substratum-attached membrane). There was no demonstrable difference in the extent of clathrin-coated pit formation between resident and exudate macrophages.