Degrading and non-degrading pathways in fluid-phase (non-adsorptive) endocytosis in human blood platelets

A morphological study of the endocytic pathways in non-activated human blood platelets has been made using fibrinogen-stabilized colloidal gold as a fluid-phase marker, and acid phosphatase as a marker for lysosomes. Two pathways of constitutive endocytosis were demonstrated: one pathway was mediated by clathrin coated vesicles which formed at the platelet surface and the surface-connected, canalicular membrane system and which fuse with and deliver their contents to the platelets' secretory alpha-granules. The other pathway follows a degrading, endosomal-lysosomal route and appears to be clathrin independent. Endosomes containing marker particles originate from the surface-connected canalicular system, through which ambient fluid percolates. An account is given of the pleomorphism of platelet lysosomes, and evidence is presented for a lysosomal, focal autophagia in platelets which may be linked to platelet senescence. Lysosomes in non-activated platelets are thus involved in the same processes as lysosomes in other cells.