Acetylcholinesterase (AchE), an essentially specific and early marker of rodent thrombocytic series, in several tissues acts both as a constituent of the cellular membrane and as a secretory enzyme. This study presents the ultrastructural transport and fate of this substance in the megakaryocytes of mouse bone marrow, using Tranum-Jensen and Behnke's adaptation of the indirect thiocholine method. It is shown that megakaryoblasts and megakaryocytes undergo a typical secretory cycle consisting of (1) enzyme synthesis and segregation on the endoplasmic reticulum and nuclear envelope, (2) enzyme concentration in AchE-vesicles and AchE-granules formed from the cisternae of the Golgi apparatus, and (3) discharge in the demarcation membrane system and extracellular space. The AchE-vesicles and granules appear to be hitherto unrecognized megakaryocytic organelles as they do not resemble alpha nor the dense granules, and their mode of formation and fate differ from those of primary lysosomes and peroxidase granules. Released platelets reveal AchE activity in the open canalicular system. The data are compatible with the hypothesis that by controlling acetylcholine concentration in hematopoietic tissues, the secretion of AchE by megakaryocytes can modulate the proliferative activity of megakaryocytes progenitors.

The biosynthesis of acetylcholinesterase in mammalian erythroid cells during differentiation and maturation was studied using a cytochemical method. Acetylcholinesterase was actively synthesized in basophilic erythroblasts I and II, and polychromatophilic erythroblasts, where it was present in the nuclear membrane, endoplasmic reticulum and Golgi apparatus. In orthochromatophilic erythroblasts the enzyme was present only in the Golgi apparatus, thus suggesting the end of the biosynthesis of acetylcholinesterase at this stage of maturation. In mouse and human marrow, evidence was also found suggesting the secretion of acetylcholinesterase into the extracellular compartment. No acetylcholinesterase reaction product was found in cat erythroblasts.