Storage of 5-Hydroxytryptamine in Megakaryocytes

The blood platelets of various species, including man, contain specific, highly osmiophilic subcellular organelles storing 5-hydroxytryptamine (5HT) . Strong evidence exists that the osmiophilia of the organelles is directly due to the presence of large concentrations of 5HT (1, 2, 3) . It has not yet been established whether the 5HT organelles are formed in the circulating blood platelets or whether the megakaryocytes are already capable of storing the amine at specific sites . In fact, except in very rare instances, the megakaryocytes, the stem cells of the platelets, do not contain typical 5HT organelles, even in species, e .g . the rabbit, whose platelets are rich in these ultrastructural elements (2, 3) . This paper deals with the problem of whether megakaryocytes are capable of storing exogenous 5HT at specific subcellular sites .