Summary
5-HT storage organelles were observed by electron microscope analysis in human megakaryocytes.
They were less numerous per unit of surface than in platelets. Their number depended
on the visualization technique employed. Thus after fixation with calcium-enriched
glutaraldehyde a higher number of very opaque organelles was observed than of uranaffin-positive
organelles after the cytochemical uranaffin reaction. With conventional electron microscopy
deep black granules characteristic of dense bodies were not observed.
Fluorescent microscopy showed greenish-yellow granules distributed throughout the
whole cytoplasm in 96 ± 1.4% of normal megacaryocytes incubated with mepacrine. In
85.6 ± 5%, 5.28 ± 1.28 granules per 10 ¼m2 were observed.
With the mepacrine labelling test, 74% of the megakaryocytes of a patient with Hermansky-Pudlak
syndrome contained no granules. A similar finding was made in the platelets of the
same patient. This suggests that mepacrine also stains the dense bodies in the megakaryocytes
and that in the Hermansky-Pudlak syndrome the platelet anomaly is secondary to a megakaryocyte
anomaly.
Key words
Megakaryocyte dense bodies - Hermansky-Pudlak syndrome - Mepacrine-labelling test
