Frozen sections of maturing MKs, immunolabeled for the detection of dense granule components.

(A) Frozen culture MKs can be identified by their large size, indented nucleus (N), numerous secretory granules (g), and prominent demarcation membrane system (dm). Numerous organelles (about 40), which were identified as MVBs by electron microscopy at a higher magnification, are present in this section. Arrowheads indicate dense granules (at least 5), which were morphologically identifiable (original magnification ×2640). (B, C) Double labeling for the 2 dense granule markers is depicted. Arrowheads indicate granulophysin (20-nm gold particles), and arrows indicate serotonin (10-nm gold particles). (B) Serotonin is detected within the (C) MVBs and (D) dense granules. Both structures are also labeled for granulophysin (original magnification ×49 450). (D, E) Simple immunogold labeling for granulophysin. (D) Granulophysin (arrowheads) labels the dense granule limiting membranes (d), which appear like vacuolar structures that represent 17% of total labeling. Internal vesicles of MVBs represented 80% of the total labeling. Minimal labeling (approximately 3%) was associated with immature α-granules of large size and odd shape (iA), whereas mature α-granules (A) were deprived of labeling. Go indicates the Golgi complex (original magnification ×46 300). (E) Granulophysin in mature MKs is mainly found within the dense granules (d). Labeling is also present on some intermediate MVBs (im), which have lost part of their vesicles and have an ultrastructure similar to that of dense granules (original magnification ×46 300).