TSP2 inhibits MK differentiation.

MKs were allowed to take up TSP2 for a period of 24 hours and were then placed in media containing 10% plasma to induce differentiation. Uptake of TSP2 was detected by FITC-conjugated IgG. Nuclei and the actin cytoskeleton were visualized with DAPI and phalloidin, respectively. Panels A and B represent images of the same field showing a combination of phalloidin and DAPI stain (A) and FITC-conjugated IgG and DAPI (B). Small arrows in panels A and B indicate cells that contain bundled actin but lack TSP2, whereas arrowheads indicate cells that lack bundled actin but contain TSP2. Yellow arrow (A-B) shows proplatelet-forming MKs, with numerous pseudopodia, that are devoid of TSP2. The effect of the uptake of TSP2 by MKs on their differentiation was quantified at 24 and 48 hours following the replacement of plasma-containing media (C). At both time points, uptake of TSP2 resulted in a reduction in the number of proplatelet-forming MKs. Addition of 10% plasma also resulted in a reduction in the number of MKs that contain TSP2 (D). Bar represents 50 μm (A-B). *P ≤ .05