Frames from a video of a terminally differentiated mouse megakaryocyte forming proplatelets in vitro

Selected images are shown from a video-enhanced light microscopy series of proplatelet formation.

(A) The megakaryocyte spreads and its cortical cytoplasm begins to unravel at one pole (asterisk). Bar, 20 mm.

(B) The cell spreads and remodels the cytoplasm at the erosion site is remodeled into large pseudopodia that elongate and become thinner over time, forming narrow tubes of 1 to 4 mm diameter. The proplatelet extensions frequently bend and bifurcate, converting the entire cytoplasm into anastomosing proplatelet extensions. Proplatelets also develop segmented constrictions along their length that impart a beaded appearance.

(C) The process of proplatelet elaboration culminates in a rapid retraction that separates the many strands of proplatelets from a residual nuclear mass. Small cytoplasmic fragments (5 to 10 mm) at the ends of some of the proplatelet extensions (arrowhead) begin crawling when the end of the proplatelet adheres and flattens, forms a leading lamellipodia, and migrates away from the cell center, dragging a trail of proplatelets behind.