Fig. 13.

Megakaryocytes are attached to the endocardial endothelium within the lumen of a ventricle. Confocal microscopic panels (a–d, f–i) demonstrating sections of 13 dpc hearts stained with anti-CD41 (green) (c, h), anti-NP1 (b) or anti-Flk1 (g) (white) and anti-Fli1 (red) (f) antibodies; merged images that include Hoechst staining of nuclei (blue) are presented in a, d, i. The area boxed in a is enlarged in d. A CD41⁺ cell (green arrow) (d) and a CD41⁺ cell co-expressing the Fli1 transcription factor (white arrow) (i) adhere to the endocardial endothelium from the side of a cardiac cavity. White stars point to the lumen of a ventricle (d, i). Electron microscopy images (e, j) demonstrating the adherence of megakaryocytes and/or platelets (Me) to the endocardial endothelium (black arrows); prominent cytoplasmic protrusions toward the endothelium suggest penetration of a megakaryocyte to the subendocardium (e); the black star indicates a marked thinning of endothelial cell cytoplasm (e). Scale bars (panels a–d, f–i) 50 µm