The Mks in the spleen from TPO-treated wild-type mice expressed several morphologic abnormalities characteristic of GATA-1^low Mks. Semithin (A) and ultrathin (B-C) sections of the spleen from TPO-treated wild-type mice. The semithin section (A) shows a cluster of light- and heavy-electron dense Mks (Mk) surrounded by numerous neutrophils (arrowheads), fibroblasts (F plus arrows), and connectival fibers (CF). Upon ultrastructural examination (B), heavy-electron dense Mks present a morphology of cytoplasm and nucleus compatible with that of cells undergoing para-apoptosis.²²  Furthermore, heavy-electron dense Mks contain neutrophils (Neu) embedded in their cytoplasm (insert in C), as proved by immunogold staining for myeloperoxidase (C) that shows gold particles (arrowheads) in the cytoplasm both of the neutrophil and of the Mks. Similar results were observed in the spleen of at least 3 separate TPO-treated wild-type mice. Magnification: × 100 (A), × 3000 (B, insert in C), and × 30 000 (C). Numerical aperture of the objective lenses: 0.1.