Microscopic view of control and TPO-mouse bone marrow.

(Ai) Sample from a normal mouse. This is a representative light-microscopic field with respect to the frequency and appearance of the normal megakaryocyte (Mk) population. Mks are easily distinguished from the other bone marrow cells by their large size and lobulated nuclei (arrowheads). (Aii) Light-microscopic view of sample from a thrombopoietin (TPO) mouse obtained 7 weeks after transplantation shows that Mks are more numerous and larger and have more lobulated nuclei than in those in normal samples. Mks are often arranged in clusters (arrowheads). (B) Average electron-microscopic appearance of an Mk from a TPO mouse. The cell is large, with a large nucleus of increased lobularity (N). α-Granules and demarcation membranes are scarce. Other characteristics of cytoplasmic immaturity are present, eg, conspicuous rough endoplasmic reticulum (er) and numerous ribosomes (r) (original magnification ×3300). (C) Electron-microscopic view of a polymorphonuclear (PMN) cell in the demarcation membrane system (dm) of a TPO mouse Mk. In the left corner, a PMN cell (PN1) is penetrating the cytoplasm of the Mk. Another PMN cell (PN2) is totally surrounded by demarcation membranes (dm) and shows signs of activation (pseudopods on the plasma membrane and cytoplasmic vacuole [arrowhead]). This figure illustrates the selectivity of the cell types retained in Mks. The cytoplasm organelles of the host Mk show alterations and vacuolation (v) (original magnification ×3300). (D) Electron-microscopic view of 3 PMN cells (1, 2, and 3) in the cytoplasm of a TPO mouse Mk that have important signs of alteration. An empty membrane seems to be all that is left of a fourth PMN cell (4). An intact PMN cell is on the left (5). The Mk boundary is indicated by a dotted line. The cytoplasm of the host Mk shows alterations, with the organelle limits poorly recognizable (original magnification ×3300).