Atypical localization of MPN MKs close to bone areas. (A) A general structure of a long bone. (B) An enlarged area showing the cortical and trabecular bone. (C) An enlarged area depicting the organization of the bone marrow with its zones and the distribution of cells in healthy steady state. (D) An enlarged area depicting the organization of the bone marrow with its zones and the distribution of cells in MPNs. MKs are shown in red. The paratrabecular zone shown in panels C and D is adjacent to trabecular bone and normally has mostly myeloid precursor cells. The intermediate zone usually includes erythroid colonies and maturing myeloid cells. The central zone contains erythroid cells, maturing myeloid cells, sinusoids, and MKs. However, as cited in the text, MPNs are characterized by an increased number of MKs that tend to atypically localize closer to the paratrabecular zone and the endosteal border.