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At low power, bone marrow trephine biopsy in myelofibrosis shows marked architectural distortion with loss of normal marrow organization. There is a characteristic “streaming” pattern of haemopoietic cells aligned along increased reticulin fibres, along with areas of cellular distortion or crushed cells due to fibrosis. The marrow may appear hypercellular in early stages, with expansion of granulocytic and megakaryocytic lineages, while erythropoiesis is relatively reduced. Sinusoidal and vascular spaces are often dilated with evidence of intra-sinusoidal haemopoiesis. Megakaryocytes are increased and seen in clusters or sheets, frequently in paratrabecular and peri-sinusoidal locations. There is also progressive reduction of intertrabecular space due to new bone formation and thickened osteosclerotic trabeculae, accompanied by increasing reticulin and collagen deposition, which becomes more diffuse with disease progression.