Megakaryocytes and megakaryocyte fragments
Circulating micromegakaryocytes or megakaryoblasts (often associated with highly abnormal platelet morphology) may be found in blood. The description of these cells is not well defined. Very rarely micromegakaryocytes or their nuclei may be found in the blood of normal individuals, but are infrequent. Where present in large numbers they indicate the possible presence a significant neoplastic disorder of the megakaryocyte cells: either a myelofibrosis, megakaryoblastic leukaemia or related disorder. It is important therefore to have terms that allow us to describe their presence.
The following terms can be used as working definitions:
1. Megakaryocytic fragment: this term is generally reserved for very large cellular fragments of platelet lineage but with very abnormal shape, granulation or both
2. Circulating micromegakarocytes are cells with relatively mature nuclei, but cytoplasmic features partly resemble mature platelets or megakaryocytic fragments on the film
3. Isolated megakaryocytic nuclei resemble micromegakaryocytes but with absent cytoplasm
4. Megakaryoblasts primitive cells which have morphological or immunophenotypic features of megakaryocytic lineage and which may show budding of platelet-like fragments
Megakaryocyte fragments: Large platelet forms with atypical shape or granulation
The distinction between a giant platelet and a megakaryocyte fragment can be subjective - here the very large size and basophilic agranular appearance favours the term megakaryocyte fragment in both examples (A&B) while the irregularity (A) further contributes. In each case note also the dysplastic change affecting other lineages.
Circulating micromegakaryocytes and megkaryocyte nucleli
A: Circulating micromegakaryocytes (note the mature nuclear appearance and the cytoplasm that is almost identical to that of the platelets); this image also has an isolated megakaryocyte nucleus (lower cell) where the cytoplasm appears absent. B: A circulating micromegakaryocyte with limited 'budding' cytoplasm; note the similarity nuclear appearance with the previous example (A). C: Shows a large multinucleate form with faintly basophilic cytoplasm, but nuclear appearance typical of a dysplastic megakaryocyte from bone marrow.
A&B: Megakaryoblasts with more primitive nuclei from a child with an acute megakaryoblastic leukaemia. Note that the blast cells have basophilic cytoplasm that in some cases resembles that of mature platelets. The cytoplasmic appearances of the abnormal megakaryoblasts show very similar features to the circulating abnormal platelets.