Bone marrow nonneoplastic


Mast cells

Last author update: 3 May 2023
Last staff update: 3 May 2023

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PubMed Search: Bone marrow mast cells

Ramya Gadde, M.D.
Rose Beck, M.D., Ph.D.
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Cite this page: Gadde R, Beck R. Mast cells. website. Accessed February 22nd, 2024.
Definition / general
  • Mast cells are important cells of the hematopoietic lineage and play a crucial role in regulating the immune system, causing allergy and autoimmunity
Essential features
  • Mast cells are oval, irregular and sometimes show elongated shapes with cytoplasmic extensions
  • Dense, granular cytoplasm packed with basophilic granules is seen
  • Neoplastic mast cells may have an atypical spindled shape or decreased granulation
  • Mast cell granules show metachromatic staining with Wright, Giemsa and Leishman stains
  • Wright stain of bone marrow shows deep pink to dark purple chunky granules that obscure the nucleus
  • On H&E they exhibit bland nuclei and pale pink, granular cytoplasm; single cells are often difficult to visualize on H&E
  • Also positive for toluidine blue and positive for immunostains tryptase and CD117 (bright expression)
  • Reference: Immunol Allergy Clin North Am 2014;34:315, Leuk Res 2001;25:537, Int Arch Allergy Immunol 2018;176:55
  • Mastocyte; labrocyte
  • Mast cells originate from the same multipotent hematopoietic progenitor cells of the bone marrow as neutrophils, eosinophils, basophils and monocytes (CD34+, KIT+, CD13+) and mature under the influence of the c-kit ligand, thrombopoietin, FLT3 ligand and IL6
  • Under normal conditions, mature mast cells do not circulate in the bloodstream
  • Mast cells are usually found in mucosal and epithelial tissues, all vascularized tissues except for the central nervous system and retina as well as in normal bone marrow (Blood 2008;112:946, J Histochem Cytochem 2014;62:698)
  • Cytoplasm of the mast cell contains numerous large granules that store predominantly histamine, heparin, eicosanoids, cytokines, chondroitin sulfate and neutral proteases (J Histochem Cytochem 2014;62:698)
  • Mast cells are known to participate in IgE mediated allergic reactions through FcεRI receptors and activation takes place via cross linking of antigen / IgE / FcεRI (Eur J Immunol 2014;44:2558)
  • Mast cells play a key role in the regulation of physiological processes (e.g., vasodilation), enhancement of angiogenesis and elimination of bacteria and parasites (Front Immunol 2016;6:620)
Clinical features
  • Reactive mast cell hyperplasia is often associated with chronic inflammatory, fibrogenic conditions and solid tumors (Am J Gastroenterol 2020;115:224)
  • Mast cell hyperplasia in bone marrow may be seen in cases of hematologic malignancies such as lymphoplasmacytic lymphoma, chronic lymphocytic leukemia (CLL), myelodysplasia and acute myeloid leukemia (Cytometry B Clin Cytom 2021;100:331)
  • Mastocytosis is a hematologic neoplasm defined by expansion and accumulation of neoplastic mast cells (MC) in the skin or in internal organs, such as the bone marrow (BM), spleen, lymph nodes, liver and gastrointestinal tract
  • World Health Organization, 5th edition (WHO) and International Consensus classification (ICC) of myeloid neoplasms delineates mastocytosis into cutaneous mastocytosis (CM), systemic mastocytosis (SM) and mast cell sarcoma (MCS) (Leukemia 2022;36:1703, Blood 2022;140:1200)
    • Based on disease specific features, systemic mastocytosis is further divided into indolent (ISM), smoldering (SSM), aggressive (ASM), systemic mastocytosis with an associated hematopoietic neoplasm (SM AHN) and mast cell leukemia (MCL) (Hemasphere 2021;5:e646)
Microscopic (histologic) description
  • Rare in normal marrow, usually concentrated within particles on aspirate smears and perivascular on biopsy sections
  • Mast cells are oval or irregularly shaped cells
  • Up to 100% larger than basophils with irregular, elongated shapes and cytoplasmic extensions
  • Dense, granular cytoplasm packed with basophilic granules is seen, often obscuring the nucleus and other organelles
  • When it can be visualized, the nucleus is round and central (StatPearls: Histology, Mast Cells [Accessed 2 March 2023])
Microscopic (histologic) images

Contributed by Ramya Gadde, M.D. and AFIP
Mast cells

Scattered mast cells


Spindle shaped mast cells



Scattered mast cells with basophilic granules

Scattered mast cells with basophilic granules

Positive stains
Negative stains
Electron microscopy description
Electron microscopy images

Images hosted on other servers:

Degranulating mast cell

Mast cells with abundant granules

Differential diagnosis
  • Systemic mastocytosis:
    • Major criteria:
      • Multifocal dense infiltrates of mast cells (≥ 15 mast cells in aggregates) in bone marrow biopsies or in sections of other extracutaneous organ(s)
    • Minor criteria:
      • ≥ 25% of all mast cells are atypical cells or spindle shaped mast cell infiltrates detected in sections of bone marrow or other extracutaneous organs
      • KIT activating KIT point mutation(s) at codon 816 or in other critical regions of KIT in bone marrow or another extracutaneous organ
      • Mast cells in bone marrow, blood or another extracutaneous organ express 1 or more of CD2, CD30 or CD25
      • Baseline serum tryptase concentration > 20 ng/mL (in cases of SM AHN, an elevated tryptase does not count as an SM criterion; in the case of a known hereditary alpha-tryptasaemia, the tryptase level should be adjusted)
    • If at least 1 major and 1 minor or 3 minor criteria are fulfilled → the diagnosis is SM can be made (Leukemia 2022;36:1703, Blood 2022;140:1200)
Board review style question #1
What are the common immunostains used to differentiate reactive and neoplastic mast cells?

  1. CD2, CD25 and CD30
  2. CD13 and CD33
  3. CD34 and CD117
  4. CD117 and tryptase
Board review style answer #1
A. CD2, CD25 and CD30 show aberrant expression in neoplastic mast cells and are useful to differentiate reactive mast cells from neoplastic mast cells. CD13, CD33 and CD34 are negative in mast cells; CD117 and tryptase are positive in both reactive and neoplastic mast cells.

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Reference: Mast cells
Board review style question #2

Detection of which of the following activating mutations is used as one of the minor criteria for diagnosis of systemic mastocytosis?

  1. JAK2
  2. KIT
  3. NPM1
  4. RUNX1
Board review style answer #2
B. KIT. Presence of KIT mutation is one of the minor criteria for diagnosis of systemic mastocytosis. JAK2, NPM1 and RUNX1 are associated with myeloid neoplasms and not with mastocytosis.

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Reference: Mast cells
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