Stains & CD markers
Actin, alpha smooth muscle type

Last author update: 20 January 2022
Last staff update: 24 June 2022

Copyright: 2003-2024,, Inc.

PubMed Search: Alpha smooth muscle actin

Kemal Kösemehmetoğlu, M.D.
Gina Johnson, M.D.
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Cite this page: Kösemehmetoğlu K, Johnson G. Actin, alpha smooth muscle type. website. Accessed July 13th, 2024.
Definition / general
  • Actin is a 43000 kDa ubiquitous protein found in all cells
  • Actins are involved in cell motility (alpha, smooth muscle) and the maintenance of the cytoskeleton (beta and gamma, all cells)
  • Antibodies to alpha smooth muscle actin do not detect the other actin isoforms
Essential features
  • Involved in cell motility
  • Identifies pericytes, myoepithelial cells, smooth muscle cells and myofibroblasts in normal, reactive or neoplastic tissue
  • Myofibroblastic staining (tram track) versus smooth muscle staining (block cytoplasmic)
  • Also called smooth muscle actin, SMA; clone ASM1 / 1A4 or sm 1
  • 3 types: alpha, beta and gamma
  • Alpha actins are found in muscle tissues and required for contraction, whereas the beta and gamma actins function as components of the cytoskeleton in many cells
  • Expression correlates with the activation of myofibroblasts (Mol Cell Biochem 2008;308:201)
  • May play a role in epithelial mesenchymal transition of carcinomas (Rom J Morphol Embryol 2014;55:1383)
Clinical features
  • Membranous or cytoplasmic staining
Uses by pathologists
Prognostic factors
  • Myogenic differentiation (either only SMA or SMA+ desmin) in dedifferentiated liposarcoma significantly decreases 5 year disease free survival (Am J Surg Pathol 2020;44:799)
Microscopic (histologic) description
  • Myofibroblastic staining (tram track) versus smooth muscle staining (block cytoplasmic)
Microscopic (histologic) images

Contributed by Kemal Kösemehmetoğlu, M.D.


SMA expression in leiomyoma


Diffuse cytoplasmic block SMA expression

Nodular fasciitis

SMA in nodular fasciitis


Focal SMA expression in myxofibrosarcoma

Glomus tumor

SMA in glomus tumor

Undifferentiated pleomorphic sarcoma

Focal SMA in undifferentiated pleomorphic sarcoma

Inflammatory myofibroblastic tumor

Myofibroblastic type SMA in IMT

Atypical apocrine adenosis

SMA in myoepithelial layer of breast adenosis

Radial scar / complex sclerosing lesion

SMA in radial scar / complex sclerosing lesion

Sclerosing papilloma

Pseudoinvasion in sclerosing papilloma

SMA positive myoepithelial layer

Tram track (myofibroblastic) staining pattern

Block (smooth muscle-like) staining pattern

Virtual slides

Images hosted on other servers:
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SMA decorating vessels of tufted hemangioma

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Glomangioma with diffuse and strong SMA expression

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SMA expression in embryonal rhabdomyosarcoma

Positive staining - normal
Positive staining - disease
Negative staining
Sample pathology report
  • Right 3rd intercostal space, wide excision:
    • Leiomyosarcoma, grade 3 (see comment)
    • Comment: Immunohistochemically, neoplastic cells showed diffuse strong cytoplasmic staining for SMA, desmin and h-caldesmon.
  • Right thigh, excisional biopsy:
    • Nodular fasciitis (see comment)
    • Comment: Immunohistochemically, neoplastic cells were positive for SMA in myofibroblastic pattern (tram track staining) and negative for desmin.
Board review style question #1

Which statement is correct about the SMA immunostaining shown above?

  1. Consistent with the diagnosis of nodular fasciitis
  2. Demonstrates smooth muscle type of SMA staining
  3. Demonstrates tram track SMA staining
  4. Excludes the diagnosis of spindle cell rhabdomyosarcoma
Board review style answer #1
B. Demonstrates smooth muscle type of SMA staining

Comment Here

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