Actin, alpha smooth muscle type

Author: Gina Johnson, M.D. (see Authors page)

Revised: 3 November 2015, last major update November 2013

Copyright: (c) 2003-2015,, Inc.

PubMed Search: alpha smooth muscle actin[title]
Cite this page: Actin, alpha smooth muscle type. website. Accessed December 13th, 2018.
Definition / general
  • Discovered in 1986 (J Cell Biol 1986;103:2787)
  • Antibodies to alpha smooth muscle actin do not detect the other actin isoforms
  • Also called smooth muscle actin, SMA; clone 1A4 or sm-1
Clinical features
  • Membranous or cytoplasmic staining
Uses by pathologists
Microscopic (histologic) images

Images from Pathout server:

Leiomyosarcoma: cervix

Sinonasal hemangiopericytoma

Images from other servers:

Ameloblastic carcinoma

Bladder: PEComa

Bone: glomus tumor

Breast: myoepithelial cells (fig C/G)

Breast: leiomyoma

Colon (left to right): carcinosarcoma; leiomyoma (top); Peutz Jeghers polyp (fig 4)

Colon and small intestine: normal and intestinal pseudoobstruction

Crohn disease: obliterative muscularization

Eye: MFH of conjunctiva (top left)

Heart: fetus with heart block

Heart scar tissue

Inflammatory myofibroblastic tumor (left to right): bone, gallbladder, salivary gland (fig 3b)

Kidney (left to right): atypical epithelioid angiomyolipoma (two images), focal segmental glomerulosclerosis

Leiomyosarcoma (left to right): bladder (fig C), esophagus

Liver: glomangioma (fig 5)

Lung: COPD

Lung: lymphangio-leiomyomatosis

Myofibroblastic sarcoma
(left: smooth muscle actin,
right: calponin)

Myofibroblastic sarcoma
(left: smooth muscle actin,
right MIB-1)

Normal: breast myoepithelial cell disruptions

Normal: breast myoepithelial cells

Normal: decidual stromal cells

Normal: eye choroid and sclera

Normal: liver (fetal)

Placenta accreta (fig 3)

Plexiform fibrohistiocytic tumor

Pleomorphic sarcoma / MFH

Salivary gland duct carcinoma

Salivary gland sialometaplasia in parotid node

Stomach: left-gastric carcinoma-diffuse type (fig A); right-gastric carcinoma-intestinal type (fig A)

Submandibular gland - adenoid cystic carcinoma

Images from

Breast: basal-like carcinoma (fig A)

Breast: cellular fibroadenoma

Breast: adenoid cystic carcinoma
(fig B) vs collagenous spherulosis
(fig G)


Lung: adenocarcinoma, subepithelial myofibroblasts

Liver: epithelioid angiomyolipoma

Positive staining - normal
Positive staining - disease
Negative staining
Normal tissue: