A case of breast carcinosarcoma with associated myoepitheliosis-like change and morule-like squamous metaplasia. Discussion of possible pathogenetic links

Metaplastic breast carcinomas are histologically heterogeneous and rare tumours constituting <1% of breast malignancies. We present here a case of carcinosarcoma consisting of a predominant sarcomatous component and a minor component of squamous cell carcinoma, a combination seen in <5% of metaplastic breast carcinomas. The presence of myoepitheliosis-like change and morule-like foci of early squamous differentiation within partially uninvolved residual breast tissue provided an opportunity for a detailed immunohistochemical analysis, with intention to look into possible pathogenetic links with the invasive elements. In situ evidence implicating the myoepithelial cell as cell of origin is presented and discussed in view of recent in vitro studies using immortalized human mammary stem/progenitor cell lines.