Fig. 1

Representative images of metaplastic breast carcinomas. a–c Pure spindle cell carcinomas were entirely composed of spindled tumor cells without other heterologous or conventional ductal elements (Sp1, Sp2, and Sp4, respectively). d–f Metaplastic carcinomas with squamous differentiation (Sq1, Sq3, and Sq4, respectively). g–i Metaplastic carcinomas with mixed squamous and spindle cell differentiation (SS1, SS3, and SS4, respectively). j–l Matrix-producing metaplastic carcinomas with chondroid differentiation (C1, C6, and C7, respectively). Carcinoma cells directly transition to tumor cells embedded within chondroid matrix without associated spindled tumor cells or other heterologous components. m–o Metaplastic carcinomas with mixed heterologous differentiation (M1, M2, and M3, respectively). These tumors demonstrated admixed areas of differentiation along multiple heterologous lineages. Areas containing osseous differentiation were sequenced and are shown here, along with foci of spindled tumor cells (m-o), chondroid (o), and squamous (o) elements. All images hematoxylin and eosin