ACTA1 and neoplasm: On the other hand, tumoral masses developed in mice co-injected with BJAB and MSC cells showed a completely different aspect, characterized by: areas containing stromal bridges admixed with BJAB cell sheets, as clearly documented by both H&E and Masson's trichromic stainings, and intra-tumor stellate mesenchymal cells often characterized by positivity to α-SMA (Figure 3B).