Lymphangiogenesis in mammary tumours of the cat

Introduction: Transport of tumour cells via lymphatics (Lcs) is the most common pathway of initial dissemination of many carcinomas. Notwithstanding the availability of lymphatic endothelium markers, the distinction between lymphatic and blood endothelium is difficult, because these markers do not always cross-react with animal tissues. Aim of this study is to test lymphangiogenesis in feline mammary gland tumours. Materials and methods: An immunohistochemical anti-laminin/anti-VEGFR3 double stain has been used to identify Lcs when negative for laminin.The Lcs were classified into positive and negative for VEGFR3 expression. The following samples have been tested: normal mammary glands (NMG)(10 cases), benign (BT) (10 cases) and malignant (MT) tumours (40 cases), these latter grouped by histological stage (stage 0 or non-infiltrating malignant tumour – 10 cases; stage I or infiltrating with stromal invasion – 10 cases; stage II or infiltrating with emboli in lymphatic or blood vessels and/or regional lymph node metastasis – 20 cases). IHC stained sections have been evaluated excluding blood vessels (showing positive laminin stain) and including lymph vessel (negative laminin stain), then furtherly subgrouped for presence and absence of VEGFR3. The count has been carried out on 10 intratumoural (intramammary in the NMG) and 20 extratumoural (extramammary in NMG) fields (0.789 square mm) for each section. The data obtained have been subjected to statistical analysis. Results: In all groups (NMG, BT, MT) the content of Lcs in the extratumoral stroma (extramammary in the NMG) was significantly higher than in the intratumoral (intramammary) stroma: intratumoral vs extraumoral counts in tumours, Lcs with VEGFR3 P<0.001, total number of Lcs P<0.001; intramammary vs extramammary counts in NGM, Lcs without VEGFR3 P<0.05, Lcs with VEGFR3 P<0.001, total number of Lcs P<0.01. The count of Lcs with or without VEGFR3 expression did not show any significant difference in intratumoral and extratumoral stroma in both BT and MT compared respectively to the intramammary and extramammary counts in NMG. Comparing the three groups (NMG, BT, MT) no differences in the number of Lcs (with or without VEGFR3 expression) was recorded in the progression from NMG to MT. No variation was found in the number of Lcs (with or without VEGFR3 expression) in the three groups of carcinomas classified for histological stage. Discussion and conclusions: On the basis of our results, the number of Lcs seems not to be increased in malignant compared to benign tumours and NMG. The ability of mammary carcinomas to spread via the lymphatic pathway is known, and it seems to be not a consequence of a true increase of Lcs in intratumoral or extratumoral stroma, but a ligand/receptor interaction between neoplastic cells and lymphatic endothelium. A significantly higher number of Lcs is present in the extratumoral than in the intratumoral stroma, as known in human breast cancer. Extratumoral stroma, rich in Lcs, seems the major site of interaction between neoplastic cells and lymphatic endothelium.