Veterinary Research Communications, 30 (2006) 613–621 DOI: 10.1007/s11259-006-3309-1 C  Springer 2006 Canine Transmissible VenerealTumour: Asessment of Mast Cell Numbers as Indicators of the Growth Phase S. Mukaratirwa ∗ , T. Chiwome, S. Chitanga and E. Bhebhe Department of Paraclinical Veterinary Studies, University of Zimbabwe, Harare, Zimbabwe ∗ Correspondence: E-mail: tmukaratirwa@vet.uz.ac.zw Mukaratirwa, S., Chiwome, T., Chitanga, S. and Bhebhe, E., 2006. Canine transmissible venereal tumour: Asess- ment of mast cell numbers as indicators of the growth phase. Veterinary Research Communications, 30(6), 613–621 ABSTRACT Mast cells are immune cells that are involved mainly in type 1 hypersensitivity reactions, and they have been implicated in tumour angiogenesis. In this study we assessed the presence of mast cell numbers and microvessel density during the progression and regression stages of natural spontaneous canine transmissible venereal tumours (CTVT). Mast cells were demonstrated by histochemical staining with toluidine blue, alcian blue and safranin O. Microvessel counts were demonstrated by immunohistochemical labelling with an antibody against the endothelial cell marker factor VIII. Mitotic cells, apoptotic cells and tumour infiltrating lymphocytes were counted from haematoxylin–eosin-stained sections. Tumour fibrosis was evaluated on Masson’s trichome- stained sections. The results showed that progressing tumours had significantly higher mast cell counts and microvessel counts at the invasive edges of the tumours than did regressing tumours. In both the progressing and regressing tumours, microvessel counts were significantly positively correlated with mast cell counts. Regressing tumours had significantly higher mast cell counts of the whole tumour than progressing tumours. The results also showed that progressing tumours had significantly higher mitotic rate than regressing tumours, and fibrosis and apoptosis were significantly higher in regressing tumours than progressing tumours. There were no significant differences between the biochemical and haematological values of dogs with progressing and regressing tumours. These results suggests that mast cells play a role in CTVT progression probably by promoting vascularization at the invasion front during the progression phase, and that mast cell count could be used as one of the histological factors to indicate growth stage of CTVT. Keywords: angiogenesis, canine transmissible venereal tumour, mast cells, progression, regression Abbreviations: CTVT, canine transmissible venereal tumour; H&E, haematoxylin and eosin; TILs, tumour infiltrating lymphocytes INTRODUCTION Canine transmissible venereal tumour (CTVT) is a round-cell tumour affecting dogs and other canids such as coyotes, foxes and jackals. It is a naturally occurring allograft, and is the only known tumour that can be transplanted from one animal to another through major histocompatibility complex barriers (Gonzalez et al., 2000). It is transmitted from dog to dog by the transplantation of viable neoplastic cells, primarily during coitus. Canine transmissible venereal tumour follows a predictable growth pattern. In both nat- ural and experimental cases, CTVT grows rapidly (progressive stage) after transmission, followed by indolent growth (static phase) and spontaneous regression (regression phase) in immunocompetent animals (Perez et al., 1998). Metastasis is rare, occurring only in 613