Melanocytic tumours are a well-known clinical and pathological entity in ho
rses, but further phenotypic characterization of these rumours is lacking.
Six melanocytic tumours from five horses (two metastatic and four benign) w
ere examined by Ki67, PCNA and p53 immunostaining, DNA nick end labelling (
Tunel) and Feulgen staining. The stainings were evaluated using quantitativ
e image analysis. The resulting parameters of growth fraction (Ki67), S-pha
se index (PCNA), p53 index, apoptotic index, DNA index, nuclear diameter, p
loidy balance, proliferation index (Feulgen) and hyperploidy were analysed.
The metastatic melanomas showed overexpression of p53 in a large portion o
f the cells. Apoptosis was also found in the metastatic melanomas. No diffe
rences were found in growth fraction, S-phase index (PCNA) nor in DNA confi
guration between the metastatic and the benign tumours. No immunohistochemi
cal evidence of mutant p53 could be found in the rumours. In conclusion, me
lanocytic rumours in horses seem to have different phenotypic characteristi
cs in comparison with melanocytic rumours in dogs, cats and humans, especia
lly with respect to proliferative activity of the benign tumours. Therefore
, markers put forward in these other species for predicting the clinical be
haviour of the melanomas seem to be of no value in the horse. Moreover, qua
ntitative DNA changes or p53 mutations do not seem to be involved in tumour
ogenesis in these cases.