Tumour spheroids are common in vitro experimental models of avascular tumour growth.,Compared with traditional two-dimensional culture, tumour spheroids more closely mimic the avascular tumour microenvironment where spatial differences in nutrient availability strongly influence growth.,We show that spheroids initiated using significantly different numbers of cells grow to similar limiting sizes, suggesting that avascular tumours have a limiting structure; in agreement with untested predictions of classical mathematical models of tumour spheroids.,We develop a novel mathematical and statistical framework to study the structure of tumour spheroids seeded from cells transduced with fluorescent cell cycle indicators, enabling us to discriminate between arrested and cycling cells and identify an arrested region.,Our analysis shows that transient spheroid structure is independent of initial spheroid size, and the limiting structure can be independent of seeding density.,Standard experimental protocols compare spheroid size as a function of time; however, our analysis suggests that comparing spheroid structure as a function of overall size produces results that are relatively insensitive to variability in spheroid size.,Our experimental observations are made using two melanoma cell lines, but our modelling framework applies across a wide range of spheroid culture conditions and cell lines.
Despite several therapeutic advances, malignant melanoma still remains a fatal disease for which novel and long-term curative treatments are needed.,The successful development of innovative therapies strongly depends on the availability of appropriate pre-clinical models.,For this purpose, several mouse models holding the promise to provide insight into molecular biology and clinical behavior of melanoma have been generated.,The most relevant ones and their contribution for the advancement of therapeutic approaches for the treatment of human melanoma patients will be here summarized.,However, as models, mice do not recapitulate all the features of human melanoma, thus their strengths and weaknesses need to be carefully identified and considered for the translation of the results into the human clinics.,In this panorama, the concept of comparative oncology acquires a priceless value.,The revolutionary importance of spontaneous canine melanoma as a translational model for the pre-clinical investigation of melanoma progression and treatment will be here discussed, with a special consideration to the development of innovative immunotherapeutic approaches.