Reappraisal of the Hansemann-Boveri hypothesis on the origin of tumors.

Cancer is now known to be a genetic disease. In tumor development, cell nuclei undergo mutations, which can result in cytologically visible chromosome aberrations. The aneuploid errors may involve amplification or deletion of whole chromosomes or segments thereof. David Hansemann [1858-1920] and Theodor Boveri [1862-1915] were major contributors to early debates on the relationship between chromosomal defects, tumorigenesis and malignancies. In 1890, Hansemann observed asymmetrical nuclear divisions in human epithelial cancers. In these abnormal, but bipolar, divisions, a fraction of the chromosomes fails to segregate properly. Hansemann carefully documented the occurrence of asymmetric divisions in a wide variety of tumors. However, he remained a lifelong skeptic with regard to whether such events could be considered the underlying cause of tumors. Almost a quarter of a century after Hansemann's initial observations, Boveri considered the origin of tumors based on his earlier recognition of the functional specificity of each chromosome. He also explicitly drew on Hansemann's observations in proposing a model for tumorigenesis. Its central tenet was that a tumor typically originates from a single cell that has inherited a defined, but incorrectly combined, set of chromosomes. The rare occurrence of a pluripolar spindle represented Boveri's paradigm for a type of abnormal mitosis that can produce a host of random chromosomal combinations. He suggested that some of these combinations will induce tumorous transformation, and will inevitably arise occasionally. Since pluripolar and unbalanced bipolar divisions fail to distribute the hereditary chromatic material correctly, both of these mechanisms can give rise to tumor progenitors.