ABSTRACT
The maintenance of tissue size and therefore of tissue function in the normal renewal tissues of the body depends upon the existence of a small number of primitive ‘stem cells’ – cells that have the capacity to maintain their own numbers while at the same time producing cells that can differentiate and proliferate to replace the rest of the functional cell population. Stem cells are at the base of the hierarchy of cells that make up the haemopoietic and epithelial tissues. Carcinomas are derived from such hierarchical epithelial tissues, and our ability to recognise this in histological sections derives from the fact that these tumours often maintain many of the features of differentiation of the tissue within which they arose. Well-differentiated tumours do this to a greater extent than anaplastic (poorly differentiated) tumours. It follows that not all the cells in a tumour are neoplastic stem cells: some have embarked on an irreversible process of differentiation. In addition, carcinomas also contain many cells that make up the stroma (fibroblasts, endothelial cells, macrophages, etc.). Stem cells thus may comprise only a small proportion of all the cells within a tumour.
