Pituitary tumour clonality revisited.
Allelotype analysis and X chromosome inactivation analysis in women enables the assessment of tissue clonality, and has demonstrated that the majority of sporadic human pituitary adenomas are monoclonal. This implies that these tumours arise from de novo somatic genetic change(s) in a single pituitary cell. However, clonality within any given tumour may be multiple or single, multiple tumours arising on the background of hyperplasia may be of identical or different clonality, multiple 'sporadic' tumours in a tissue may be of differing clonal origin, and finally morphology cannot predict genetic makeup. These general principles may also apply to the pituitary so it is simplistic to assume that monoclonality is inevitable and that pituitary tumours cannot be multiclonal in origin. Indeed, these observations would be entirely compatible with the initiating stimulus resulting in hyperplasia of specific cell subtypes in the pituitary giving rise to a number of different clones each with variable potential to develop into a discrete tumour depending on their rate of cell division/rate of apotosis. Stimuli might include pituitary-specific oncogenes, intrapituitary growth factors, or extrapituitary trophic factors (e.g. hypothalamic releasing hormones).