| Pituitary physiology. |  |
The anterior pituitary is formed by 5 different cell types that store and release important hormones controlling largely the entire endocrine system. Release of anterior pituitary hormones is driven by a series of hypothalamic releasing factors that stimulate exocytosis mediated by increases in calcium concentration elicited in target cells. One particular characteristic of the anterior pituitary is its plasticity or the ability of the gland to change its size, cell composition and activity according to hormone demans. We have found that a large population of anterior pituitary cells display a mixed phenotype with cells storing multiple anterior pituitary hormones (polyhormonal cells) and/or expressing multiple receptors for hypothalamic releasing factors (multiresponsive cells). Evidence indicate that multifunctional cells are the expression of pituitary plasticity and of the ability of the gland to undergo cell transdifferentiation, a phenotypic change without cell division. We aim at investigating the cell phenotypes present in normal and transformed anterior pituitaries (from human pituitary adenomas) as well as their contribution to cell plasticity processes and tumorigenesis. For this end we have developed a new methodology that enables typing indvidual anterior pituitary cells by all the hormones stored and all hypothalamic releasing factor receptors expressed.