A New Perspective on Regulation of Pituitary Plasticity: The Network of SOX2-Positive Cells May Coordinate Responses to Challenge.

Plasticity of function is required for each of the anterior pituitary endocrine axes to support alterations in the demand for hormone with physiological status and in response to environmental challenge. This plasticity is mediated at the pituitary level by a change in functional cell mass resulting from a combination of alteration in the proportion of responding cells, the amount of hormone secreted from each cell, and the total number of cells within an endocrine cell population. The functional cell mass also depends on its organization into structural and functional networks. The mechanisms underlying alteration in gland output depend on the strength of the stimulus and are axis dependent but in all cases rely on sensing of output of the functional cell mass and its regulation. Here, we present evidence that the size of pituitary cell populations is constrained and suggest this is mediated by a form of quorum sensing. We propose that pituitary cell quorum sensing is mediated by interactions between the networks of endocrine cells and hormone-negative SOX2-positive (SOX2+ve) cells and speculate that the latter act as both a sentinel and actuator of cell number. Evidence for a role of the network of SOX2+ve cells in directly regulating secretion from multiple endocrine cell networks suggests that it also regulates other aspects of the endocrine cell functional mass. A decision-making role of SOX2+ve cells would allow precise coordination of pituitary axes, essential for their appropriate response to physiological status and challenge, as well as prioritization of axis modification.