A single population of cells in the male rat anterior pituitary responds mitotically to both gonadectomy and adrenalectomy

The male rat anterior pituitary responds highly reproducibly to specific hormonal stimuli in terms of the extent and timing of mitotic and apoptotic (ie. trophic) activity, as well as changes in hormone secretion. Unlike the latter, which is easily attributable to a specific cellular subpopulation, the nature of the cells that respond trophically, and therefore the extent of the contribution of hormonally identifiable cells to the mitotic response is much harder to define. The mitotic response to bilateral adrenalectomy and bilateral gonadectomy are very similar. Each is characterized by a gradual 2- to 3-fold increase in mitotic activity to a peak 3 to 4 days post-surgery followed by a gradual return to baseline levels over the subsequent 10 days. As far as apoptotic responsiveness, the short lived burst of apoptotic activity induced by dexamethasone in cells formed within the previous 10-14 days is enhanced in amplitude 2- to 3-fold not only by prior adrenalectomy, but also, somewhat surprisingly, by gonadectomy. We have shown in the present study that the amplitude of the mitotic response when surgical adrenalectomy and gonadectomy are combined, is no bigger than that seen after adrenalectomy or gonadectomy alone. Similarly, combining gonadectomy and adrenalectomy does not enhance the size of the dexamethasone-sensitive apoptotically responsive cell population generated by recent mitotic activity. Unless there is a substantial cross-axis inhibitory effect on mitotic activity, these data strongly suggest that the same population of cells respond mitotically to both adrenalectomy and gonadectomy. This finding supports the concept of a progenitor cell population that responds to several different hormonal stimuli, and weakens the prevailing concept that hormonally identifiable cells with specific mitotic and apoptotic response profiles contribute significantly to trophic activity in the pituitary.