The anterior pituitary gland, a small organ found in the brain, regulates hormones and puberty. Because the gland grows and remodels itself after birth, it was thought, reasonably, to contain a population of multipotent stem cells, but such cells had not previously been found. Now, a collaboration between the labs of Michael G. Rosenfeld at the University of California, San Diego, and Grigori Enikolopov at Cold Spring Harbor Laboratory, in Cold Spring Harbor, New York, has identified a discrete set of cells that contribute to the growth of the mouse pituitary gland after birth1.

The discovery relied on the realization that many adult, lineage-specific stem cells express a protein called nestin, though its role is unknown. The researchers used transgenic mice that express green fluorescent protein when nestin is present, which causes the affected cells to appear green; observation of embryos revealed a small population of nestin-positive cells in the pituitary gland 11.5 days into embryogenesis. Excitingly, the cells expressed Lhx3, a marker of all six pituitary lineages, as well as Sox2, a transcription factor associated with stem cells. During the postnatal 'second wave' of pituitary growth, these nestin-expressing cells migrated and divided. Could they be the elusive adult stem cells?

Using an in vivo system that marks descendents of nestin-positive cells, the group showed that at birth the nestin-positive population accounted for two per cent of pituitary cells. This indicated that the majority of pituitary tissue had never expressed nestin and was therefore derived from a completely different stem cell lineage. After the postnatal growth spurt, this percentage increased tenfold, proving that nestin-expressing cells contribute significantly to the pituitary structure. Both in vivo and in vitro, the cells were capable of self-renewal and could differentiate into all six pituitary lineages. Further work hinted that the adult stem cells may, under specific circumstances, act as cancer stem cells.

One question is why the pituitary gland switches from one source of new cells to another after birth. Enikolopov suggests that the stem cells could be better suited to dealing with dynamic changes in the endocrine system, meaning that they are likely to differ subtly from their embryonic counterparts. The next steps will be to examine how external stimuli such as hormones, stress and drugs affect the contribution of nestin-positive cells to the pituitary gland. Enikolopov also hopes to begin looking at a role for nestin in the stem cells of other tissues.