In rats, a shift from somatotroph dominance to lactotroph dominance during pregnancy and lactation is well reported. after weaning, and concurrent changes in CHR2797 manufacturer cell size were estimated. At all stages, the EYFP reporter was expressed in 80% of the lactotrophs, but in fewer than 1% of other pituitary cell types, indicating that transdifferentiation from those lactotrophs where reporter expression was activated is extremely rare. Contrary to expectations, no increase in the lactotroph/somatotroph ratio was seen during pregnancy and lactation, whether assessed by immunochemistry for the reporter or PRL: findings confirmed by PRL immunochemistry in non-transgenic mice. Mammosomatotrophs were rarely encountered at the age CHR2797 manufacturer group studied. Individual EYFP+ve cell volumes increased significantly by mid-lactation compared Hapln1 with virgin animals. This, in combination with a modest and non-cell type-specific estrogen-induced increase in mitotic activity, could account for pregnancy-induced changes in overall pituitary size. Introduction Estrogen is believed to be responsible both for increased anterior pituitary mitotic variability in estrous-cycling females compared with male rats (Oishi gene, PRL-Cre/ROSA26-YFP mice express EYFP from the ROSA26 promoter in any cell that has expressed the PRL transgene promoter at any time (Srinivas gene expression on day 15 of fetal life (Matsubara em et al /em . 2001, Ogasawara em et al /em . 2009). Integrating what appear to be mutually exclusive findings into a single model is complicated as technical constraints, unpredictable bystander effects, and different congenic footprints of the various transgenic models, including the one used in the present study, are difficult to predict or quantify. The implication, however, is that there may be two broad lineages of PRL cells: a GH cell-independent lineage and a GH cell-dependent lineage that contribute differently to overall PRL cell mass in the pituitary at different ages. In early neonatal life where in both mice (Doll em et al /em . 1990) and rats (Watanabe & Haraguchi 1994) the expression of PRL and GH has been observed in different cells, the GH cell-dependent PRL lineage perhaps accounts for a relatively larger proportion of all PRL-containing cells. In later life, the proportion of PRL cells that have expressed GH is reduced to 10% of the total, perhaps through continuing activity of oligopotent stem cells that while not responsible for organogenesis, are nevertheless the principal source of cell renewal and plasticity (Fauquier em et al /em . 2008, Gleiberman em et al /em . 2008). In the present study, we sought to determine whether the potential transdifferentiation from somatotrophs to lactotrophs may occur in reverse by tracing the fate of lactotrophs with a transgenic mouse with the expression of Cre recombinase driven by the rat PRL promoter. Cre recombinase activity was monitored by crossing with ROSA26-EYFP mice, which have a targeted insertion of EYFP into the ROSA26 locus (Srinivas em et al /em . 2001). The EYFP sequences are preceded with a strong CHR2797 manufacturer termination sequence flanked with loxP sites, and Cre recombination leads to the removal CHR2797 manufacturer of the termination sequence and expression of the fluorescent reporter. Once recombination has occurred, it is permanent, and reporter expression from the ROSA26 promoter is usually no longer dependent on the expression of the recombinase. Thus, EYFP identifies cells which have expressed the transgene at any time. By crossing these mice with ROSA26-EYFP Cre-reporter mice, we have found that the Cre transgene leads to reporter activity almost entirely in lactotrophs, with only a very low proportion of somatotroph (GH+ve but PRL?ve) cells expressing the EYFP reporter. Not all lactotrophs were positive for the Cre-reporter, a obtaining unlikely to result from a lack of reporter expression or detection in some lactotrophs following Cre-mediated recombination, since in other transgenic models expressing Cre in the pituitary, EYFP is usually expressed in all lactotrophs (Arnaud Jaubert, personal.