Progenitor cells in the embryonic anterior pituitary abruptly and concurrently depress mitotic rate before progressing to terminal differentiation

The control of progenitor cell proliferation in concert with terminal diffe rentiation during embryonic development is poorly understood. The present p aper examines this issue in the different cell lineages of the fetal mouse pituitary. Mouse fetuses were pulse-exposed to H-3-thymidine (H-3-T) on a s ingle day between embryonic day (E) 10 and E16 (prior to the onset of hormo ne phenotype expression) and the H-3-T labeling index of each cell type det ermined 3 or 4 days later (E13-19), when hormone phenotypes were detectable . In the pars tuberalis primordium, TSH beta appeared from E13. Of these ce lls 75.5% were labeled when H-3-T had been administered on E10. Label decre ased to 40.8% when it had been incorporated on E11 and was negligible (4.2% ) when it had been taken up on E12. In the pars distalis, ACTH appeared on E13, TSH beta, and PRL on E14, LH beta/FSH beta on E15 and GH on E16. When examined on E16, all these cell types were labeled for 50-60% if H-3-T had been injected on E12, but this number dropped to about 15% when H-3-T had b een given on E13. Only 5-10% of the hormonal cells had taken up label when E14, 15, and 16 were the days of H-3-T administration. The decline in overa ll labeling index (LI) within both parts of the pituitary was significantly smaller than that in the hormone expressing cells. It is concluded that an outspoken decline in proliferation of the cells destined to become hormone -expressing cell types occurs one to several days before these hormones com e to expression. In the pars distalis, this decline occurs at a common time point i.e, between E12 and E13 for each cell type. Pars tuberalis and pars distalis TSH beta cells show distinct H-3-T labeling profiles, suggesting distinct cell lineage sources for each. (C) 1999 Elsevier Science Ireland L td. All rights reserved.