BackgroundThe adult endocrine pituitary is known to host several hormone-producing cells regulating major physiological processes during life. Some candidates to progenitor/stem cells have been proposed. However, not much is known about pituitary cell renewal throughout life and its homeostatic regulation during specific physiological changes, such as puberty or pregnancy, or in pathological conditions such as tumor development.Principal FindingsWe have identified in rodents and humans a niche of non-endocrine cells characterized by the expression of GFRa2, a Ret co-receptor for Neurturin. These cells also express b-Catenin and E-cadherin in an oriented manner suggesting a planar polarity organization for the niche. In addition, cells in the niche uniquely express the pituitary-specific transcription factor Prop1, as well as known progenitor/stem markers such as Sox2, Sox9 and Oct4. Half of these GPS (GFRa2/Prop1/Stem) cells express S-100 whereas surrounding elongated cells in contact with GPS cells express Vimentin. GFRa2+-cells form non-endocrine spheroids in culture. These spheroids can be differentiated to hormone-producing cells or neurons outlining the neuroectoderm potential of these progenitors. In vivo, GPSs cells display slow proliferation after birth, retain BrdU label and show long telomeres in its nuclei, indicating progenitor/stem cell properties in vivo.SignificanceOur results suggest the presence in the adult pituitary of a specific niche of cells characterized by the expression of GFRa2, the pituitary-specific protein Prop1 and stem cell markers. These GPS cells are able to produce different hormone-producing and neuron-like cells and they may therefore contribute to postnatal pituitary homeostasis. Indeed, the relative abundance of GPS numbers is altered in Cdk4-deficient mice, a model of hypopituitarism induced by the lack of this cyclin-dependent kinase. Thus, GPS cells may display functional relevance in the physiological expansion of the pituitary gland throughout life as well as protection from pituitary disease.

Endogenous opioid peptides subserve a wide range of physiological adaptations to stress. Prominent among these functions are inhibition of reproduction (1), modulation of the hypothalamic-pituitary-adrenal (HPA) axis (2), and maintenance of homeostasis in response to autonomic challenge (3). Opioids, together with neurotransmitters in parallel nonopioid neural circuits, also mediate stress-induced analgesia (4). The existence of an intrinsic pain-inhibition system was first demonstrated by the induction of analgesia by electrical stimulation in periventricular, periaqueductal, and medial brainstem loci (5, 6). A possible opioid mechanism for central pain modulation was suggested by the ability of opioid antagonists to block (7) (19)(20)(21). Of the three classes of opioids, 3-endorphin is particularly noteworthy because of its high potency and a nearly one-to-one correspondence between sites supporting electrical stimulation-produced analgesia and high concentrations of endorphinergic fibers in the human brain (22).To further investigate the physiological roles of P-endorphin, we used a genetic approach of homologous recombination in embryonic stem cells to produce mice that are unable to synthesize 13-endorphin. Because 3-endorphin is posttranslationally processed from a larger multifunctional precursor, we introduced a point mutation into the proopiomelanocortin (POMC) gene that translates to a truncated prohormone lacking the entire C-terminal amino acid region encoding /3-endorphin.

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We used 35S-labe1ed oligonucleotides and cRNAs (riboprobes) to detect the temporal order and spatial pattern of anterior pituitary hormone gene expression in (BfiCBF1 x B6CBPi)F2 fetal mice from embryonic Day 9.5 (E9.5) to posmatal D a y 1 (Pl). Pro-opiomelanmrtin (POMC) mRNA was expressed in the basal diencephalon on D a y E10.5, in the ventromedial zone of the pars distalis on Day E12.5, and in the pars intermedia on Day E14.5. The common a-glycoprotein subunit (a-GSU) mRNA fmt appeared in the anterior wall of Rathke's pouch on Day E11.5 and extended to the pars tube& and ventromedial zone of the pars distalis on Day E12.5. Thyroid-stimulating hormone-P (TSHP) subunit mRNA was expressed initially in both the pas tuberalis and ventromedial pars distalis on D a y E14.5, with an identical spatial distribution to a-GSU at the time. In contrast, luteinizing hormone+ (LHP) subunit and folliclestimulating hormone P (FSHB) subunit mRNAs were detected initially only in the ventromedial pars distalis on Days E16.5 and E17.5, respectively, in an identical distribution to each other. POMC-, a-GSU-, 'ISH@, LHP, and FSHPpositive cells within the pars distalis all increased in number and autoradiographic signal with Mering degrees of spatial expansion posteriorly, laterally, and dorsally up to Day P1. POMC expression was typically the most intense and extended circumferentially to include the entire lateral and dorsal surfaces of the pars distalis. The expression of both growth hormone (GH) and prolactin (PE) started coincidentally on D a y E15.5. However PRL ceh localized in the ventromedial area similarly to POMC and the glycoprotein hormone subunits, whereas GH cells were found initially in a more lateral and central distribution within the lobes of the pars distalis. Somatotrophs increased dramatically in number and autoradiographic signal, extending throughout the pars distalis except for the most peripheral layer of cells on Day E17.5. Mammotrophs also increased in number but less abundantly than somatotmphs, and F' RL expression remained more confined to central-medial and ventrolateral areas of the pars distalis up to Day PI. These data demonstrate distinctive patterns of expression for each of the major anterior pituitary hormone genes during development of the mouse pituitary gland and suggest that different groups of committed cells are the immediate precursors to the terminally differentiated hormone-secreting cell types. (JHiStochem IntroductionThe anatomic development ofthe murine adenohypophysis has been described in detail (1). Briefly, an invagination of the stomodeal ectoderm forms Rathke's pouch, which becomes a closed structure at embryonic Day E14.5 in rat (1) and El2 to E12.5 in mouse (2,3).The cells of this immediate pituitary precursor proliferate, possibly in response to stimuli from the surrounding structures, mesenchyme and diencephalon. Cell proliferation in a particular spatial and temporal sequence establishes the three parts of the adenohypophysis: the pars tuberalis, pars distalis (anterior ...