pitx3 defines an equivalence domain for lens and anterior pituitary placode

Hedgehog signaling is required for formation and patterning of the anterior pituitary gland. However, the role of Hedgehog in pituitary precursor cell specification and subsequent placode formation is not well understood. We analyzed pituitary precursor cell lineages and find that pitx3 and distal-less3b (dlx3b) expression domains define lens and pituitary precursor positions. We show that pitx3 is required for pituitary pre-placode formation and cell specification, whereas dlx3b and dlx4b are required to restrict pituitary placode size. In smoothened mutant embryos that cannot transduce Hedgehog signals, median pituitary precursors are mis-specified and form an ectopic lens. Moreover, overexpression of sonic hedgehog (shh) blocks lens formation, and derivatives of lens precursors express genes characteristic of pituitary cells. However, overexpression of shh does not increase median pituitary placode size nor does it upregulate patched (ptc) expression in pituitary precursors during early somitogenesis. Our study suggests that by the end of gastrulation, pitx3-expressing cells constitute an equivalence domain of cells that can form either pituitary or lens, and that a non-Hedgehog signal initially specifies this placodal field. During mid-somitogenesis, Hedgehog then acts on the established median placode as a necessary and sufficient signal to specify pituitary cell types.     

Notch signaling regulates endocrine cell specification in the zebrafish anterior pituitary

The vertebrate pituitary gland is a key endocrine control organ that contains six distinct hormone secreting cell types. In this study, we analyzed the role of direct cell-to-cell Delta-Notch signaling in zebrafish anterior pituitary cell type specification. We demonstrate that initial formation of the anterior pituitary placode is independent of Notch signaling. Later however, loss of Notch signaling in mind bomb (mib) mutant embryos or by DAPT treatment leads to increased numbers of lactotropes and loss of corticotropes in the anterior pars distalis (APD), increased number of thyrotropes and loss of somatotrope cell types in the posterior pars distalis (PPD), and fewer melanotropes in the posterior region of the adenohypophysis, the pars intermedia (PI). Conversely, Notch gain of function leads to the opposite result, loss of lactotrope and thyrotrope cell specification, and an increased number of corticotropes, melanotropes, and gonadotropes in the pituitary. Our results suggest that Notch acts on placodal cells, presumably as a permissive signal, to regulate progenitor cell specification to hormone secreting cell types. We propose that Notch mediated lateral inhibition regulates the relative numbers of specified hormone cell types in the three pituitary subdomains.     

Cytology of the pituitary gland of the Plains viscacha (Lagostomus maximu)

The anatomy of the pituitary gland of the Plains viscacha ( Lagostomus maximus ) is described. The pituitary gland in various physiological states has been investigated in order to localise the secretory sites of the trophic hormones. The various cell types of the pituitary gland were distinguished on the basis of their morphological characteristics, tinctorial properties and differential solubilities of the protein hormones in trichloroacetic acid solutions. Two cell types were found in the pars tuberalis, three cell types in the pars intermedia and six chromophi1 cell types in the pars anterior. Their possible functions are discussed.     

WNT5A signaling affects pituitary gland shape

Wnt signaling is important in organogenesis, and aberrant signaling in mature cells is associated with tumorigenesis. Several members of the Wnt family of signaling molecules are expressed in the developing pituitary gland. Wnt5a is expressed in the neuroectoderm that induces pituitary gland development and has been proposed to influence pituitary cell specification. We discovered that mice deficient in Wnt5a display abnormal morphology in the dorsal part of the developing pituitary. The expression of downstream effectors of the canonical Wnt pathway is not altered, and expression of genes in other signaling pathways such as Shh, Fgf8, Fgf10 and Fgfr2b is intact. Prop1 and Hesx1 are also important for normal shape of the pituitary primordium, but their expression is unaltered in the Wnt5a mutants. Specification of the hormone-producing cell types of the mature anterior pituitary gland occurs appropriately. This study suggests that the primary role of Wnt5a in the developing pituitary gland is in establishment of the shape of the gland.     

The role of flow cytometry in the study of cell growth in the rat anterior pituitary gland

Flow cytometry is a suitable technique for studying in vivo and in vitro the cell cycle kinetics of different animal and human tissues, both in normal and tumoral conditions. The rat anterior pituitary gland is a model to investigate cell growth and replication of differentiated, neuroendocrine cells, and we report current evidence on its cell cycle kinetics as well as on the role played by flow cytometry in this type of study. The proliferation potential of normal anterior pituitary cells is related to a number of different conditions, including heterogeneity of cell types, age and sex of donors, and circadian influences. In addition, the trend of cell proliferation in both in vivo and in vitro studies is similar, suggesting that cultured anterior pituitary elements may, at least in part, retain growth features analogous to those of the intact gland. Sorting of selective cell types and analysis of the relation between proliferating anterior pituitary cells and the light-dark cycle have shown that flow cytometry may be useful to investigate the replication process of the gland. By using a combination of flow cytometry, light microscopic immunocytochemistry and morphometry, we have reported a peculiar trend of proliferation in primary monolayer cultures of rat anterior pituitary gland, characterized by a non-linear reduction in their proliferation rate with advancing age, primarily dependent on a reduced transition of cells from the G0/G1- to the early S-phase pool. These studies indicate that flow cytometry offers insights into cell cycle check points of anterior pituitary cells, and suggest that it might be applied to the study of growth of selective pituitary elements, both in normal and tumoral conditions.     

Non-hormonal cell types in the pituitary candidating for stem cell

Hormone balances in the body are primarily governed by the hypothalamus-pituitary system. For its pivotal role, the pituitary gland relies on an assortment of different hormone-producing cell types, the proportions of which dynamically change in response to fluctuating endocrine demands. Mechanisms of pituitary cellular plasticity are at present far from understood, and may include proliferation and transdifferentiation of hormonal cells. Whether new cells also originate by recruitment from stem cells is unsettled, although this idea has frequently been proposed. Here, I will review these data by focusing on the non-hormonal cell types that have been advanced as candidates for the pituitary stem cell position.     

Adrenomedullin expression in pituitary adenomas and nontumoral adenohypophyses

Adrenomedullin (ADM) is a novel peptide originally identified in extracts of human pheochromocytoma. It is produced by several tissues, including the pituitary gland. The presence of ADM has been immunohistochemically demonstrated in pathologic pituitary glands, but no systematic study of ADM expression in human pituitary adenomas has been reported. Thus, we investigated ADM immunoexpression in 88 various hormone-secreting and clinically nonfunctioning pituitary adenoma types as well as 30 nontumoral adenohypophyses. Furthermore, ADM immunoreactivity was assessed on a 0 to +3 scale in all samples. We found strong immunoreativity for ADM in normal gonadotrophs also expressing FSH and LH whereas in the other adenohypophysial cell types expression of ADM was mild. Results showed that normal adenohypophyses were strongly immunopositive for ADM (2.18+/-0.11). Our findings demonstrate that ADM expression in the anterior pituitary is diminished in tumors as compared to the normal gland. The physiologic function of ADM is unknown, but it could act as a paracrine or autocrine factor in the adenohypophysis.     

Are folliculo-stellate cells in the anterior pituitary gland supportive cells or organ-specific stem cells?

Folliculo-stellate cells (FS-cells) in the anterior pituitary gland are star-shaped cells and form tiny follicles. FS-cells are positive for S-100 protein and produce many cytokines or growth factors, such as interleukin-6 (IL-6), leukemia inhibitory factor (LIF), basic fibroblastic growth factor (bFGF) and vascular endothelial cell growth factor (VEGF). Therefore, it is generally accepted that FS-cells regulate endocrine cells through these growth factors. FS-cells also exhibit a phagocytotic activity and are known to work as scavenger cells. In addition to these functions, FS-cells are considered to have some unknown functions. In order to reveal the biological significance of FS-cells in the anterior pituitary gland, we performed a morphological study and obtained some new findings. First, we were interested in the colloid formation in the senescent porcine pituitary gland. We analyzed the colloids and found that clusterin is a major protein in them. We also found that the accumulation of clusterin in the colloids is related to the phagocytotic activity of FS-cells. In our next study, we found that FS-cells have the potential to differentiate into striated muscle cells. From FS-cells show multi-potent cell character and other cytological evidence, we propose that FS-cells are candidate of organ-specific stem cells in the anterior pituitary gland.     

Functional Morphology of the Teleost Pituitary Gland

The current state of our knowledge of the morphology and histo-and cytophysiology of the teleost pituitary gland is presented.The zonation that characterizes the adenohypophysis is basedon the regional distribution of specific cell types. As a resultthere is also a functional zonation indicated by specific hormonesemanating from clearly identifiable parts of the gland. Theseobservations make the teleost hypophysis ideal for the studyof problems that may be basic to all vertebrates. The ultrastructure of hypophysial cells is presented and relatedto observations at the light microscope level. The manner inwhich secretory granules are released from specific cells iscompared, and the problem of evaluating "cell activity" discussed. The application of immunohistochemical techniques to the studyof the teleost pituitary gland is reviewed, and its use as atool of investigation for future studies evaluated. These methodshave helped to elucidate the cellular source of teleost pituitaryhormones and have confirmed the findings of the histophysiologists.     

Follicle formation in the embryonic chick thyroid. III. Initiation of follicle formation

It has been proposed that the follicular spaces in the thyroid form by either the coalescence of intracellular droplets or by separation of cell apices by secretion into the extracellular space. On the basis of examination of thyroid primordia in early chick embryos this study provides evidence that in the chick, at least, follicle formation conforms to the second model. The first indications of change in the chick thyroid is the appearance of interdigitations of the cell apices. These interdigitations form microvilli as the two surfaces become separated and the follicular space is established. Vesicles with two types of contents can be identified in proximity with the cell surface during follicle formation, but it is not clear if either the dense particulate or the more electron-lucid materials that they contain actually enter the follicular space. Neither removal of the pituitary gland by decapitation nor inhibition of collagen synthesis and a concomitant failure of the invasion of capsular mesenchyme prevents the initiation of normal follicle formation.     

N-cadherin loss in POMC-expressing cells leads to pituitary disorganization

Pituitary tumors are the third most common intracranial tumor in humans and can cause altered hormone secretions leading to hypercortisolism, acromegaly, and infertility. Reduced expression of the cell adhesion molecule N-cadherin has been linked with the formation of pituitary tumors, but its role in normal pituitary gland physiology or tumor initiation is unknown. In the murine pituitary, N-cadherin expression is detected in virtually all cells of the posterior, intermediate, and anterior lobes. N-cadherin may function to initiate important cues such as controlling proliferation, directing cell placement, and promoting formation of cell networks that coordinately release hormones into the bloodstream. To address this, we generated mice lacking N-cadherin in proopiomelanocortin-expressing melanotrope and corticotrope cells of the intermediate and anterior lobes of the pituitary. We observed that intermediate lobe cells can aberrantly displace SOX2-containing progenitor cells in the N-cadherin conditional knockout mice at postnatal d 1. By postnatal d 30, although a reduction in α- and β-catenin membrane staining occurs, there is little effect on intermediate lobe architecture with N-cadherin loss. Also, despite these changes in adherens junction molecules, no alterations in cell proliferation occur. In contrast, loss of N-cadherin in the corticotropes leads to aberrant cell clustering and a reduction in Pomc mRNA. Taken together, our data reveal important roles of N-cadherin in pituitary cell placement and that loss of N-cadherin alone does not lead to pituitary tumor formation.     

Ultrastructural and functional characteristics of anterior pituitary cells in the Indian fruit bat, Rousettus leschenaulti (Desmarest)

The pituitary glands of normal and experimental male and female bats were examined by light and electron microscopy. Six cell types were identified in the anterior pituitary by differential staining techniques, ultrastructural characteristics and changes brought about during the different phases of the sexual cycle. Conventional methods like removal of thyroids, testes and adrenals, and animals in lactation withdrawal and treatment with propylthio-uracil, cyproterone acetate and metyrapone were employed. A marked predominance of somatotrophin and luteotrophin (LTH) cells were present in the intact adult female bat pituitary gland. LTH cells were also observed in milk retention experiments. The two gonadotrophic cell types were randomly distributed throughout the gland. Hypertrophy of two gonadotroph cells was observed in response to the physiological conditions of the animals, gonadectomy and administration of the male antifertility drug cyproterone acetate. Thyrotrophin and adrenocorticotrophin cells were identified by ablation of the respective target organs, thyroids and adrenals, and after treatment with propylthio-uracil and metyrapone. On the basis of the pathological conditions of the bats, the possible functional significance of the different cell types is also discussed.