A scanning electron-microscopic study of "supramarginal cells" in the pituitary cleft of the rat

Expression of Retinaldehyde Dehydrogenases in the Pituitary Glands of Fetus and Adult Mice

Retinoic acid (RA) plays a critical role in cell growth and tissue development. RA is synthesized from retinoids through oxidation processes by the retinaldehyde dehydrogenase (Raldh) family. However, the expression of Raldhs during pituitary development and the identification of Raldh-expressing cells in the adult pituitary have not been fully elucidated. Here, we performed in situ hybridization to localize the three Raldh isoforms (Raldh1-3) in fetal and adult mouse pituitary glands. The results showed that Raldh2 expression was observed in Rathke's pouch from embryonic day 13.5 (E13.5), and this expression was sustained in the anterior lobe of the pituitary primordium from E15.5 to E17.5. In contrast, Raldh1 and Raldh3 were rarely detectable. Real-time PCR analysis revealed that Raldh2 was the predominant isoform expressed in the adult pituitary, although Raldh1 was also expressed to a lesser extent. In the adult pituitary, Raldh1-expressing cells were primarily observed in the posterior lobe. Raldh2-expressing cells were found in the marginal cell layer and parenchyma of the anterior lobe and were immunopositive for aldolase C (folliculostellate cells), but not for anterior pituitary hormones. These results suggest that RA is an important regulatory factor in the functions of the pituitary throughout its development in mice.

The multiciliated cells in Rathke's cleft express CYP26A1 and respond to retinoic acid in the pituitary

The adenohypophysis consists of the anterior and intermediate lobes (AL and IL). The marginal cell layer (MCL), including the ventral region of the IL and the dorsal region of the AL lining the Rathke's cleft, acts as the primary stem/progenitor cell niches in adult adenohypophysis. The cells of the MCL on the IL side consisted of cluster of differentiation 9 (CD9)-positive stem/progenitor cells with or without motile cilia. However, any additional cellular properties of multiciliated CD9-positive cells are not known. The present study aimed to identify the character of the multiciliated cells in stem cell niche of the pituitary gland. We observed the fine structure of the multiciliated cells in the MCL of male Wistar rats at an early stage after birth and in adulthood (P60) using scanning electron microscopy. Since the previous study showed that the MCL cells of adult rats synthesize retinoic acid (RA), the present study determined whether the multiciliated cells are involved in RA regulation by the expression of retinal aldehyde dehydrogenase 1 (RALDH1) and CYP26A1, an enzyme synthesizing and degrading RA, respectively. Results showed that 96% of multiciliated cells in adult male rats expressed CYP26A1, while 60% expressed RALDH1. Furthermore, the isolated CD9-positive cells from the IL side MCL responded to RA and activated the degradation system of RA by increasing Cyp26a1 expression. These findings indicated that multiciliated cells are involved in RA metabolism in the MCL. Our observations provide novel insights regarding the stem cell niche of the adult pituitary.

Expression and localization of forkhead box protein FOXJ1 in S100β-positive multiciliated cells of the rat pituitary

S100β-positive cells exist in the marginal cell layer (MCL) of the adenohypophysis and follicle structure in the parenchyma of anterior lobe (ALFS) in pituitary. They have multiple functions as phagocytes or cells that regulate hormone secretion. Majority of S100β-positive cells in the adenohypophysis express sex determining region Y-box 2 protein (SOX2), a stem cell marker; therefore, S100β/SOX2 double positive cells are also considered as one type of stem/progenitor cells. MCL and ALFS are consisting of morphologically two types of cells, i.e., multiciliated cells and non-ciliated cells. However, the relationship between the S100β-positive cells and multiciliated cells in the pituitary is largely unknown. In the present study, we first immunohistochemically verified the feature of multiciliated cells in MCL and ALFS. We then examined the expression patterns of FOXJ1, an essential expression factor for multiciliated cell-differentiation, and SOX2 in the S100β-positive multiciliated cells by in situ hybridization and immunohistochemistry. We identified anew the S100β/SOX2/FOXJ1 triple positive multiciliated cells, and revealed that they were dispersed throughout the MCL and ALFS. These results indicate that the MCL and ALFS are consisting of morphologically and functionally distinct two types of cells, i.e., S100β/SOX2 double positive non-ciliated cells and S100β/SOX2/FOXJ1 triple positive multiciliated cells.

Pituitary Cell Turnover: From Adult Stem Cell Recruitment through Differentiation to Death

The recent demonstration using genetic tracing that in the adult pituitary stem cells are normally recruited from the niche in the marginal zone and differentiate into secretory cells in the adenopituitary has elegantly confirmed the proposal made when the pituitary stem cell niche was first discovered 5 years ago. Some of the early controversies have also been resolved. However, many questions remain, such as which are the markers that make a pituitary stem cell truly unique and the exact mechanisms that trigger recruitment from the niche. Little is known about the processes of commitment and differentiation once a stem cell has left the niche. Moreover, the acceptance that pituitary cells are renewed by stem cells implies the existence of regulated mechanisms of cell death in differentiated cells which must themselves be explained. The demonstration of an apoptotic pathway mediated by RET/caspase 3/Pit-1/Arf/p53 in normal somatotrophs is therefore an important step towards understanding how pituitary cell number is regulated. Further work will elucidate how the rates of the three processes of cell renewal, differentiation and apoptosis are balanced in tissue homeostasis after birth, but altered in pituitary hyperplasia in response to physiological stimuli such as puberty and lactation. Thus, we can aim to understand the mechanisms underlying human disease due to insufficient (hypopituitarism) or excess (pituitary tumor) cell numbers.

Localization of Notch signaling molecules and their effect on cellular proliferation in adult rat pituitary

Notch signaling is a cell-to-cell signaling system involved in the maintenance of precursor cells in many tissues. Although Notch signaling has been reported in the pituitary gland, the histological characteristics of Notch receptors and ligands in the gland are unknown. Here, we report the histological gene expression pattern of Notch receptors and ligands and the role of Notch signaling in cellular proliferation in adult rat pituitary gland. In situ hybridization detected transcripts of Notch1 and 2 and Jagged1 and 2. Double-staining with a combination of in situ hybridization and immunohistochemistry revealed that their mRNAs were localized in almost half of the S100-protein-positive cells, which are generally regarded as marginal layer cells and folliculo-stellate cells. In primary culture of anterior pituitary cells, proliferation of S100-protein-positive cells was modulated by Notch signaling inhibitor and solubilized Notch ligand. Furthermore, quantitative analysis revealed that the inhibition of Notch signaling led to the down-regulation of mRNA for the Notch target gene Hes1 and the up-regulation of p57 gene expression. These findings suggest that Notch signaling is involved in the proliferation of S100-protein-positive cells, presumably precursor cells, in adult rat pituitary gland.

Components of the basal lamina and dystrophin-dystroglycan complex in the neurointermediate lobe of rat pituitary gland: different localizations of beta-dystroglycan, dystrobrevins, alpha1-syntrophin, and aquaporin-4

The so-called neurointermediate lobe is composed of the intermediate and neural lobes of the pituitary. The present immunohistochemical study investigated components of the basal lamina (laminin, agrin, and perlecan), the dystrophin-dystroglycan complex (dystrophin, beta-dystroglycan, alpha1-dystrobrevin, beta-dystrobrevin, utrophin, and alpha1-syntrophin), and the aquaporins (aquaporin-4 and -9). Glia markers (GFAP, S100, and glutamine synthetase) and components of connective tissue (collagen type I and fibronectin) were also labeled. In the neurohypophysis, immunostaining of basal lamina delineated meningeal invaginations. In these invaginations, vessels were seen to penetrate the organ without submerging into its parenchyma. On the parenchymal side of the invaginations, beta-dystroglycan was detected, whereas utrophin was detected in the walls of vessels. Immunostaining of alpha1-dystrobrevin and alpha1-syntrophin did not delineate the vessels. The cells of the intermediate lobe were fully immunoreactive to alpha1-dystrobrevin and alpha1-syntrophin, whereas components of the basal lamina delineated the contours of the cells. GFAP-immunoreactive processes surrounded them. Aquaporin-4 localized at the periphery of the neurohypophysis, mainly adjacent to the intermediate lobe but not along the vessels. It colocalized only partially with GFAP and not at all with alpha1-syntrophin. Aquaporin-9 was not detected. These results emphasize the possibility that the components of the dystrophin-dystroglycan complex localize differently and raise the question about the roles of dystrobrevins, alpha1-syntrophin, and aquaporin-4 in the functions of the intermediate and neural lobes, respectively.

Expression of retinaldehyde dehydrogenase 1 in the anterior pituitary glands of adult rats

Retinoic acid (RA) plays a critical role in cell growth and tissue development and is also a regulatory factor of pituitary function. However, whether RA is generated in the pituitary gland and plays a role as a paracrine and/or autocrine factor is generally unknown. RA is synthesized from retinoids through oxidation processes. Dehydrogenases that catalyze the oxidation of retinal to RA are members of the retinaldehyde dehydrogenase (RALDH) family. Recently, we demonstrated that RALDH2 and RALDH3, but not RALDH1, were expressed in the developing anterior pituitary gland of rats, but the expression of RALDHs in the adult pituitary gland was not determined. Therefore, we have now examined the expression of RALDH1, RALDH2, and RALDH3 mRNAs in the pituitary gland of adult rats. Analysis by quantitative real-time polymerase chain reaction of adult pituitary glands has revealed a high level of RALDH1 mRNA but not of RALDH2 mRNA or RALDH3 mRNA. We have also detected mRNA expression for RALDH1 in the anterior pituitary gland by in situ hybridization with digoxigenin-labeled cRNA probes. Double-staining for RALDH1 mRNA and pituitary hormones or S-100 protein, a marker of folliculo-stellate cells (FS-cells), has revealed RALDH1 mRNA expression in a portion of prolactin-producing cells, marginal layer cells, and FS-cells. Our results suggest that RA is generated in the adult anterior pituitary gland, and that it may act locally on pituitary cells.

The structure and function of folliculo-stellate cells in the anterior pituitary gland

The folliculo-stellate cells (FS cells) in the anterior pituitary gland are characterized by their star-like appearance and their ability to form follicles. Although FS cells do not produce any pituitary hormones, their special tendency to surrounding endocrine cells with their long cytoplasmic processes suggests that they regulate endocrine cells by intercellular communication. In spite of many morphological and cytophysiological studies recently performed, a precise understanding of the major functions of FS cells in the pituitary gland remains obscure. We review here the morphological characteristics of FS cells and their suspected functions in the anterior pituitary gland. It is well established that the FS cell produces many kinds of growth factors, i.e., fibroblast growth factor, vascular endothelial cell growth factor and interleukin 6. The biological significances of these growth factors in the anterior pituitary gland are also discussed in this paper. The origin and differentiation of FS cells, especially the possibility that the FS cell is a kind of stem cell which has the potential to differentiate into endocrine cells, is also presented.