Occurrence of colloid-containing follicles in the pars distalis of pituitary glands from aging guinea pigs

Summary Colloid-containing follicles in the pars distalis of pituitary glands from guinea pigs at various ages ranging from 5 days to 36 months were examined by the periodic acid-Schiff (PAS) reaction, immunohistochemistry, and electron microscopy. The follicles storing PAS-positive colloid were first detected in 6-month-old animals, in which only a few follicles were present and mean diameters of colloid deposits were small: 4.3±1.0 m in males and 4.1±0.4 m in females. Thereafter, the follicles gradually increased in number and size with age. The largest number of follicles was observed in the senile groups: 410.5±92.3 in males, 454.7±84.7 in females. Mean diameters of colloid masses in the senile groups were more than 2 times larger than those in 6-month-old animals: 10.0±0.1 m in males, 9.7±0.1 m in females. These findings suggest that the formation of colloidcontaining follicles in the guinea-pig pars distalis is an aging phenomenon. The follicular lumina were mainly surrounded by thin cytoplasmic processes or cell bodies of folliculo-stellate cells immunoreactive for S-100 protein. The lining folliculo-stellate cells showed aggregations of intermediate-sized filaments, numerous lysosomes and colloid-like inclusions. Granulated cells in contact with colloid were occasionally encountered. Intracellular cavities storing colloid-like and fibrous materials were detected in the syncytial formation of GH cells.     

Localization of an endocannabinoid system in the hypophysial pars tuberalis and pars distalis of man

The hypophysial pars tuberalis (PT) acts as an important interface between neuroendocrine brain centers (hypothalamus, pineal organ) and the pars distalis (PD) of the hypophysis. Recently, we have identified an endocannabinoid system in the PT of hamsters and provided evidence that 2-arachidonoylglycerol is a messenger molecule that appears to play an essential role in seasonal reproduction and prolactin release by acting on the cannabinoid receptors in the PD. We now demonstrate the enzymes involved in endocannabinoid synthesis and degradation, namely sn-1-selective diacylglycerol lipase α, N-acylphosphatidylethanolamine-specific phospholipase D, and monoacylglycerol lipase, in the PT of man by means of immunohistochemistry. High-performance liquid chromatography coupled with tandem mass spectrometry revealed 2-arachidonoylglycerol and other endocannabinoids in the human PT. Furthermore, we detected the expression of the cannabinoid receptor 1 (CB1), a primary receptor for endocannabinoids, in the PD. Double-immunofluorescence staining for CB1 and various hypophysial hormones or S-100, a marker for folliculostellate (FS) cells, revealed that CB1 immunoreactivity was mainly localized to corticotrophs and FS-cells. A limited number of lactotrophs and somatotrophs also showed CB1 immunoreactivity, which was however absent from gonadotrophs and thyrotrophs. Our data thus indicate that the human PT comprises an endocannabinoid system, and that corticotrophs and FS-cells are the main target cells for endocannabinoids. The functional significance of this newly discovered pathway remains to be elucidated in man; it might be related to the control of stress responses and/or reflect a remnant seasonal control of hypophysial hormonal secretion.     

An endocannabinoid system is localized to the hypophysial pars tuberalis of Syrian hamsters and responds to photoperiodic changes

The hypophysial pars tuberalis (PT), an important interface between neuroendocrine brain centers (hypothalamus, pineal organ) and the pars distalis (PD) of the hypophysis, plays a central role in regulating seasonal reproduction and prolactin release. However, the signaling molecules that transmit photoperiodic information from the PT to the PD and control prolactin release (the so-called “tuberalins”) have not yet been identified, despite an intense search for more than three decades. Here, we demonstrate an endocannabinoid system in the PT of the Syrian hamster, a photoperiodic species. By means of in situ hybrization, the PT was found to express N-acylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD), fatty acid amide hydrolase (FAAH), sn-1-selective diacylglycerol lipases (DAGLα and DAGLβ), and monoacylglycerol lipase (MAGL), enzymes involved in endocannabinoid synthesis and degradation. The expression of NAPE-PLD, FAAH, and DAGLα was confirmed by immunohistochemistry. Expression and protein levels of DAGLs controlling the synthesis of 2-arachidonoyl glycerol (2-AG), a major endocannabinoid, were upregulated in the PT of Syrian hamsters kept under long-day conditions. Consequently, 2-AG levels were increased in the PT of these hamsters. A primary target of 2-AG, the cannabinoid receptor 1 (CB1), was expressed in the PD. Double-immunolabeling revealed that most of the CB1-immunoreactive cells in the PD were folliculostellate cells that were also immunoreactive for S-100 protein. Thus, the PT comprises an endocannabinoid system, and 2-AG may act as a photoperiodic messenger from the PT to the PD for the regulation of hypophysial hormonal secretion.     

Influence of photoperiod on the ultrastructure of the hypophysial pars tuberalis of the Djungarian hamster,Phodopus sungorus

Summary Conspicuous cytological differences are found between specific secretory cells of the hypophysial pars tuberalis of Djungarian hamsters exposed to long and short photoperiods. The cells differ with respect to the shapes of perikarya and nuclei and show diverse amounts of secretory granules, lysosome-like bodies and glycogen.Dedicated to Professor H. Rollhäuser, Münster, on the occasion of his 65th birthday     

Ultrastructure of melatonin-responsive cells in the ovine pars tuberalis

Summary Functional receptors for melatonin have been localized and characterized on the pars tuberalis (PT) of a number of mammalian species, but the cell-type responsive to melatonin is unknown. The ultrastructure of the ovine pars tuberalis has been examined and these findings correlated with the functional response of the gland to melatonin. This study revealed that two secretory cell types predominate in the ovine PT, which differ in the abundance of dense-core granules. The most abundant of the cells are either agranular or very sparsely granulated and represent 90% of the total population, with the remaining 10% being composed of cells with abundant dense-core vesicles. Few follicular cells were observed. This ratio of secretory cell-types persisted in primary culture, with the two types non-separable by Percoll gradient centrifugation. Using forskolin, as a non-specific stimulant of adenylate cyclase, melatonin was shown to inhibit the formation of cyclic AMP by 80–90% in cells both before and after Percoll centrifugation. The results demonstrate that the agranular secretory cells of the ovine pars tuberalis are the melatonin responsive cell-type of this gland.     

Cell organization of the rat pars tuberalis. Evidence for open communication between pars tuberalis cells, cerebrospinal fluid and tanycytes

The pars tuberalis (PT) is the only pituitary region in close contact with the medial-basal hypothalamus and bathed by cerebrospinal fluid (CSF). Although PT has long been recognized as an endocrine gland, certain aspects of its structure remain obscure. The present investigation has been designed to gain information concerning (1) the cellular organization of PT, (2) the PT/median eminence spatial relationship and (3) the exposure of various cell compartments of PT to CSF. Non-endocrine cells (S100-reactive) appear as the organizer of the PT architecture. The apical poles of these cells line large cistern-like cavities and the processes of these cells establish a close spatial relationship with PT-specific secretory cells, portal capillaries and tanycytes. The cisterns are also endowed with clusters of ciliated cells and with a highly electron-dense and PAS-reactive content. The unique spatial organization of endocrine and non-endocrine cells of the PT supports a functional relationship between both cell populations. PT endocrine cells display a hallmark of PT-specific cells, namely, the paranuclear spot, which is a complex structure involving the Golgi apparatus, a large pool of immature secretory granules and a centriole from which originates a single 9+0 cilium projecting to the intercellular channels. Horseradish peroxidase (HRP) injected into the CSF readily reaches the intercellular channels of PT and the inner channel of the single cilium and is incorporated by the endocytic machinery of the secretory cells. The PT endocrine cells, through their single 9+0 cilium, may act as sensors of the CSF. HRP also reaches the lumen of the cisterns, indicating that this PT compartment is also exposed to CSF. PT endocrine cells establish direct cell-to-cell contacts with hypothalamic β₁ tanycytes, suggesting a second means of brain-PT communication.     

The pars tuberalis as a target of the central clock

The pars tuberalis (PT) of the pituitary has emerged from being a gland of obscure and unknown function to a tissue of central importance to our understanding of how photoperiod regulates seasonal responses. The discovery of melatonin receptors on this gland first pointed to its involvement in seasonal physiology. However, the more recent demonstration of the expression of clock genes in the PT, such as Per1, has heightened interest in the gland. Recent work shows how photoperiod, through the hormone melatonin, affects the timing and amplitude of expression of the Per1 gene, as well as other genes such as Icer. The effect of photoperiod and melatonin on the expression of Per1 in the PT is distinct to its effects on the SCN, and this probably reflects distinct functions of the clock genes in the two tissues - acting as part of the biological clock in the SCN, but as an interval timing system within the PT. The changes in amplitude of Per1 gene expression in response to altered length of photoperiod have provided the first clues as to how the durational melatonin signal is decoded within the neuroendocrine system.     

Characterization of small intestinal mucus glycoproteins from pigs of various ages.

1. The mucus from 2-day-old and 3-week-old pigs was separated into polymeric mucus glycoprotein, degraded mucus glycoprotein subunit, and low-molecular-weight glycoprotein. 2. The low-molecular-weight glycoprotein fraction contained significant amount of mannose. 3. The degraded mucus glycoprotein subunit and the low-molecular-weight glycoprotein decreased in the mucus from older pigs.     

A comparative in vitro study on LHRH responsiveness of LH cells of the pars tuberalis and pars distalis

Summary The aim of the present study was to test whether the luteinizing-hormone (LH) cells in the pars tuberalis (PT) of the rat and mouse respond to LH-releasing hormone (LHRH) as do those of the pars distalis. A part of the basal hypothalamus containing the pituitary stalk, median eminence and the pars tuberalis (H-PT), was dissected out and incubated in vitro.The LH-secreting capacity of the PT was investigated after removal of the pituitary body (i.e., partes distalis, intermedia and nervosa). First, some rat and mouse H-PT tissues were treated with synthetic LHRH (100ng/ml), while others were incubated without LHRH. After 24 h of incubation, variable amounts of LH release were detected in the medium. This LH discharge, however, was not LHRH-dependent but proportional to the number of PT LH cells that were immunohistochemically detected in each incubated tissue. Since there was marked individual variation in the number of LH cells in the PT, the LH levels in the incubation medium were next compared before and after LHRH treatment using the same H-PT of the rat. An effect of LHRH could not clearly be shown in this experiment.Finally, the cytological response of the PT to LHRH was investigated by incubating both the H-PT and pituitary body connected to the intact pituitary stalk. Immunohistochemical examination of LHRH-treated tissues after 24 h revealed that, in females of both rats and mice, hormone depletion occurred in LH cells of the pars distalis but not in those of the PT. These results indicate that although LH cells in the PT can release LH in vitro, their mode of hormone synthesis and/or discharge differs from that of LH cells in the pars distalis. Since there was a marked individual variation and small LH-secreting capacity by the PT tissue, it seems unlikely, at least in rats and mice, that LH of PT origin plays an important role in the normal physiological state.     

Embryonic development and secretory differentiation in the pars tuberalis of the mouse hypophysis

Summary The pars tuberalis (PT) of the mouse, like that of other mammals, consists mainly of glandular cells rich in glycogen and peculiar to this lobe. In the mouse, the glandular cells are characterized by large, dense secretory vesicles (up to 300 nm in diameter), the abundance of which indicates a marked secretory activity. The PT develops from a distinct antero-ventral area of Rathke's pouch. The border between the anlagen of the PT and the pars distalis is formed by Atwell's recessus which represents the access for the vessels afferent to the pars distalis. The pedicle of Rathke's pouch is incorporated into the PT anlage, thus contributing to its formation. The entire PT anlage is characterized by glycogen accumulation from the commencement of its formation and persisting in the adult tuberal lobe. Secretory differentiation of the glandular cells of the PT occurs at day 12 of gestation, preceding that of all other adenohypophysial cell types. The secretory features of these cells (development of ergastoplasm and Golgi apparatus, abundance of dense secretory vesicles) appear at an early stage of the embryonic life (14 days) comparable to those of mature cells. These results confirm earlier observations in the foetal rat where hypophysial secretion also begins in the PT. The existence of peculiar glandular cells speaks in favour of a specific but still unknown function of the PT during foetal and adult life.This work is dedicated to Professor F. Stutinsky     

Hes1 regulates formations of the hypophyseal pars tuberalis and the hypothalamus

The hypophyseal pars tuberalis surrounds the median eminence and infundibular stalk of the hypothalamus as thin layers of cells. The pars tuberalis expresses MT1 melatonin receptor and participates in mediating the photoperiodic secretion of pituitary hormones. Both the rostral tip of Rathke’s pouch (pars tuberalis primordium) and the pars tuberalis expressed αGSU mRNA, and were immunoreactive for LH, chromogranin A, and TSHβ in mice. Hes genes control progenitor cell differentiation in many embryonic tissues and play a crucial role for neurulation in the central nervous system. We investigated the Hes1 function in outgrowth and differentiation of the pars tuberalis by using the markers for the pars tuberalis. In homozygous Hes1 null mutant embryos, the rostral tip was formed in the basal-ventral part of Rathke’s pouch at embryonic day (E)11.5 as well as in wild-type embryos. In contrast to the wild-type, the rostral tip of null mutants could not extend rostrally with age; it remained in the low extremity of Rathke’s pouch during E12.5–E13.5 and disappeared at E14.5, resulting in lack of the pars tuberalis. Development of the ventral diencephalon was impaired in the null mutants at early stages. Rathke’s pouch, therefore, could not link with the nervous tissue and failed to receive inductive signals from the diencephalon. In a very few mutant mice in which the ventral diencephalon was partially sustained, some pars tuberalis cells were distributed around the hypoplastic infundibulum. Thus, Hes1 is required for development of the pars tuberalis and its growth is dependent on the ventral diencephalon.     

Neuroglandular contacts between vasopressin-reactive fibers and cells of the pars tuberalis in the rat

Summary In electron micrographs fibers containing vasopressin-immunoreactive elementary granules 100–120 nm in diameter are observed within the basal lamina of the adenohypophyseal pars tuberalis adjacent to the rostral portion of the median eminence. The concept of a neuroglandular transmitter function of vasopressin is discussed.Supported by the Deutsche Forschungsgemeinschaft (Grant Nr. Kr. 569/2) and the Stiftung VolkswagenwerkThe excellent technical assistance of Mrs. Helga Prien is thankfully acknowledged