The ultrastructure of the cell types and of the neurosecretory innervation in the pituitary of Mugil cephalus L. from fresh water, the sea, and a hypersaline lagoon. I. The rostral pars distalis

Functional Pituitary Networks in Vertebrates

The pituitary is a master endocrine gland that developed early in vertebrate evolution and therefore exists in all modern vertebrate classes. The last decade has transformed our view of this key organ. Traditionally, the pituitary has been viewed as a randomly organized collection of cells that respond to hypothalamic stimuli by secreting their content. However, recent studies have established that pituitary cells are organized in tightly wired large-scale networks that communicate with each other in both homo and heterotypic manners, allowing the gland to quickly adapt to changing physiological demands. These networks functionally decode and integrate the hypothalamic and systemic stimuli and serve to optimize the pituitary output into the generation of physiologically meaningful hormone pulses. The development of 3D imaging methods and transgenic models have allowed us to expand the research of functional pituitary networks into several vertebrate classes. Here we review the establishment of pituitary cell networks throughout vertebrate evolution and highlight the main perspectives and future directions needed to decipher the way by which pituitary networks serve to generate hormone pulses in vertebrates.

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.

Stellate Cell Networks in the Teleost Pituitary

The folliculostellate cells of the mammalian pituitary are non-endocrine cells that are implicated in long-distance communication and paracrine signaling, but to date, these cells have yet to be characterized in teleosts. We found that the stellate cells of the teleost pituitary share many common attributes with mammalian folliculostellate cells. By labeling of stellate cells in live preparations of tilapia pituitaries we investigated their distribution, association with other endocrine cells and their anatomical and functional coupling. In the pars intermedia, stellate cells were arranged around neuronal bundles and their processes extended into the pars distalis. Within the pars distalis, stellate cells formed close associations with FSH cells and, to a lesser degree, with GH and LH cells, suggesting differential paracrine regulation of the two gonadotrope populations. The production of follistatin by stellate cells further corroborates the notion of a paracrine role on FSH release. We also found stellate cells to form gap junctions that enabled dye transfer to neighboring stellate cells, implicating that these cells form a large-scale network that connects distant parts of the pituitary. Our findings represent the first wide-scale study of stellate cells in teleosts and provide valuable information regarding their functional roles in pituitary function.

Immunocytochemical demonstration of melanotropic and adrenocorticotropic cells from the gilthead sea bream (Sparus aurata L., Teleostei) by light and electron microscopy: an ontogenic study

In the pituitary of gilthead sea bream, Sparus aurata, melanotropic (MSH) and adrenocorticotropic (ACTH) cells were identified at the light and electron microscopic levels using rabbit anti-synthetic alphaMSH (MSH) and anti-human ACTH (1-24) (ACTH) sera. The distribution of these cell types was followed from hatching to 48 months. The techniques used included the peroxidase anti-peroxidase (PAP) method, conventional electron microscopy, and an immunogold technique. Using PAP, MSH (immunoreactive to both anti-MSH and anti-ACTH) and ACTH (immunoreactive to anti-ACTH) cells were detected from hatching onward. These cells were distinguished ultrastructurally in 1-day-old larvae. Immunogold labeling was first detected in MSH cells in 5-day-old larvae, while ACTH cells were only immunogold labeled in adults. In newly hatched larvae, MSH cells were located from the middle to the posterior region of the adenohypophysis, while ACTH cells were found in the dorsoanterior region, next to the hypothalamus. At this age, both cell types were scarce. As the fish developed, these cell types progressively increased in number: MSH cells made up a layer surrounding the neurohypophysis (NH) in the pars intermedia (pi), whereas ACTH cells bordered the developing NH in the rostral pars distalis (rpd). From 82 days onward, a few MSH cells were observed in the proximal pars distalis (ppd) next to the pi and some ACTH cells were seen in the ppd next to the rpd. In adult specimens, both MSH and ACTH cells were adjacent to the stellate cells and showed processes and synaptic-like structures. MSH cells exhibited numerous round secretory granules with a granular content and of varying electron density and compactness. These granules were immunogold labeled with anti-MSH serum. Electron-dense secretory granules near the Golgi complex immunoreacted with anti-MSH, anti-ACTH, or with both antisera. ACTH cells exhibited round secretory granules with a homogeneous, high electron-dense core and a narrow, clear halo. These granules immunoreacted with anti-ACTH serum. The main ultrastructural features that characterize the MSH and ACTH cells of adults appeared early during ontogeny. Involutive MSH and ACTH cells were only observed in adult specimens.

An immunocytochemical study of the pituitary gland of the white seabream (Diplodus sargus)

The adenohypophysis of the white seabream (Diplodus sargus) was studied using histochemical and immunocytochemical techniques. The adenohypophysis was composed of rostral pars distalis, proximal pars distalis and pars intermedia. Prolactin (anti-chum salmon prolactin positive) and adrenocorticotropic (anti-human ACTH positive) cells were found in the rostral pars distalis. Prolactin cells were organized into follicles, while ACTH cells were arranged in cords around neurohypophyseal tissue branches that penetrated the rostral pars distalis. In the proximal pars distalis, somatotropic (anti-chum salmon and anti-gilthead seabream growth hormone positive), gonadotropic (anti-chum salmon beta-gonadotrophin II and anti-carp beta-gonadotrophin II positive, but anti-chum salmon beta-gonadotrophin I negative) and thyrotropic (anti-human beta-thyrotropin positive) cells were observed. Growth hormone cells were restricted to the dorsal and ventral part of the proximal pars distalis. They were clustered or surrounded the neurohypophyseal branches. Only one type of gonadotrophin cell was identified and they were clustered or isolated in the proximal pars distalis. Scattered groups of thyrotropin cells were located throughout the proximal pars distalis. In the pars intermedia somatolactin (anti-chum salmon and anti-gilthead seabream somatolactin positive) and melanotropic (anti-alpha-melanotropic hormone positive) cells were localized. In addition, gonadotrophin cells surrounded the pars intermedia or distributed evenly between somatolactin and melanotropic hormone cells. Somatolactin cells were periodic acid-Schiff negative and surrounded the neurohypophyseal branches intermingled with melanotropic cells. These cells were also immunoreactive to anti-human ACTH antiserum.

Immunocytochemical and ultrastructural characterization of melanotropin and adrenocorticotropin cells from the Mediterranean yellowtail (Seriola dumerilii, Risso 1810)

BACKGROUND

Melanotropin (MSH) and adrenocorticotropin (ACTH) are pituitary hormones derived from a common precursor: the proopiomelanocortin (POMC), which is processed differently in the melanotropic and corticotropic cells of several vertebrates. While ACTH is a major final product in corticotropes, it is further processed into alpha-MSH and corticotropin-like intermediate lobe peptide (CLIP) in melanotropes. Cells which are immunoreactive to ACTH (ACTH cells) and to both alpha-MSH and ACTH (MSH cells) have been described in a number of teleosts, including the Mediterranean yellowtail, by light microscopic immunocytochemistry. However, these cells have been ultrastructurally characterized only in a few species. In this paper, we use electron microscopy to identify and characterize the cells producing MSH and ACTH in M. yellowtail (Seriola dumerilii).

METHODS

Pituitaries from adult specimens were dissected and processed for conventional and immunocytochemical electron microscopy. An immunogold technique was performed using anti-synthetic alpha-MSH and anti-human (h) ACTH (1-24) sera.

RESULTS

MSH cells had round secretory granules with a granular content of varying electron density and compactness, which were immunogold-labeled with anti-alpha-MSH. Homogeneous and electron-dense secretory granules found in the Golgi area of these cells reacted with both anti-alpha-MSH and anti-hACTH (1-24). ACTH cells had round secretory granules with a homogeneous and medium or high electron-dense core and narrow clear halo, which were grouped in the cell area near the neurohypophysis (NH). Some granules showed an osmiophilic semicore in the medium electron-dense content, which has not been described in other teleost pituitary cells. Immunogold-labeling over the secretory granules only was obtained with all the antisera used. Some ACTH cells showed involutive features.

CONCLUSIONS

MSH and ACTH are respective final products of the POMC in two ultrastructurally different cells of the pituitary of M. yellowtail, MSH and ACTH cells. The immature granules in the Golgi area of MSH cells seem to be the site of proteolitic cleavage of ACTH into alpha-MSH and CLIP.

Effect of osmotic pressure on prolactin release in rainbow trout: in vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

Ultrastructural changes in the adenohypophysis during the ovarian cycle of the viviparous teleost Poecilia latipinna. III. The growth hormone, adrenocorticotrophic, and prolactin cells and the pars intermedia

During the monthly cycle of vitellogenesis, intraovarian gestation, and parturition, the pituitary growth hormone (GH) cells show ultrastructural changes indicative of an increase in secretory activity related to vitellogenesis. In contrast, the pituitary adrenocorticotrophic (ACTH) cells are relatively inactive during vitellogenesis, but become active during late pregnancy in the few days before parturition. The prolactin cells and the two cell types of the pars intermedia do not appear to change their secretory activity during the cycle. In discussing these findings it is suggested that GH may play a metabolic role related to vitellogenesis, and that corticosteroids, secreted in response to elevated output of ACTH, may facilitate follicular rupture and/or the expulsion of the brood at the end of pregnancy.

Cellular composition of the rostral pars distalis of the anterior pituitary gland of the alewife, Alosa pseudoharengus, during the spawning run

The rostral pars distalis of the anterior pituitary gland of the marine alewife, Alosa pseudoharengus, during its annual spawning run to fresh water was examined histologically. The rostral pars distalis is composed of many interconnecting follicles of various sizes. Contrary to earlier reports, the follicular epithelium contains not only prolactin (PRL) cells but corticotropic (ACTH) cell and thyrotropic (TSH) cells (in addition to two nonendocrine cell types). Basally all three endocrine cell types make direct contact with the basement membrane which separates the follicles from the neurohypophysial processes. Apically, however, only the prolactin cells, the largest of the three, protrude into the follicular lumen by means of the small ciliated apical protruberance. All other cellular elements are sealed from the follicular lumen by a layer of covering cells which have properties of transitional epithelial cells. In the follicular epithelium, the slender TSH cells are intercalated between the large conspicuous prolactin cells. The ACTH cells, the smallest of the three endocrine cells, lie in deep invaginations in the basal regions of the individual PRL cells in such a way that on cursory examination they can be mistaken for the nuclei of the latter. Only a small portion of the cellular surface of the ACTH cell escapes the enveloping prolactin cell to make contact with the basement membrane of the follicle. In teleosts, prolactin, ACTH, and TSH have all been implicated in the regulation of hydromineral metabolism and reproductive development. The intimate spatial relation between the three endocrine cells in the alewife rostral pars distalis thus raises the possibility of some functional interactions at the adenohypophysial level, perhaps as an adaptation of this anadromous teleost whose reproductive development and behavior is associated with large changes in ambient salinity. The functional significance of the follicular lumen is discussed together with possible sensory functions of the PRL cells.

The extravascular channel system in the rostral pituitary of Mugil cephalus (Teleostei) as revealed by use of horseradish peroxidase

The interlobular spaces, separating the neuro- and the adenohypophysis can be visualized in the pituitary of the Grey Mullet by using the horseradish peroxidase technique. On the one hand, these spaces communicate with the pericapillary spaces. On the other, they are linked with the pericellular spaces in the adenohypophysis and the periaxonal spaces in the neurohypophysis, thus providing a pathway for the exchange of metabolites and hormones between blood system, epithelial cells and the terminal region of the neurosecretory axons. The interlobular spaces branch off between the tips of the corticotrop cells and form a network adjacent to the adeno- and neurohypophyseal interface. It is possible that branches of this network penetrate into the interior of the corticotrops. The probable role of the network is to permit close contact between neurosecretory hormones and the corticotrops and also to enable the corticotrop hormone to reach the capillaries.