Ultrastructure of the nongranulated cells and morphology of the extracellular spaces in the pars distalis of adult and pouch-young tammar wallabies (Macropus eugenii)

Early onset of ghrelin production in a marsupial

Ghrelin regulates appetite in mammals and can stimulate growth hormone (GH) release from the pituitary. In rats and humans, ghrelin cells appear in the stomach during late fetal life. Nevertheless, the role of ghrelin in early mammalian development is not well understood. Marsupials deliver highly altricial young that weigh less than 1g so they must feed and digest milk at a comparatively immature stage of development. Since they complete their growth and differentiation while in the pouch, they are accessible models in which to determine the time course of ghrelin production during development. We examined the distribution of gastric ghrelin cells, plasma ghrelin concentrations and pituitary expression of the ghrelin receptor (ghsr-1alpha) and GH in the tammar wallaby, Macropus eugenii. There were ghrelin immunopositive cells in the developing mesenchyme of the stomach from day 10 post partum (pp) to day 150pp. Subsequently ghrelin protein in the fore-stomach declined and was absent by day 250pp but remained in the gastric cells of the hind-stomach. Ghrelin was detected in the developing pancreas from day 10pp but was absent by day 150pp and in the adult. Pituitary ghsr-1alpha expression and plasma concentrations of ghrelin increased significantly up to day 70-120pp while GH expression was also elevated, declining with GH to reach adult levels by day 180pp. These results demonstrate an early onset of gastric ghrelin expression in the tammar in concert with a functional stomach at a relatively earlier stage than that of developmentally more mature eutherian young.

Folliculostellate cells of the pituitary

In addition to the hormone-producing granulated cells, the so-called folliculostellate (FS) cells of the adenohypophysis represent a population of nongranulated cells extensively described in a large number of species. They show distinctive morphological features including a star shape with thin cytoplasmic projections extending between granulated cells and well-developed junctional complexes. FS cells are joined together surrounding irregular microcavities and project microvilli into the lumina. The immunocytochemical localization of S-100 protein, glial fibrillary acidic protein, and vimentin constitutes a reliable and easy method for investigating their presence and distribution in the normal pituitary gland and in pituitary adenomas. Although the expression of glial cell markers raised the hypothesis of a neuroectodermal origin of FS cells, most evidence supports that they derive from the epithelium of the Rathke's pouch, as do granulated adenohypophyseal cells. Morphological studies indicate that FS cells are involved in phagocytosis and possess sustentacular functions. Investigations using cell cultures show that FS cells play important roles in the paracrine regulation of adenohypophy-seal secretion by their ability to liberate several growth factors and regulate the ionic composition of the extracellular fluid. Further research using novel immunocytochemical markers and ceil culture techniques may clarify the origin and the role of this enigmatic cell type in the normal and pathological pituitary gland.

Occurrence of colloid-containing follicles and ciliated cysts in the hypophysial pars tuberalis from guinea pigs of various ages

Colloid-containing follicles and ciliated cysts in the hypophysial pars tuberalis of guinea pigs at various ages ranging from 5 days to 36 months were examined by periodic acid-Schiff (PAS) reaction, immunohistochemistry, and electron microscopy. The follicles storing PAS-positive colloid were encountered in the pars tuberalis of all guinea pigs examined, although only a few were present in young animals. The follicles gradually increased in number with age. The largest number of follicles was found in the senile male group: 141.3 +/- 11.9, about 10 times the number in the 5-day-old male group. The follicles were scattered throughout the entire length of the pars tuberalis. Follicles with enlarged luminal cavities were concentrated in the ventral caudal region surrounding the infundibular stem and merges with the pars distalis. Three different types of follicles were found by electron microscopy: 1) those surrounded by nongranulated follicular cells that may correspond to the stellate-follicular cells in the pars distalis, 2) those surrounded by specific cells that were packed with vesicular inclusions, and 3) those surrounded by granulated cells that may be gonadotropes. In the follicles lined by non-granulated follicular cells, long, prominent microvilli and cytoplasmic processes protruding into the lumen and invaginations of colloid were often observed at the apical cell region. The follicles lined by the specific cells having numerous vesicles were localized only in the ventral caudal portion. The vesicles ranged from 200 to 700 nm in diameter, and the outer surface of their limiting membrane was partly studded with ribosomes. Gonadotropes immunoreactive to the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) antisera were distributed in the guinea pig pars tuberalis. As well as the typical follicles described above, the follicles composed solely of granulated cells showed microvilli protruding into the cavities and junctional complexes at the apical lateral surface. They stored heterogeneous materials in the lumina. Some secretory granules gave the appearance of being discharged into the lumen. Ciliated cysts were frequently observed in the pars tuberalis; their incidence was 71.7%. The ciliated cysts were much larger than colloid-containing follicles. Cystic cavities were only partly filled with heterogeneous materials showing colloid-like, flocculent, and granular features.

Morphological, developmental and immunohistochemical observations on the opossum pituitary with emphasis on the pars intermedia

Development of the pituitary in Didelphis follows the general pattern of that described for most mammalian species. The dorsal region of a multichambered pituitary vesicle, which forms from Rathke's pouch, comes to lie adjacent to the presumptive infundibulum by the 10 1/2 d of gestation. The epithelial wall of this vesicle consists of spindle-shaped cells. The dorsal wall of the upper chamber of the pituitary vesicle forms the pars intermedia; the ventral wall of this chamber gives rise to cells of the pars distalis. Corticotropes, somatotropes, and lactotropes are seen in the presumptive adenohypophysis of the 11 1/2 d embryo. Gonadotropes and thyrotropes appear about 1 d later, shortly after birth. By the 2 postnatal week, the adult distribution of all 5 cell types within the pars distalis appears to have been established. The wall bounding the pituitary cleft in the adult represents an epithelial continuum limited by a basal lamina and corresponds to the upper chamber of the original pituitary vesicle. Ultrastructurally, the limiting walls of the pituitary cleft consists of stellate (marginal) cells, large, bulbous cells, and granulated cells. The latter correspond to the various endocrine cell types normally associated with the pars distalis. Non-granular folliculo-stellate cells also are observed within the epithelial cords of the pars distalis.

The development and morphogenesis of the human pituitary gland

In order to clarify the environmental factors which are involved in the development of the primordium of the pituitary gland such as cell-cell interactions, a three-dimensional reconstruction of this organ and its surrounding tissues was carried out. Pituitary material was obtained from human fetuses mainly during the period of organogenesis. Rathke's diverticulum was found to stretch rostrally from the stomodeal epithelium to the middle of the mesoderm, and already by the 5th week of fetal growth, it was clearly seen to be involved with the diencephalon. The area of contact between Rathke's pouch and the diencephalon gradually moved from the rostral to caudal regions and, after 13 weeks of development, had a position similar to that found in the newborn infant. Among the cells forming Rathke's pouch, it was found that the closer their relationship was to the diencephalon, the greater were their epithelial characteristics. When the relationship of such cells to the diencephalon was weaker, their differentiation to endocrine cells occurred earlier. Immunohistochemically, that portion of the pituitary primordium which has a close relationship with the diencephalon, later to become the pars intermedia, showed an adrenocorticotropic hormone (ACTH) immunoreactivity later than that of the pars anterior. On the other hand, in the 21st fetal week, nearly all of the cells of the pars intermedia were found to be ACTH-positive. This finding is thought to indicate a close connection between the physical contact between the brain (diencephalon) and the pituitary primordium and the development of the pars intermedia; the differentiation of ACTH cells. The surface of the epithelium of Rathke's cavity continues to increase at least until the 21st fetal week, so the growth of the epithelium of Rathke's pouch is thought to be heavily involved in the growth of the primordium of the pituitary gland in the early stages of development.

Cystlike structures derived from the marginal cells of Rathke's cleft in rat pituitary grafts

Hemipituitary glands from 30 rats were isotransplanted under renal capsules. At 1, 2, 4, 7 and 20 days after transplantation, the grafts were examined by light and electron microscopy. Two days after transplantation, the central area of graft showed necrosis; however, the peripheral area, where marginal cells of Rathke's cleft were ingesting the remnants of necrotic glandular cells, survived. Four days after transplantation, mitotic figures of marginal cells were observed. Seven days after transplantation, the damaged area of the graft disappeared, and Rathke's cleft was completely lined by marginal cells. Remnants of necrotic glandular cells were not seen in intercellular spaces or in the cytoplasm of marginal cells. Cystlike structures formed in the grafts; some were connected to Rathke's cleft by narrow cavities. The cavity-lining cells of the cysts were agranular and similar to those that lined Rathke's cleft. At 20 days after transplantation, granular cavity-lining cells appeared. It is suggested that marginal cells of immature rats can differentiate into granular cells.

The origin of ciliated cell cysts of the anterior pituitary. An experimental study in the rat

Rats were decapitated and the heads were stored at 4 degrees C for 24 h. The anterior pituitaries were then removed and incubated for 5 days. On the 1st, 3rd and 5th days of incubation, explants were studied by electron microscopy and immunohistochemistry using antiserum against S-100 protein. In the explant many granulated cells underwent necrosis; folliculo-stellate (FS) cells formed many cyst-like structures (CLSs) and became squamous epithelioid cells (CLS-forming cells). After incubation for 5 days the explants were isotransplanted under the renal capsules of male rats in order to observe morphological changes in the CLSs. Immediately after transplantation, the CLSs were encircled by a basement membrane, but from the 8th to 14th day, ciliation occurred in CLS-forming cells and "ciliated cell cysts" were formed. The ciliated cells were immunostained with antiserum against S-100 protein. The present study suggests that FS cells are related to CLS formation and have the potential to trans-differentiate to ciliated cells.

Proliferating potential of folliculo-stellate cells in human pituitary adenomas. Immunohistochemical and electron microscopic analysis

Folliculo-stellate cells (FS cells) in 40 pituitary adenomas and portions of anterior pituitary adjacent to the tumor in 26 cases were investigated immunohistochemically, using polyclonal antisera to S-100 protein (S-100) and glial fibrillary acidic protein (GFAP). The objective was to clarify the histological behavior of the FS cells. In most pituitary adenomas there were few or no S-100- or GFAP-positive cells, in comparison with numerous positive cells in the parts of the adenohypophyses compressed by adenomas. However, positive FS cells were observed in some types of pituitary adenomas. Growth hormone and prolactin producing adenomas frequently contained significant amounts of FS cells. In non-functioning adenomas, an unique case of FS cell adenoma was present. The adenoma was composed mainly of FS cells and immature glandular cells. The FS cells were sometimes located around follicles containing Periodic acid Schiff-positive material. Therefore, the FS cell adenoma is characterized by S-100- and GFAP-positive FS cells and PAS-positive follicles. In this type of adenoma, FS cells seemed to be the main proliferating component. In parts of the adenohypophyses adjacent to the adenomas, GFAP-positive FS cells were numerous. In the pathological conditions FS cells may possess the potential of reactive proliferation.

The embryological development and cytodifferentiation of the anterior pituitary in the marsupial, Isoodon macrourus

Light and electron microscopy were used to study the development of the anterior pituitary gland from fetal stages to the end of pouch life in the marsupial I. macrourus. The early morphological development of the anterior pituitary in I. macrourus follows a similar pattern of events to that described for eutherians. Rathke's and Seessel's pouches were present in 10 1/2 day old embryos. At birth these pouches had formed a multi-chambered vesicle which was still connected to the stomodeum by a thin cord of tissue. A small number of granules (200-400 nm dia.) were found in cells at birth. These cells could not be classified on ultrastructural features but alcian blue-periodic acid Schiffs-orange G staining suggested one cell type was possibly a presumptive thyrotroph. There were no capillaries in the pars distalis at birth. The cords connecting Rathke's and Seessel's pouches to the stomodeum were located at the site of the periosteal bud of the developing basisphenoid which commenced to ossify at 7 days. At this stage presumptive thyrotrophs, gonadotrophs, and somatotrophs could be distinguished using alcian blue-periodic acid Schiffs-orange G staining. However, five cell types could be categorised at the same age using ultrastructural characteristics alone. Precise names for these cells are unavailable but two closely resemble presumptive mammotrophs and thyrotrophs described for another marsupial M. eugenii. By 13 days after birth the anterior pituitary of I. macrourus had become vascular and acidophils were concentrated in a posterior zone. There was little gross morphological change from 13 to 66 days after birth by which the time weaning has occurred. Cilia were seen in cells of the anterior pituitary and mitosis of granulated cells was observed from birth onwards. There is a considerable range of variation in pituitary cytogenesis amongst marsupials, and its functional significance awaits further investigation.

Structure of the pars distalis in pouch-young tammar wallabies (Macropus eugenii)

The pars distalis of pouch-young wallabies (Macropus eugenii) aged 1 to 50 days was studied by means of light-microscopic immunohistochemistry and electron microscopy. In the pars distalis of these pouch-young presumptive somatotrops, which constituted up to 70% of the gland, and nongranulated cells were the most numerous cell types. Small numbers (together representing less than 30% of the pars distalis) of immunoreactive mammotrops, thyrotrops, gonadotrops and corticotrops were also found. The presumptive mammotrops, gonadotrops and thyrotrops increased in number and apparent activity between 1 and 50 days postpartum. Presumptive corticotrop cells in 25 to 30 day-old animals were relatively most numerous, and apparently more active than at any other stage of pouch life; these cells decreased in apparent activity and relative number in older animals. The changes in number and activity of cell types in the pars distalis correlated well with major developmental events such as the onset of adrenal activity, the rapid growth phase in the first 100 days postpartum, and the generally low thyroid activity in pouch-young of less than 50 days of age.