Glycoconjugate localization with lectin and PA-TCH-SP cytochemistry in rat hypophysis

Glycoconjugates were localized by light microscopy with lectin-peroxidase conjugates and by electron microscopy with the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) sequence in immunocytochemically or morphologically identified cell types in rat pituitary. Lectin histochemistry demonstrated sialic acid and glycoconjugates with N-glycosidically linked oligosaccharides in gonadotrophs, thyrotrophs, and corticotrophs. Galactose penultimate to sialic acid was observed mostly in gonadotrophs. The terminal galactose-N-acetylgalactosamine disaccharide was detected in a few gonadotrophs and in a moderate number of mammotrophs. Fucose was localized in only corticotrophs with two fucose-binding lectins and in thyrotrophs with another. Several different monosaccharides were seen in glycoconjugates in melanotrophs and in Herring bodies. Melanotrophs displayed heterogeneous staining with fucose-binding lectins. A small number of nonsecretory cells were also visualized in the pars distalis by virtue of their glycogen content. PA-TCH-SP staining revealed complex carbohydrates in secretory granules and some Golgi cisternae in all types of hormone-producing cells in the pars distalis except for the somatotrophs. Melanotrophs of pars intermedia exhibited stained secretory granules and irregular dense bodies containing a stained meshwork. Corticotrophs of the pars distalis lacked the latter bodies, although they form the same glycoprotein precursor hormone as melanotrophs. Lectin conjugates and the PA-TCH-SP sequence stained some groups of secretion granules in Herring bodies, possibly representing vasopressin-containing granules as well as other cell types in the pars nervosa.     

Topographical distribution of the gonadotrophs,mammotrophs, somatotrophs and thyrotrophs in the pituitary gland of the baboon (Papio cynocephalus)

The identification and regionalization of four pituitary parenchymal cell types, gonadotrophs, mammotrophs, somatotrophs and thyrotrophs, were studied in the baboon (Papio cynocephalus) hypophysis using immunocytochemistry. The gonadotrophs were homogeneously distributed throughout the entire pars distalis. Both mammotrophs and somatotrophs predominate at the superior and inferior poles of the organ. The medial and anteromedial regions are populated by mammotrophs and thyrotrophs, while the lateral and posterior portions of the pars distalis contain large numbers of somatotrophs.     

Functional cytology of the pituitary gland of the teleost Clarias batrachus (L.).

The RPD (rostral pars distalis) is formed of two cell types. The erythrosinophilic acidophils were hypertrophied in response to ammonium sulphate treatment and dark background adaptation. The response to former may be due to osmoregulatory involvement. The PbH positive corticotrophs were markedly hypertrophied and degranulated in response to metopirone and ammonium sulphate treatments. They were moderately activated in the dark and white background adapted fish. The PPD (proximal pars distalis) is composed of two types of cyanophils and acidophils. The thyrotrophs are identified by their hypertrophic response to radiothyroidectomy and thiourea treatment. They are distributed in the ventral part of the PPD, posterior neurohypophysis and PI (pars intermedia). The gonadotrophs which largely occupy the dorsal aspect of the PPD increase numerically as the gonads mature and become the major cell type before spawning. The acidophilic somatotrophs are distributed all over the PPD. They are the predominant cell type in the PPD of fish with immature gonads. They gave strong reaction to Baker's acid haematein indicating the presence of phospholipid. Both PbH and PAS cells of PI were stimulated in fish kept in dark background whereas they did not exhibit any obvious change under white background adaptation.     

Rathke's pouch grafts in adult brain sites

Donor tissue containing Rathke's pouch (RP) with its associated mesenchyme and neural lobe was isolated from 15-day fetal rats and stereotaxically transplanted either to hypothalamic hypophysiotropic sites or to cerebral cortex of adult females for 30 days. Hosts either were intact or had been hypophysectomized 2-4 weeks prior to transplantation of Rathke's pouch. Grafts in the hypothalamus of either intact or hypophysectomized hosts were pleomorphic and large, often as wide as 1-2 mm, and occasionally larger. Grafts in the cortex of either intact or hypophysectomized hosts were nodular and occasionally projected upward in association with the meninges (cortex/meninges grafts). Certain features were characteristic of the grafts in all experimental groups, i.e., development of histotypic pars distalis with cell cords and fenestrated capillaries. In all experimental groups gonadotrophs and somatotrophs, when present, were localized at the graft margin adjacent to the connective-tissue interface; mammotrophs, when present, were distributed throughout the graft. Features specific to each experimental group also were apparent. Grafts in the hypothalamus of both intact and hypophysectomized hosts typically were encapsulated by a labyrinthine meshwork of cell processes, whereas cortex/meninges grafts directly abutted dense connective tissue or neural tissue. In hypothalamic grafts in intact hosts, moderately differentiated mammotrophs, gonadotrophs, and somatotrophs could be identified by their cytological features and immunopositivity for prolactin, luteinizing hormone, and growth hormone, respectively. In hypothalamic grafts in hypophysectomized hosts, mammotrophs were absent, and gonadotrophs and somatotrophs were poorly granulated and not abundant. Grafts in the cortex of intact hosts contained numerous, well-differentiated mammotrophs, gonadotrophs, and somatotrophs. Many of the mammotrophs in these grafts were hypertrophied, and profiles of exocytosis were common. In grafts in the cortex of hypophysectomized hosts, mammotrophs were either absent or very few, whereas gonadotrophs and somatotrophs were numerous. Gonadotrophs in these grafts were dramatically hypertrophied, although exocytosis was rare. The results indicate that development of histotypic pars distalis may occur in hypophysiotropic and non-hypophysiotropic brain sites and that the hormonal state of the host as well as implantation site modulate cytodifferentiation of specific pars distalis cell types.     

Cytological changes in hypophysis of immatur rats after administration of anti-LH antibodies (author's transl)]

In the present work, cytological changes produced in rats' pars distalis were studied by administration of anti-LH antibodies. The anti-LH sera was obtained by active immunization of adult rabbits using bovine LH hormone with Freund's complete adjuvant. Newborn albino Wistar male and female rats were daily and subcutaneously inoculated with 0.1 ml. to 0.8 ml of anti-LH sera from the first 24 hr. on during 5 weeks. For controls, a similar schedule of inoculations with normal rabbit serum was used. At weekly intervals during the treatment, 5 lots of rats were sacrificed and the rest after a recovery period of 95 days. At the end of the 2nd week of immunization, both male and female animals showed degranulated gonadotroph cells in the central part of the pars distalis; at the 3rd week these cells were hypertrophied and presented developed Golgi complex. At the 4th week, the first large vacuoles of "castration" were seen in the gonadotroph cells of male pars distalis, besides in the females the degranulation of gonadotrophs continued. During the last week of treatment, the gonadotrophs of the male animals presented a highly dilated Golgi complex, more "castration-like cells" and numerous mitosis figures. The gonadotrophs of females pars distalis presented degranulation, but not vacuoles. After the recovery period the gonadotrophs of male rats were similar to those of the control hypophisis and did not show castration vacuoles in their cytoplasm. In female rats, the gonadotrophic cells showed "castration-like vacuoles" and a raised number of mitosis cells compared to control pituitaries. The significance of these findings will be discussed.     

Cellular associations of pituitary gonadotrophs in a rodent (Lagostomus maximus maximus) with photoperiod-dependent reproduction

The morphological characteristics and percentage of the cellular associations between gonadotrophs (LH- and FSH-secreting cells) and other cellular types were studied in pituitary pars distalis of adult male viscachas (Lagostomus maximus maximus) by double immunohistochemistry using specific antibodies to LH, FSH, PRL, GH, ACTH, TSH and S-100 protein (by folliculostellate cells; FSC), during long and short photoperiods. Bihormonal gonadotrophs were observed in ventro-medial and dorsal regions, interspersed between monohormonal gonadotrophs, and their number increased in short photoperiod. LH- and FSH-gonadotrophs were found around lactotrophs, enclosed by somatotrophs in the dorsal region, and associated with irregular corticotrophs. Gonadotrophs and thyrotrophs were associated along blood vessels and follicular structures. The cytoplasmic prolongations of FSC were in contact with both gonadotrophs. The percentage of LH–FSH, LH–ACTH, LH–FSC, FSH–LH, FSH–PRL, FSH–GH, FSH–ACTH, FSH–TSH and FSH–FSC associations decreased, whereas LH–PRL increased in short as compared to long photoperiod. The most abundant associations were LH–GH and LH–TSH during long photoperiod, but LH–GH and LH–PRL during short photoperiod. FSH–GH and FSH–PRL were the most numerous associations, and LH–FSC and FSH–FSC were the less abundant ones in both photoperiods. These results provide the morphological evidence for specific cellular associations between gonadotrophs and other cellular types of viscacha pituitary.     

The pituitary of non-pregnant and pregnant viscachas (Lagostomus maximus maximus): a comparative study by immunohistochemistry and morphometric analysis

The neuroendocrine hypothalamic-pituitary axis undergoes morphological and biochemical changes throughout gestation. In viscacha, pregnancy lasts approximately 154 days, and three stages can be described: early, mid- and late pregnancy. The aim of this work was to study the pituitary LH-gonadotrophs, FSH-gonadotrophs, somatotrophs, corticotrophs and thyrotrophs of non-pregnant and pregnant adult viscachas by immunohistochemistry and morphometric analysis. Immunopositive percentage area (%IA), cell percentage in the pars distalis (%PDC), number of cells per reference area (n°cell/RA), and major cellular (MCD) and nuclear (ND) diameters were analyzed. The different cell populations showed a well-defined morphology, immunolabeling patterns and regionalization in the pars distalis (PD). In the early pregnancy of animals the morphometric analysis demonstrated that %IA, %PDC and n°cell/RA increased in the FSH-gonadotrophs and decreased in the somatotrophs in relation to non-pregnant animals. In mid-pregnancy, there was an increase in %IA, %PDC, and n°cell/RA of LH-gonadotrophs, FSH-gonadotrophs, somatotrophs, and thyrotrophs. The MCD of LH-gonadotrophs and FSH-gonadotrophs increased. In late pregnancy, the %IA, %PDC and n°cell/RA of LH-gonadotrophs, FSH-gonadotrophs, somatotrophs and corticotrophs decreased whereas the values of the thyrotrophs remained constant. The MCD of LH-gonadotrophs, FSH-gonadotrophs and corticotrophs decreased. No significant changes were observed in the ND of the studied cell types. In conclusion, this work provides evidence for histological and morphometric changes in the different cell types of the pituitary PD in viscachas during pregnancy, probably according to the requirements of this physiological stage.     

Aminoglutethimide-stimulated corticotrophs

Thirty-six rats of both sexes in two groups were treated with aminoglutethimide (AG), a steroid synthesis inhibitor, for 1 and 8 weeks respectively. The anterior pituitaries were investigated by light and electron microscopy, the techniques used including immunocytochemical and morphometric methods. Corticotrophs were identified by the avidinbiotin-peroxidase complex technique at the light and electron microscopic levels. AG administration resulted in hyperplasia of ACTH-containing cells. The increase in the volume density of corticotrophs was prevented by simultaneous medication with 3 mg corticosterone in male rats, whereas in female rats a larger dose of corticosterone was required to suppress pituitary corticotroph hyperplasia. Electron microscopy revealed that in AG-treated rats, corticotrophs were larger and contained more secretory granules than those in controls, the mean secretory granule diameter increasing from 100 nm to 165 nm. Under AG stimulation, corticotrophs showed considerable variations in structural features probably reflecting differences in their functional state. It was apparent that only one ACTH-producing cell type existed in the pars distalis of the rat adenohypophysis, although this cell type may undergo substantial morphologic alterations due to changes in endocrine activity. Gonadotrophs and thyrotrophs showed morphologic signs of stimulation and exhibited hyperplasia following AG treatment indicating that the effect of the drug was not restricted to corticotrophs.     

Structure of the adenohypophysis in parturient female and neonate harp seals,Pagophilus groenlandicus

Summary The pars distalis of parturient harp seals (Pagophilus groenlandicus) is divisible into three distinct zones based on the major cellular components of the different regions. A clear zonation is lacking in neonate seals, the medial basophilic wedge and the anterior gonadotrophic were small and difficult to identify. Five cell types were identified based on cytological characteristics: somatotrophs, mammotrophs, thyrotrophs, gonadotrophs and stellate cells; corticotrophs could not be identified, with any certainty, in any of the preparations, nor could the gonadotrophs be classified into LH, FSH, and ICSH cells.The pars intermedia enlarged during the early post-partum period and was large in the parturient females.Abbreviations Used LH luteinizing hormone - FSH follicle stimulating hormone - ICSH interstitial cell stimulating hormone - PAS periodic acid-Schiff We wish to thank Mrs. Lucy Lin for her technical assistance. The work was supported by grants-inaid of research to J.F.L. and K.R. from the National Research Council of Canada and a grant from the Donner Canadian Foundation to K.R.     

Ultrastructural immunocytochemistry of gonadotrophs in the goldfish pituitary gland

Summary The immunocytochemical distribution of gonadotropin (GTH) in the goldfish pituitary gland was studied applying the peroxidase-antiperoxidase (PAP) method and the protein A-gold technique at lightand electron-microscopic levels, respectively, with an antiserum raised against silver carp GTH. In the light-microscopic immunocytochemistry, PAS-positive cells in the proximal pars distalis showed strong reaction with the antiserum. Gold particles were concentrated both on globules (large) and on granules (small) of the gonadotrophs (PAS-positive cells) in the electron-microscopic immunocytochemistry. Other cells in the pituitary gland, including thyrotrophs, displayed no immunoreactivity with the antiserum at the dilutions tested. These results indicate, not only immunocytochemical distribution of GTH both in globules and in granules in the gonadotrophs, but also the high purity of the antigen (silver carp GTH) and specificity of the antiserum.     

A typology of the gnathostome adenohypophysis with some emphasis on its gonadotropic function.

Recent publications show that the average gnathostome adenohypophysis consists of four parts and contains six morphological cell types. Histophysiological and immunocytochemical data prove the secretion of one or two gonadotropins by one type of basophils that in numerous species in characterized by the presence of large globules besides secretory vesicles. The existence of a second type of gonadotropin secreting cells remains doubtful. Thyrotropin is produced by a separate type of basophilic cells, prolactin by erythrosinophils of the rostral pars distalis, and somatotropin by orangeophils of the proximal pars distalis. At rostral and caudal places of contact with the neurohypophysis, cells are concentrated that may be lead-haematoxylin-positive, and produce a protein consisting of peptides with hormonal and opiate activities. Among the hormones secreted by these cells, situated in the pars distalis and the pars intermedia, are corticotropin and melanotropin respectively. The factors that cause the differentiation of the primordium of the adenohypophysis are almost entirely unknown.     

Structure of the adenohypophysis in juvenile harp seal,Pagophilus groenlandicus

Summary A study was made of the morphology of the adenohypophysis in immature harp seals and the fine structure of cellular components of the pars tuberalis, pars intermedia and pars distalis was described.The pars intermedia composed 8–15% of the hypophysis and contained colloid filled vesicles similar to those found in the other mammalian species.The pars distalis cells were grouped into more or less well defined regions, thus facilitating the correlation of cellular identification from both light and electron micrographs. Five chromophilic cell types were tentatively identified, one acidophil (putative somatotroph), four basophils (3 putative gonadotrophs and one cell type with the characteristics of both corticotrophs and thyrotrophs) and non-granulated stellate cells. The absence of a positive prolactin cell identification was thought to be due to the immaturity of the seals used in the study.The mercury exposure experiment was supported by a contract grant from the Halifax Laboratory of Fisheries and Marine Service. Drs. Uthe and Freeman of that laboratory also carried out the methyl mercury analyses. We recognize the support in maintaining the seals provided by Mrs. C. Rae, Mrs. H. Pedersen, Mr. S. Tessaro and Dr. J. M. Terhune. We also wish to thank Mrs. L. Lin for her technical assistance. Further financial support was provided through operating and development grants. The paper is number 134 in the physiology of migration series     

Lectin cytochemistry of cell types in human and canine pituitary

Labeled lectins specific for different sugars were employed to identify different cell types in pituitaries from six human autopsies and seven dogs. To determine the lectins bound by each cell type, fixed-paraffin embedded sections serial to those stained with lectins were immunostained for specific hormones and the serial pairs were examined in a comparison microscope. In human pituitaries corticotrophs stained selectively with lectins having affinity for alpha-L-fucose and the core region of complex type N-glycosyl-proteins. Some corticotrophs also stained for the presence of terminal beta-galactose. Thyrotrophs stained selectively with a periodate oxidation-borohydride reduction-concanavalin A sequence. Some mammotrophs evidenced content of glycoconjugate with terminal beta-galactose. Dendritic cells stained selectively for abundant glycogen with the periodate-reduction-concanavalin A sequence and a lectin from Griffonia simplicifolia. Adenohypophyseal cells of dog pituitary differed in showing absence of terminal beta-galactose in corticotrophs, presence of terminal beta-galactose in thyrotrophs, presence of glycoconjugate with N-glycosidically bound oligosaccharide in thyrotrophs and gonadotrophs and presence of terminal beta-galactose with a different lectin affinity in mammotrophs. The main contributions of lectin histochemistry applied to the pituitary include providing an additional histologic method for identification of some cell types, and localizing glycosylated prohormone or other biochemically unrecognized non-hormone glycoconjugates whose function in pituitary cells remains to be explained.     

Ultrastructure of the adenohypophysis in the teleost Poecilia latipinna

Summary In the sailfin molly, Poecilia latipinna, seven morphological endocrine celltypes could be distinguished with the electron microscope. Each of these was identified with one of the seven cell-types distinguished with the light microscope, to most of which endocrine functions have previously been allocated. Corticotrophs and prolactin cells form the rostral pars distalis, and the proximal pars distalis consists of an outer layer of gonadotrophs and an inner zone containing growth hormone cells and thyrotrophs. The pars intermedia contains two cell-types, of uncertain function. Stellate cells (interstitial cells) occur throughout the adenohypophysis, but are most numerous and prominent in the rostral pars distalis. The inner proximal pars distalis contains a cell-type not previously distinguished in this species with the light microscope, the Z-cell, which could be aminergic.The ultrastructural features of each cell-type are described in detail, and discussed in comparisons with the homologous cells described in other teleosts. There is good agreement for different teleosts in the ultrastructural details of each cell type.We thank Mr. L. Ethridge, Mr. M. P. Hancock, Mr. D. Hollingworth and Mr. W. Thomson for technical assistance, and Mr. D. Taylor of the Nuffield Institute of the Zoological Society of London for permission to use the Nuffield Institute electron microscope. We are grateful to Dr. Harry Grier, who collected and embedded glands from P. latipinna in its natural fresh-water habitat in Florida, U.S.A. T. Batten is in receipt of an S.R.C. Research Studentship.     

The Cell Types in the Adenohypophysis of the Marine Teleost Crenilabrus melops

Sagittal sections from pituitaries of winter and spring fish were stained with a number of methods including alcian blue-PAS-orange G, Herlant's tetrachrome, Luxol fast blue-PAS-orange G and lead haematoxylin. By these methods seven different endocrine cell types were readily demonstrated in the adenohypophysis. In the rostral pars distalis (RPD), there were conspicuous vacuoles (intra- and possibly extracellular) in the prolactin cell zone of all animals. The ACTH cells in the RPD were very numerous. In the proximal pars distalis (PPD), the growth hormone cells were arranged in coiling bands that were predominantly dorsal and juxtaposed to neurohypophysis. Two types of basophils were readily distinguished: both were predominantly ventral in the proximal pars distalis, though the type I basophil also formed a constant group in the dorsal region of the pituitary, between the PPD and the RPD. The pars intermedia contained two cell types — a lead haematoxylin-positive cell and a PAS-positive cell. The pituitary morphology is compared with that of other teleosts.     

Cyclical changes in the pituitary gonadotrophs (GTH cells) in relation to the testicular cycle in Puntius sarana (Hamilton)

The pituitary gland of Puntius sarana is of the leptobasic type and has three subdivisions--a proximal pars distalis (PPD), a rostral pars distalis (RPD) and a pars intermedia (PI) arranged vertically one below the other. The gonadotrophs (GTH cells) of the PPD were identified by their PAS, ATh, AF, aniline blue and alcian blue positivity and by their immunoreactivity to ovine anti-LH beta-subunit, anti-salmon GTH and anti-silver carp GTH. The testes of Puntius sarana have been classified as the "unrestricted type". The morphological, histological and histometric findings indicate that the testing passes through four distinct stages--preparatory, fully mature, spawning and spent. Spermatogenesis begins in January and continues up to July. The spawning season lasts from late July until October. Cyclical changes in the gonadotrophs correlate well with changes in the testes. The histometric data show that the GTH cells attain maximum values (size and number) during the breeding season. The peak values for testicular weight, lobule diameter, the gonadosomatic index (GSI) and the sperm concentration in the testes, and the number, size and amount of secretory material in the cytoplasm of the gonadotrophs, coincide with the time of maximum environmental temperature and the longest days.     

Cytology and distribution of secretory cell types in the mouse hypophysis as demonstrated with immunocytochemistry.

Since differential chemical staining has been unsatisfactory for demonstration of specific secretory cell types in the hypophyseal pars distalis of the mouse, the objective of this study was to determine whether the peroxidase-antiperoxidase immunocytochemical procedure might be more effective. Accordingly, representative sections from the hypophyses of 17 female and 15 male adult mice of the Swiss-Webster strain were immunostained, 16 antisera to 5 pituitary hormones or their subunits being utilized. Five secretory cell types were demonstrated. Somatotropes were ovoid to spheroidal and distributed quite generally in the gland except for the "sex zone" where they were scarce. Somatotropes were larger and more numerous in the male than in the female. Mammotropes were polyhedral and also generally distributed in the gland except for the "sex zone" where few were observed. Mammotropes were larger and more numerous in the female than in the male. Corticotropes were small, stellate and few. They were most common near the ventral surface of the gland and formed bilateral centromedial groups in the lateral wings. Thyrotropes were usually large and polyhedral. They were restricted almost solely to the ventral region of the pars distalis. Gonadotropes were polyhedral, and generally distributed, except for aggregation in the cephalomedian "sex zone." Most gonadotropes appeared to contain both luteinizing hormone and follicle-stimulating hormone. Thus, all secretory cell types recognized in other species can be demonstrated readily in the mouse hypophysis with immunocytochemistry.     

The utility of antisera to different synthetic adrenocorticotrophins (ACTH) and melanotrophins (MSH) for immunocytochemical staining of the dog pituitary gland

Using the immunoperoxidase technique and specific antisera to synthetic ACTH beta (1-24), ACTH beta (17-39) and bMSHbeta1, selective immunocytochemical staining was localized in a distinctive cell type in the pars distalis and pars tuberalis of the dog pituitary gland. Except for a rare cell, the pars distalis and pars tuberalis did not stain with an anti-bMSH alpha serum. In the pars intermedia immunoreactive cells containing ACTH beta(1-24), ACTHbetap(17-39), bMSHbeta and/or bMSH alpha were observed. The specificity and validity of the antisera were demonstrated by elimination of their immunostaining capacity after prior absorption with their respective antigens, while absorption with other antigens failed to decrease staining intensity. The cytoplasm of the ACTH/MSH cells showed a positive reaction to periodic-acid-Schiff and assumed a pale aniline blue colour, whilst the granules were stained with carmoisine L and acid alizarine blue. These ACTH/MSH cells were further differentiated from other functional cell types of the pars distalis on the basis of their typical cytological features, intraglandular distribution and by immunochemical double staining. It is concluded that ACTH and MSH beta were present and most probably produced by the corticomelanotrophs of the pars distalis and pars tuberalis. In addition to corticomelanotrophs analogous to those of the pars distalis and pars tuberalis, the pars intermedia showed many cells which contain MSH alpha alone or together with MSH beta and/or ACTH.     

Gonadotroph-lactotroph associations and expression of prolactin receptors in the equine pituitary gland throughout the seasonal reproductive cycle

An interaction between gonadotroph and lactotroph cells of the pituitary gland has long been recognized in several species. The current study was conducted to investigate whether an association between gonadotrophs and lactotrophs occurs in mares and whether prolactin receptors are expressed within the pituitary gland of this species. The effects of both reproductive state and season on these variables were examined in pituitary glands obtained from sexually active mares in July (breeding season), sexually active mares in November (non-breeding season) and anoestrous mares in November. Pituitaries were dissected out immediately after death and immunofluorescent staining was carried out on 6 micrometer sections using specific antibodies to the LHbeta subunit, FSHbeta subunit, prolactin and prolactin receptor. Gonadotrophs were observed in both the pars distalis and pars tuberalis; although they appeared mostly as isolated cells, small groups of gonadotrophs were also identified in the pars distalis. In contrast, lactotrophs were observed only as clusters of cells exclusively in the pars distalis of sexually active and anoestrous mares in November and in most of the sexually active mares in July. A specific gonadotroph-lactotroph association was identified only between large isolated gonadotrophs and lactotroph clusters. Double immunofluorescent staining for FSHbeta and prolactin revealed a similar gonadotroph-lactotroph association to the one detected for LH gonadotrophs. No statistical difference in the gonadotroph:lactotroph ratio was observed as a result of changes in reproductive status or season. However, a tendency for a simultaneous decrease in the number of gonadotrophs and an increase in the number of lactotrophs was detected in anoestrous animals. Prolactin receptor immunoreactivity was found in the pars distalis, but not in the pars tuberalis, of sexually active (July and November) and anoestrous animals for both long and short forms of the receptor. No prolactin receptor co-localization for either form of the receptor was observed in LH or FSH gonadotrophs in either of the reproductive states examined during both summer and winter seasons. Furthermore, no significant difference was apparent in the proportion of cells expressing prolactin receptors between mares of different reproductive state or season. The specific anatomical association between gonadotroph and lactotroph cells and the expression of prolactin receptors in the equine pituitary gland indicate a potential role of prolactin in the regulation of gonadotrophin secretion. However, the absence of evidence for co-localization of prolactin receptors in LH or FSH cells does not support the hypothesis of a direct effect of prolactin on the gonadotroph as reported in a short day breeder. The results raise the possibility that, in horses, an intermediate regulatory cell may mediate the action of prolactin on gonadotroph function.     

Histology of the pituitary gland of the grey mullet, Mugil cephalus L.

The present communication deals with a histological study of the pituitary gland of the teleost fish Mugil cephalus , found in the estuarine waters of Cochin area. Six different cell types were identified in the pituitary gland on the basis of their grouping, distribution and staining properties. The prolactin and the TSH cells (thyroid stimulating hormone producing cells or thyrotrops) were identified in the rostral pars distalis and the ACTH cells (adrenocorticotropic hormone producing cells or corticotrops) in the interphase between the neurohypophysis and the rostral pars distalis. The STH cells (somatotropic hormone producing cells or somatotrops) and the gonadotropic cells were distinguished in the proximal pars distalis and the MSH cells (melanin stimulating hormone producing cells or melanotrops) in the pars intermedia.