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.     

Cytology and cytochemistry of the hypophysis of the freshwater teleost Rhamdia hilarii (Val., 1840).

The characterization of cell types in the rostral pars distalis, proximal pars distalis and pars intermedia of the Bagre fish hypophysis during the reproductive cycles, as well as a cytochemical study of mucosubstances were undertaken. The cytophysiological relationship among the different acidophilic, basophilic and chromophobic cells with the several hypophyseal hormones was also considered.     

Immunocytochemical identification and ontogeny of adenohypophyseal cells in a cave fish,Phreatichthys andruzzii (Cypriniformes: Cyprinidae)

The morphogenesis of the pituitary gland and the chronological appearance of adenohypophyseal cells were investigated for the first time in the Somalian cave fish Phreatichthys andruzzii by immunocytochemistry. The adult adenohypophysis contained: a rostral pars distalis, with prolactin (PRL) cells arranged in follicles and adrenocorticotropic (ACTH) cells, a proximal pars distalis with somatotropic (GH), β‐thyrotropic (TSH), β‐gonadotropic type I (FSH) and type II (LH) cells and a pars intermedia with α‐somatolactin (SL), α‐melanotropic (MSH) and β‐endorphin (END) cells. All regions were deeply penetrated by neurohypophyseal branches. At hatching (24 h post‐fertilization) the pituitary was an oval cell mass, close to the ventral margin of diencephalon. The first immunoreactive cells appeared as follows: PRL at 0·5 days after hatching (dah), GH and SL at 1·5 dah, END at 2 dah, TSH, ACTH and MSH at 2·5 dah, FSH at 28 dah and LH at 90 dah. The neurohypophysis appeared at 5 dah and branched extensively inside the adenohypophysis at 130 dah, but there was no boundary between rostral pars distalis and proximal pars distalis at this stage. The potential indices of prolactin and growth hormone production increased until 28 and 60 dah, respectively. The potential index of growth hormone production correlated positively with total length. Activity of PRL and GH cells, measured as ratio of cell area to nucleus area, was significantly higher in juveniles than in larvae.     

Fine structure of the adenohypophysis in immature sockeye salmon,Oncorhynchus nerka

Summary The ultrastructure of the secretory cells of the adenohypophysis of juvenile sockeye salmon was investigated. Pituitary glands were collected from immature fish transferred experimentally to sea water and subsequently returned to fresh water. The rostral pars distalis contained three cell types: ACTH cells, prolactin cells, and non-secretory cells. The prolactin and non-secretory cells were joined together in the form of follicles by desmosomes and they both had cilia and microvilli projecting into the follicle lumen. Various follicular structures such as lumen, multivesicular structures, and peripheral basement membrane are discussed as possible sites of prolactin cell granule release. The columnar ACTH cells were found at the junction of the rostral pars distalis and the neurohypophysis. The cytoplasmic granules in these cells were characteristically separated from their limiting membrane by a clear space. Multivesicular structures were also found in association with this cell type. The caudal pars distalis also contained three cell types: one acidophil (putative somatotrop) and two basophils (putative thyrotrops and gonadotrops), all of which were similar to those described in adult fish. The pars intermedia contained only one cell type. They appeared to be active cells and were characterized by containing membrane-bounded granules similar to those found in the ACTH cells. Changes in ambient salinity had no apparent effect on any cell type described.The work was supported by a grant in aid of research from the National Research Council of Canada. We wish to thank Mr. R. Lindsay, Mr. C. Cooper, and Mr. G. Longworth for their technical assistance. We would also like to thank Mr. S. Killick of the International Pacific Salmon Fisheries Commission for his assistance in the collection of fish and Dr. H. Cook for his helpful discussion of the project. This paper is No. 058 in the University of Guelph Migration Series.     

Immunohistochemical identification of cells in the pars distalis of the pituitary of the lizard Anolis carolinensis

Immunocharacteristics of the pars distalis cells of the pituitary of the male lizard A. carolinensis are determined by employing the immunoperoxidase technique with antisera to mammalian pituitary hormones. On the basis of their immunoreactivity, 5 different cell types with characteristic anatomical distribution are recognized. ACTH cells are found in the rostral half of the pars distalis, and PRL cells in the rostral two thirds of the pars distalis. GH and TSH cells are located in the caudal half of the pars distalis. GTH cells are distributed throughout the gland. When consecutive sections are stained with antiserum to ovine FSH or its beta-subunit and to ovine LH, the same cells show immunoreactivity to all the three antisera. None of the GTH cells show positive immunoreactivity to ovine beta-LH antiserum. The results suggest the existence of one gonadotropic cell type in the pituitary of this lizard.     

The Pituitary Gland of the Roach Leuciscus rutilus

Three different endocrine cell types were identified by electron microscopy in the rostral pars distalis (RPD) of the roach, Leuciscus rutilus. The cells which are characterized by the morphology and sizes of their secretory granules are assumed to secrete prolactin, ACTH and TSH. Prolactin cells occur in two forms which could be separated on account of the sizes of their granules. Intra-peritoneal injections of reserpine had a strong and persistent effect 24 hours after the treatment. The prolactin cells with small granules reacted with a marked release of secretion. The border of the RPD is composed of a so-called double basement membrane in contact with the perivascular spaces and forms extensive ramifications into the rostral neurohypophysis (RNH). Nerve fibers of different categories terminate at the border and its network in the RNH. Substances released from these terminals may influence the endocrine cells of the RPD. The ACTH cells and the prolactin cells are innervated by axons with granules less than 100 nm. The nomenclature of type ‘A’ fibers and ‘B’ fibers is discussed because it was difficult to make a reliable discrimination of the fibers and their nature by the sizes of their granules. However, four types of nerve fibers were distinguished in connection with the RPD although the presence of more types was indicated. In all four types the size variations of the granules are great and overlapping. The average sizes of the granules in these fibers are 157, 114, 119 and 81 nm. The origin of the nerve fibers innervating the RPD is discussed. All morphological data indicate a complex hypothalamic control of the RPD of the roach.     

The Thyrotropic Cell in the Atlantic Salmon,Salmo salar

The thyrotropin (TSH) producing cells are distributed in the rostral and proximal pars distalis. This cell type is the smallest and most infrequent cell of the adenohypophysis. Its cytology is similar to the smallest gonadotropic (GTH) cells although the two cell types can be separated by the size of the small secretory granules (diameter less than 200 nm) in the TSH cells. In presmolts and smolts the cells are more numerous than in parr and adult salmon and have cytological features indicating an increased activity. This was also the case after intraperitoneal injections of synthetic TRH. Antisera to carp GTH and salmon GTH cross-reacted with both the GTH and the TSH cells. Anti-human TSH cross-reacted only with the TSH cells which confirms the assumption of antigenic similarity between human and fish TSH.     

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 -gonadotrophin II and anti-carp -gonadotrophin II positive, but anti-chum salmon -gonadotrophin I negative) and thyrotropic (anti-human -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--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.     

Immunohistochemical identification of cells in the pars distalis of the pituitary of the lizard anolis carolinensis

Summary Immunocharacteristics of the pars distalis cells of the pituitary of the male lizard A. carolinensis are determined by employing the immunoperoxidase technique with antisera to mammalian pituitary hormones. On the basis of their immunoreactivity, 5 different cell types with characteristic anatomical distribution are recognized. ACTH cells are found in the rostral half of the pars distalis, and PRL cells in the rostral two thirds of the pars distalis. GH and TSH cells are located in the caudal half of the pars distalis. GTH cells are distributed throughout the gland. When consecutive sections are stained with antiserum to ovine FSH or its -subunit and to ovine LH, the same cells show immunoreactivity to all the three antisera. None of the GTH cells show positive immunoreactivity to ovine -LH antiserum. The results suggest the existence of one gonadotropic cell type in the pituitary of this lizard.Supported by U.S. Council for International Exchange of Scholars (to D.R.N.) and PHS Grant NS09914     

Pituitary cytology of radiothyroidectomised teleost Heteropneustes fossilis (Bloch.).

The pituitary of Heteropneustes fossilis is divisible into rostral (RPD) and proximal (PPD) pars distalis, and pars intermedia (PI). Neurohypophysis extensively ramifies in the PI. The RPD is formed of acidophilic prolactin cells and lead hematoxylin (H. Pb) positive ACTH cells. The latter is arranged around the neurohypophysial extensions. The PPD is comprised of thyrotrophs (TSH) and gonadotrophs which are stainable with PAS, AF, and aniline blue. Acidophilic somatotrophs (STH) and chromophobes are scattered throughout the PPD. In the pars intermedia PAS positive and H. Pb positive cells are present. The latter is believed to secrete MSH whereas the function of the former is yet to be confirmed. In the partly and completely thyroidectomised H. fossilis the TSH cells exhibited marked hypertrophy. They are located in the middle of the PPD flanked by the gonadotrophs on either side. Hypertrophy of the ACTH cells were also evident. But the other cell types did not show any marked changes as compared to the controls.     

Immunohistochemical Localization of Growth Hormone and Prolactin in the Pituitary Gland of Acipenser güldenstaedti Brandt (Chondrostei)

Abstract Prolactin (LTH) and growth hormone (GH) containing cells in A. güldenstaedti have been localized by means of anti-ovine prolactin and anti-bovine growth hormone respectively, coupled indirectly to peroxidase, and localized histochemically with hydrogen peroxide as substrate and 3,3′-diaminobenzidine as capturing agent. The distribution of the anti-prolactin positive cells has been demonstrated and correlated histologically with the acidophilic cells both in the rostral and proximal pars distalis. This cell type is elongated and arranged in follicles in the rostral pars distalis; in the proximal pars distalis they are smaller and oval, without any special orientation. Neither of the other cell types which are scattered among these acidophils contain prolactin. The anti-bovine growth hormone positive cells are evenly distributed in the proximal pars distalis above the hypophysial cleft, and some are also found in the pars intermedia. The anti-GH positive cells have been correlated histologically with the amphiphilic cells in the proximal pars distalis. These cells are arranged in cell cords in close contact with the secondary capillary plexus, near its origin from the primary capillary plexus covering the median eminence.     

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.     

Distribution of immunoreactivities for adenohypophysial hormones in the pituitary gland of the polypteriform fish, Polypterus endlicheri

Polypteriform fish constitutes the most primitive living descendent of the ancient bony fish. In polypteriform fish, only proopiomelanocortin (POMC) has been identified so far in the adenohypophysis, which is surprising in view of their evolutionary importance. In the present study, distribution of immunoreactive adenohypophysial hormones was examined in juvenile individuals of Polypterus endlicheri. Antisera to tetrapod and fish adenohypophysial hormones were used as immunostaining probes. Adrenocorticotropin (ACTH)-like cells were detected by antisera to salmon POMC N-terminal peptide, porcine ACTH and mammalian alpha-melanotropin (MSH), and were distributed in the rostral pars distalis in close proximity to the hypophysial duct. MSH-like cells were found in the pars intermedia, and were stained by anti-salmon N-Ac-beta-endorphin II as well as anti-mammalian alpha-MSH and anti-salmon POMC-N terminal peptide. Prolactin (PRL)-like cells were detected only after application of anti-sturgeon PRL, and were distributed in the rostral pars distalis, where PRL-positive material was found in columnar mucinous cells lining the diverticuli of the hypophysial duct. Growth hormone (GH)-like cells were stained with antisera to sturgeon GH, human GH, salmon GH and blue shark GH, and were distributed in the proximal pars distalis. Somatolactin (SL)-like cells were stained with anti-salmon SL, and were distributed in the pars intermedia. Two types of glycoprotein hormone-positive cells were detected in the proximal pars distalis. Although both types of cells were stained with several antisera to glycoprotein hormones, such as sturgeon LHbeta and salmon LHbeta, it was difficult to know which types of cells produce LH, FSH, or TSH. Thus, the present study revealed seven types of adenohypophysial hormone-like cells in the Polypterus pituitary gland, which may provide the morphological basis for better understanding on evolution of the pituitary gland and the adenohypophysial hormones in vertebrates.     

Effects of Castration and Androgen-treatment on Pituitary and Testes of the Three-spined Stickleback,Gasterosteus aculeatus L., in the Breeding Season

Methyltestosterone (MT) (35 μg/l aquarium water) suppresses steroid production in the testes of sham-operated fish, as studied enzyme histochemically. This treatment also diminishes the volume of the proximal pars distalis, as well as the area of the gonadotropic (GTH) cell nuclei. Such effects were not found in MT-treated castrated fish. Castration alone led to a partial degranulation of the GTH-cells as compared with sham-operated controls. Extensively dilated granulated endoplasmic reticulum was found in the GTH-cells of some of the castrated animals and also in GTH-cells of some castrated fish that had been treated with MT. The possibility of a non-steroidal feedback mechanism from testes to pituitary is discussed. No effects of the treatments were observed on prolactin cells and on the second basophilic cell type (GTH II?, TSH?).     

Immunohistochemical Identification of the Adenohypophysial Cells in the Indian Paddy Field Frog,Rana limnocharis

Cells of the pituitary pars distalis (PD) and pars intermedia (PI) in the frog Rana limnocharis have been identified by an unlabelled antibody enzyme method using antisera developed in rabbit against mammalian hypophysial hormones. On the basis of their immunoreactivity, six types of cells, viz. thyrotropic (TSH), gonadotropic (GTH), prolactin (PRL), growth hormone (GH), corticotropic (ACTH) and melanotropic (MSH), cells have been recognized. GTH and PRL cells are distributed throughout the PD. GH cells usually occur in the anterodorsal and central region of the gland. Immunoreactive TSH cells are fewer in number and are localized in the ventromedian region of the PD. Cells showing immunoreactivity to ACTH 1–24 antiserum are encountered in the rostroventral part of the PD. Cells of the PI also show immunoreactivity to ACTH 1–24 antiserum. PI cells cross-react with α-MSH antiserum at all dilutions up to 1: 50 000. However, when the same antiserum was used at dilutions up to 1: 20 000, the ACTH cells of the PD also showed cross-reactivity.     

Cell types in the adenohypophysis of the Japanese quail and effects of injection of luteinizing hormone-releasing hormone

The cells of the adenohypophysis of the Japanese quail were studied by both light and electron microscopy after exposure to long photoperiods or injection of lutenizing hormone-releasing hormone (LRH). Six cell types were identified in the adenohypophysis by examining alternate thick and thin sections by light and electron microscopy. In the cephalic lobe, there are four types of glandular cells. They are the prolactin cells, ACTH cells, TSH cells, and gonadotropic cells (FSH?). In the caudal lobe, there are two types of cells, STH cells and gonadotropic cells (LH?). After exposure to long daily photoperiods, gonadotropic cells in both lobes were strongly activated. They became larger and accumulated many granules. ACTH cells became vacuolated; granules were sparse. Synthetic LRH injection (10 mug/0.2 ml/day) for 10 days to the non-photostimulated quail stimulated certain numbers of the gonadotropic cells in the both lobes, although the response of the cells was less than that induced by photostimulation. No response was seen in the other cell types.     

The Pituitary Gland of the Roach Leuciscus rutilus

Three endocrine cell types were recognized in the proximal pars distalis (PPD) of the roach, Leuciscus rutilus, by electron microscopy. Two of these cell types are basophilic and assumed to be gonadotropic. The third cell type is acidophilic and its similarity to somatotropic (STH) cells is evident. Green fluorescent cells corresponding to cells with dense core vesicles of about 85 nm are scattered between the PPD and the proximal neurohypophysis (PNH). Their possible content of a catecholamine was demonstrated with microspectrofluorometric analyses. Four axon types with granules 65, 81, 110 and 137 nm in average diameter terminate at the basement membrane. Only fibers with granules of 81 nm enter the PPD to make synapses with the STH cells. Extensions of the basement membrane are continuous with the perivascular space and penetrate the endocrine tissue. It is postulated that axon terminals on the basement membrane are functionally equivalent to synaptic contacts on the endocrine cells. Axon type 4 has granules which in sizes correspond to those of cells in lateral and ventrolateral parts of the nucleus lateralis tuberis (NLT). A similar correlation is possible between axon type 3 and other cells in the NLT. This is in accordance with earlier conclusions of a regulation of the gonadotropic cells from the lateral (posterior) parts of the NLT.     

Extravascular circulation in the pituitary of Mugil cephalus (Teleostei)

Summary Extravascular circulation in the pituitary of Mugil cephalus was investigated by injecting live fish with horseradish peroxidase and studying the distribution of the enzyme in the gland. The principal components of the extravascular circulatory system are the pericapillary spaces, and, arising from them, the interlobular and circumhypophyseal spaces. Extensions of these spaces penetrate the glandular parenchyma of the pars distalis, where they merge with pericellular spaces. In the neurohypophysis, pericapillary spaces are connected to the periaxonal spaces.Capillaries penetrating from the proximal neurohypophysis into the pars distalis are accompanied by neurosecretory axons. These axons form a mass of tissue which is limited near the capillaries by the pericapillary spaces and near the adenohypophysis by the interlobular spaces. Toward the interior of the adenohypophysis the amount of nervous tissue accompanying the capillaries progressively diminishes, thus reducing the distance between pericapillary and interlobular spaces. Within the pars distalis, the neurosecretory axons accompanying the capillaries are sparse, and the secretory and stellate cells are mostly located directly adjacent to the pericapillary spaces. In the neuro-intermediate lobe, interlobular spaces outline the neuro-adenohypophyseal boundary.The relationship between extravascular spaces and hormone-secreting cells varies in the different regions of the adenohypophysis depending upon the type of neurosecretory innervation in the respective region. In the regions of prolactin and gonadotropin cells, where neurosecretory axons are in direct contact with the secretory cells, the hormone-secreting and stellate cells are adjacent to the pericapillary spaces. In the regions of ACTH and STH cells, secretory and stellate cells are found adjacent to the interlobular spaces, which are interposed between the cells and the neurosecretory axons.Abbreviations AH adenohypophysis - CH circumhypophyseal - DNH distal neurohypophysis - HRP horseradish peroxidase - NH neurohypophysis - NS neurosecretory - PD pars distalis - PI pars intermedia - PPD proximal pars distalis - RNH rostral neurohypophysis - RPD rostral pars distalis This research was supported by a grant from the National Council for Research and Development, Israel, and the GKSS Geesthacht-Tesperhude, Federal Republic of Germany     

Light- and electron-microscopic studies on the secretory cytology of the adenohypophysis of the Japanese quail,Coturnix coturnix japonica

The effects of thyroidectomy, adrenalectomy, and castration on the pars distalis of male Japanese quail, and of injection of LH-RH on sexually inactive females, were investigated by light and electron microscopy. Correlation between light and electron microscopy was attained by use of alternate thin and thick sections. Six types of secretory cells were identified and the ultrastructural characteristics described. Putative endocrine functions have been designated on the basis of responses to experimental interventions and on other criteria. The putative STH cells are characterized by the presence of large dense secretory granules (250-300 nm) that are stained with orange-G by the trichrome method. They occur only in the caudal lobe and appear to be unchanged by castration, thyroidectomy, adrenalectomy and LH-RH injection. The putative prolactin cells are characterized by large (400-600 nm), spherical or polmorphic, dense secretory granules stainable with acid fuchsin and aniline blue; prominent Golgi apparatus and well developed endoplasmic reticulum with densely packed, regularly parallel lamellae. They are found mainly in the cephalic lobe. The prolactin cells develop some vacuolization after adrenalectomy and undergo some degeneration after castration. The ACTH cells, which are restricted to the cephalic lobe, are identified by the dense, spherical granules (250-300 nm) that are stained with acid fuchsin. After adrenalectomy, they lose their secretory granules and are transformed into large, chromophobic adrenalectomy cells. TSH cells are so designated by their response to thyroidectomy including loss of their fine secretory granules and transformation to large, vacuolated thyroidectomy cells. We have found TSH cells and thyroidectomy cells only in the cephalic lobe. Basophilic cells, considered to be gonadotropes, occur in both the cephalic and caudal lobes. The gonadotropes of the cephalic lobe appear to have slightly larger (120-200 nm) granules than the caudal lobe (120-150 nm). However, after castration, the gonadotropes in both lobes become hypertrophied and vacuolated and are transformed into mutually indistinguishable castration cells. Twenty minutes after injection with LH-RH, the gonadotropes of both lobes increase in size and number, degranulate, develop vacuoles in the cytoplasm, and appear very similar to castration cells.     

Immunocytochemical characterization of adenohypophyseal cells in the greater weever fish (Trachinus draco)

The adenohypophysis of the greater weever fish (Trachinus draco) was studied using histochemical and immunocytochemical methods. The adenohypophysis comprised the rostral pars distalis (RPD), the proximal pars distalis (PPD), and the pars intermedia (PI). Neurohypophysis showed a patent hypophyseal stalk which was divided into several branches intermingled with the adenohypophysis. Salmon prolactin (PRL)-immunoreactive (ir) cells, arranged in follicles, resided in the RPD and the most rostral part of the ventral PPD. Human adrenocorticotropin (ACTH)-ir cells were located in the RPD between PRL-ir cells and the neurohypophyseal processes. Salmon and seabream somatotropin (GH)-ir cells were located in both the dorsal and the ventral PPD. Some GH-ir cells were seen in surrounding and in contact with neurohypophyseal branches, whereas other isolated or clustered GH-ir cells were embedded in adenohypophyseal cells of the PPD. In addition, isolated or clustered GH-ir cells were also detected in the tissue of the PPD covering the most rostral part of PI. Only one class of salmon and carp gonadotropin (GTH)-ir cells was detected. Isolated or clustered GTH-ir cells resided in both the dorsal and the ventral PPD and were seen surrounding the PI and in the tissue of the PPD covering the most rostral part of PI. In addition, a few scattered GTH-ir cells were observed in the ventral RPD. Scattered groups of thyrotropin (TSH)-ir cells were present in the anteroventral PPD. Salmon and seabream somatolactin (SL)-ir and bovine melanotropin (MSH)-ir cells were intermingled surrounding the neurohypophyseal tissue. SL-ir cells were negative to periodic acid-Schiff technique. MSH-ir cells showed a very weak immunoreactivity to anti-human ACTH((1-24)) serum. In addition to the PI location, few isolated or clustered SL- and MSH-ir cells were observed in the dorsal PPD.