The hypothalamo-neurohypophysial system in Indian fresh-water goby, Glossogobius giuris (Ham.)

The hypothalamo-neurohypophysial neurosecretory system in Indian fresh-water goby, Glossogobius giuris (Ham.) has been described. The tractus preoptico-hypophyseus serves the function of a morphological and physiological connection between the hypothalamus and pituitary gland. In addition to main mass of the nucleus preopticus cells (cystine/cysteine bearing), a group of few cells in the hypothalamus has also been observed. These cells are situated posterior to the position of the nucleus preopricus and are CH Ph + ve and AF + ve. The neurosecretory material in the cells of nucleus preopticus is in the form of fine granules. The nucleus lateralis tuberis is absent in the fish under study. The disposition of neurosecretory material is heaviest along the fibres of the neurohypophysis in the region of pars intermedia with which it forms a profuse interdigitation. The fibres usually terminate over the blood vessels. The Herring bodies are noticeable at different levels in the neurohypophysis and pars-distalis. Besides the neurosecretory fibres, Herring bodies, non-stainableneurosecretory fibres and blood vessels, the pituicytes are also present in the neurohypophysis (SAKSENA 1974a, b). The intraaxonal flow of neurosecretory material, the vascularization of the nucleus preopticus and hypothalamo-hypophysial regulatory mechanism have been also discussed.     

Monoamine oxidase in the hypothalamo-hypophysial region of the teleosts,Anguilla japonica and Oryzias latipes

Summary Distribution of monoamine oxidase (MAO) was histochemically examined in the hypothalamo-hypophysial region of the eel (Anguilla japonica) and the medaka (Oryzias latipes) with a modified Glenner's tryptamine-tetrazolium method. The hypothalamic neurosecretory cells showed very weak MAO activity in their perikarya. MAO-positive fibers were present in close contact with the neurosecretory cells, suggesting that monoaminergic fibers participate in the control of neurosecretory cell activity. The nucleus lateralis tuberis (NLT) contained cells exhibiting strong MAO activity. These cells must be monoaminergic neurons.In the anterior region of the neurohypophysis of both eel and medaka, two bundles of MAO-positive fibers originating from the NLT proceed down along each side of the third ventricle into the pars distalis. This suggests that monoaminergic neurons of the NLT are involved in the release of hormones from the pars distalis. In addition to these tracts, numerous MAO-positive fibers proceed backward from the post-optic area and end around the blood capillaries located between the neurohypophysis and the pars intermedia in both species.I wish to express my gratitude to Prof. H. Kobayashi for his valuable advice during the course of this study. I am indebted to Prof. S. Uchida, Ocean Research Institute, University of Tokyo, for supplying the eels.     

The hypothalamo-hypophysial vascular relationship in Indian fresh-water goby, Glossogobius giuris (Ham.).920u.

The hypothalamo-hypophysial vascular relationship and intra-hypophysial vasculatisation have been described in order to understand the regulatory mechanism of hypothalamic control over the functions of the pituitary gland. In Glossogobius giuris, the disposition of the blood vessels in the head region is on typical teleostean pattern with certain modifications. The nucleus preopticus is supplied through the nucleus preopticus artery, a small blood vessel arising from the anterior branch of the posterior cerebral artery, whereas the pituitary gland receives blood through a pair of hypophysial arteries. The blood from the pituitary is drained off by the pituitary veins whch pour their blood into the supra-orbital sinus. The anterior cerebral vein after taking the blood from anterior part of the brain including the hypothalamus and the nucleus preopticus joins with the supra-orbital sinus. The hypothalamo-hypophysial portal system is absent in this fish. The saccus vasculosus receives blood from the posterior cerebral artery through a small blood vessel and is collected by a prominent saccus vasculosus vein which pours blood into the supra-orbital sinus before it joins the infra-orbital sinus to form the heat vein. There seems to be no physological connection between the saccus vasculosus and pituitary gland. The highly vascularised neurohypophysis interdigitate with the pars intermedia and extends upto the proximal pars distalis. The blood vessels are restricted to the neurohypophysial extensions only. However, in the rostral pars distalis the blood vessels are present but the neurohypophysis does not extend to this part. The blood capillaries enter the rostral pars distalis from the capillary network on the surface of pituitary gland along with the connected tissue covering of the pituitary. The neurohypophysis shows a greater vascularisation in comparison to that of the other glandular part of the pituitary gland. In the present study of Glossogobius giuris, though an extensive ramification of neurohypophysis occurs with the pars intermedia and the proximal pars distalis, the neurosecretory axons do not innervate the endocrine cells of the pituitary gland and the blood vessels are found restricted to the neurohypophysial extensions except that of the rostral pars distalis. The neuro-vascular way of hypothalamic control over the functions of the pituitary gland seems to be justified as the neurosecretory fibres have been found associated with the blood vessels.     

Aminergic Innervation of the Cranial and Caudal Neurosecretory Systems in the Teleost Gillichthys mirabilis

Abstract Numerous fluorescent varicosities surround most of the caudal neurosecretory neurons and also regularly occur among pars intermedia cells of the adenohypophysis in the teleost, Gillichthys mirabilis. The color of the varicosities, as well as their responses to pharmacological treatments, is diagnostic of catecholaminergic neurons and processes. No fluorescence characteristic of monamines is found in the rostral pars distalis, in the proximal pars distalis or in the cells of the nucleus lateralis tuberis (NLT), although fluorescent varicosities are found within the ventral hypothalamus in the vicinity of the NLT. Bilateral clusters of fluorescent cell bodies are located in the ventral hypothalamus (posterior to the NLT); some of these cells border the neurohypophysis. Fluorescent tracts from these cell clusters extend to a pair of elongate nuclei of nonfluorescent neurons which are surrounded by fluorescent varicosities. Alteration of osmotic conditions did not effect the fluorescence, except for the caudal neurosecretory cells of fish exposed to fresh water for long periods. Adrenergic nervous input thus seems to be an important component of both the cranial and caudal neurosecretory systems.     

Galanin-like immunoreactivity in the brain and pituitary of the "four-eyed" fish, Anableps anableps

The distribution of galanin (GAL)-like immunoreactivity was investigated in the brain and pituitary of the "four-eyed" fish, Anableps anableps. GAL-immunoreactive (GAL-ir) perikarya were located in the area ventralis telencephali pars supracommissuralis, nucleus preopticus periventricularis, nucleus preopticus pars parvocellularis, nucleus preopticus pars magnocellularis, nucleus lateralis tuberis ventralis, nucleus lateralis tuberis lateralis, and nucleus lateralis tuberis posterior. A few scattered, GAL-ir neurons were also observed in or adjacent to the nucleus recessus lateralis, nucleus recessus posterioris and lobus facialis (VII). GAL-ir fiber networks were widespread in the brain, with a comparatively higher density in the ventral telencephalic, preoptic and infundibular regions. The neurohypophysis showed GAL-ir innervation and there were GAL-ir cells in the adenohypophysis. The presence of GAL-ir cells in the hypothalamus and in the pituitary is an important asset for the supposed role of GAL-like peptide in neuroendocrine regulation of brain and pituitary functions.     

Development of melanin-concentrating hormone-immunoreactive elements in the brain of gilthead seabream (Sparus auratus)

The development of the hypothalamic melanin-concentrating hormone (MCH) system of the teleost Sparus auratus has been studied by immunocytochemistry using an anti-salmon MCH serum. Immunoreactive perikarya and fibers are found in embryos, larvae, and juvenile specimens. In juveniles, most labeled neurons are present in the nucleus lateralis tuberis; some are dispersed in the nucleus recessus lateralis and nucleus periventricularis posterior. From the nucleus lateralis tuberis, MCH neurons project a conspicuous tract of fibers to the ventral hypothalamus; this penetrates the pituitary stalk and reaches the neurohypophysis. Most fibers end close to the cells of the pars intermedia, and some reach the adenohypophysial rostral pars distalis. Immunoreactive fibers can also be seen in extrahypophysial localizations, such as the preoptic region and the nucleus sacci vasculosi. In embryos, MCH-immunoreactive neurons first appear at 36 h post-fertilization in the ventrolateral margin of the developing hypothalamus. In larvae, at 4 days post-hatching, perikarya can be observed in the ventrolateral border of the hypothalamus and in the mid-hypothalamus, near the ventricle. At 26 days post-hatching, MCH perikarya are restricted to the nucleus lateralis tuberis. The neurohypophysis possesses MCH-immunoreactive fibers from the second day post-hatching. The results indicate that MCH plays a role in larval development with respect to skin melanophores and cells that secrete melanocyte-stimulating hormone. Received: 4 April 1995 / Accepted: 17 July 1995     

Pituitary gland of the exotic carp, Carassius carassius Linn., with reference to the histocytology of the adenohypophysis in conjunction to male and female rhythms of reproduction

The pituitaries of the exotic carp (Carassius carassius) are studied at the light microscopic level, for the characterization of the adenohypophysial cell-types with particular emphasis to the gonadotropic potency of the pituitary in relation to the annual reproductive patterns. The gland in the fish is of the cranioleptobasic type. Based on the classical staining methods and localization of the differential cell-types, the adenohypophysis of the fish can be subdivided into the rostral (RPD) and proximal (PPD) pars distalis and the pars intermedia (PI). The pars nervosa (PN) of the fish consists of the neurosecretory (NS) fibres which show varied accumulations of the NS material in concert with the seasonal reproductive cycle. The NS tract is constituted of AF--ve nucleus praeopticus and AF + ve nucleus lateralis tuberis. 2 tinctorially different cell-types, corresponding to corticotrops and lactotrops of other teleosts, are identifiable in the RPD of the present species. In the PPD, 3 cell-types can be distinguished with various granule-staining procedures: they represent somatotrops, thyrotrops, and gonadotrops. Further, 2 gonadotropic cells with apparently different staining properties are discernible. In the PI are dispersed besides a few agranular cells, islets of 2 kinds of cells separable with combined PAS-PbH technique. Seasonal changes in the gonadosomatic index and diameter of the testicular lobules and egg diameter as indices of gonadal maturation are found to be closely interwoven with the morphometric alterations in the basophilic cell area (BCA) chosen as criterion of gonadotropic potency of the PPD. The maximum BCA values are preceded with the time of greatest atresia of the eggs indicating the influence of the pituitary gonadotropins in the follicular resorption in the test fish.     

An in situ Study of the Hypothalamic Neurosecretory System of the Freshwater Catfish Ailia coila (Ham.)

The hypothalamo-neurohypophysial complex of Ailia coila is well demonstrated with the help of in situ staining procedure. Both pars magnocellularis and pars parvocellularis components of the nucleus preopticus contribute to the formation of the right and the left main neurosecretory tracts. Anterior one third of these tracts are loosely set and posteriorly they became more compact. From the posterior two thirds of the main tracts several pairs of lateral tracts were given off which join at the midline to form the paired median tracts. The median and the main tracts jointly enter the pituitary as the common tract. The common tract on entering the pituitary often divides into two or more branches and enter the pars intermedia independently. The rostral pars distalis is least innervated by the neurosecretory axons. Since the proximal pars distalis has varying amount of AF-positive cells, and the pars intermedia has the bulk of the neurosecretory axons both these regions are stained dark in the in situ preparations. Bulk preparations provide a clear topographic picture of the entire neurosecretory system, which is very difficult to visualise in tissue sections and in their reconstructions.     

Immunocytochemical identification of CRF-like and SRIF-like peptides in the brain and the pituitary of cyprinid fish

Summary Single and double immunocytochemical techniques were applied to the brain and pituitary of carps and goldfish. With the use of antiserum raised against synthetic corticotropin-releasing factor (CRF), immunoreactive perikarya were observed in the nucleus praeopticus and the nucleus praeopticus periventricularis. CRF-like-immunoreactive hypothalamic nerve fibers reach the pituitary. In cyprinids, some fine fibers enter the rostral neurohypophysis bordered by prolactin- and ACTH cells. Other thicker fibers extend ventrocaudally into the neurointermediate lobe. This CRF-like system appears to differ from the SRIF-like system, which is restricted to the proximal pars distalis of the pituitary containing somatotrophs.The technical assistance of Jacqueline Olivereau, Biologiste-Adjointe from the CNRS, is gratefully acknowledged     

Hypothalamo-hypophysial neurosecretory system of the teleost Trichogaster fasciatus (Bloch & Schneider) with a note on it's vascularization

The hypothalamo-hypophysial neurosecretory (HN) complex of Trichogaster fasciatus is described with the help of in situ staining technique and tissue sections. The neurons of the nucleus preopticus (NPO) give rise to fine neurosecretory axons which form the left and right main tracts. The bulk of the tracts come laterally and enter the middle portion of the neurohypophysis. However, some of them spread out and penetrate the rostral neurohypophysis as well. Hypophysial artery contributes to the formation of primary capillary plexus (PCP) which extends from the subterminal region to the extremity of the anterior neurohypophysis. Structurally, the subterminal region and the anterior neurphoyophysis can be compared to the median eminence (ME) of tetrapods as it is differentiated into ependymal, fibrous and reticular layers. Also, these areas have aboundant neurosecretory and silver positive axons suggesting the possibility of direct transmission of neurohormones into the blood.     

Hypothalamo-hypophysial Neurosecretory System in Normal and Hypophysectomised Catfish,Clarias Batrachus L

Two neurosecretory centers, nucleus preopticus (NPO) and nucleus lateralis tuberis (NLT), are distinguished in the hypothalamus of Clarias batrachus L. The cell bodies of NPO are grouped into nucleus preopticus magnocellularis (NPM) and nucleus preopticus parvocellularis (NPP). The NLT is recognised into three divisions: pars rostralis, pars medialis and pars ventrolateralis. The neurons of both the NLT and NPO are aldehyde-fuchsin (AF) positive. The axons from NPO run into four or five bundles ventrolaterally and caudally for some distance before all of them from either side join ventromedially into a single thick cord of neurosecretory fibers which finally enters the pars intermedia to form the neurointermediate lobe. The fibers from NLT, which are separate from the neurosecretory fibers of the NPO, anastomose in the region of the pars distalis. The neurosecretory material (NSM) of NPO is transported by axonal route whereas that of NLT is by axonal as well as ependymovascular pathways. Hypophysectomy results in the increase of AF-positive material in the neurons of NPO shortly after the operation, but later on, the material begins to deplete in them. The AF-positive material at the cut end of neurosecretory fibers, however, accumulates. The AF-positive material in the cell bodies of NLT is also depleted, but the nuclei increase in size, after hypophysectomy.     

Hypothalamo-neurohypophysial neurosecretory system and its vasculature in the freshwater catfish Mystus vittatus (Bloch)

The architectural pattern of the hypothalamo-neurohypophysial (HN) system of M. vittatus basically resembles that of other catfishes described earlier (Sathyanesan 1969 a, b). Some of the neurons of the nucleus preopticus of the hypophysectomised fish exhibit degeneration, whereas the viable ones were degranulated. The neurosecretory tract (NT) is in close association with the ependymal lining, both in the third ventricle and infundibular recess extending into the neurohypophysis. The pituicytes of the neurohypophysis are in close association with the NT which is of functional significance. In this species the NT entering the pituitary could be demonstrated with aldehyde fuchsin (AF), Palmgren's silver impregnation technique, alkaline and acid phosphatase and ascorbic acid tests. At least some among them may constitute separate tracts and their origin needs to be confirmed. As all those tracts pass through the infundibular base they come in direct contact among themselves as well as with the nucleus lateralis tuberis (NLT) neurons which lie on their pathway. In M. vittatus, in addition to the presence of neuro-adeno interface vasculature which is the median eminence (ME) equivalent of the teleosts, there is morphological evidence for the presence of a ME of tetrapodan type. The presence of strong alkaline and acid phosphatase and ascorbic acid activity in both the sites further supports the above view. However, additional ultrastructural evidence is needed to prove the functional status of the extra-hypophysial ME.     

Somatostatin in the brain and the pituitary of some teleosts

Summary Immunocytochemical investigations show that somatostatin (SRIF)-like immunoreactive material is present in the brain and the pituitary of nine different species of teleosts. In the brain, immunoreactive perikarya and fibers are observed in the preoptic periventricular nucleus, the entopeduncular nucleus, the anterior periventricular nucleus, and the nucleus lateralis tuberis. In the pituitary, SRIF-like-immunoreactive fibers occur in the proximal pars distalis (PPD), which contains the growth hormone (GH)-secreting cells. Nerve fibers are scattered among GH cells (cyprinids), or end on the basal lamina at the neuroglandular interface of the PPD (eel, salmonids). In the eel, the proximal neurohypophysis does not penetrate deeply into the PPD that is very poorly vascularized. In some species, e.g. Myoxocephalus, SRIF-like immunoreactive fibers are also observed in the caudal neurohypophysis, and even among MSH cells of the pars intermedia.In long-term starved carps and eels, the amount of SRIF-like material in the pituitary is clearly reduced. A possible role of SRIF in the concomitant stimulation of GH cells is discussed.     

The distribution of neuropeptide Y and dynorphin immunoreactivity in the brain and pituitary gland of the platyfish,Xiphophorus maculatus,from birth to sexual maturity

Summary Immunoreactive neuropeptide Y and dynorphin have been localized in the brain and pituitary gland of the platyfish, Xiphophorus maculatus, at different ages and stages of development from birth to sexual maturity. Immunoreactive neuropeptide Y was found in perikarya and tracts of the nucleus olfactoretinalis, telencephalon, ventral tegmentum and in the neurohypophysis and in the three regions of the adenohypophysis. Immunoreactive dynorphin was found in nerve tracts in the olfactory bulb and in cells of the pars intermedia and the rostral pars distalis of the pituitary gland.     

Immunocytochemical identification of melanin-concentrating hormone in the brain and pituitary gland of the teleost fishes Oncorhynchus keta and Salmo gairdneri

Summary Melanin-concentrating hormone (MCH) has been purified from the chum salmon pituitary. Its complete amino acid sequence has recently been established. To identify the precise site of origin of MCH, immunostaining was performed in the brain and pituitary gland of the chum salmon and the rainbow trout using a highly sensitive and specific antiserum raised against synthetic MCH. In these two salmonid species immunoreactivity for MCH was detected in neurons and neuronal processes in the pars lateralis of the nucleus lateralis tuberis (NLT) in the basal hypothalamus. Numerous positive-staining processes of these MCH-neurons project to the pituitary gland, extending into neurohypophysial tissues within the pars intermedia and, to a lesser extent, into the pars distalis. No pituitary cells showed cross-reactivity. These results suggest that MCH is biosynthesized in the neurons of the NLT/pars lateralis and released in the neurohypophysis. On the other hand, prominent but less numerous MCH-positive processes could be traced to the pretectal area in which projection of both optic and pineal fibers has been detected using tracers. This observation suggests that the synthesis and/or release of MCH might be under the influence of either of these photosensory neurons. Moreover, the existence of an extrahypothalamic projection from MCH-positive neurons suggests that, in addition to melanin-concentration, MCH might be involved in other neuronal functions, perhaps serving as neuromodulator in the brain.     

An in situ study of the preoptico-neurohypophysial complex of the freshwater teleost Ciarias batrachus (L.)

Summary The complicated architectural pattern of the preoptico-neurohypophysial neurosecretory system of Clarias batrachus is described using bulk staining procedure. The nucleus preopticus is U-shaped; its limbs constitute the PMC and PPC. The difference in staining intensities noticed among the PMC and PPC might suggest a possible functional difference within the system. Axons of the PMC diagonally pass through the PPC and emerge along with those of the latter to form the right and the left main tracts. The folds and recurrent curves of the neurosecretory axonal tracts, the formation of several pairs of lateral tracts and their union to form median tracts might be devices to increase the storing and releasing capacity for the neurosecretory material. The presence of prominent Herring bodies, and diffuse areas in the neurosecretory tract suggest possible sites of release of neurohormones along the highly vascular hypothalamic region. The common neurosecretory tract is divided into several smaller branches at the level of the pars distalis prior to entering the pars intermedia. Branches do penetrate into the pars distalis, but those entering the rostral component of the pituitary are relatively scarce.Regeneration of the anterior stump of the severed hypophysial stalk to form a neurohypophysis-like organ is also demonstrated in toto with the aid of the bulk staining procedure. A demonstrable increase of stainable neurosecretory material was noticed all along the hypothalamo-neurohypophysial complex of some specimens subjected to complete darkness. Total staining methods could be advantageously used to map the course of recurrent tracts and extra-hypophysial axonal endings. Since the neurosecretory system seems to exhibit a varied structural pattern in different fishes, extension of this study to a larger number of species may throw additional light on the function and the evolution of this important system.I am indebted to Dr. K. N. Udupa, Principal, College of Medical Sciences and Director, Surgical Research Laboratory and Dr. L. M. Singh, Officer in-charge of the Laboratory for providing all facilities and encouragements.     

The distribution of monoamines in the hypothalamus of the Japanese quail Coturnix coturnix japonica

Summary The distribution of monoamines in the hypothalamus of the Japanese quail (Coturnix coturnix japonica) has been studied using a histochemical fluorescence technique. In the posterior hypothalamus catecholamine-containing nerve fibres are localised in the nucleus tuberis and nucleus hypothalamicus posterior medialis and are linked by fluorescent tracts running in the stratum cellulare internum. Further tracts may be traced from the nucleus tuberis around the base of the third ventricle to the sub-ependymal layer of the median eminence, where they then appear to pass through the hypothalamo-hypophysial neurosecretory tract to terminate in the palisade zone on the portal vascular bed. The innervation of the palisade layer by catecholamines is sparse. The fluorescent terminals are spread evenly throughout both the anterior and posterior divisions of the median eminence. There is no monoamine innervation of the pars nervosa. The paraventricular organ has both 5-hydroxytryptamine- and catecholamine-containing cell bodies and axons may be traced into the region of the nucleus hypothalamicus posterior medialis. In the anterior hypothalamus the neurosecretory paraventricular nucleus contains many catecholamine nerve fibres and terminals. These are linked by fibre tracts to the nucleus basalis and to the nucleus hypothalamicus posterior medialis. The supraoptic nucleus is less well innervated although a dense accumulation of fibres lies in the preoptic recess. The latter is thought to give rise to long axons which pass in association with the neurosecretory tract to end in the nucleus tuberis.Supported by a Grant (AG 24/36) from The Agricultural Research Council. We are indebted to Dr. G. A. Clayton, Institute of Animal Genetics, University of Edinburgh, for supplying the birds.     

Immunohistochemical Localization of C-RFamide, a FMRF-related Peptide, in the Brain of the Goldfish, Carassius auratus

Carassius RFamide (C-RFa) is a novel peptide found in the brain of the Japanese crucian carp. It has been demonstrated that mRNA of C-RFa is present in the telencephalon, optic tectum, medulla oblongata, and proximal half of the eyeball in abundance. Immunohistochemical methods were employed to elucidate the distribution of the peptide in the brain of the goldfish (Carassius auratus) in detail. C-RFaimmunoreactive perikarya were observed in the olfactory bulb, the area ventralis telencephali pars dorsalis and lateralis, nucleus preopticus, nucleus preopticus periventricularis, nucleus lateralis tuberis pars posterioris, nucleus posterioris periventricularis, nucleus ventromedialis thalami, nucleus posterioris thalami, nucleus anterior tuberis, the oculomotor nucleus, nucleus reticularis superior and inferior, facial lobe, and vagal lobe. C-RFa immunoreactive fibers and nerve endings were present in the olfactory bulb, olfactory tract, area dorsalis telencephali pars centralis and medialis, area ventralis telencephali, midbrain tegmentum, diencephalon, medulla oblongata and pituitary. However, in the optic tectum the immunopositive perikarya and fibers were less abundant. Based on these results, some possible functions of C-RFa in the nervous system were discussed.     

Hypophysiotropic neurons in the goldfish hypothalamus demonstrated by retrograde transport of horseradish peroxidase

Summary The horseradish-peroxidase (HRP) technique was used to visualize the cell bodies of axons projecting to the goldfish pituitary. Following intravenous injections of HRP, HRP reaction products were observed in axons of the rostral pars distalis, proximal pars distalis, neurointermediate lobe, pituitary stalk and in axons coursing from the pituitary into the hypothalamus. HRP-labelled cells in the brain were localized in two regions only — the nucleus preopticus (NPO) pars magnocellularis and pars parvocellularis, and the nucleus lateralis tuberis (NLT) of the hypothalamus. These observations suggest that the NPO and NLT are the source of the neurosecretory innervation of the goldfish pituitary.     

Ultrastructure of the hypothalamic neurosecretory system of the dog

Summary The hypothalamic neurosecretory system of normal dogs was studied by light and electron microscopy after perfusion-fixation. In the supraoptic nucleus most neurons are loaded with elementary neurosecretory granules having a content of low electron density. Neurons with less neurosecretory material and signs of enhanced synthetic activity, as recognized by the changes in the endoplasmic reticulum, were also observed.The vesiculated neurons ofJewell were studied under the electron microscope and various stages of development were described. It was postulated that they originate by a localized process of cytoplasmic cytolysis which ends in the formation of a large aqueous intracellular cavity limited by a plasma membrane. The possible significance of these vesiculated neurones is discussed. Some few myelinated neurosecretory axons are found in the supraoptic nucleus.The neurons of the paraventricular nucleus are smaller and contain less neurosecretory material. This is abundant and very pale in the axons. The median eminence consists of an inner zone, mainly occupied by the neurosecretory axons of the hypothalamic-neurohypophysial tracts, and an outer zone in which some neurosecretory axons end on the capillary of the portal system. This outer zone contains numerous axons with the axoplasm rich in neurofilaments and some containing granulated and non-granulated synaptic vesicles. Some neurons with granulated vesicles were observed in this region. The adrenergic nature of these neurons and axons is postulated.The infundibular process of the neurohypophysis shows small axons with discrete amounts of elementary granules and vesicles of synaptic type at the endings. Some enlarged axons having, in addition, large polymorphic bodies are observed and related to the Herring bodies.The size and morphology of the granules are analyzed along the entire hypothalamic-neurohypophysial system. The changes in diameter and electron density are related to the maturation of the granules and the possible significance of such evolution.Supported by grants from the Consejo Nacional de Investigaciones Cientificas y Técnicas and by the Air Force Office of Scientific Research (AF-AFOSR 963-66).