Neurosecretory Fibers Research Articles

Paraventricular-hypophysial neurosecretory pathways in the guinea-pig.

SUMMARY The neurosecretory pathways from the paraventricular (PV) nuclei have been identified in the guinea-pig and plotted according to stereotaxic coordinates for the guinea-pig brain. The magnocellular neurosecretory neurones are concentrated in the lateral tip of the PV nucleus approximately half-way between the third ventricle and the fornix. Some neurosecretory fibres pass between the cranio-ventral extension of the PV nucleus and the supraoptic (SO) nucleus. Neurosecretory fibres leave the rostral region of the main part of the PV nucleus and pass ventral to the fornix, those from the middle region pass both ventral and dorsal to the fornix and those from the caudal region pass mainly dorsal to the fornix. After passing around the fornix, these fibres merge to form a broad band, the paraventricular—hypophysial tract, which curves ventrolaterally and then ventromedially in a gentle arc around the lateral edge of the anterior hypothalamic area and the hypothalamic ventromedial nucleus before turning caudomedially to run over the dorsal surface of the optic chiasma towards the median eminence. Neurosecretory fibres were not observed in the anterior hypothalamic area or in the ventromedial nucleus. An accessory PV nucleus was found dorsolateral to the fornix. Neurosecretory fibres pass over the fornix in both directions between the PV and the accessory PV nuclei, also fibres leave the accessory PV nucleus and pass ventrally to merge with those of the main paraventricular—hypophysial tract.

DETERMINATION OF THE DETAILED HYPOTHALAMIC ROUTE OF THE MILK-EJECTION REFLEX IN THE GUINEA-PIG

SUMMARY The effect of various types of surgical damage to the forebrain on the release of oxytocin in response to electrical stimulation of the discrete ascending milk-ejection reflex pathway in the mid-brain was investigated in 99 anaesthetized lactating guinea-pigs. Oxytocin release was measured by comparison of experimental milk-ejection responses with the response to i.v. injection of known amounts of synthetic oxytocin. Removal of the entire telencephalon, including cerebral cortex, hippocampi, amygdalae and forebrain rostral to the hypothalamus, did not affect the subsequent release of oxytocin after electrical stimulation of the pathway in the mid-brain, from which it was concluded that the reflex pathway within the forebrain is entirely diencephalic. Transection of the hypothalamus immediately rostral to the paraventricular (PV) nuclei was without effect, while transection immediately caudal to the PV nuclei blocked the release of oxytocin. Destruction of the PV nuclei by a radiofrequency lesion which spared the supraoptic (SO) nuclei blocked the release of oxytocin. Undercutting both PV nuclei so as to isolate them from the ventral hypothalamus blocked the release of oxytocin. Undercutting the PV nucleus ipsilateral to the stimulated side of the mid-brain blocked the release of oxytocin, while undercutting the contralateral PV nucleus had no effect. The PV nuclei, therefore, lie on the ascending path of the milk-ejection reflex, the SO nuclei do not, and, from the mid-brain forwards, the ascending pathway remains uncrossed. The course of the reflex pathway was traced rostrally from the mesodiencephalic junction by making narrow transverse knife-cuts and determining which cuts reduced or blocked the release of oxytocin after mid-brain stimulation. At this level, the pathway on each side of the brain is represented by separate dorsal and ventral paths and in the present study it was found that the ventral path is more important than the dorsal path in terms of oxytocin release. The ventral path passes forward in the medial forebrain bundle, in the far-lateral hypothalamus, while the dorsal path enters the posterior hypothalamus dorsally in the periventricular region at the top of the third ventricle and impinges on the thalamic reuniens nucleus. Shortly afterwards the dorsal path swings abruptly in the lateral direction to join the ventral path in the lateral hypothalamus. The reunited pathway then moves forward in this position until it is level with the PV nuclei, where it swings dorsomedially to relay with the lateral tip of the ipsilateral PV nucleus, and in doing so intermingles with the descending neurosecretory fibres from this nucleus.

A possible neuroendocrine system in polynoid polychaetes.

AbstractWithin the supraesophageal ganglion of polynoids is a vertical fiber tract which has the appearance of a “Y” in transverse sections of the brain, and contains the axons of many neurosecretory cells. The granule‐filled terminals of these neurosecretory fibers are found at the base of the tract where they are in contact with the inner surface of the sheath covering the ventral surface of the brain. This sheath separates these neurosecretory endings from an underlying pericapsular epithelium which is thicker in this region. Beneath this pericapsular epithelium is a coelomic sinus. The dorsal blood vessel is located within this sinus and is “innervated” by a pair of fiber bundles that pass out of the brain at the base of the vertical fiber tract. The outer surface of the vessel is covered by epithelioid cells which contact these fiber bundles and the thickened pericapsular epithelium, and sometimes contain granular cytoplasmic inclusions. The lumen of the vessel is continuous with the lumina of a pair of cellular, thickwalled structures of unknown function which are attached to the ventro‐lateral margins of the brain. The relationship between neurosecretory endings, enlarged pericapsular cells, coelomic sinus and blood vessel provides morphological evidence for the hypothesis that these structures are elements of a neuroendocrine system, similar in some respects to the brain‐infracerebral gland complex of nereid and nephtyid polychaetes.

On neurosecretory and adrenergic fibers in the intermediate lobe of the hypophysis in albino mice

Relations between the neurosecretory and adrenergic fibers and the glandular cells of the intermediate lobe of the hypophysis in albino mice were studied by means of light, fluorescence, and electron microscopy. A thin connective tissue layer is situated between the neural and the intermediate lobes. A few connective tissue septa leaving a connective tissue layer penetrate deep into the tissue of the intermediate lobe. The connective tissue layer and septa usually contain capillaries. Basement membranes separate this layer from the tissue elements of both lobes. The neurosecretory fibers enter the intermediate lobe of the hypophysis along the connective tissue septa as well as directly from the tissue of the neural lobe. The neurosecretory fibers entering the tissue of the intermediate lobe form some small enlargements contacting with the glandular cells. In the zones of contact, plasma membranes of the neurosecretory terminals and the glandular cells retain their constant thickness and osmiophilia on their whole length. They are separated from each other only by a 150–200-Å space. A concentration of elements of the granular endoplasmatic reticulum and of polysomes is observed in the cytoplasm of the glandular cells of the intermediate lobe of the hypophysis at the place of such contacts. Using the histochemical fluorescence method for the detection of monoamines, a network of adrenergic fibers showing a yellow-green fluorescence was found around the glandular cells of the intermediate lobe. The intensity of the fluorescence varied markedly from animal to animal. Fibers containing dense core vesicles of 500–900 Å in diameter have been observed in the same region as the adrenergic fibers. The former make synaptic contacts with the cells of the intermediate lobe. Structural peculiarities and functional significance of contacts of neurosecretory and adrenergic fibers and the glandular cells of the intermediate lobe are discussed.

Structure and ultrastructure of the neurohypophysis of the viviparous teleost, Cymatogaster aggregata Gibbons

The neurohypophysis (NH) of the viviparous seaperch Cymatogaster aggregata Gibbons is composed of an anterior region made up of predominantly B-type neurosecretory fibers and small numbers of A-type fibers and a posterior region of predominantly A-type and relatively few B-type fibers. The anterior region interdigitates with the pars distalis and the posterior region with the pars intermedia. Glial cells and blood capillaries are common to both regions of the NH. In the posterior region glial cells and capillaries are largely situated along the perimeter of a zone of degranulated A-type (and some B-type) fibers; the zone of fibers is immediately adjacent to the 'boundary membrane' of the pars intermedia.The 'boundary membrane' is formed of two membranes separated by an amorphous matrix. In the posterior NH, the membrane completely separates neurosecretory fibers and adenohypophysial cells, whereas in the anterior NH, the separation is not complete and some neurosecretory fibers directly contact cells of the pars distalis. The 'boundary membrane' is closely connected with blood capillaries and may be involved in the transportation of adenohypophysial and (or) neurohypophysial hormone(s) to the capillary system.Terminals of A-type fibers with "synaptoid" structures are found adjacent to blood vessels, glial cells, and the 'boundary membrane' in the pars intermedia. Terminals of B-type fibers are found adjacent to the 'boundary membrane' in all regions of the pituitary and also close to glial cells: B-type fiber terminals adjacent to acidophils of the proximal pars distalis and blood vessels are less common.

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.

The activity of the pituitary cells of the teleost Poecilia during the gestation cycle and the control of the gonadotropic cells

The cytological changes occurring in the pituitary gland during the 24-day gestation cycle of Poecilia are described. Experimental treatments indicate that active gonadotropic cells have enlarged nuclei and nucleoli, increased vacuolation and PAS-positive granulation, and decreased amounts of aldehyde thionin staining granulation. The gonadotropic cells are active from days 1 to 4; decreased activity follows until days 10 and 11. There is then a slight increase in activity which is rapidly reversed to give inactive cells again by day 14 or 15. From day 16 to day 24, the end of the cycle, there is a gradual increase in activity. The neurosecretory fibres of the neurohypophysis that are close to the gonadotropic cells accumulate neurosecretion whenever the gonadotropic cells are inactive. On this basis, a model for the control of the gonadotropic cells is proposed. The cycle of thyrotropic cell activity parallels that previously described for the thyroid. The cells are active during the first 8 days and inactive from days 9 to 16. They are then variable but active until the end of the cycle. The somatotropic cells also show cyclical changes, but there is not sufficient evidence to interpret these changes. It is concluded that the changes seen in the pituitary reflect cycles of activity related to the maturation of ova and to the presence of a gestation cycle.

Ultrastructure and hormonal content of the proximal stump of the transected hypothalamo-hypophysial tract of the frog (Rana pipiens).

Several types of neurosecretory fibers were observed in the normal infundibulum of the frog. After transection of the median eminence, these neurosecretory fibers of the proximal stump reacted asynchronously, but followed approximately the same pattern: a “passive accumulation” of granules observed early after the transection was followed by an “active axonal reaction” with the appearance of numerous tubular formations which are thought to be related to the Golgi apparatus. They filled the axon almost completely, and then became dilated and filled with an electron dense material. Subsequently these dilatations pinched off and gave origin to new neurosecretory granules. These locally packed granules plus others which were probably formed in more proximal parts of the axon, and the perikaryon and then transported distally, accumulated in the proximal axonal stumps and started to fill the fibers retrogradely.

Pituitary-adrenal relationships in mice with hereditary nephrogenic diabetes insipidus, with special emphasis on the neurohypophysis and pars intermedia.

The extremely hypertrophic and hyperplastic pars intermedia (PI), of the hereditary nephrogenic diabetes insipidus (DI Os/+) mice, which is correlated to the increased neurosecretory activity of the hypothalamo-hypophysial system on one hand, and the hypertrophy of the adrenal cortex on the other, is compared to these structures in normal VII+/+ mice. In DI mouse, in addition to the moderate hypertrophy of the pituitary gland as a whole, other unique features include a great penetration of neurohypophysial fibres in PI and the presence of a great number of light glandular cells. Very often, these glandular cells are surrounded by neurosecretory fibres, pituicyte-like fibres or AF+ granules. The small marginal cuboidal cells bordering the hypophysial cleft contain and/or are surrounded by AF+ granules. These features are lacking in normal VII+/+ mice. The excess release of NSM from the neurohypophysis by different stresses such as dehydration, formalin injection, surgery and adrenalectomy is accompanied by a hypertrophy of PI and of the light glandular cells as well as of the adrenal cortex in the normal and DI mice. In DI mice under different conditions of stress, all the AF+ granules in different parts of PI disappear in addition to the above changes. From the present experimental evidence and from the fact that DI mice have a hypertrophied hypothalamus (neurohypophysis), pituitary and adrenal cortex, it appears that these three endocrine entities are functionally related. The role played by vasopressin in augmenting MSH, CRF and ACTH is discussed. The probable role of light glandular cells of PI in synthesis of ACTH or ACTH-like substances is also discussed.

The arrangement of neurosecretory and catecholamine fibres in relation to the pituitary intermedia cells of the skate, Raja radiata.

The innervation pattern of the intermediate lobe of the skate (Raja radiata) was studied with histological and fluorescence histochemical methods. Neurosecretory fibres, stained with i.a. pseudo-iso-cyanine, were found running in bundles in the central parts of the cell cords. They terminated partly around the perinuclear parts of the intermedia cells, partly around the apices of the cells close to the vascular walls.

Ultrastructure and histochemistry of neurosecretory cells and neurohaemal areas in the pond snail Lymnaea stagnalis (L.).

In the central ganglia of Lymnaea stagnalis neurosecretory cell groups have previously been identified by means of chrome-haematoxylin or paraldehyde-fuchsin stains. In the present study seven types have been distinguished within the class of Gomori-positive cells on the basis of different staining reactions with the alcian blue/alcian yellow technique. Five types are located in the cerebral ganglia and in the lateral lobes, whereas two cell types occur in the ganglia of the visceral ring. No neurosecretory cells have been observed in the buccal and pedal ganglia. The staining technique used proved to be superior to the classic neurosecretory stains, because with this method the secretory substances can easily be distinguished from nonsecretory Gomori-positive tissue constituents. One of the two Gomori-negative neurosecretory cell types of the cerebral ganglia react positively with the alcian blue/alcian yellow technique. In addition, two Gomori-negative neurosecretory cell types, which had not been described before, were identified in the visceral ring. The ultrastructure of the four neurosecretory cell types in the visceral ring is described. The electron microscope revealed that each of the histochemically distinguished secretory substances consists of elementary granules which differ in size and appearance from each other and from the neurosecretory elementary granules which have been described by other authors in the cerebral ganglia and in the lateral lobes. The neurohaemal areas of the neurosecretory cells in the visceral ring are very extensive and include the peripheral parts of the nuchal nerves and of the connectives and nerves of the ganglia of the ring. The perineurium and the adjacent parts of the connective tissue which surround the ganglia, the connectives and the nerves are regarded as additional neurohaemal zones, because in these regions many tiny nerves occur, which consist mainly of neurosecretory axons ending non-synaptically near parts of the vascular system. In the perineurium surrounding the cerebral ganglia and their neurohaemal area a similar network of neurosecretory fibres was observed. Indications of release of the secretory material were regularly observed. Release apparently takes place by exocytosis. A circadian rhythmicity was observed in the release activity of some of the neurosecretory cell types.

Hypothalamo-hypophyseal system in the minnow, Phoxinus phoxinus L. with a note on the effects of hypophysectomy.

AbstractThe developmental differentiation of the hypothalamo‐hypophyseal system and its disposition in the adult normal minnow is described. Neurosecretory stainable material is present either as granules or droplets in the nucleus preopticus and it is suggested that they are related with one another and follow a sequence in their developmental appearance. This differentiation of the neuorsecretory material and its distribution along the preoptico‐hypophyseal tract suggests a strong basis in support of the “transport‐hypothesis” of Bargmann and Scharrer ('51). The presences of the Herring bodies and peripheral movement of the neurosecretory material along the axon is suggestive of the functional role performed by the tract in the process of storage and release of hormone into the circulation.The presence of intrathalamic branches of the optic nerve and their close association with neurosecretory fibers of the tract seem to be of significance in the important problem of retina‐pituitary relationship.Injections of synthetic oxytocin, synthetic lysine vasopressin and a suspension of acetone‐dried hypothalmus containing nucleus preopticus and the neurosecretory material have no effect on the color response in the minnow.The effect of hypophysectomy clearly confirms that the axons of the nucleus preopticus pass down to the neurohypophysis. After severe operational interruptions of the fibers, their trophic centers (the cell bodies of the nucleus preopticus) degenerate. The accumulation of the neurosecretory material in the stalk region after hypophysectomy supports the “transport‐hypothesis.”

Structural similarities between the neurohypophyses of brook trout and tetrapods

The vascularization and prcoptic innervation of the hypophysis of the brook trout ( Salvelinus fontinalis) indicate that the rostral neurohypophysis is structurally similar to the median eminence of tetrapods and that the caudal neurohypophysis is comparable with the neural lobe of higher vertebrates. These conclusions are supported by three observations. 1. The vascular supply to the rostral hypophysis is largely independent of that to the caudal hypophysis. 2. Neurosecretory fibers of the preoptic tract appear to terminate on a rostral capillary plexus whose extensions form the chief blood supply of the pars distalis. 3. Stainable neurosecretory material accumulates in the extremities of the caudal neurohypophyseal processes which often abut on venous channels. It is suggested that some of this material passes directly into the systemic circulation.

Investigations on the hypothalamo-neurohypophysial neurosecretory system of the grass frog (Rana pipiens) after transection of the proximal neurohypophysis. II. Light- and electron microscopic findings in the disconnected distal neurohypophysis with special emphasis on the pituicytes.

With the onset of degeneration of the neurosecretory nerve fibers following transection of the proximal neurohypophysis, the pituicytes phagocytize these nerve fibers. Concomitant with a considerable increase in the size of the pituicytes, which reaches a peak between 8 and 10 days after the transection, the following sequence of events can be observed: reduction of the amount of intergranular axoplasm, increase in the size of some granules, partial or total loss of the electron density of the neurosecretory granules, loss of granule membranes, fusion of some granules, polymorphous axonal content in digestion vacuoles, formation of multilamellate bodies, digestion vacuoles with moderately electron dense peripheral material, empty digestion vacuoles. At about 12 days after the transection many vacuoles appear which subsequently disappear as the pituicytes shrink. Free neurosecretory granules resulting from the disappearance of the axolemma remain intact in the intercellular and perivascular connective tissue spaces and are eventually phagocytized by pituicytes and pericytes. Phagocytosis is considered to be a basic function of pituicytes. The problems related to this function as well as the possible implications for the interpretation of Herring bodies are discussed.

Argyrophil Adenohypophysial Cells in the Rainbow Trout (Salmo Irideus), Demonstrated Simultaneously With Chromophil Cells and With Intracellular Argyrophil Fibrils By a New Technique

Argyrophil cells were revealed in the proximal pars distalis of the hypophysis in the rainbow trout (Salmo irideus). For the demonstration of the cells a new silver technique was used. Simultaneously with the argyrophil cells, an argyrophil fibrillar structure-which is supposed to be functional in the process of nuclear amitosis-was shown in cells of the pars intermedia and of the proximal pars distalis. The copper sulphate-silver protein technique can be combined with azan or aldehyde-fuchsin with very good results for remaining adenohypophysial cell types or for neurosecretory fibres. A complete histological picture of the adeno (and neuro-)hypophysis is thus obtained.

The urophysis and the caudal neurosecretory system of fishes.

Summary1. The caudal neurosecretory system is defined in teleosts as a complex of secretory neurones (Dahlgren cells) in the caudal spinal cord leading by a tract to neurohaemal tissue organized as a typical neurosecretory storage‐release organ: the urophysis.2. The teleost urophysis is generally a distinct, easily recognizable, lobate structure of variable external form. Significant morphological variations lie in the organization of the neurosecretory fibres in relation to the vascular bed and in the degree of penetration of the meninx by the neurosecretory fibres to form an organ external to the spinal cord proper.3. The elasmobranch caudal system is composed of large cells with short axons projecting to a diffuse vascular bed; there is no organized urophysis.4. The caudal neurosecretory system and its urophysis appear late in post larval development by comparison with the hypothalamic neurosecretory system. The Dahlgren cells originate from the ependyma in development and also during regeneration of the caudal system in adult life.5.The elasmobranch system may represent the more primitive condition, and stages in the evolution of the advanced urophysial types can be visualized. The particular histology shown by the caudal system appears to have taxonomic significance.6.The cytology of the Dahlgren cell and its neurosecretory material is described. The proteinaceous neurosecretory material has an affinity for acid stains but not for the Gomori stains or reagents demonstrating SH/SS groups. The inclusions visible at the light‐microscope level are aggregates of elementary neurosecretory granules, 800–2500A diameter, which originate from Golgi centres. The possible participation of preterminal axonal regions–and tubular systems evident therein—in the formation of neurosecretory material is considered.7.The structure of the axon terminals raises questions about the way in which neurohormone may be released into the blood. Small vesicles have been variously interpreted as cholinergic synaptic vesicles and as products of the fragmentation of membranes of elementary neurosecretory granules. Evidence for the release of ‘neuro‐secretion centripetally’ into the cerebrospinal fluid also exists.8.Functional analysis of the caudal neurosecretory system has proven most difficult, The bulk of earlier data and more recent information indicate a role in ionic regulation. Increased sodium uptake by the gills of goldfish has been reported, as a result of administration of urophysial extract, and electrophysiological studies indicate a responsiveness of the system to variations in blood sodium ion concentration. The urophysis also has a definite pressor effect in eels and will stimulate water retention in anurans. The early claim of Enami that the system was involved in buoyancy regulation has never been substantiated. It must be admitted that the function of this system, virtually ubiquitous in teleost and elasmobranch fishes at least, has been anything but established and still represents a major challenge to comparative physiologists.

The vascularization of the hypothalamic-hypophyseal region of the Eastern Brook Trout, Salvelinus fontinalis.

AbstractThe hypophysis of the brook trout is irrigated by blood vessels which originate from the internal carotid arteries. These are: (1) branches of the ventral hypothalamic arteries that give rise to an extensive capillary plexus in the neurohypophysis from which vessels extend into all regions of the adenohypophysis; (2) branches of the caudal hypothalamic artery that irrigate the saccus vasculosus and continue anteriorly to supply the ventral areas of the meta‐adenohypophysis; (3) a caudal hypophyseal artery which vascularizes a portion of the meta‐adenohypophysis however, this vessel is not always present; (4) a pair of small arteries which supply the peripheral regions of the gland directly from the carotids.Most of the neurosecretory fibers of the preoptic‐hypophyseal tract terminate close to capillaries in the neurohypophysis. A few axons extend into meso‐adenohypophyseal tissue. It is suggested that the secretory activities of the pro‐, meso‐ and metaadenohypophyses are governed by hypothalamic factors that are chiefly transmitted to the gland cells via the vascular system (indirect control). However, the activity of the meso‐adenohypophysis may also be regulated by factors which are transmitted directly to the cells from the endings of neurosecretory fibers which have traversed the neurohypophysis (direct control). The distribution and abundance of neurosecretion in the ventral hypophysis suggest the possibility of storage of hypothalamic products within this region.

The infundibular process of rodents of the genus Meriones.

The infundibular process of the desert-living rodents Meriones differs from those of other mammals particularly in having extremely large perivascular spaces. These may extend between nerve fibres which are also much more loosely packed than in other species. Typical neurosecretory fibres are present and may project into the perivascular spaces. Both microvesicles and neurosecretory granules may occur in clumps bound by a membrane, forming multivesicular or multigranular bodies.

Monoamine-containing fibres in the pituitary neuro-intermediate lobe of the pig and rat.

A rich system of monoamine-containing fibres is described in the neural lobe and pars intermedia of the pig and rat. a) A rich network of delicate varicose fibres is evenly distributed throughout the parenchyma of the neural lobe and surrounds the cells of the pars intermedia. b) Droplets or clusters of droplets are scattered throughout the neural lobe. Most of them probably constitute terminal swellings or end-apparatuses of smooth or varicose fibres. The number of droplets varies from animal to animal; they are found also in the pars intermedia. c) Coarse varicose fibres are mainly localized around larger vessels. At least some of these fibres are nerve fibres of sympathetic origin. A combination of fluorescence microscopy and aldehyde-fuchsin staining on the same sections demonstrated that the majority at least of these monoamine-containing structures were not identical with aldehyde-fuchsin positive neurosecretory fibres.

The subfornical organ and the area postrema of the dromedary (Camelus dromedarius).

The subfornical organ (SFO) and area postrema (AP) are both topographically closely related to the choroid plexus of the respective ventricles. Vascular connections exist between the organs and the plexus. The vascular network which shows a definite zoned arrangement in the area postrema is surrounded by a connective tissue sheath. The center of the SFO is covered by very flat ependymal cells. The adjacent ventricular surface of the organ is lined by a simple cuboidal ependymal epithelium. The layer beneath the ependymal layer is a loosely textured layer of ependymal and neuroglial cell processes with large intercellular spaces. The so-called parenchymal cells were considered as nerve cells; two types, small and large ones, could be identified. The small nerve cells are predominantly located within the center of the SFO, whereas the large ones are mainly found in its dorsal part. The nerve cells as well as various neuroglial cells (astrocytes, tanycytes, and microglial), with the exception of the microglial cells, contain paraldehyde fuchsin-positive inclusions. Larger granules which can be demonstrated with chrome-alum-haematoxylin-phloxin were found in nerve cells exclusively. Neurosecretory nerve fibers were constantly found in the SFO. The area postrema is also predominantly lined by a simple squamous ependymal epithelium; toward the central canal of the medulla a stratified columnar epithelium could be observed. Small nerve cells were found throughout the organ, very large nerve cells mainly in the dorsal and ventral parts; their location among ependymal cells suggests a receptor function. All cell elements, except microglial cells, contain paraldehyde fuchsin positive granules. In both, the SFO and the AP, hyaline, colloid material of undetermined nature can be observed.