Ultrastructural studies on the localization of neurohypophysial hormones and their carrier proteins

Neurophysin, vasopressin and oxytocin were localized in different portions of the supraopticohypophysial tract (SHT) using the unlabeled antibody enzyme technique at the ultrastructural level. In vasopressin-positive supraoptic perikarya, vasopressin and neurophysin were present in all neurosecretory granules. Within the zona interna of the median eminence, vasopressin and neurophysin were present in two populations of axons, one with granules of 1300-1500 A and one with granules of 900-1300 A. Following exposure of thin sections of median eminence to antiserum to neurophysin, reaction products were present in granules and in the extragranular cytoplasm in the axons with larger granules; in all other cases reaction product was confined to the granules. Vasopressin-positive fibers were also presented in large numbers of the zona externa of the median eminence and many terminated on the pituitary primary portal plexus. A few oxytocin fibers were present on the portal capillaries in the infundibular stalk. In the posterior pituitary all axon profiles were neurophysin positive. Neurophysin was present as both a granular and cytoplasmic pool. Vasopressin-containing axons account for 90% of the neuronal elements in the posterior pituitary and oxytocin for the remaining 10%. Findings on the subcellular distribution of these peptides are related to current theories on transport and release of neurohormones.

Ultrastructural immunocytochemical localization of luteinizing hormone-releasing hormone (LH-RH) in the median eminence of the guinea pig

LH-RH was localized at the ultrastructural level in axons and nerve terminals of the median eminence of the male guinea pig. LH-RH positive neuronal profiles were most concentrated in the medial-dorsal aspect of the infundibular stalk and in the post-infundibular median eminence at the level immediately following separation of the stalk from the base of the brain. LH-RH containing axon profiles were most abundant in the palisade zone; nerve terminals in contact with the hypophysial portal vasculature were relatively rare. The hormone was present within granules that measured 900-1,200 A in axons of the palisade zone and 400-800 A in nerve terminals abutting on the portal plexus. The differently sized granules represent heterogeneous populations.

Fine structure of neurons of the arcuate nucleus and median eminence of the hypothalamus of the golden hamster following immobilization

The fine structure of arcuate neurons of the arcuate nucleus, the ependymal tanycytes and the contact zone of the median eminence was examined following immobilization, an acute stress which significantly activated the hypothalamo-pituitary-adrenal (HPA) axis. Arcuate neurons of immobilized adult male hamsters displayed morphological indications of heightened activity; the number of lysosomes and dense core vesicles (80-120nm) was increased. A markedly greater number of dense core vesicles was present in axon terminals of the contact zone of the mid-central median eminence and the ventral proximal stalk. Tanycytes of the median eminence exhibited an augmented number of electron dense bodies in both perikarya and end processes. These results indicate that the arcuate neurons, the axons of the contact zone, and the ependymal tanycytes of the hamster medial basal hypothalamus (MBH) may be involved in the response to immobilization.

Microfluorimetric quantitation of catecholamine fluorescence in rat median eminence. II. Turnover changes in hormonal states

Catecholamine nerve terminals in the rat median eminence have been studied using the fluorescence histochemical technique of Falck and Hillarp in combination with quantitative microfluorimetry. The catecholamine fluorescence intensities recorded from various parts of the median eminence were all found to be within the linear part of the dopamine or noradrenaline concentration-fluorescence relationship as studied in an agar-albumin model system. The catecholamine fluorescence was also found to disappear with time in an exponential manner following tyrosine hydroxylase inhibition produced by alpha-methyl-p-tyrosine methylester (H44/68). Similar results were obtained when measuring the dopamine decline by mass fragmentography in the median eminence after H44/68 treatment. These results and analysis of fluorescence frequency histograms strongly indicate that the catecholamine fluorescence values recorded are proportional to the catecholamine concentration. It is concluded that the microfluorimetric technique used is a reliable method for catecholamine quantitation in discrete nerve terminal areas of the median eminence. The main advantages of the technique are that a high sensitivity and quantitative data on the transmitter content can be obtained in strict relation to the neuroanatomy. Measurement of the catecholamine fluorescence disappearance after H44/68 was used to evaluate catecholamine turnover during various endocrine states. The results showed that two dopamine systems with different transmitter turnover may be distinguished. Tuberinfundibular dopamine neurons projecting to the lateral palisade zone were thus shown to have a slower turnover than those projecting medially to the capillary loops. No definite changes in catecholamine turnover were observed after adrenalectomy and castration in the male, although there was a tendency toward increased noradrenaline turnover in both states. During pregnancy an increase in noradrenaline as well as dopamine turnover was noted. The present results therefore give further evidence for the view that catecholamine nerve terminals in the median eminence may participate in the regulation of gonadotrophin secretion.

Efferent projection of the nucleus praeopticus medialis to the median eminence in rats

Efferent projection from the medial preoptic nucleus, especially to the median eminence, was studied in the rat by light and electron microscopy, in combination with 3H-leucine labelling and electrical coagulation of the nucleus. In the light microscopy autoradiographs of the brain, an ipsilateral distribution of silver grains was followed caudally and dorsally in the periventricular area, into the triangular area between the ventromedial nucleus and the arcuate nucleus, and, further, into the paraventricular and supraoptic nuclei and median eminence. After electrical coagulation of the medial preoptic area, electron microscopy showed numerous degenerated nerve terminals in the external layer of the median eminence. Occasionally, cored vesicles (90 nm mean diameter with a core of 70 nm mean diameter) and clear vesicles (40-50 nm diameter) were encountered in the axoplasm of the degenerated nerve terminals. These findings indicate that the axons of the nerve cells of the medial preoptic nucleus extend caudally, terminating in the external layer of the median eminence.

Neurosecretory axo-axonic synapses in the median eminence of the turtle, Geoclemys reevesii

Neurosecretory axo-axonic synapses were found in the median eminence of the turtle. Most of the presynaptic axons contain granules approximately 95 nm in diameter, and the postsynaptic elements have granules approximately 110 nm in diameter. The functional significance of axo-axonic synapses in the median eminence is discussed in relation to the discharge of releasing hormones from the axon terminals.

The role of the blood-brain barrier in perfusion fixation of the brain for electron microscopy

Regions of the brain vascularized by capillaries of the blood-brain barrier (BBB) type require a different fixative from regions which have capillaries of the endocrine type. Fixative with isotonic buffer gives excellent ultrastructural preservation in the BBB regions, but cause severe shrinkage of cells in the endocrine regions. This is evidently due to the difference in the permeability of the capillary walls to solutes in the fixative. In the BBB regions in less permeable capillaries do not allow outflow of osmotically active particles to a harmful extent, whereas in the endocrine regions osomotic imbalances are created between the intra- and extracellular compartments. The diffusion rate of the fixative and the final volume of the fixed brain depend on the balance between the intravascular and intersitial hydrostatic and oncotic pressures across the capillary wall during the perfusion fixation, as those pressures regulate the amount of perfusate that will enter the parenchyma. Generally, as high a perfusion pressure as possible is recommended to obtain effective wash-out of blood and rapid diffusion of fixative into the tissue. Addition of macromolecules (2% PVP, mol. wt. 40,000) into the fixative slightly improved the ultrastructural preservation in the BBB regions of the central nervous system.

Gomori-positive elementary granules in inner and outer layer of the infundibulum

With the gallocyanine technique (Wittkowski, Bock and Franken, 1970) Gomori-positive substances of the infundibulum can be stained for light- and electron-microscopic examination. In various mammalian species, the size of Gomori-positive elementary granules in the outer layer is markedly different from that in the inner layer of the infundibulum. In general, the granules of the outer layer have less then half the diameter of those of the inner layer. In birds and fish, however, only small differences were found between the granules of both layers. The significance of the results is discussed.

Post-natal development of the median eminence of the guinea pig

The ultrastructure of the median eminence of neonatal (newborn) 1-, 3-, 5-, and 10-day old) and adult guinea pigs was studied to determine the dynamic changes occurring in this structure during early life. At birth the portal vasculature consists of the Mantelplexus and a few, non-fenestrated capillary loops. The number of ansae and the degree of fenestration increase rapidly after birth. The abundance of cytoplasmic and ciliary projections into the ventricular recess and the large numbers of organelles indicate that the ependymal cells are more active in the neonatal period than in the adult male. Moreover, the ependymal endfeet cover most of the surface area of the primary portal plexus during this time. The neuronal layers of the median eminence are difficult to distinguish at birth due to the lack of myelinated fibers in the zona interna. Significant myelination appears on day 3 but is not complete until day 10. There is a progressive increase in the numbers of Herring bodies and large neuro-secretory granules (1,500-1,700 A) during this same time period. In the zona externa, few nerve terminals abut on the perivascular space until day 3. Increases in numbers of granules per axon profile were noted for each day after birth. Despite the relatively long gestation period of the guinea pig (68-72 days), the morphologic appearance of the median eminence at birth suggests that the neurovascular link controlling anterior pituitary function is not yet complete.

Ultrastructural localization of luteinizing hormone-releasing hormone in the median eminence of the rat

The nature of cells in the hypothalamus that produce hypothalamic hypophysiotropic hormones remains unsettled. To investigate this problem electron microscopic immunocytochemistry was performed on thin sections of the median eminence of proestrous rats using antibodies to synthetic luteinizing hormone-releasing hormone (LHRH) and the peroxidase-antiperoxidase (PAP) technique. PAP complexes indicating the presence of LHRH were found over small, homogeneously dense neurosecretory granules 75-90 nm in diameter that occurred in clusters within neurons. Positive staining was more prevalent in the palisade zone in the anterior median eminence than at more posterior levels. LHRH positive granules were not obsereved in neuronal processes in the neurohemal contact zone, but appeared closer to the pericapillary space in the posterior median eminence than at more anterior levels. Occasional groups of LHRH positive granules were also found in the internal layer and hypendymal zone. No staining was observed in tanycytes or glial elements. These results support the hypothesis that hypothalamic hypophysiotropic hormones are produced by neurons and are stored in granules of homogeneous size and density in nerve processes located in the median eminence in the proximity of the hypophysial portal plexus.

Biosynthesis of thyrotropin-releasing hormone in organ cultures of the guinea pig median eminence

Guinea pig median eminence tissue was put into organ culture for periods up to 13 days. The cultured tissue was examined in 3 ways at various ages: a) morphologically, by transmission electron microscopy and light microscopic autoradiography; b) biochemically, by determination of the ability of the cultures to accumulate [3H]L-proline and to incorporate this isotope into [3H]TRH; and c) by bioassay, in which the content of TRH in tissue and culture medium was determined by the ability of extracts of both to release radioimmunoassayable TSH from rat hemi-pituitaries incubated in vitro. It was found that the cultures exhibited a high degree of preservation of ependymal cell morphology and a sustained ability to accumulate [3H]L-proline over the entire 13-day time course. Neuronal elements showed a progressive degeneration with time in culture, and the ability of the culture to produce [3H]TRH was lost concomitantly with the loss in neuronal integrity.

The primate median eminence. I. Correlative scanning-transmission electron microscopy

A combined scanning/transmission electron microscopic (SEM/TEM) technique was used to analyze the third cerebral ventricle and underlying tissue of the median eminence of 6 mature rhesus monkeys. The same sample of the ventricular wall was subjected to both SEM and TEM. This technique demonstrates two basic subpopulations of supraependymal cells on the surface of the supraoptic, infundibular and mammillary recesses. Type 1 cells are definitely neuron-like in their surface configuration and internal fine structural organization. Type 2 cells are more similar to histiocytes and are not as numerous as type 1 cells. The functional capacity of type 1 cells is discussed in the context of their potential role as a neuronal network that may serve as a short loop autoregulatory mechanism controlling the synthesis of releasing hormones or biogenic amines.

Scanning electron microscopy of the wall of the third ventricle of the brain of Rana temporaria. III. Electron microscopy of the ventricular surface of the median eminence

From combined scanning and transmission electron microscopic studies it is concluded (1) that all surface specializations of the median eminence of Rana temporaria are protrusions of ependymal cells; (2) that the microvillus-like structures of the median eminence are transient differentiations of the ependymal cell surface, associated with the activity of the ependymal cells.

Ultrastructure of the neurohemal hypothalamic floor of the frog, Rana catesbeiana

The fine structure of the hypothalamic floor was studied in the frog, Rana catesbeiana. The regions slightly anterior and posterior to thw swollen hypothalamic floor part, which has been classically designated as the median eminence, contained neurosecretory axon terminals abutting on the capillary walls. The region relatively far anterior to the swollen part did not show neurosecretory axons terminating on capillary walls. The neural stalk contiguous to the rostral border of the pars intermedia had few neurosecretory axon terminals which end on the terminal portions of the ependymal processes covering the capillary wall. The functional significance of the regional differentiation of the frog neurohypophysis is discussed in connection with the regional differentiation of various secretory cells in the adenohypophysis.

Different populations of granules and their distribution in the hypothalamo-neurohypophysial tract of the rat under various experimental conditions. II. The median eminence

In connection with previous studies (Krisch, 1974), a morphologic analysis of the median eminence of male, female, pregnant, and thirsting rats has been made, in order to evaluate: 1. whether the median eminence is an additional area where oxytocin and vasopressin may be released from neurosecretory axons in the zona interna; 2. whether it is possible to distinguish different populations of axons according to the mean diameters of their granules in the external zone of the median eminence. In the zona interna of the male and the female control animals, the oxytocin- and vasopressin-containing granules have already reached their final diameters, i.e. their size remains constant until they reach and are stored in the neurohypophysis. However, the relative proportions of these populations in the fibre layer of the median eminence indicate certain differences between male and female animals, concerning the turnover of oxytocin vasopressin. In pregnant, as well as in thirsting animals, there is an apparent decrease of the mean granule diameters in oxytocin- and vasopressin-containing axons. In particular in the latter group, ballooned axons in the fibre layer of the median eminence, containing disintegrating granules, tubular structures filled with electron dense material, and fine-flocculent material in the axoplasm, together with the morphometric results, support our concept that in case of highly increased peripheral demand the contents of the mature, large granules are released into the axoplasm of the fibre layer in the median eminence. In addition, axons containing granules smaller than 115 nm may be classified into four populations with mean diameters of 84 nm, 96 nm, 103 nm, and 100n. The same populations are found in the palisade zone of the median eminence as well as in the pericapillary zone, but in contrast to the zona interna there is a fifth population of axons containing very small granules with a mean diameter of about 70-75 nm. The relative proportions of these five populations differ between the zona palisadica and the pericapillary zone on the one hand and between the experimental groups on the other hand. Some morphological findings concerning the tanycytes are discussed with respect to previously published data.

Ultrastructural immunocytochemical localization of neurophysin and vasopressin in the median eminence and posterior pituitary of the guinea pig

With the use of tissue prepared by freeze-substitution and the unlabelled antibody enzyme technique, neurophysin and vasopressin were localized at the ultrastructural level in the pofterior pituitary and median eminence of the guinea pig. In the posterior pituitary neurophysin was found in the large neurosecretory granules (1300-1500 A) of axons, Herring bodies, and nerve terminals. In some of these axons immunoreactive neurophysin was found outside of granules in the axoplasm. By light microscopy neurophysin was found in both the zona interna and zona externa of the median eminence; this was confirmed by electron microscopy. In the zona interna as in the posterior pituitary, neurophysin was localized both inside and outside the large neurosecretory granules. In the zona externa, immunoreactive deposit was primarily located in granules with a diameter of 900-1100 A in nerve terminals abutting on the primary portal plexus. The distribution of vasopressin paralleled that of neurophysin except that the hormone was rarely extragranular. These results demonstrate for the first time that both neurophysin and vasopressin are present in granules of axons that are in contact with the hypophysial portal vasculature.

Influence of castration on incorporation of exogenous peroxidase into "synaptic" vesicles of the median eminence. II. The perivascular part of the palisade zone in male rats

In the median eminence of male rats, nerve profiles in the immediate vicinity of portal capillaries have been divided into 4 categories on the basis of their vesicular content: profiles a with agranular "synaptic" vesicles of about 50 nm, b with similar agranular vesicles and also with granular vesicles of mainly 60-140 nm, c with granular vesicles alone and d without vesicles. Twenty-four hours after castration, the percentage of profiles of category a was significantly increased when compared with sham-operated animals, whereas the percentage of profiles of category b was significantly decreased. After intravenous injection of exogenous peroxidase, especially the nerve profiles located in direct contact with the outer basement membrane of the portal capillaries contained peroxidase positive "synaptic" vesicles. Injection off peroxidase after castration resulted in a significant increase in the percentage of nerve profiles containing both peroxidase positive and peroxidase negative "synaptic" vesicles (category a). It is suggested that, in certain nerve terminals, castration may lead to release off the content of granular vesicles, which may contain gonadotropin releasing factor. This release then may cause that nerve terminals with agranular vesicles and granular vesicles (category b) change in their vesicular content and are therefore classified as terminals with only agranular vesicles (category a). An increased turnover rate in the ultrastructurally affected terminals may be reflected in increased uptake of exogenous peroxidase. The observations imply that certain neurones projecting to portal capillaries in the median eminence are, directly or indirectly, sensitive to changes in the level of gonadal steroids.

Retrograde transport of horseradish peroxidase in the magnocellular neurosecretory system of the rat

Horseradish peroxidase (HRP) was injected into the pituitary in adult rats. After 2-3 days, the neurones of the supraoptic nuclei, the magnocellular parts of the paraventricular nuclei, and the various accessory neurosecretory hypothalamic nuclei showed accumulation of HRP. The HRP reaction product consisted of fine, discrete cytoplasmic granules, and in electron micrographys it was seen to be located in the lysosome-like dense bodies of 0.4-0.6 mum diameter which are normally present in the cytoplasm of the neurosecretory neuron.es. Very little reaction product was found in the neurosecretory axons. Reaction product was also found in the hypothalamic arcuate nuclei. This was the result of an endogenous peroxidase-like activity, since it occurred in control animals which had not received HRP. This endogenous reaction product is non-neuronal. Morphologically, it takes the form of distinctive clusters of coarse granules which are seen in electron micrographs to be characteristic angular bodies of 0.7-1.0 mum diameter located in the cytoplasm of astrocytes or their processes.

Immuno-electron microscopic localization of luteinizing hormone-releasing hormone in the arcuate nuclei and median eminence of the rat

Luteinizing hormone-releasing hormone (LH-RH) was localized in neurons of the arcuate nucleus and median eminence, and in many of the nerve fibres of the median eminence of the rat, by using antisera to synthetic LH-RH with the immunoglobulin-peroxidase technique or by using a soluble peroxidase-antiperoxidase complex at the electron microsc opic level. Many of the arcuate neurons and some of the median eminence neurons contained numerous densely stained granules elsewhere in the cytoplasm. Similar granules, positive for LH-RH, were observed in many nerve profiles around the capillaries of the primary portal plexus of zona palisadica and zona granulosa of the medain eminence, and such profiles may represent the release sites of LH-RH into the portal vessels. About 20% of the nerve profiles around the primary portal plexus were LH-RH positive and the rest of the nerve profiles were negative. The negative profiles may belong to other hypophysiotropic hormones and/or to catecholaminergic fibers. Some nerve endings at the ventricular surface of the ependymal cells, in contact with the ventricular fluid, were also LH-RH positive. These nerve endings probably release their secretion into the cerebrospinal fluid. There was no staining of the mitochondria, the nuclei or the axonal membranes. Stain of varying intensity was seen in the secretory granules, and within and around the cisternae of the Golgi complex. The stained granules in the cisternae wer pleomorphic. From these observations it was concluded that the LH-RH is packaged or condensed into granules in the Golgi cisternae.

Projections to the median eminence and the arcuate nucleus with special reference to monoamine systems: effects of lesions

The external layer of the median eminence (ELME) and the arcuate nucleus of male rats were studied with the Falck-Hillarp technique and electron microscopy of aldehyde-OsO4 or KMnO4 fixed material after various types of hypothalamic deafferentation experiments with the Haláz knife. Special reference was paid to the monoamine systems and the results can be summarized as follows. 1. The main monoaminergic input to the ELME comes from the arcuate nucleus-periventricular area via a dorsal approach. A horizontal transection through the arcuate nucleus decreases the percentage of monoamine boutons i.e. boutons with small granular vesicles, from 31.6% in the controls to 4.4% in the lesion group, whereas only a small effect is seen after anterior (or complete) deafferentations. 2. A major input to the ELME enters the basal hypothalamus at the anterior-lateral aspects (see Réthelyi and Halsáz, 1970). The fibers cut after anterior deafferentations in all probability mainly come from cell bodies localized in the anterior hypothalamus or even further rostrally but some may represent Na axons ascending from the lower brain stem. 3. The degeneration course of nerve endings in the ELME both after anterior deafferentations as well as after lesions in the arcuate nucleus is rapid (within 2-3 days) and morphologically characterized by an initial aggregation of large dence cored vesicles seemingly to electron dense bodies within the boutons and probably also to a closer spacing of the small electron lucent synaptic vesicles (see Raisman, 1972). This type of degeneration seems to take place both in monoamine and non-monoamine neurons. 4. Degenerating boutons are found in the arcuate nucleus after anterior and complete deafferentations. Thus, the anterior hypothalamus may exert an "indirect" control of the pituitary gland via synapses on arcuate neurons although quantitavely the direct influence through the projection to the ELME is of more importance. 5. After anterior deafferentations enlarged axons containing large amounts of large dense cored vesicles and other organelles are found caudally of the cut indicating the existence of rostral projections from the medial hypothalamus.

Ultrastructural study of the uptake of peroxidase by the rat median eminence

An active role of the ependymal cells (tanycytes) of the median eminence in the transport of hypothalamic hormones has been recently suggested. In order to investigate the fate of material present in the cerebrospinal fluid, a protein tracer, horse-radish peroxidase (HRP) was injected into the left lateral ventricle of rats. Two minutes after the injection, HRP had largely diffused between tanycytes and hypendymal cells. As soon as 5 min after the injection, HRP had completely penetrated all the layers of the median eminence. A few labelled vesicles and lysosomes were occasionally seen in ependymal and glial cells. At longer time intervals (20 min, 1 and 4 hrs), a reaction was observed in the lumen of fenestrated capillaries of the pituitary portal plexus. In many nerve endings of the external zone, vesicles and lysosomes were seen to contain HRP. An interesting observation was the localization of HRP between nerve endings and cells in both the pars nervosa and the pars intermedia of the pituitary gland. No reaction was recorded in the anterior pituitary and the kidney. Seventeen hours after the injection, the extracellular space was free of reaction but a few positive intracellular structure were still found. These results clearly indicate that some material from the third ventricle can rapidly diffuse between cells and axons of the median eminence to reach the fenestrated capillaries of the pituitary portal plexus and the posterior pituitary without involving an active transport by tanycytes.