Ultrastructural changes in the gracile nucleus of the spontaneously diabetic BB rat.

The present study describes the structural changes in the gracile nucleus of the spontaneously diabetic BB rat. At 3-7 days post-diabetes, axons, axon terminals and dendrites showed electron-dense degeneration. Degenerating axons were characterized by swollen mitochondria, vacuolation, accumulation of glycogen granules, tubulovesicular elements, neurofilaments and dense lamellar bodies. Degenerating axon terminals consisted of an electron-dense cytoplasm containing swollen mitochondria, vacuoles and clustering of synaptic vesicles. These axon terminals made synaptic contacts with cell somata, dendrites and other axon terminals. Degenerating dendrites were postsynaptic to normal as well as degenerating axon terminals. At 1-3 months post-diabetes, degenerating electron-dense axons, axon terminals and dendrites were widely scattered in the neuropil. Macrophages containing degenerating electron-dense debris were also present. At 6 months post-diabetes, the freshly degenerating neuronal elements encountered were similar to those observed at 3-7 days. However, there were more degenerating profiles at 6 months post-diabetes compared to the earlier time intervals. Terminally degenerating axons were vacuolated and their axoplasm appeared amorphous. It is concluded that degenerative changes occur in the gracile nucleus of the spontaneously diabetic BB rat.     

The Fine Structure of Degenerating Nerve Fibers, their Sheaths, and their Terminations in the Central Nerve Cord of the Cockroach (Periplaneta americana)

The connectives above and below the second thoracic ganglion and nerves to and from the mesothoracic leg were severed in Periplaneta americana. Isolated ganglia and severed nerve cord were examined in the electron microscope. In the connectives, sheaths of degenerating fibers remain continuous but become thicker and more dense. There is increase in number and more haphazard disposition of the neuroglial processes which ensheath the axons. The cytoplasm contains vacuoles. Dense droplets normally intercalated between the layers of neuroglial processes ensheathing the axons are strikingly increased in number. The axoplasm with its organelles forms dense clumps. Mitochondria in axons are enlarged, the intramitochondrial matrix is more dense, and the internal folds are disorganized. In ganglia, mitochondrial changes in terminal parts of the axons appear similar to those described in the parent axons in the connective. The synaptic portions of nerve fibers appear very dense. Alterations of the sheath are minimal. Synaptic particles in the degenerating axoplasmic coagulum undergo only slight morphological changes and are still present up to 6 days after severance of their nerve fibers. It is difficult to assess whether there are any alterations in the total number of synaptic particles during degeneration.     

Fine structure,origin, and distribution density of the autonomic nerve endings in the tarsal muscle in the eyelid of the mouse

Summary The fine structure, origin, and distribution density of the autonomic nerve endings in the tarsal muscle of the mouse were studied by histochemistry and electron microscopy. With histochemical methods, the fine nerve plexus in the normal muscle shows both catecholamine-positive varicose fibers and acetylcholinesterase-active varicose fibers. The former are distributed more densely than the latter. After superior cervical ganglionectomy, the catecholamine-positive fibers disappear, while after pterygopalatine ganglionectomy, the acetylcholinesterase-active fibers vanish. In electron micrographs, the varicosities appear as expansions containing many synaptic vesicles. The axonal expansions partly lack a Schwann sheath and directly face the pinocytotic vesicle-rich zones of the smooth muscle cells. A relatively wide space, 0.1 to 1.0 m in width, lies between nerve expansion and muscle cell. The expansions can be classified into two types: Type I having small granular synaptic vesicles, and Type II having agranular vesicles instead of small granular synaptic vesicles. Type I undergoes degeneration after superior cervical ganglionectomy, while Type II degenerates after pterygopalatine ganglionectomy. This indicates that Type I corresponds to the synaptic ending of the adrenergic fiber originating from the superior cervical ganglion, and Type II to the synaptic ending of the cholinergic nerve fiber derived from the pterygopalatine ganglion. Type I is more frequent (88/10⁴ m² area of muscle) than Type II (17/10⁴ m²).     

Reversible ultrastructural changes of arcuate neurons after vinblastine injection into the median eminence of the rat

Summary The ultrastructural effects of vinblastine on the arcuate neurons and median eminence were studied in the rat. The animals were stereotaxically injected with solutions of 1 mM and 5 mM vinblastine into the median eminence and killed 3, 8 and 21 days after injection. Eight days after injection of 1 mM vinblastine the neurons of the arcuate nucleus showed marked changes. The Golgi complex was more distinct and considerable increases in the populations of dense bodies, granulated vesicles and coated vesicles were observed. Changes in the axo-somatic synapses and degenerating fibers in the surrounding neuropil were also characteristic of the experimental animals. The outer zone of the median eminence showed numerous degenerated nerve fibers and fibers engulfed by glial cell processes. Eight days after injection of 5 mM vinblastine arcuate neurons and median eminence showed similar changes, but quantitative differences were noted. A striking ultrastructural recovery of the arcuate neurons and axons in the outer zone of the median eminence was observed 21 days after injection of either 1 mM or 5 mM vinblastine. The results are discussed in relation to axoplasmic transport and axonal degeneration.Supported by CONICET and National University of Cuyo, Argentina.Members of the Scientific Research Career of the Consejo Nacional de Investigaciones Cientificas y Tecnicas, Argentina.     

Nerve ultrastructure of the arteries of the brain stem]

Ultrastructure of the nerve apparatus in the arteries of the brain base has been studied in cats. The structure of peri- and adventitial nerves has been investigated electron microscopically. Three types of efferent axons and four types of synaptic vesicles (small agranular and granular, large granular, large electron opaque vesicles) have been revealed. Vesicle-containing axons in the brain arteries approach the external smooth muscle cells of about 80 nm. Terminal axonal dilatations possessing direct and mediated connections with muscular cells of the middle tunica have been revealed.     

Fine structure of synapses in the dorsal nucleus of the lateral geniculate body of normal and blinded rats

Summary Degenerating boutons, observed from 2 to 60 days after eye enucleation, displayed decreased plasma membrane density, increased axoplasmic density, and enlarged mitochondria with deformed cristae when compared with boutons from normal animals. There was also a loss of synaptic plasma membrane specialization and the boutons abnormally indented contiguous dendrites. The number and appearance of synaptic vesicles in some degenerating boutons were notably altered. Phagocytosis of boutons in most instances appeared to be accomplished by astrocytes. When degeneration was first apparent in some boutons, the subsynaptic organelle in the adjacent dendritic cytoplasm was enlarged, somewhat less dense and was associated with small granular and circular profiles. Subsynaptic organelles in experimental animals were absent from contiguities between dendrites and other cell processes, except in a few instances when only small portions of boutons remained at their synaptic sites, suggesting that the organelles disappeared when boutons had been completely phagocytized.Degenerating myelinated axons, observed from 2 to 300 days after enucleation, exhibited the same triad of features as degenerating boutons. They appeared to be phagocytized in most instances by dense glial processes, presumably oligodendrocytic, which were normally situated between the axon and its myelin sheath and were related to the inner mesaxon.This investigation was supported by U.S.P.H.S. Training Grants Nos. 2 T1 GM 202 T1 CA 505506, and 2RO 1 AM 368806.The author expresses his appreciation to Dr. A. J. Ladman for acquainting him with the techniques used in the study and to Dr. R. J. Barrnett for valuable criticism of this report. Gratitude is also extended to Mr. E. Z. Rutkowski for making the drawing.     

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.     

The origin,structure and occurrence of corpora amylacea in the bovine mammary gland and in milk

Summary Fibres growing from neurons of explanted dorsal root ganglia from 10 day chick embryos were transected and subsequently observed by light and electron microscopy after periods of a few to fifty minutes. Changes immediately proximal and distal to the cut together with alterations further away from the site of injury on both sides of the cut were recorded. Observations were also made on the growth cones of damaged axons and on changes in associated glial cells.Reactive and degenerative changes including the rotation, retraction and swelling of cut axons occurred rapidly. Electron microscopy revealed tracts of filamentous material close to the sealed-off ends of axons, swollen organelles such as mitochondria, and lamellar bodies of varying dimensions.Proximal to the injury and closer to the expiant, damaged and degenerating axons mingled with normal processes. Many contained only a fine granular material, others clumps of organelles, particularly mitochondria.Distal to the cut, microspikes were lost from some growth cones. The dense granular material filling microspikes and growth cones remained unchanged. Clumps of large clear vesicles, lamellar bodies and swollen degenerating mitochondria were present, not only within growth cones, but also in all parts of the axon distal to the cut.Glial cells associated with transected axons soon developed an electron dense cytoplasm containing swollen organelles. Large numbers of vesicles filled with a particulate substance were also found.The possible significance of the changes observed after transection are considered and discussed.The author wishes to thank Prof. D.W. James in whose laboratory at University College London these studies were initiated, Dr. A.R. Lieberman for his expert help and advice and the University of London Central Research Fund and Wellcome Trust for financial assistance     

Programmed cell death: Cytochemical and X-ray microanalytical characterization of calcium compartments in neuromuscular junctions during the normal breakdown of the intersegmental muscles in the giant silkmothAntheraea polyphemus

Summary Calcium stores were cytochemically demonstrated using a combined oxalate—pyroantimonate method in the neuromuscular junctions of the degenerating intersegmental muscles in the giant silkmothAntheraea polyphemus. The elemental composition of punctate precipitates of the reaction product was determined by electron probe X-ray microanalysis of unstained thin sections by energy-dispersive spectrometry and wavelength-dispersive spectrometry. The wavelength-dispersive spectra collected over terminal axons demonstrate a significant calcium signal and a trace of antimony.During the rapid lytic phase of spontaneous muscle degeneration, the calcium punctate deposits were detected in presynaptic terminals in the following sites: the synaptic vesicles and the mitochondria. Calcium precipitates were also found in the dense bodies and the mitochondria encountered in the glial convolutions. No calcium deposit was seen in the synaptic clefts and intercellular spaces of the subsynaptic reticulum of type I and type II. A comparison of calcium to antimony ratios between the terminal axons and the sarcoplasmic lysosomes revealed highly significant differences (P<0.001). Such a variability of the calcium to antimony ratio may be related to different conditions of precipitation or antimony diffusion in the different cell compartments. It was concluded that such synaptic terminals do not appear damaged in spite of the muscle degeneration and presumably continue to perform vital functions while the muscles are no longer contractile 20 h after adult ecdysis.     

Distribution and ultrastructure of thalamic afferents in the cat auditory cortex

Ultrastructural features of thalamic afferent fibers were studied in the cat auditory cortex in the early stages (on the 4th day) of experimental degeneration produced by destruction of the medial geniculate body. A coordinate grid was used in conjunction with an electron micro-scope to study the topography of the degenerating elements over wide areas of sections, so that the density of degeneration could be determined quantitatively in different layers of the cortex. Degenerating axons were found in all layers. Most of the large (5–7 µ) degenerating axons are located in layer VI; their diameters were smaller in the upper layers of the cortex. Degenerative changes affecting synaptic terminals of thalamo-cortical afferents were of the "dark" type. Fibers of the geniculo-cortical tract were shown to terminate mainly in cortical layer IV. A few degenerating synapses were found in the molecular layer. Terminals with sperical synaptic vesicles are found mainly on the spines of dendrites where they form "asymmetrical" contacts. A few degenerating axo-somatic synapses were observed on stellate neurons in layer IV. The results are discussed in connection with electrophysiological investigations of the cat auditory cortex during stimulation of specific afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 612–620, November–December, 1972.     

Structural changes associated with parotid “Degeneration secretion” after post-ganglionic sympathectomy in rats

Parotid glands of rat have been examined 12, 24 and 48 hours after avulsion of the cervical sympathetic ganglion and compared with normally innervated left glands. Formaldehyde-induced fluorescence showed a relatively normal complement of adrenergic nerves at 12 hours but most of the nerves had lost their noradrenaline content by 24 hours and no fluorescent nerves were detected at 48 hours. Ultrastructural degenerative changes in axons were rare at 12 hours, common at 24 hours, and the degenerating axons appeared to have disappeared by 48 hours. The glands looked whitish and pale and similar to the controls at 12 and 48 hours but were pinkish and oedematous on the sympathectomised side at 24 hours. Correspondingly the acini were loaded with secretory granules at 12 and 48 hours but were extensively depleted of granules at 24 hours. This loss of granules is considered to be due to sympathetic "degeneration secretion" caused by the release of noradrenaline from the degenerating adrenergic nerves between 12 and 24 hours after ganglionectomy. This is thought to be the first example of morphological change resulting from "degeneration activation" to be recorded microscopically.     

Fine structure and innervation of the avian adrenal gland

Summary Apart from cholinergic nerve fibers, which make up the main part of efferent fibers to the avian adrenal gland (Unsicker, 1973b), adrenergic, purinergic and afferent nerve fibers occur. Adrenergic nerve fibers are much more rare than cholinergic fibers. With the Falck-Hillarp fluorescence method they can be demonstrated in the capsule of the gland, in the pericapsular tissue and near blood vessels. By their green fluorescent varicosities they may be distinguished characteristically from undulating yellow fluorescent ramifications of small nerve cells which are found in the ganglia of the adrenal gland and below the capsule. The varicosities of adrenergic axons exhibit small (450 to 700 Å in diameter) and large (900 to 1300 Å in diameter) granular vesicles with a dense core which is usually situated excentrically. After the application of 6-hydroxydopamine degenerative changes appear in the varicosities. Adrenergic axons are not confined to blood vessels but can be found as well in close proximity of chromaffin cells. Probably adrenergic fibers are the axons of large ganglion cells which are situated mainly within the ganglia of the adrenal gland and in the periphery of the organ and whose dendritic endings show small granular vesicles after treatment with 6-OHDA.A third type of nerve fiber is characterized by varicosities containing dense-cored vesicles with a thin light halo, the mean diameter (1250 Å) of which exceeds that of the morphologically similar granular vesicles in cholinergic synapses. Those fibers resemble neurosecretory and purinergic axons and are therefore called p-type fibers. They cannot be stained with chromalum-hematoxyline-phloxine. Axon dilations showing aggregates of mitochondria, myelin bodies and dense-cored vesicles of different shape and diameter are considered to be afferent nerve endings. Blood vessels in the capsule of the gland are innervated by both cholinergic and adrenergic fibers.Supported by a grant from the Deutsche Forschungsgemeinschaft (Un 34/1).     

Distribution and ultrastructure of pyramidal tract endings in the cat rhombencephalon

The distribution and ultrastructure of terminals of corticofugal fibers in the cat rhombencephalon were investigated under the optical and electron microscopes at different periods (2–6 days) of experimental degeneration evoked by destruction of the sensomotor cortex. It was shown by the Fink–Heimer method that most degenerating fibers are distributed in the reticular nuclei of the pons and medulla. Massive degeneration of corticofugal fibers also was observed in the nuclei of the dorsal columns (nuclei of Goll and Burdach). Most of the degenerating (the "pale" type of degeneration) axo-dendritic and axo-somatic synapses in the gigantocellular reticular nucleus and the nucleus of Goll retained spherical vesicles. Small endings were found on the branches of the dendrites in which degenerative changes were of the "dark" type. The topography of the degenerating elements and axo-axonal synapses was studied in large areas of sections by the coordinate grid method. The dimensions of most degenerating axons in the gigantocellular reticular nucleus were greater (1.5 µ) than those of the degenerating axons (0.5 µ) in the nucleus of Goll. Most endings of pyramidal fibers and axo-axonal synapses are located in the central part of the nucleus of Goll at a depth of 0.5–1.2 mm from the brain surface. The results are discussed in connection with electrophysiological studies of the mechanisms of cortical control over unit activity of the reticular formation of the brain stem and nuclei of the dorsal columns.     

Myoneural junctions and neurosecretory endings on spermathecal muscle fibers of two weevils,Sitophilus granarius and Hypera postica

The fine structure of muscle fibers connecting the two arms of the spermatheca and their innervating axons was studied with the electron microscope. The muscle fibers appear to be a sub-set of skeletal and not visceral muscles. Neurosecretory axons with electron dense granules are adjacent to the muscle fibers in young females O-day post-eclosion but not in the ovipositing adult. The typical nerves form synaptic junctions with muscle fibers at all ages but the nerves are divided into two types based on the morphology of the synaptic vesicles they contain, either spherical or flattened.     

On degeneration of peptidergic neurosecretory fibres in the albino rat

Three types of degenerating peptidergic neurosecretory fibres have been found in the posterior pituitary of chronically dehydrated albino rats. "Dark" neurosecretory fibres and their swellings contain neurosecretory granules, neurotubules, shrunken mitochondria and diffusely distributed fine dense material. Some swellings are filled with synaptic vesicles and/or conglomerations of dense membranes. The transitional forms exist between these fibres and extracellular accumulations of electron dense material. Synaptic vesicles, single neurosecretory granules, lipid-like droplets and lamellar bodies occur in the latter. Some neurosecretory fibres and swellings have numerous polymorphous inclusions arising due to degradation of secretory inclusions and organelles, mitochondria and neurotubules in particular. "Dark" neurosecretory elements and those with numerous polymorphous inclusions are enveloped by pituicyte cytoplasm. Sometimes the plasma membranes both of the pituicytes and neurosecretory fibres are destroyed or transformed into a multi-membrane complex. It is assumed that pituicytes may phagocytize degenerating neurosecretory elements. N urosecretory fibres with a locally dissolved neuroplasm and/or large lucent vacuoles seem to be due to axonal degeneration by the "light" type. These neurosecretory elements, the largest of them in particular, may transform into large cavities bordered by a membrane and containing flake-like material and single-membrane vacuoles. Degeneration of neurosecretory elements seems to occur mainly due to hyperfunction of the hypothalamo-hypophysial neurosecretory system.     

Ultrastructural studies on neuromuscular contacts and the formation of junctions in the flight muscle of Antheraea polyphemus (Lep.)

In the moth Antheraea polyphemed at the onset of adult development. The subsequent breakdown of the isolated motor stulongated vesicles similar in structure to channels of smooth ER, appear in large numbers in the axoplasm. Their nature as well as the functional aspects of early axonal changes are discussed. From the 7th day onward two types of axonal breakdown become prominent. The first is characterized 0y swelling axon profiles, distorted vesicles and strongly shrunken mitochondria, uhile shrinking axon profiles containing tightly packed mitochondria and unaltered vesicles are typical of the second. Both types presumably take place independently of each other in different axon terminals. Axons and the contents of at least the first type are finally removed by transformation into lamellar bodies. Glial processes obviously behave independently of degenerating terminals; they loose any contact with them and never act as phagocytes for axon remnants. During the whole period of breakdown undifferentiated contacts between nerve fibers and muscle anlagen are present but synaptic structures as in normal developing dlm have never been observed. This fact, in comparison with earlier studies, suggests a lack of trophic nervous activity on the muscle anlagen tissue. A short time after removal of the isolated stumps new nerve tracts appear between dlm-fibers (which are, of course, strongly retarded in development). They are presumably sensory wing nerves which lack a guide structure to the central target, due to axotomy. Neuromuscular contacts or even junctions formed by axons of these nerves have occasionally been detected on the dlm. Their nature is discussed. Wallerian axon degeneration is compared to the normal, metamorphic breakdown of the innervation of the larval dlm-precursor. In contrast to the former, glial processes here remain in contact with the terminals. Glia and axons first swell. Then most glial processes are transformed into lamellar bodies whereas neurites shrink and become electron-dense. Axonal organelles remain intact for a long period.     

Ultrastructure of celiac plexus nodes in the dog

A normal structure of the celiac plexus nodes has been studied in 12 mature dogs. As demonstrate the results of the investigation, gangliocytes of the celiac plexus are characterized with a well developed granular cytoplasmic reticulum and a large number of Golgi complexes. In perikaryon of the gangliocytes, an essential number of mitochondria, microtubules, free ribosomes and polysomes, lysosomes, multivesicular bodies, agranular and granular vesicles and neurofilaments are found. The gangliocyte has one nucleus which occupies about 1/3 of the whole area of the cell. The nucleus is rich in chromatin. The nucleolus makes about 1/5 of the whole area of the nucleus and is intensively rich in heterochromatin. In the celiac plexus nodes amyelinated neural fibers predominate. Dendrites in the gangliocytes differ from axons by a higher electron density of their matrix and contain the same organells that does the perikaryon of the gangliocyte. Rather complex glyoneuronal interrelations are observed in the canine celiac plexus nodes. Synapses are revealed in all ganglionar zones. The axodendritic synaptic contacts predominate over the axosomatic ones.     

Localization of catecholamines and acetylcholinesterase in the terminal nerve fibres of the rabbit myometrium

Summary The autonomic nerves of the myometrium of the rabbit were studied in order to demonstrate simultaneously the adrenergic nature of an axon and the localization of acetylcholinesterase (AChE) in the same axons. The synaptic vesicles of the adrenergic axons and nerve terminals remained partially filled with the electron dense material typical for them after formaldehyde fixation and short incubation time for AChE. AChE stain was localized regularly on the axons which contained agranular synaptic vesicles and also on axons which contained dense cored synaptic vesicles beeing probably adrenergic. The role of AChE on the adrenergic axons is discussed.     

Observations on the fine structure of the baroreceptors and adrenergic innervation of the guinea-pig carotid sinus

We examined the fine structure of the baroreceptors and the adrenergic innervation of the guinea-pig carotid sinus. The tunica adventitia contained many nerve bundles whose perineuria enclosed unmyelinated nerve fibers, alone or together with myelinated nerve fibers. Baroreceptors, which lay close to elastic and collagen fibers in the adventitia and media, were surrounded by “terminal” cells with ultrastructural features characteristic of Schwann cells and contained inclusions of various types. Morphologic features of the baroreceptors included densely packed mitochondria, osmiophilic lamellated and homogeneous bodies, clear and granular vesicles, lamellar systems, glycogen granules, neurofilaments, neurotubuli, and vacuolated mitochondria. In animals that had been treated with 6-hydroxydopamine, occasional electrondense endings (or fibers) were observed in the adventitial layer. The baroreceptors in the guinea-pig carotid sinus appear to have most of the morphologic features reported for other species.     

Electron microscopic analysis of tyrosine hydroxylase, dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase immunoreactive innervation of the hypothalamic paraventricular nucleus in the rat

The catecholaminergic innervation of the hypothalamic paraventricular nucleus (PVN) of the rat was studied by preembedding immunocytochemical methods utilizing specific antibodies which were generated against catecholamine synthesizing enzymes. Phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive terminals contained 80-120 nm dense core granules and 30-50 nm clear synaptic vesicles. The labeled boutons terminated on cell bodies and dendrites of both parvo- and magnocellular neurons of PVN via asymmetric synapses. The parvocellular subnuclei received a more intense adrenergic innervation than did the magnocellular regions of the nucleus. Dopamine-beta-hydroxylase (DBH)-immunopositive axons were most numerous in the periventricular zone and the medial parvocellular subnucleus of PVN. Labeled terminal boutons contained 70-100 nm dense granules and clusters of spherical, electron lucent vesicles. Dendrites, perikarya and spinous structures of paraventricular neurons were observed to be the postsynaptic targets of DBH axon terminals. These asymmetric synapses frequently exhibited subsynaptic dense bodies. Paraventricular neurons did not demonstrate either PNMT or DBH immunoreactivity. The fibers present within the nucleus which contained these enzymes are considered to represent extrinsic afferent connections to neurons of the PVN. Tyrosine hydroxylase (TH)-immunoreactivity was found both in neurons and neuronal processes within the PVN. In TH-cells, the immunolabel was associated with rough endoplasmic reticulum, free ribosomes and 70-120 nm dense granules. Occasionally, nematosome-like bodies and cilia were observed in the TH-perikarya. Unlabeled axons established en passant and bouton terminaux type synapses with these TH-immunopositive cells. TH-immunoreactive axons terminated on cell bodies as well as somatic and dendritic spines of paraventricular parvocellular neurons. TH-containing axons were observed to deeply invaginate into both dendrites and perikarya of magnocellular neurons. These observations provide ultrastructural evidence for the participation of central catecholaminergic neuronal systems in the regulation of the different neuronal and neuroendocrine functions which have been related to hypothalamic paraventricular neurons.