Ontogenetic development of TRH-like immunoreactive nerve terminals in the median eminence of the rat

Ontogenetic development of TRH-like immunoreactive nerve terminals in the median eminence of the rat was studied immunocytochemically. By light microscopy, TRH-like immunoreactivities were first detected on the 1st day after birth in the external layer of the median eminence. By electron microscopy, TRH-like immunoreactive nerve fibers and terminals were visible on the 0.5th day after birth. The nerve terminals were first found in direct contact with the perivascular basal lamina of the portal vessel on the 2nd day. TRH-like immunoreactivities were only localized on dense granular vesicles about 105 nm in diameter in the axoplasm throughout the developmental stages. The immunoreactive nerve fibers with TRH-like immunoreactive granular vesicles gradually increased in number with development. The physiological significance of TRH as a hormone is discussed in relation to the presence of TRH-like immunoreactive nerve terminals in the median eminence of the developing rat.     

Axovascular relationships in developing median eminence of perinatal rats with special reference to luteinizing hormone-releasing hormone projections

Topographical relationships of neurosecretory axons with the capillaries of the primary portal plexus were studied in the median eminence of rats from the 14th fetal till the 9th postnatal day by means of electron microscopy combined with morphometric analysis. Special attention was given to the light and electron microscopic immunocytochemical examination of luteinizing hormone-releasing hormone projections to the median eminence. Neurosecretory axons possessing secretory granules and clear microvesicles were first observed in the median eminence at the 14th fetal day. However, all of them were situated at a distance from the primary portal plexus. By the 20th fetal day, neurosecretory axons reached the external basal lamina of the primary portal plexus giving rise to so-called axovascular contacts. Some axons even penetrated into the perivascular space, apparently facilitating the neurohormone delivery into the hypophysial portal circulation. From that time on, both the number of the axons abutting on the external basal lamina and the entire area of axovascular contacts increased gradually. As to luteinizing hormone-releasing hormone axons, they grew into the median eminence from the 18th fetal day concentrating in older fetuses and neonates either over the primary portal plexus, or around the infundibular recess of the 3rd ventricle. After birth, the concentration and distribution of luteinizing hormone-releasing hormone axons within the median eminence became similar to those of adults. Luteinizing hormone-releasing hormone axons were found to arise from the neurons of septopreoptic area including the diagonal band of Broca. These data suggest the onset of neurohormone release in the median eminence from the 14th fetal day followed by the establishment of the hypothalamic control over the pituitary functions during the perinatal period in rats.     

Monosodium glutamate induced lesions of the arcuate nucleus. I. Endocrine deficiency and ultrastructure of the median eminence

Monosodium glutamate was administered daily on days 5 through 10 postnatally at a dose of 2.5 mg/gm body weight. Counts of remaining perikarya in the arcuate nucleus of adult mice indicated approximately an 80% decrease in the number of perikarya. The arcuate lesion resulted in endocrine deficits; reproductive capacity was reduced, animals were smaller in stature and obese, and the weights of the anterior pituitary, ovaries and testes were significantly decreased while adrenals were unaffected. Light microscopic studies revealed no significant changes in thickness or general histological appearance of the median eminence. At the electron microscope level, there were no alterations in the number of nerve terminals or dense core vesicles per unit area in the contact zone. These observations suggest that afferents to the median eminence from the arcuate nucleus may form a relatively small portion of its total nerve terminal population.     

Structural changes in nerve endings of rat median eminence superfused with media rich in potassium ions

In vitro fragments of male rat mediobasal hypothalami were superfused with Krebs--Ringer solution in the presence or absence of CaCl2. Infusions containing up to 60 mM potassium chloride were applied, at the end of which tissues were fixed in osmium tetroxide and prepared for transmission electron microscopy. Control superfusions were run in parallel. Quantitative measurements performed on electron micrographs of the outermost palisade region showed significant (20-30%) increase in caliber of axon endings after intensive potassium ion stimulation. Ultrastructurally, widespread depletion of granular vesicles and microvesicles was found. Vesicle shift to the outer zone of the terminals, formation of membrane-bound tubules of the same diameter as microvesicles, and images of attachment and collapse of vesicles into the axolemma were found, particularly after 1 min stimulation. These findings were interpreted as consistent with exocytosis. Longer stimulations were followed by the appearance of large pleomorphic vacuoles that are probably the result of post-exocytotic membrane retrieval. Axon enlargement and vesicle depletion were absent in specimens superfused with calcium-free medium containing high potassium. The functional significance of these ultrastructural changes are interpreted as supporting the hypothesis that exocytosis of calcium-loaded microvesicles can contribute to extrude this ion from median eminence nerve endings during secretion.     

Morphological changes in the hypothalamic arcuate nucleus and median eminence in the golden hamster during the neonatal period

The purpose of the present investigation was to study the ultrastructure of the arcuate nucleus (ARC) and median eminence of hamsters on days 1–15 of the neonatal period. From days 1–6, the neurons of the ARC had large nuclei and a small amount of cytoplasm which contained polysomes, mitochondria, RER, lysosomes and Golgi complexes. From days 7–15 there was an increase in the amount of cytoplasm as well as more extensive Golgi complexes and RER. Astrocytes were the predominant glial component in both the ARC and median eminence. Astrocytic processes were in juxtaposition to unmyelinated axons, dendrites, and synapses. Axodendritic and axosomatic synapses containing clear vesicles were observed in the neuropil on day 1. There was an increase in the number of dense-core vesicles in the axonal endings beginning on day 4. Concomitantly, there were increasing numbers of clear and dense-core vesicles (64–70 nm) in terminals of the external layer of the median eminence, whereas larger dense-core vesicles (105–140 nm) were distinguishable by day 10 immediately dorsal to the external layer. The capillaries of the median eminence were composed of nonfenestrated endothelium from days 1–9. Fenestrae began to appear about day 10. Ependymal cells lining the third ventricle had pinocytotic vesicles, microvilli, and bleb-like protrusions on their apical surfaces. Ependymal processes were adjacent to nerve processes in the neuropil of the ARC and in the external layer of the median eminence, where they contacted the perivascular space. Two types of supraependymal cells were seen in animals throughout the neonatal period. One resembled a neuron which sent processes along the ependymal surface and often between cells. The second type was similar to a macrophage. The results of this study demonstrate the maturation of the neural elements in the ARC/median eminence area of the neonatal hamster.     

弓状核和正中隆起的超微结构特征及其机能意义

本文用醛-锇酸固定法对大白鼠弓状核和正中隆起的超微结构特征进行了研究。观察结果表明:在弓状核内有暗细胞和亮细胞两种神经元,它们可能兼具产生多巴胺和释放激素的功能。第三脑室室管膜的伸张细胞纵贯正中隆起的各层,抵达门脉毛细血管的周围间隙。含神经分泌物质的神经末梢可穿行于室管膜细胞之间,也可止于正中隆起毛细血管基膜周围,甚至直接紧贴在“开放型毛细血管”的内皮窗孔上。在弓状核神经细胞的胞体和树突上有轴体突触和轴树突触。在正中隆起的纤维层内有无颗粒囊泡型及颗粒囊泡型两种轴轴突触。在正中隆起的栅状层内,伸张细胞的基突与含有大颗粒囊泡的神经末梢之间也存在着突触联系。上述超微结构特征提示:1.丘脑下部的释放(或抑制)激素可能经由两条途径释放:直接由神经末梢释放入门脉毛细血管或先释放入第三脑室的脑脊液,由伸张细胞摄取,再由伸张细胞的基突转运至门脉毛细血管;2.释放(或抑制)激素的合成、贮存、运输和释放等各个环节都受到神经的调节和控制。     

The fine structure of the equatorial regions of developing muscle spindles in the rat

Summary The fine structure of developing muscle spindles in the gastrocnemius muscles of young Wistar rats has been examined at stages from 18 days gestation to 14 days after birth. Initial simple contacts between large unmyelinated intramuscular nerve terminals and apparently undifferentiated single myotubes at 18 days gestation led to the encirclement of the myotube by the nerve terminal, with concurrent extension of the nerve sheath to form a unilamellar capsule around the innervated region. Subsequent proliferation and fusion of satellite myoblasts associated with the innervated myotube gave rise to a succession of further intracapsular myotubes. At birth the original myotube and its first formed companion had differentiated into recognizable nuclear bag intrafusal fibres, and by the third post-natal day a second generation of smaller diameter nuclear chain myotubes could be distinguished. These developing intrafusal muscle fibres remained in a tightly packed bundle, having numerous interlocking pseudopodial extensions of their superficial cytoplasm, until some 3–6 days after birth, the sensory nerve terminal enwrapping the external surface of the whole bundle. Lateral separation of the intrafusal fibres during the second post-natal week was accompanied by the acquisition of individual sensory innervation, the development of an axial sheath distinct from the now multilamellar capsule, and the first appearance of the subcapsular space. The fine structure of the developing sensory nerve terminals closely resembled that of mature spindle terminals, having dense amorphous cytoplasm containing many small mitochondria and 40–60 nm vesicles.     

Ultrastructural study on cell differentiation of the rabbit carotid body

The development of the carotid body in both fetuses from 13 to 30 days of gestation and newborn rabbits was studied by light and electron microscopy. The carotid body anlage first appeared as a cellular aggregation close to the ventral wall of the third branchial artery on the 14th day of gestation. Type-I cells were recognized as such by the presence of dense-cored vesicles in the cytoplasm on the 16th day of gestation, whereas cells destined to develop into Type-II cells became clearly distinguishable by their typical relationship to the Type-I cells on the 20th day of gestation. Afferent and efferent synapses as well as reciprocal-like synapses between Type-I cells and nerve endings were also observed in perinatal fetuses. Although cell differentiation was almost finished by birth, Type-I cells and their innervation did not seem to be fully matured. Thus, it was concluded that ultrastructurally, the rabbit carotid body during the fetal and perinatal periods was relatively inactive.     

Immunohistochemical localization of human growth hormone-like material in the median eminence of the rat: Light and electron microscopic observations

Usng the peroxidase-antipoeroxidase pre-embedding technique for immunoelectron microscopy, human growth hormone (hGH)-like material was demonstrated in large, dense core vesicles in axon terminals in the hypothalamic median eminence of the rat. The close association of immunoreactive terminals with fenestrated capillaries of the portal plexus, suggests that the hGH-like material is contained within the tuberoinfundibular neuronal system and may be a hypophysiotropic hormone.     

Ultrastructure and immunocytochemistry of the neuroepithelial bodies in the lung of the tiger salamander, Ambystoma tigrinum (Urodela, Amphibia).

Light and electron microscopy of the lungs of Ambystoma tigrinum (Urodela) revealed a relatively complex pattern of the neuroendocrine (NE) cells. In the apical parts of smaller septa single NE cells not associated with nerve fibres were covered and surrounded by pneumocytes. The larger septa possessed small areas of ciliated epithelium, in which the NE cells were grouped in a form of neuroepithelial bodies (NEB) consisting of 3-5 cells and covered by goblet cells. NE cells possessed a large nucleus with patches of condensed chromatin, clear cytoplasm, and membrane-bound vesicles of variable morphology and size, containing an electron dense interior surrounded by a lucent space. The size of these dense core vesicles (DCV) ranged from 70-140 nm, while rarely the larger ones exhibited a diameter of 300-600 nm. In some NEB a second type of NE cells was observed for the first time in an amphibian species: these cells communicated with the air space and exhibited on their surface microvilli and a single modified cilium with a 8 + 1 microtubule arrangement. Their cytoplasm contained two types of DCV: dense core granules with a diameter of 140-260 nm and vesicles 320-700 nm in diameter with a moderately electron dense interior. The NEB were associated with intracorpuscular, sensory nerve terminals morphologically afferent and efferent. By immunocytochemistry, the NE cells revealed the presence of serotonin, met-enkephalin, and leu-enkephalin. A paracrine and chemoreceptor role is proposed for NEB of Ambystoma tigrinum.     

An electron microscopic study of the mediodorsal cerebral cortex in the lizard Agama agama

Summary An electron microscopic analysis was made of the small-celled part of the mediodorsal cortex of the lizard Agama agama. This cortex consists of four layers: Superficial plexiform layer, cellular layer, deep plexiform layer and fiber layer. In the superficial plexiform layer one type of solitary neuron with smooth dendrites is present.Three types of axon terminals can be observed: terminals with a moderately electron dense matrix packed with spherical vesicles (S1 type), axon terminals with an electron lucent matrix containing fewer spherical synaptic vesicles than the S1 type (S2 type) and axon terminals with an electron lucent matrix and scattered pleomorphic synaptic vesicles (F type). F type axon terminals are larger than S terminals. At the pial surface endfeet of tanycytic processes form a limiting glial layer, contacting one another by means of gap junctions. In the cellular layer perikarya of pyramidal neurons are densely packed. The karyoplasm of these neurons shows either evenly dispersed or discretely clumped chromatin. Spiny dendrites arise from the perikarya and extend into both the superficial and deep plexiform layers. The structure of the deep plexiform layer is roughly similar to that of the superficial plexiform layer. The fiber layer contains the majority of the afferent and efferent axons of the mediodorsal cortex. The axons are myelinated and unmyelinated. Between the fibers, scattered solitary neurons are present, often accompanied by glial cells.The lateral ventricle beneath the fiber layer is lined by a single row of ependymal tanycytes. Tanycytic processes traverse the cortical layers and may form endfeet at the pial surface. Protoplasmic excresenses from some ependymal cells protrude into the ventricle.     

Reinnervation of Pacinian corpuscles by CNS axons after transplantation to the dorsal column: incidence and ultrastructure

Summary We have investigated the capacity of injured axons in the spinal dorsal columns of young adult rats to reinnervate grafted Pacinian corpuscles. A branch of the hindlimb interosseous nerve with a group of crural Pacinian corpuscles attached to it was autotransplanted to the surface of the spinal cord and the nerve stump was implanted into the dorsal column. Two to three months later 16 grafts were removed for examination by light and electron microscopy. By 3 months after transplantation almost all Schwann cell columns of the grafted nerve branch were occupied by regenerated myelinated and unmyelinated axons. Of 41 corpuscles examined by electron microscopy 24 were reinnervated by 1–3 myelinated fibres which gave rise to multiple terminals in the inner core. The remaining corpuscles appeared to be denervated. Only two of the reinnervated corpuscles contained regenerated endings which reiterated the distinct ultrastructure of normal presynaptic terminals of CNS axons, characterized by clusters of lucent vesicles and paramembranous densities. All other corpuscles were reinnervated by terminals which resembled peripheral mechanosensory endings as they contained mitochondria and very few vesicles. One such corpuscle was coinnervated by small terminals filled with large dense cored vesicles. We assume that the majority of grafted Pacinian corpuscles have been reinnervated by dorsal column axons and that the regenerated terminals with the ultrastructure of peripheral mechanosensory endings derive from central axons of primary sensory neurons, which are apparently capable of constructing mechanosensory-like terminals in response to signals from the Pacinian corpuscles. The vesicle-filled endings are probably formed by second order sensory neurons, corticospinal neurons and small peptidergic neurons unable to adjust their terminals to the new target.     

猫丘脑腹后内侧核内三叉丘系纤维终末的超微结构及其突触联系

本文采用顺行溃变法对猫丘脑腹后内侧核内发自三叉神经尾侧脊束核的三叉丘系纤维终末的超微结构及其突触联系进行了研究。在电灼损毁三叉神经尾侧脊束核四天后 ,在电镜下发现丘脑腹后内侧核内的三叉丘系终末存在三种溃变形式 ,即 :电子致密型溃变 ,电子透明型溃变及神经微丝型溃变 ,以电子致密型溃变终末最为常见。溃变的三叉丘系终末常较大 ,含有大量密集的圆形无颗粒小泡 ,在丘脑腹后内侧核内主要形成不对称型轴—树突触 ,多数溃变轴终末还参与形成以树突为中心的汇聚型突触复合体     

The synaptic organization of the cerebello-oiivary circuit

Summary Section of the superior cerebellar peduncle just rostral to the deep cerebellar nuclei results in degenerating axon terminals within the contralateral inferior olive. The nuclear origin of this fiber system and its distribution within the subdivisions of the inferior olive were described in a companion study (Martin et al., 1976). Precise localization of these degenerating terminals within the nucleus was accomplished by the examination of 1 plastic sections cut from each tissue block prior to thin sectioning. Degenerating axon terminals are present in all the nuclear subdivisions and when seen with the electron microscope they frequently are localized in the previously described synaptic clusters (King, 1976). These terminals demonstrate an electron dense reaction at survival times of 2 and 3 days. By day 4, they are shrunken and irregular in shape, and typically are surrounded by astrocyte processes. Cerebello-olivary axon terminals measure 1–3 , contain spherical, clear synaptic vesicles and typically contact spiny appendages within the synaptic clusters (glomeruli). Thus, we have demonstrated that one of the primary axon systems which terminates within the synaptic clusters is from the cerebellar nuclei. We have yet to determine the origins of the remaining terminals within the synaptic clusters which include endings with either smaller spherical, pleomorphic or numerous dense core vesicles.This investigation was supported by United States Public Health Service Grant NS 08798.     

Definitive evidence for the existence of morphological plasticity in the external zone of the median eminence during the rat estrous cycle: implication of neuro-glio-endothelial interactions in gonadotropin-releasing hormone release

Despite intense investigation, the demonstration of morphological plasticity in the external zone of the median eminence concerning the gonadotropin-releasing hormone system has never been reported. In this study, we investigate whether dynamic transformations of the gonadotropin-releasing hormone nerve terminals and/or tanycytes in the external zone of the median eminence of the hypothalamus occurred during the rat estrous cycle, by following individual gonadotropin-releasing hormone-immunoreactive nerve terminals on serial ultrathin sections observed by electron microscopy. Female rats were killed at 16.00 diestrus II (n = 3), i.e. when estrogen levels are basal and gonadotropin-releasing hormone release is low, and at 16.00 proestrus (n = 4), i.e. when estrogen levels peak and the preovulatory gonadotropin-releasing hormone surge occurs. Our results show that, in the median eminence obtained from proestrus rats, 12+/-2% of the gonadotropin-releasing hormone nerve terminals were observed to make physical contact with the parenchymatous basal lamina, i.e. the pericapillary space. In the median eminence obtained from diestrus II rats, no contacts were observed. On proestrus, numerous physical contacts between gonadotropin-releasing hormone nerve terminals and the basal lamina occurred by evagination of the basal lamina and/or by emerging processes from gonadotropin-releasing hormone nerve terminals. The quantification of the evagination of the basal lamina revealed that the basal lamina was at least twofold more tortuous in appearance during proestrus. These results demonstrate for the first time the existence of dynamic plastic changes in the external zone of the median eminence, allowing gonadotropin-releasing hormone nerve terminals to contact the pericapillary space on the day of proestrus, thus facilitating the release of the neurohormone into the pituitary portal blood.     

Pituitary adenylate cyclase-activating polypeptide (PACAP)-like immunoreactive neuronal elements in rat hypothalamus and median eminence with special reference to morphological background of its effect on anterior pituitary — light and electron microscopic immunocytochemistry

Pituitary adenylate cyclase-activating polypeptide-like immunoreactive (PACAP-LI) neuronal elements in the rat hypothalamus including the median eminence (ME) were investigated by light and electron microscopic immunocytochemistry. PACAP-LI neuronal perikarya with well-developed cell organelles and dense granules were distributed mainly in the magnocellular portion of the paraventricular nucleus and throughout the entire supraoptic nucleus. In the ME, numerous PACAP-LI neuronal processes were found in the internal layer (IL), and immunoreactive terminals containing dense granules, vesicles and mitochondria were detected around portal capillaries which penetrated into the IL from the external layer. Thereafter, PACAP is released into the portal capillaries in the IL, transported to the anterior pituitary and plays a role in the stimulation of adenylate cyclase of anterior pituitary cells.     

Origin of the nerve endings in the surface zone of the median eminence of the rat hypothalamus

Summary In order to study the origin of the nerve endings in the surface zone (zona palisadica) of the median eminence and the proximal part of the pituitary stalk, experiments were designed to determine the extent of the smallest region of the brain, isolation of which does not result in terminal degeneration in this zone. Various hypothalamic areas were partially or totally separated from the brain by means of a stereotaxically manipulated bayonet-shape knife and the zona palisadica was examined for secondary degeneration of axon terminals under the electron microscope. If the medial basal hypothalamus, including the suprachiasmatic area was disconnected from the rest of the brain, degenerated nerve endings do not occur in the palisade zone. Axon degeneration is observed if any part of the medio-basal area was separated from the median eminence or became necrotic. — These findings indicate that the nerve terminals in the zona palisadica arise exclusively from neurons in the medial basal hypothalamus, and that fibers of other origin do not terminate in this zone. The present observations support the assumption that the hypothalamic releasing (and inhibiting) factors are produced by, and the neural control of the anterior pituitary is exercised over a short final neuronal link, localized in the medial basal hypothalamus.     

生长抑素样神经元及其纤维在人胎下丘脑的分布与发育研究

本文用免疫细胞化学技术观察了人胎下丘脑内含生长抑素样神经元及其纤维的分布与发育。胎龄14周,见浅染的生长抑素免疫反应阳性神经元散布于下丘脑前部、室周带、第三脑室壁室管膜上皮深面。15～16周胎龄,含生长抑素样神经元除见于上述部位外,还见于腹内侧基底部、室旁核、视上核和室管膜上皮内,阳性纤维散布于正中隆起,室管膜上皮。17—18周胎龄,含生长抑素样结构在上述部位变化明显,神经元更加密集,分布范围扩大,反应染色加深,阳性突起延长。21周胎龄至足月胎儿,含生长抑素神经元及其纤维在下丘脑的分布变化不明显。生长抑素样神经元以中、小型细胞为主,室旁核内见有少量大细胞。电镜观察21周和29周胎龄正中隆起发现:毛细血管为窗孔型,基底膜外有生长抑素样阳性终末靠近。在阳性终末及非阳性终末内可见分泌囊泡。本结果提示人下丘脑内生长抑素系统在出生前已经发育并可能参与内分泌活动的调节。