Ultrastructural studies on the secretory cycle of the neurosecretory cells and the formation of herring bodies in the paraventricular nucleus of the rat

Summary The ultrastructure of the neurosecretory cells in the paraventricular nucleus of the normal male rat was studied by electron microscopy during various functional states. Four morphologically distinct types of neurosecretory cells were observed. It appears that they do not represent different classes of cells but different phases of secretory activity of a single cell type. The perikarya of the neurosecretory cells show a definite cycle of formation and transportation of secretory granules. We have designated the phases of this cycle as: (1) phase of synthesis, (2) phase of granule production, (3) phase of granule storage and (4) phase of granule transport. The neurosecretory granules appear to be moved in bulk into the axons, forming a large axonal swelling filled with granules as a result of one cycle in the neurosecretory process. Thus it may be postulated that a secretory cycle in the perikaryon of the neurosecretory cell seems to result in the formation of a Herring body in its axon, and that its content is then conveyed to the posterior pituitary.     

中华绒螯蟹窦腺神经末梢及X-器官神经分泌细胞的类型

在电子显微镜下观察了性未成熟的中华绒螯蟹黄蟹的窦腺及X－器官的超微结构。X－器官位于眼柄神经节终髓的腹外侧,与窦腺位置斜相对,窦腺主要由神经分泌细胞的末梢和胶质细胞组成。神经末梢含有大量的膜结构包围的颗粒、线粒体、粗面内质网和许多电子透明的小泡,末梢外周有时可见指状突起。依据颗粒的大小、形状、电子致密度以及胞质特征,可区分出6种类型的窦腺神经末梢及7种X－器官神经分泌细胞。观察了末梢中神经分泌颗粒的胞吐作用方式的释放过程,并且尝试对窦腺不同末梢中的颗粒及X－器官神经分泌细胞中的颗粒作了比较,发现二者之间具有较好的对应性,即电子致密度无大的变化,形态特征相似,只是大小稍有增加。     

Occurrence of immunoreactive enkephalins in a neurohemal organ and other nervous structures in the eyestalk of the shore crab,Carcinus maenas L. (Crustacea,Decapoda)

Summary Light-microscopical observations with immunofluorescence and peroxidase staining procedures revealed leu-enkephalin-like immunoreactivity in axon profiles of the sinus gland (SG) and in single small neurons in the optic ganglia of the eyestalk of Carcinus maenas. Electron microscopy of the SG showed reactivity to be associated with neurosecretory granules 82±23 nm in diameter. High performance liquid chromatography of SG-extracts revealed radioimmunoreactive substances with the retention times of synthetic met- and leu-enkephalin and met-enkephalin-Arg⁶-Phe⁷, respectively.     

The ultrastructure of the sinus gland of Carcinus maenas (Crustacea: Decapoda)

Summary The sinus gland of Carcinus maenas consists of the swollen axonal endings of the neurosecretory cells of the major ganglia and acts as a storage release centre for the membrane bound neurosecretory material. These neurosecretory granules fall into five different types based on size and electron density. Their contents are released by exocytosis of the primary granules or smaller units budded from the primary granules.I thank Professor E. Naylor for his constant advice and Professor E. W. Knight-Jones, Department of Zoology, University College, Swansea, for the provision of laboratory facilities. I am grateful to the Science Research Council for the financial support. Finally, I thank the Electron Microscope Unit, Southampton General Hospital, where the work was completed.     

An electron microscopical investigation of the follicle gland (cerebral gland) and of some neurosecretory cells in the lateral lobe of the cerebral ganglion of the pulmonate gastropod Lymnaea stagnalis L.

Summary The ultrastructure of the follicle gland and of some Gomori-positive neurosecretory cells in the lateral lobes of the cerebral ganglia of Lymnaea stagnalis is described.The follicle wall consists of epithelial cells containing secretion granules, and of processes of lateral lobe nerve cells bearing cilia, including the processes of bipolar neurosecretory B-cells. The ultrastructure of the follicle of Ancylus fluviatilis and Planorbarius corneus appeared to be very similar to that of L. stagnalis. From the ultrastructure of the follicle wall and lumen, no conclusions can be drawn as to its formerly proposed possible functions (sense organ, endocrine organ, neuroendocrine organ). Of three neurosecretory cell types (B-cells, canopy cells, droplet cells), the positivity to chrome-haematoxylin and paraldehyde-fuchsin appeared to be due to elementary granules of different size and appearance. This finding supports in a particular sense the view that results obtained with the neurosecretory stains, should be judged carefully. Also in ordinary neurons elementary granules can be found, at times in great numbers. Several types of elementary granules (at least 6) could be distinguished. Their character and function (neurohormones, neurotransmitters) are not clear.     

The primate median eminence II

Summary The ultrastructure of the normal median eminence of the male rhesus monkey (Macaca mulatta) is described using high-voltage electron microscopy. Surface specializations of ependymal cells lining the infundibular recess included cilia, apical extrusions, and microvilli. Supraependymal cells were predominantly macrophage-like, but examples of lymphocytic types were also seen. Tanycytes had long, branching, basal processes filled with numerous microtubules, some lipid droplets, and granules. The zona interna was composed of large unmyelinated neurosecretory fibers. A few myelinated fibers were also seen, but their character as neurosecretory fibers could not be established. The zona externa was composed of densely-packed profiles of neurosecretory fibers of small diameter, was well-vascularized and contained the terminations of tanycytes. Perivascular glial cells, vesiculated elements, pituicytes, and cellular elements common to connective tissue were observed. The intricate relationships between both the cellular and fibrous elements of the median eminence can be appreciated with the capability of high-voltage electron microscopy to discern ultrastructure in sections 10 times thicker than those used for low-voltage electron microscopy. The median eminence of this primate species has an ultrastructural organization similar to that described for most other species.Supported by USPHS Program Project Grant NS-11642 and USPHS HD-08867. The authors appreciate the excellent technical advice and assistance of Mr. George Wray in operation of the HVEMCareer Development Awardee K04-GM-70001     

Ultrastructure of the corpus cardiacum and corpus allatum of the house cricket Acheta domesticus

Summary The ultrastructure of the corpus cardiacum (CC) and corpus allatum (CA) of the house cricket, Acheta domesticus, is described. Axon profiles within the CC contain neurosecretory granules 160–350 nm in diameter which are indistinguishable from those found in type I neurosecretory cells of the pars intercerebralis and in the nervus corporis cardiaci I. The CC itself contains two cell types: intrinsic neurosecretory cells and glial cells. Intrinsic NSC cytoplasm contains Golgi bodies and electron dense neurosecretory granules 160–350 nm in diameter. Synaptoid configurations with 20–50 nm diameter electron lucent vesicles were observed within axon profiles of the CC. The structure of the CA is relatively uniform with one cell type predominating. Typical CA cells possess large nucleoli, active Golgi complexes, numerous mitochondria, and occassional microtubules. Groups of dark staining cells scattered throughout the CA of some animals were interpreted as evidence of cellular death.This work was done while JTB was supported by USPHS Training Grant HD-0266 from NICHDI wish to express my thanks to Dr. Richard A. Cloney for sharing his expertise in electron microscopy     

The ultrastructure of the sinus gland of the crayfish Astacus leptodactylus (Nordmann)

Summary An ultrastructural study of the sinus gland of the crayfish Astacus leptodactylus demonstrates that this gland is mainly composed of glial cells, axons and axon terminals. On the basis of the size, shape and electron density of the neurosecretory granules, we could distinguish five different types of axon terminals.     

The ultrastructure of the sinus gland of Gammarus oceanicus (Crustacea: Amphipoda)

Summary The sinus gland of Gammarus oceanicus, like that of other crustaceans, is composed of three elements: neurosecretory axons, glial cells and stromal sheath. Five neurosecretory axon types are identified on the basis of granule diameter, shape, and electron density, and axon matrix density. Exocytosis appears to be the major release mechanism of neurosecretory material. The preterminal regions of neurosecretory axons contain axoplasmic reticulum and neurotubules. Their arrangement in the axon and relationship with one another suggest a transport function. Multilamellar bodies are found in the terminal regions of neurosecretory axons. They arise from mitochondria and may be involved in granulolysis.The technical assistance of G.A. Bance, statistical assistance of D. MacCharles and D.W. Hagen, and financial support provided by the University of New Brunswick Research Fund to K.H. are gratefully acknowledged     

The ultrastructure of corpus allatum of the earwig Euborellia annulipes lucas (Dermaptera: Labiduridae)

The ultrastructure of corpus allatum of the earwig, Euborellia annulipes has been described. The corpus allatum (CA) is an oval body. The gland is covered by a thin stromal sheath which is wavey and acellular in composition. The gland consists of either parenchymal cells with distinct, double-layered, smooth or slightly undulated plasma-membranes. Golgi bodies and endoplasmic reticulum are poorly developed. The mitochondria are found in abundance and are oval, spherical, elongate, and Y-shaped, with oblique and longitudinal cristae. The gland is innervated by neurosecretory and non-neurosecretory axons. The neurosecretory axons are of two types. The type NS-II, contain electron dense granules of 40...120 nm in diameter and the type NS-I, small less electron dense granules (of mixed nature) 40...90 nm in diameter. Branches of tracheoles also occur that penetrate the gland. The large electron dense granules 100...220 nm in diameter occuring abundantly in aorta, and representing the secretions of the medial A-cells are absent in the CA.     

Distinct structural features of caprin-1 mediate its interaction with G3BP-1 and its induction of phosphorylation of eukaryotic translation initiation factor 2alpha, entry to cytoplasmic stress granules, and selective interaction with a subset of mRNAs

Caprin-1 is a ubiquitously expressed, well-conserved cytoplasmic phosphoprotein that is needed for normal progression through the G(1)-S phase of the cell cycle and occurs in postsynaptic granules in dendrites of neurons. We demonstrate that Caprin-1 colocalizes with RasGAP SH3 domain binding protein-1 (G3BP-1) in cytoplasmic RNA granules associated with microtubules and concentrated in the leading and trailing edge of migrating cells. Caprin-1 exhibits a highly conserved motif, F(M/I/L)Q(D/E)Sx(I/L)D that binds to the NTF-2-like domain of G3BP-1. The carboxy-terminal region of Caprin-1 selectively bound mRNA for c-Myc or cyclin D2, this binding being diminished by mutation of the three RGG motifs and abolished by deletion of the RGG-rich region. Overexpression of Caprin-1 induced phosphorylation of eukaryotic translation initiation factor 2alpha (eIF-2alpha) through a mechanism that depended on its ability to bind mRNA, resulting in global inhibition of protein synthesis. However, cells lacking Caprin-1 exhibited no changes in global rates of protein synthesis, suggesting that physiologically, the effects of Caprin-1 on translation were limited to restricted subsets of mRNAs. Overexpression of Caprin-1 induced the formation of cytoplasmic stress granules (SG). Its ability to bind RNA was required to induce SG formation but not necessarily its ability to enter SG. The ability of Caprin-1 or G3BP-1 to induce SG formation or enter them did not depend on their association with each other. The Caprin-1/G3BP-1 complex is likely to regulate the transport and translation of mRNAs of proteins involved with synaptic plasticity in neurons and cellular proliferation and migration in multiple cell types.     

The fine structure of the carapace integument of Daphnia magna straus (Crustacea branchiopoda)

Summary The structure of the two integumental layers comprising the carapace of female D. magna was examined at several points through the molt cycle. The epicuticle and procuticle are simple in organisation; pore canals are absent but intracuticular fibres are present, forming complexes with invaginations of the epidermal plasma membrane similar to such complexes described in the literature for other arthropods. The epidermis consists almost entirely of cuticle-secreting cells. Secretion of the new cuticle begins when 50–67% of the instar has elapsed by which time the epidermal cells have increased in height and their nuclei have become more rounded. However, other presumed secretory phenomena observed viz. the formation of dense core vesicles by Golgi bodies, and the occurrence of these and coated vesicles near the apical plasma membrane are not restricted to any particular period during the molt cycle. This suggests that the mechanisms of cuticle secretion do not undergo marked changes in activity as they do in decapods; presumably this relative continuity is related to the much shorter molt cycle of cladocerans.The technical assistance of G.A. Bance, and the financial support provided by the National Research Council of Canada are gratefully acknowledged     

Acidophilic granular cells in the epidermis of the brown trout,Salmo trutta L.

Summary Acidophilic cells occur in the epidermis of several species of salmonid fish, although their abundance fluctuates considerably between individuals within the same population and at different times during the life cycle. The histology, histochemistry and ultrastructure of an acidophilic, granular celltype in the epidermis of the brown trout, Salmo trutta L., is described. At the light microscope level this cell type is easily distinguished from the large, mucus-secreting, epidermal goblet cells by its acidophilic, proteinaceous secretion. At the ultrastructural level this secretion consists of membrane-bound granules formed by the very active Golgi region. It is argued that the acidophilic, granular cell is not a transformed blood cell but constitutes a normal epidermal component of the brown trout. Possible roles of this cell in the function(s) of the epidermis are discussed.     

Multicellular antennal sensilla containing a sensory cell with a short dendrite and dense-core granules in the insect,Hypogastrura socialis (Collembola): intermolt and molting stages

Summary At the antennal tip of the collembolan insect Hypogastrura socialis two terminal-pore sensilla are located, which, in addition to normally structured and most probably chemosensitive sensory cells, also contain aberrant sensory cells. Portions of these cells resemble chemoreceptors but also shown are features that, as a rule, occur in mechanoreceptors. One cell in each sensillum is remarkable in two characteristics: (1) Its dendritic outer segment does not reach the cuticular outer structures of the sensillum; (2) it contains dense-core granules (diameter 60–110 nm) within its perikaryon, its dendritic inner segment and its axon. Additionally, these two cells do not show lengthening of their dendritic outer segment during molt as do all other sensory cells. Among the fibers of one major branch of the antennal nerve within the head capsule a single axon was observed to contain dense-core granules. This axon was traced to its termination where normal synaptical contacts were found. Based on the assumption that the axon belongs to one of the granule-containing sensory cells two alternative hypotheses are proposed: (1) an individual sensory cell of a sensillum may synthesize a transmitter that is different from that of the other sensory cells of this sensillum; (2) the aberrant cells have lost exteroceptive functions but act as neuromodulatorsSupported by the Deutsche Forschungsgemeinschaft (SFB 4/G1)     

Identification of adenohypophysiotropic neurohormone producing neurosecretory cells in Rana temporaria

Summary Electron microscopic investigation of neural isolated and normal pars ventralis of the tuber cinereum showed the presence, in Rana temporaria, of a tubero-hypophysial neurosecretory system. A striking structural resemblance between this parvicellular, aldehydefuchsin negative, tubero-hypophysial neurosecretory system and the magnocellular, aldehydefuchsin positive, hypothalamo-hypophysial neurosecretory system was observed. Six different neurosecretory cell types are described, characterized by different shape and size of their respective secretory granules. The nature of the secretory product of these cells is briefly discussed.Of the results of this investigation, a preliminary note has been published (Dierickx, 1971).The authors wish to thank Prof. Dr. H. Steyaert and Dr. W. Moerdijk of the Laboratory for Mathematical Statistics, Ghent University, for invaluable advice and assistance with their computers. They thank Dr. G. De Waele for his help with the scanning electron microscopy.     

Ultrastructure of the neurosecretory system of the Colorado potato beetle,Leptinotarsa decemlineata (Say)

Summary The ultrastructure of seven types of neurosecretory cells (NSC) in the medial and lateral groups of the protocerebrum is described. The differences among cell types established earlier by light microscopy parallel differences in size and appearance of the neurosecretory particles observed in electron micrographs. No relationship was found between the affinity for Gomori's paraldehyde fuchsin stain and the nature of the particles.The secretions of the A-, A₁-, and C-types of NSC of the medial group are characterized by electron-dense neurosecretory granules of 1250 Å dia., medium-dense granules of 2100 Å, and electron-lucent vesicles of 1700 Å, respectively. The L-type NSCof the lateral group contain smaller (1300 Å) or larger (1700 Å) neurosecretory granules. The medial B- and E-types of NSC and the lateral L_B-type contain granulated vesicles (1200 Å) of the same appearance. These cell types differ in other respects and most likely have separate functions.The author wishes to thank the Laboratory of Virology of the Agricultural University for the use of the electron microscope, Mr. J. Groenewegen and Miss J. van Rinsum for technical assistance, and Professor J. Lattin for correcting the English text. Part of the work has been done while the author was in the service of the Netherlands Organization for the Advancement of Pure Research (ZWO, grant 942-48), and the National Council for Agricultural Research (TNO).     

Immunogold labelling of serotonin-like and FMRFamide-like immunoreactive material in neurohaemal areas on abdominal nerves of Rhodnius prolixus

The ultrastructure of neurohaemal areas on abdominal nerves of the blood-sucking bug Rhodnius prolixus was investigated. Four types of axon terminals were found, distinguished by the morphology of their neurosecretory granules. By use of post-embedding immunogold labelling, granules in Type I axon terminals were shown to contain serotonin-like immunoreactive material, and granules in Type II axon terminals were shown to contain FMRFamide-like immunoreactive material. There was no colocalization of these materials. It is suggested that Type III terminals contain peptidergic diuretic hormone, which has previously been reported to be present in electron-dense neurosecretory granules in this neurohaemal area. The identity of material in Type IV terminals is unknown.     

Neurosecretion in the sinus gland of the fiddler crab,Uca pugnax

The ultrastructure of the sinus gland of the fiddler crab, Uca pugnax, was investigated and found to be similar to that in other crustaceans. Five types of neurosecretory axon terminals were tentatively identified on the basis of the size, shape, and electron density of granules within the axons. Release of neuro-secretory material appears to be by exocytosis.     

An ultrastructural in vitro study on the regulation of neurosecretory activity in the freshwater snail Lymnaea stagnalis (L.) with particular reference to Caudo-dorsal cells

The neurosecretory Caudo-Dorsal Cells (CDC) in the cerebral ganglia of the freshwater pulmonate snail Lymnaea stagnalis produce an ovulation stimulating hormone. Previously it has been shown that neuronal and non-neuronal inputs are involved in the regulation of their activity. The degree of autonomy of these cells has been investigated by studying with morphometric methods the ultrastructure of CDC maintained in vitro. CDC of isolated cerebral ganglia which were cultured for 7 days show a considerable rate of synthesis, transport and release of neurohormone. Apparently these processes can proceed in the absence of neuronal and hormonal inputs from outside the cerebral ganglia. Completely isolated CDC, however, do not show neurosecretory activity in vitro; active Golgi zones, indicating the formation of neurosecretory elementary granules, are absent from such cells. Isolation does not seem to affect general cell functions such as protein synthesis and respiration. It is suggested that a neuronal input, originating within the cerebral ganglia, is necessary for the stimulation of CDC neurosecretory activity. Techniques are described for the isolation and culture of neurosecretory cells of L. stagnalis.     

Structural re-evaluation of the neurosecretory system in the crayfish eyestalk

Summary The topography of the neurosecretory system in the decapod eyestalk has not been precisely delineated with light microscopy. Cobalt iontophoresis and electron microscopy have proved useful in clarifying the microstructure of this system. The sinus gland (sg) of the crayfish eyestalk consists of aggregated axon terminals which end at or near the blood space, lontophoresing cobalt back through the cut base of the sinus glands reveals proximal cell bodies in the eyestalk only in the X organ (Xo) region. Electron microscopy demonstrates that axons from about 115 neurosecretory cell bodies in the Xo form the Xo-sg tract. Intermingled with these Xo somata are smaller non-neurosecretory cell bodies which do not send axons into the sinus gland. One of these exhibits catecholamine fluorescence. Backfilling also reveals a second group of fibres which run from the brain along the optic tract and into the sinus gland. These brain-sg fibres are smaller in diameter than Xo-sg axons and lack neurosecretory vesicles. From these fibres collaterals extend into the eyestalk neuropil, especially in the proximity of the visual elements. The possible function of these non-neurosecretory processes within the sinus gland is discussed.This work was supported by a National Research Council of Canada grant