玉米螟幼虫侧单眼的神经鞘和胶质细胞

用透射电镜观察了欧洲玉米螟Ostrinia nubilalis(Hbner)5龄幼虫侧单眼神经的神经围膜、周神经细胞和其他神经胶质.神经围膜与若干周神经细胞包围若42根轴突.周神经细胞的原生质膜在它们的侧面和内面高度卷曲,并与相邻细胞交错对插,这是细胞与细胞间的特殊连接方式;它们的外面以桥粒和半桥粒固定在神经围膜内面.周神经细胞由神经胶质细胞演化而来,所形成的膜称神经束膜,它与神经围膜组成围在侧单眼神经外面的神经鞘.侧单眼神经内的神经胶质细胞大而平整,具有许多突起物(相当于脊椎动物的少突神经胶质细胞),每一个突起物包被一个感光轴突.神经胶质细胞包被轴突的形式有三种不同的类型:一种是相邻轴突间插入15层神经胶质细胞突起物所形成的普通轴系膜形式,另两种是神经胶质细胞突起物在一个轴突的周围,由一些褶所形成的不同形式.最后,对这些神经胶质细胞以不同形式包被轴突的功能意义进行了讨论.     

ACETYLCHOLINE CONTENT OF THE SCHWANN CELL AND AXON IN THE GIANT NERVE FIBRE OF THE SQUID

Abstract— Acetylcholine and choline were identified and their concentrations measured, by means of gas chromatography/mass spectrometry, in extracts obtained from nerve fibers of the hindmost stellar nerve of the squid Sepioteuthis sepioidea. These compounds were quantitated in samples of stellar nerve devoid of giant fiber, intact giant nerve fiber, extruded axoplasm, and axoplasm-free giant nerve fiber sheaths. In 11 samples of stellar nerve devoid of giant fiber, weighing an average of 20.8 ± 2.3 mg ( s.e.m. ), 756 ± 91 pmol ACh and 8.65 ± 0.62 nmol of choline were found. The total ACh content of the largest fibre in this group (10 μ m in diameter), for a 5 cm length of nerve, is in the order of 0.16 pmol. The average wet weights of a single giant nerve fiber (270-420 μ m in diameter) and its separate components ( s.e.m .; in mg; number of fibers in parentheses) were: intact fiber, 4.58 ± 0.19 (25); extruded axoplasm, 3.38 ± 0.13 (20); sheaths, 1.21 ± 0.11 (16). The average ACh content per unit weight of sample was about 2-3 times higher in the sheaths (5-13 pmol-mg⁻¹) than in the axoplasm (2-4 pmol mg⁻¹), whereas the ACh concentrations estimated per unit volume of cellular water were about 40 times higher in the Schwann cell (107-222 μ m ) than in the axon (2-5 μ m ). These experimental findings establish the presence of ACh in the giant nerve fiber of S. sepioidea. They also indicate the Schwann cells themselves as the main source for the release of ACh, responsible for their long-lasting hyperpolarizations following the conduction of nerve impulse trains by the axon.     

ELECTRON MICROSCOPE AND LOW-ANGLE X-RAY DIFFRACTION STUDIES OF THE NERVE MYELIN SHEATH

1. A close correlation has been obtained between high resolution electron microscopy and low-angle x-ray diffraction studies of the myelin sheath of frog and rat peripheral and central nerves. Extensive studies were performed by application of both techniques to the same specimens, prepared for examination by OsO₄ or KMnO₄ fixation, and embedding either in methacrylate or in gelatin employing a new procedure. Controlled physical and chemical modifications of the myelin sheath prior to fixation were also investigated. 2. A correspondence was established between the layer spacings observed in electron micrographs and the fundamental radial repeating unit indicated by the low-angle x-ray diffraction patterns. The variations in relative intensities of the low-angle x-ray reflections could be related to the radial density distributions seen in the electron micrographs. 3. An analysis of the preparation procedures revealed that OsO₄ fixation introduces a greater shrinkage of the layer spacings and more pronounced changes in the density distribution within the layers than KMnO₄ fixation. The effects of methacrylate and gelatin embedding are described, and their relative merits considered in relation to the preservation of myelin structure by OsO₄ fixation. 4. The experimental modifications introduced by freezing and thawing of fresh whole nerve are described, particularly the enhancement of the intermediate lines and the dissociation of the layer components in the myelin sheath. A characteristic collapsing of the radial period of the sheath is observed after subjecting fresh nerve trunks to prolonged and intense ultracentrifugation. 5. Controlled extraction of fresh nerve with acetone at 0°C., which preferentially removes cholesterol, produces characteristic, differentiated modifications of the myelin sheath structure. Electron microscopy reveals several types of modifications within a single preparation, including both expanded and collapsed layer systems, and internal rearrangements of the layer components. Alcohol extraction leads to a more extensive structural breakdown, but in certain areas collapsed layer systems can still be observed. The components of the lipide extracts could be identified by means of x-ray diffraction. These modifications emphasize the importance of cholesterol in the myelin structure, and disclose a resistance of the dense osmiophilic lines to lipide solvents. 6. The significance of these structures is discussed in relation to present concepts of the molecular organization of myelin. The available evidence is consistent with the suggestion that the primary site of osmium deposition is at the lipoprotein interfaces and that the light bands probably represent regions occupied by lipide chains. The electron microscope and x-ray diffraction data also indicate the possibility of a regular organization within the plane of the layers, probably involving units of 60 to 80 A. The myelin sheath is regarded as a favourable cell membrane model for detailed analysis by combined application of x-ray diffraction and electron microscopy.     

SYNTHESIS OF RNA IN NEURONS OF THE HYPOGLOSSAL NERVE NUCLEUS, AFTER SECTION OF THE AXON, IN MICE

Synthesis of RNA in neurons of the hypoglossal nerve nucleus after axonal section was studied by means of [5-³H]uridine administration and radioautographic counting techniques in mice. The results of the experiments were evaluated by counts of silver grains over the nucleoplasm and cytoplasm of the neurons. RNA synthesis was greater in neurons after axonal section, and this increase was evident from 12 hr after the operation. The greatest increases in the operated side were observed in the 1st, 2nd and 3rd days after operation. In the 7th and 14th days RNA synthesis was still greater in the hypoglossal nucleus of the sectioned nerve but the difference in the control nucleus was not so striking. In the 30th day synthesis of RNA in left and right hypoglossal nuclei was comparable.     

THE ULTRASTRUCTURE AND DISPOSITION OF VESICULATED NERVE PROCESSES IN SMOOTH MUSCLE

The walls of the gastrointestinal tract and urinary bladder of rats were fixed in osmium tetroxide, embedded in methacrylate, and sectioned for electron microscopy. The examination of sections of smooth muscle tissue with the electron microscope reveals the presence of bundles of unmyelinated nerve fibers within the intercellular spaces. In addition, vesiculated nerve processes, bounded on their outer surfaces by delicate plasma membranes and typically containing varying quantities of synaptic vesicles and mitochondria, make intimate contact with the surface of smooth muscle cells. These nerve processes are similar in structure and disposition to nerve endings previously described in skeletal muscle, in the central nervous system, in peripheral ganglia, in receptors, and in glands. It is concluded that the relationships existing between vesiculated nerve processes and the surface of smooth muscle cells constitute neuromuscular junctions. Profiles of protrusions of smooth muscle cells are often seen protruding into the intercellular spaces. Here they occur singly or in groups, originating from one or more cells. Because of the plane of section the protrusions may sometimes appear as individual entities between the muscle cells. In such cases care must be exercised in their identification because they have characteristics similar to sectioned nerve processes which also occur in the intercellular spaces.     

THE ULTRASTRUCTURE OF ADULT VERTEBRATE PERIPHERAL MYELINATED NERVE FIBERS IN RELATION TO MYELINOGENESIS

Adult chameleon myelinated peripheral nerve fibers have been studied with the electron microscope in thin sections. The outer lamella of the myelin sheath has been found to be connected as a double membrane to the surface of the Schwann cell. The inner lamella is connected as a similar double membrane with the double axon-Schwann membrane. The relations of these double connecting membranes suggest that the layered myelin structure is composed of a double membrane which is closely wound about the axon as a helix. These findings support the new theory of myelinogenesis proposed recently by Geren. The possible significance of these results with respect to cell surface membranes and cytoplasmic double membranes is discussed.     

STUDIES ON THE PROBLEM OF PRESERVATION OF MYELIN SHEATH ULTRASTRUCTURE: EVALUATION OF FIXATION, DEHYDRATION, AND EMBEDDING TECHNIQUES

Currently accepted methods of tissue preparation for electron microscopy result in alterations of myelinated nerve fibers. In an attempt to minimize distortion of myelin, various fixation techniques, dehydration schedules, and embedding methods have been evaluated. It was found that the major damage to myelinated nerves occurs in the embedding procedure. A technique for embedding nerve tissue using the polyester Vestopal W is described which was found to result in improved preservation of myelin.     

LONGITUDINAL IMPEDANCE OF THE SQUID GIANT AXON

Longitudinal alternating current impedance measurements have been made on the squid giant axon over the frequency range from 30 cycles per second to 200 kc. per second. Large sea water electrodes were used and the inter-electrode length was immersed in oil. The impedance at high frequency was approximately as predicted theoretically on the basis of the poorly conducting dielectric characteristics of the membrane previously determined. For the large majority of the axons, the impedance reached a maximum at a low frequency and the reactance then vanished at a frequency between 150 and 300 cycles per second. Below this frequency, the reactance was inductive, reaching a maximum and then approaching zero as the frequency was decreased. The inductive reactance is a property of the axon and requires that it contain an inductive structure. The variation of the impedance with interpolar distance indicates that the inductance is in the membrane. The impedance characteristics of the membrane as calculated from the measured longitudinal impedance of the axon may be expressed by an equivalent membrane circuit containing inductance, capacity, and resistance. For a square centimeter of membrane the capacity of 1 µf with dielectric loss is shunted by the series combination of a resistance of 400 ohms and an inductance of one-fifth henry.     

ULTRASTRUCTURE AND FUNCTION OF GROWTH CONES AND AXONS OF CULTURED NERVE CELLS

Dorsal root ganglion nerve cells undergoing axon elongation in vitro have been analyzed ultrastructurally. The growth cone at the axonal tip contains smooth endoplasmic reticulum, vesicles, neurofilaments, occasional microtubules, and a network of 50-A in diameter microfilaments. The filamentous network fills the periphery of the growth cone and is the only structure found in microspikes. Elements of the network are oriented parallel to the axis of microspikes, but exhibit little orientation in the growth cone. Cytochalasin B causes rounding up of growth cones, retraction of microspikes, and cessation of axon elongation. The latter biological effect correlates with an ultrastructural alteration in the filamentous network of growth cones and microspikes. No other organelle appears to be affected by the drug. Removal of cytochalasin allows reinitiation of growth cone-microspike activity, and elongation begins anew. Such recovery will occur in the presence of the protein synthesis inhibitor cycloheximide, and in the absence of exogenous nerve growth factor. The neurofilaments and microtubules of axons are regularly spaced. Fine filaments indistinguishable from those in the growth cone interconnect neurofilaments, vesicles, microtubules, and plasma membrane. This filamentous network could provide the structural basis for the initiation of lateral microspikes and perhaps of collateral axons, besides playing a role in axonal transport.     

TRANSVERSE ELECTRIC IMPEDANCE OF THE SQUID GIANT AXON

The impedance of the excised giant axon from hindmost stellar nerve of Loligo pealii has been measured over the frequency range from 1 to 2500 kilocycles per second. The measurements have been made with the current flow perpendicular to the axis of the axon to permit a relatively simple analysis of the data. It has been found that the axon membrane has a polarization impedance with an average phase angle of 76° and an average capacity of 1.1µf./cm² at 1 kilocycle. The direct current resistance of the membrane could not be measured, but was greater than 3 ohm cm.² and the average internal specific resistance was four times that of sea water. There was no detectable change in the membrane impedance when the axon lost excitability, but some time later it decreased to zero.     

CTx促进远端视神经切断后视网膜节细胞轴突再生与c-Jun表达的关系

目的探讨霍乱毒素(CTx)促进成年金黄地鼠视神经远端切断后视网膜节细胞(RGCs)轴突再生与c-Jun的表达关系。方法远端切断视神经并对接一段自体坐骨神经,玻璃体内注射CTx及/或植入小段坐骨神经分支(SN)。动物随机分为AG CTx组;AG SN组;AG SN CTx组,各组动物分别存活4W,用荧光金(FG)逆行标记和c-Jun免疫荧光组织化学双标法观察轴突再生的RGCs内c-Jun表达情况。结果再生RGCs内有c-Jun蛋白表达,玻璃体内给予CTx或植入SN组RGCs表达c-Jun的再生RGCs分别为35·8±9·57和32·2±7·25个,约占其再生总数的94%及90%,两组相比无显著性差异(P>0·05);CTx与SN联用组c-Jun阳性再生的RGCs为150·2±43·92个,占再生总数的97%,与前两组相比,均有显著性差异(P<0·05)。结论视神经远端切断后约90%以上的再生RGCs有c-Jun表达,提示c-Jun表达与视神经远端受损后节细胞轴突再生密切相关,CTx及外周神经对RGCs轴突再生及c-Jun表达有协同促进作用。