大鼠中缝大核内5-羟色胺样P物质样和亮氨酸脑啡肽样突触结构的免疫电镜观察

用免疫电镜法在大鼠中缝大核内观察到:(1) 5-羟色胺(5-HT)样阳性轴突终末与阴性胞体、阳性和阴性树突以及阴性轴突终末,分别形成轴-体突触、轴-树突触和轴-轴突触;阴性轴突终末与阳性胞体和阳性树突分别形成轴-体和轴-树突触;(2) P物质样(SP样)和亮氨酸脑啡肽(L-Enk)样阳性轴突终末与阳性和阴性的胞体和树突,以及阴性轴突终末与阳性胞体和树突分别形成轴-体突触和轴-树突触,L-Enk样阳性轴突终末之间形成轴-轴突触;(3) 上述5-HT、SP和L-Enk样结构所形成的突触中,阴性轴突终末与阳性树突所形成的轴-树突触最多见;(4) 上述阳性轴突终末内主要含透明圆形小泡。免疫反应产物为电子密度高的物质,主要沉积于膜性细胞器的表面、透明圆形小泡和部分颗粒囊泡内和小泡膜上。     

家兔脊髓腰骶部中间外侧核区的超微结构——神经纤维网和突触

本实验用家兔7只,取腰髓2～4和骶髓2～4节中间外侧核区,做超薄切片,电镜观察。此区的神经纤维网内含树突、轴突、轴突终末、终端树突、突触和突触球。胶质细胞的突起穿行其间。树突散在,形态和大小多变。轴突则常成束分布。突触连接以轴树和轴体突轴为多见,偶见轴轴突触。多数突触单独存在,部分形成以树突或轴突为中心的突触球。突触内的突触小泡有清亮的圆形、椭圆形、扁平形和不规则形,还有相当多见的大致密核心小泡和少数有衣小泡。依终末囊内突触小泡的形态和突触前后膜的对称与不对称,所见突触可分为三类:1.圆形小泡不对称型;2.扁平小泡对称型;3.其它中间类型。     

大鼠中脑导水管周围灰质腹外侧区5-羟色胺样、P物质样和亮氨酸-脑啡肽样免疫反应阳性亚微结构的电镜观察

本文用免疫电镜技术研究了大鼠中脑导水管周围灰质腹外侧区内5-HT样、SP样和L-ENK样的免疫反应阳性亚微结构。5-HT样免疫反应阳性的胞体较多,常见5-HT样阳性树突与阴性轴突终末形成多为非对称性的轴-树突触;偶见阳性轴突终末与阴性树突以及阴性轴突终末与阳性胞体分别构成轴-树和轴-体突触.SP样阳性胞体数目较少,可见少量含多形性小泡的阴性轴突终末与之形成轴-体突触;由阴性轴突终末与阳性树突所形成的轴-树突触最常见;阳性轴突终末与阴性胞体和阳性树突分别构成轴-体突触和轴-树突触。L-ENK样阳性胞体数目也较少,L-ENK样阳性树突与阴性轴突终末所形成的轴-树突触最多见,可见L-ENK样阳性胞体与阴性轴突终末构成轴-体突触;偶见阳性轴突终末与阴性树突形成轴-树突触。上述各种突触均主要含圆形小泡,有时有少量扁平小泡、椭圆形小泡和颗粒囊泡。     

猫丘脑腹后内侧核的超微结构及突触联系

在电镜下对猫丘脑腹后内侧核内的超微结构及突触联系进行观察。该核内的轴突终末主要有３种类型：（１）含有圆形小泡的小轴突终末；（２）含有圆形小泡的大轴突终末；（３）含有扁平小泡的突终末。该核内的树突主要为不含突触小泡的Ⅰ型树突，此外还可见到少量含有突触小泡的Ⅱ型树突。     

三叉神经尾侧亚核大颗粒小泡的非突触部位胞吐及膜再循环的形态观察

在切除大鼠刚髭部皮肤的刺激下,从常规醛-锇酸固定的三叉神经脊束核尾侧亚核的超薄切片中观察到:1.大颗粒小泡轴突终末非突触部位的胞吐影像;2.含致密物质的大有衣小泡,由终末质膜内陷而成,提示大颗粒小泡在终末通过有衣小泡进行膜再循环;3.大颗粒小泡也可由含致密物质的平滑内质网样的管状结构形成。本研究支持大颗粒小泡在非突触部位,通过胞吐释放递质或神经调节物的假说。     

脊髓侧角区5-HT、TH、SP和L-ENK免疫反应纤维和终末的超微结构

本文应用免疫细胞化学ABC法,在电镜下观察脊髓侧角区单胺能和某些肽能纤维及末梢的突触组合。大鼠侧角内的5-HT、TH、SP和L-ENK免疫反应纤维均为无髓纤维。在侧角细胞簇内,这些纤维穿行于胞体之间,有的与胞体相邻,但很少与胞体形成轴-体突触。这些单胺和肽类纤维也与树突伴行,在树突束内数量最多。有时一小束无髓纤维都含同一种免疫反应物质。轴-树突触是各种免疫反应纤维终末所形成突触的主要形式。各种纤维终末所含的小泡多为圆清亮小泡,或兼有少数大颗粒泡。SP和L-ENK纤维膨体内的小泡与其终末内者不同,大颗粒泡较多,有时约占半数。各种免疫反应终末所组成参与的突触,对称或非对称型均不显现优势。     

大鼠三叉神经脊束核Ⅱ尾侧亚核层内CB样、PV样阳性结构的分布及其突触联系

本研究用免疫组织化学方法观察了 Calbindin D-2 8k( CB)样和 Parvalbumin ( PV)样胞体、纤维和终末在三叉神经脊束核尾侧亚核 ( Vc) 层内的分布及它们的突触联系。在光镜下观察到 CB样和 PV样阳性胞体、纤维和终末在 II层内侧带 ( IIi)最为密集 ,PV样阳性神经元的胞体稍大 ,但数量少于 CB样阳性神经元。在电镜下观察到 CB样或 PV样阳性结构主要形成下列 4种突触联系 :( 1)阳性轴突终末与阳性或阴性轴突终末形成对称性轴 -轴突触和少量非对称性轴 -轴突触 ;( 2 )阳性轴突终末与阳性树突形成非对称性和对称性轴 -树突触 ;( 3 ) CB样阳性轴突终末与阴性树突主要形成非对称性轴 -树突触 ,PV样阳性轴突终末与阴性树突主要形成对称性轴 -树突触 ;( 4 )阴性轴突终末与阳性树突形成非对称性和对称性轴 -树突触。另外还可见到 CB样或PV样阳性或阴性树突、轴突及终末与 CB样、PV样阳性或阴性的初级传入纤维终末形成 型和 II型突触小球。 型突触小球数量较多 ,有典型的扇贝样初级传入纤维终末和不均一的小泡 ,线粒体少 ;II型突触小球的初级传入纤维终末粗大而清亮 ,外观不规则 ,有均匀一致的小泡和丰富的线粒体。根据上述结果可以推知在面口部伤害性信息的传递和调控过程中 ,Vc II层神经元发挥着重要的作用     

大鼠三叉神经脊束核尾侧亚核胶状质突触小球的超微结构

用透射电镜观察了大鼠三叉神经脊束核尾侧亚核胶状质突触小球的各种突起成分及突触联系。突触小球的中央轴突终未与周围两类 (一、二型)树突棘,树突干形成非对称的轴树突触。含突触小泡的二型树突棘、树突干与不含小泡的一型树突棘、树突干形成对称的树树突触,并与中央轴突终末形成树轴突触。周围轴突终末(P)与中央轴突终末形成对称的轴轴突触,并与小球内的树突形成对称的轴树突触。     

大鼠三叉神经脊束核尾侧亚核Ⅱ层内CB样、PV样阳性结构的分布及其突触联系

本研究用免疫组织化学方法观察了CalbindinD-28k(CB)样和Parvalbumin(PV)样胞体、纤维和终末在三叉神经脊束核尾侧亚核(Vc)Ⅱ层内的分布及它们的突触联系.在光镜下观察到CB样和PV样阳性胞体、纤维和终末在Ⅱ层内侧带(Ⅱi)最为密集,PV样阳性神经元的胞体稍大,但数量少于CB样阳性神经元,在电镜下观察到CB样或PV样阳性结构主要形成下列4种突触联系:(1)阳性轴突终末与阳性或阴性轴突终末形成对称性轴-轴突触和少量非对称性轴-轴突触;(2)阳性轴突终末与阳性树突形成非对称性和对称性轴-树突触;(3)CB样阳性轴突终末与阴性树突主要形成非对称性轴-树突触,PV样阳性轴突转末与阴性树突主要形成对称性轴-树突触;(4)阴性轴突终末与阳性树突形成非对称性和对称性轴-树突触.另外还可见到CB样或PV样阳性或阴性树突、轴突及终末与CB样、PV样阳性或阴性的初级传入纤维终末形成Ⅰ型和Ⅱ型突触小球.Ⅰ型突触小球数量较多,有典型的扇贝样初级传入纤维终末和不均一的小泡,线粒体少;Ⅱ型突触小球的初级传入纤维终末粗大而清亮.外观不规则,有均匀一致的小泡和丰富的线粒体.根据上述结果可以推知在面口部伤害性信息的传递和调控过程中,VeⅡ层神经元发挥着重要的作用.     

SP,CGRP,ENK样阳性纤维终末在猫骶髓后连合核内的分布特征及超微结构

本实验用免疫组化电镜技术对骶髓后连合核中SP祥、CGRP样、L-ENK样阳性终末进行了观察,结果表明:SP样阳性终末主要含圆形清亮小泡,间有少量颗粒囊泡,主要与中、小树突形成不对称型轴-树突触(93%);还可见到不对称型轴-体突触(5%);也可见到少量的轴-轴突触(2%),SP样阳性终末为突触后成分。CGRP样阳性终末以含圆形清亮小泡为主,有的终末内混有颗粒囊泡。大多数终末(89%)与树突构成轴-树突触,但以远侧树突为主;也有少数(6%)的CGRP样阳性终末与胞体形成轴-体突触;还观察到由阴性终末与CGRP样阳性终末构成的轴-轴突触。L-ENK样阳性终末以含圆形清亮小泡为主,有时可见散在的颗粒囊泡,多与中、小树突形成不对型轴-树突触(92%);也观察到轴-体突触(5%)和轴-轴突触(3%)。     

Double-walled coated vesicle formation: Evidence for massive and transient conjugate internalization of plasma membranes during cerebellar development

Summary A massive and. transient increase in the formation of double-walled coafed vesicles (DWCVs) from surface membranes during late cerebellar development is reported here. These structures are characterized by an outer vesicle (65 nm in diameter), bearing a 15 nm thick spiny coat, containing an inner vesicle (30 nm in diameter). DWCVs occur free in the cytoplasm or attached to the plasma membranes. In the latter case, the membrane of the outer vesicle can be seen to be an invagination of the plasmalemma of the parent cell process while the membrane of the inner vesicle is an evagination of the plasmalemma of the adjacent cell process. DWCVs were observed in a variety of cellular elements in the granular and molecular layers of immature mouse cerebellum, including axons, dendrites, glia and cell bodies. Morphometric analysis revealed that the number of DWCVs in cerebellar mossy terminals became elevated between 16 and 37 days of age and reached a peak 45–50 times higher at 20 days than at either 10 or 70 days of age. The data suggest that a massive conjugate internalization of apposed plasma membranes occurs during late postnatal development which may serve to remodel neural membranes.     

Effect of Clostridium difficile enterotoxin A on ultrastructure of Chinese hamster ovary cells.

Electron microscopical immunocytochemistry and light microscopy were used to study the effect of Clostridium difficile enterotoxin A (EA) on cultured Chinese hamster ovary (CHO) cells. At 4 degrees C, immunocytochemically detected EA was randomly distributed along the plasma membrane; when cells were subsequently transferred to 37 degrees C, the EA moved into coated pits and coated vesicles within 2 min. Within 2 h of incubation at 37 degrees C with EA, the perinuclear cytoplasm of the CHO cells became highly refractile, and in 4 h, most of the cells were round; however, the majority of rounded cells excluded erythrosin B while remaining firmly attached to the culture dish plastic. When EA was removed from the cultured cells within 2 h, the cells returned to a normal morphology and formed a confluent monolayer. The nuclei of rounded cells were irregularly shaped; cytoplasmic intermediate filaments were collapsed toward the nucleus. Long bundles of parallel, 11-nm-diameter filaments appeared in the nuclei after 3 h of incubation with EA and disappeared by the fourth hour of incubation. Intoxicated cells did not undergo mitosis. Thus, EA was internalized, at least in part, by receptor-mediated endocytosis and subsequently affected the nuclei of CHO cells.     

Endocytic apparatus and transcytosis in epithelial cells of the vas deferens in the rat

The apex of the principal epithelial cells lining the vas deferens of the rat contains coated pits in continuity with the apical plasma membrane and large subsurface-coated vesicles (100-125 nm). In the apical cytoplasm, large, pale, uncoated vesicles (150-300 nm), small coated and uncoated vesicles (50-60 nm), uncoated vesicles about 75-90 nm, and membranous apical tubules are present, in addition to large, vacuolar, pale, multivesicular bodies, dense multivesicular bodies, and secondary lysosomes seen deeper in the cytoplasm amongst numerous ER cisternae, saccules of the Golgi apparatus, and mitochondria. The endocytic activity of these cells was investigated by using cationic ferritin (CF) as a marker of adsorptive endocytosis and native ferritin (NF) for demonstrating fluid-phase endocytosis. These tracers were injected separately into the lumen of the vas deferens, and the animals were killed at various time intervals thereafter from 2 to 90 minutes. At 2 minutes CF was seen bound predominantly to microvilli and to areas of the apical plasma membrane delimiting coated pits as well as in large, coated vesicles. At 5 and 15 minutes the tracers were seen in apical tubules and pale multivesicular bodies; at 30 minutes moderately dense multivesicular bodies were labeled. At 1 hour and longer time intervals dense multivesicular bodies and secondary lysosomes were labeled. NF followed the same pathway as CF; however, no binding to microvilli or areas delimiting coated pits was observed. The numerous other vesicular structures, i.e., the large uncoated vesicles (150-300 nm) and the small coated and uncoated vesicles (50-60 nm), never became labeled with the tracers and therefore were not involved in the endocytic process. There was, however, an exception in the case of several small (75-90 nm) uncoated vesicles seen deeper in the apical cytoplasm of these cells which were labeled exclusively with CF. With time such vesicles appeared along the lateral and basal surfaces of these cells and discharged their content of CF into the lateral intercellular space or the connective tissue space at the base of these cells. Thus the principal epithelial cells in addition to sequestering the endocytosed tracers within secondary lysosomes where they are presumably degraded also appear to be involved in the transcytosis of material from the lumen of the vas deferens to the underlying lamina propria.     

Endocytic appartus and transcytosis in epithelial cells of the vas deferens in the rat

The apex of the principal epithelial cells lining the vas deferens of the rat contains coated pits in continuity with the apical plasma membrane and large subsurface-coated vesicles (100–125 um). In the apical cytoplasm, large, pale, uncoated vesicles (150–300 nm), small coated and uncoated vesicles (50–60 nm), uncoated vesicles about 75–90 nm, and membranous apical tubules are present, in addition to large, vacuolar, pale, multivesicular bodies, dense multivesicular bodies, and secondary lysosomes seen deeper in the cytoplasm amongst numerous ER cisternae, saccules of the Golgi apparatus, and mitochondria. The endocytic activity of these cells was investigated by using cationic ferritin (CF) as a marker of adsorptive endocytosis and native ferritin (NF) for demonstrating fluid-phase endocytosis. These tracers were injected separately into the lumen of the vas deferens, and the animals were killed at various time intervals thereafter from 2 to 90 minutes. At 2 minutes CF was seen bound predominantly to microvilli and to areas of the apical plasma membrane delimiting coated pits as well as in large, coated vesicles. At 5 and 15 minutes the tracers were seen in apical tubules and pale multivesicular bodies; at 30 minutes moderately dense multivesicular bodies were labeled. At 1 hour and longer time intervals dense multivesicular bodies and secondary lysosomes were labeled. NF followed the same pathway as CF; however, no binding to microvilli or areas delimiting coated pits was observed. The numerous other vesicular structures, i.e., the large uncoated vesicles (150–300 nm) and the small coated and uncoated vesicles (50–60 nm), never became labeled with the tracers and therefore were not involved in the endocytic process. There was, however, and exception in the case of several small (75–90 nm) uncoated vesicles seen deeper in the apical cytoplasm of these cells which were labeled exclusively with CF. With time such vesicles appeared along the lateral and basal surfaces of these cells and discharged their content of CF into the lateral intercellular space or the connective tissue space at the base of these cells. Thus the principal epithelial cells in addition to sequestering the endocytosed tracers within secondary lysosomes where they are presumably degraded also appear to be involved in the transcytosis of material from the lumen of the vas deferens to the underlying lamina propria.     

Coated vesicles and pits during enhanced quantal release of acetylcholine at the neuromuscular junction

Summary Frog neuromuscular junctions were stimulated by different methods to secrete quanta of ACh, and the attendant changes in the ultrastructure of the nerve terminal were assessed by morphometric analysis of electron micrographs. Secretion was stimulated by electrical stimulation at 2 Hz or by application of the secretagogues, lanthanum, ouabain or black widow spider venom, either in the presence or in the absence of extracellular Ca²⁺. The numbers of synaptic vesicles, coated vesicles and coated pits, and the length of axolemma and area of axoplasm were measured on the micrographs. There was a significant increase (about threefold) in the total number of coated structures (vesicles plus pits) per m² of axoplasm, but the fractional increase in the number of coated pits exceeded the fractional increase in the number of coated vesicles. These increases were positively correlated with the increase in the length of axolemma per unit area and negatively correlated with the changes in concentration of synaptic vesicles, suggesting that they were due to the increases in the surface area of the terminal that accompany a loss of vesicles. However, the increase in the concentration of coated structures was not related to the number of quanta secreted or to the estimated number of vesicles recycled. The lack of correspondence between the fractional increases in the coated pits and coated vesicles and the poor correlation between the numbers of these structures and the overall parameters of the secretory process suggest that, in contrast to the situation in other secretory systems, coated pits and coated vesicles may not play a crucial role in maintaining the functional population of synaptic vesicles at rapidly secreting neuromuscular junctions.     

Neural mechanism underlying acupuncture analgesia

Acupuncture has been accepted to effectively treat chronic pain by inserting needles into the specific "acupuncture points" (acupoints) on the patient's body. During the last decades, our understanding of how the brain processes acupuncture analgesia has undergone considerable development. Acupuncture analgesia is manifested only when the intricate feeling (soreness, numbness, heaviness and distension) of acupuncture in patients occurs following acupuncture manipulation. Manual acupuncture (MA) is the insertion of an acupuncture needle into acupoint followed by the twisting of the needle up and down by hand. In MA, all types of afferent fibers (Abeta, Adelta and C) are activated. In electrical acupuncture (EA), a stimulating current via the inserted needle is delivered to acupoints. Electrical current intense enough to excite Abeta- and part of Adelta-fibers can induce an analgesic effect. Acupuncture signals ascend mainly through the spinal ventrolateral funiculus to the brain. Many brain nuclei composing a complicated network are involved in processing acupuncture analgesia, including the nucleus raphe magnus (NRM), periaqueductal grey (PAG), locus coeruleus, arcuate nucleus (Arc), preoptic area, nucleus submedius, habenular nucleus, accumbens nucleus, caudate nucleus, septal area, amygdale, etc. Acupuncture analgesia is essentially a manifestation of integrative processes at different levels in the CNS between afferent impulses from pain regions and impulses from acupoints. In the last decade, profound studies on neural mechanisms underlying acupuncture analgesia predominately focus on cellular and molecular substrate and functional brain imaging and have developed rapidly. Diverse signal molecules contribute to mediating acupuncture analgesia, such as opioid peptides (mu-, delta- and kappa-receptors), glutamate (NMDA and AMPA/KA receptors), 5-hydroxytryptamine, and cholecystokinin octapeptide. Among these, the opioid peptides and their receptors in Arc-PAG-NRM-spinal dorsal horn pathway play a pivotal role in mediating acupuncture analgesia. The release of opioid peptides evoked by electroacupuncture is frequency-dependent. EA at 2 and 100Hz produces release of enkephalin and dynorphin in the spinal cord, respectively. CCK-8 antagonizes acupuncture analgesia. The individual differences of acupuncture analgesia are associated with inherited genetic factors and the density of CCK receptors. The brain regions associated with acupuncture analgesia identified in animal experiments were confirmed and further explored in the human brain by means of functional imaging. EA analgesia is likely associated with its counter-regulation to spinal glial activation. PTX-sesntive Gi/o protein- and MAP kinase-mediated signal pathways as well as the downstream events NF-kappaB, c-fos and c-jun play important roles in EA analgesia.     

Three-dimensional reconstruction of vesicles in endothelium of blood-brain barrier versus highly permeable microvessels

In this study the three-dimensional organization of pinocytotic vesicles in mouse endothelia from permeable (choroid plexus, area postrema, and skeletal muscle) and blood-brain barrier (bbb) (cerebral gray and white matter) microvessels was examined. Reconstructions of 75 segments of endothelial cells from microvessels were done with very thin (less than 23 nm) serial sections and tracings. A total of 2,013 vesicles from five tissues were reconstructed for this study. Vesicles were classified as to whether they were attached to other vesicles (fused), connected to golgi or endoplasmic peticulum (tubule-connected), open to vessel lumen or ablumen (surface-connected) or isolated in the cytoplasm (free). The densities of tubule-connected vesicles and free vesicles were the same in all four types of vessels. It seems unlikely, therefore, that these vesicles are related to vascular permeability. Vesicular clusters and surface-connected vesicles were found in much higher densities in area postrema, choroid plexus, and skeletal muscle vessels than in bbb vessels. Single-vesicle transendothelial channels were found in attenuated endothelia of area postrema and choroid plexus. These results support the hypothesis that endothelial vesicles play a role in vascular permeability, possibility by transient fusion of vesicle clusters to the plasmalemma, to form transendothelial channels.     

An ultrastructural study of lactation in the human breast

In this study the morphological features of lactation in the human breast were examined by scanning and transmission electron microscopy. The lactating lobules comprised large numbers of interconnecting acini which were lined by a single layer of epithelial cells with underlying myoepithelial cells. Marked variations were noted in the shape of the epithelial cells. The myoepithelial cells formed an open meshwork of interconnecting cytoplasmic processes packed with myofibrils. The basal cytoplasm of the epithelial cells was packed with rough endoplasmic reticulum while the apical cytoplasm contained a hypertrophic Golgi body, numerous vacuoles (a few of which contained casein micelles), a number of lipid droplets and small coated and uncoated vesicles. The lipid droplets were released by progressive protrusion from the apical surface. They remained covered by the plasmalemma and were finally budded off into the lumen. In certain cases a portion of cytoplasm was released with the lipid droplet. The vacuoles and small vesicles fused with the plasmalemma and released their contents by exocytosis. Within the samples the majority of epithelial cells were actively lactating although examples of undifferentiated "resting" and dead (lysed) cells were also identified.     

Staphylococcal toxic shock toxin specifically binds to cultured human epithelial cells and is rapidly internalized.

Staphylococcal toxic shock syndrome toxin (TST) was labeled with 125I under mild conditions without apparent destruction of the molecule. [125I]TST bound specifically to human epithelial (Chang) cells in culture; the binding was inhibited by a 100-fold excess of unlabeled toxin. Scatchard analysis of the binding data indicated about 10(4) receptor sites per cell and a dissociation constant (Kd) of 4 X 10(-9) M. When cells pretreated with TST at 4 degrees C were swiftly transferred to 37 degrees C, the amount of surface-bound toxin rapidly declined, as determined by release of noninternalized label from the cell surface. Half-time (t1/2) of internalization was about 1.5 min. Ultrastructural studies showed that toxin labeled with ferritin-conjugated antibodies entered the cytoplasm via coated pits forming coated vesicles in the first 2 min of incubation at 37 degrees C. The coated vesicles coalesced with transport vesicles that are ultrastructurally unlike receptosomes. Thus, the unusual ultrastructural pattern of this internalization suggests that TST is initially internalized by receptor-mediated endocytosis and then enters an alternate pathway involving translocation in special transport vesicles, perhaps to other cells.     

The zinc-iodide-osmium reactive sites in the sensory epithelia of the frog labyrinth

Summary In the sensory epithelia of the vestibular system of the frog,Rana temporaria, the zinc-iodide-osmium stain reacts in the sensory cells with mitochondria, multivesicular bodies, lysosomes, coated vesicles, the smooth endoplasmic reticulum, the vesicles surrounding the synaptic bars and large numbers of morphologically similar vesicles scattered throughout the cytoplasm. In supporting cells positive reactions are given by mitochondria, multivesicular bodies, a few vesicles and areas of cytoplasm. Reactive sites in nerve fibres are the synaptic vesicles, mitochondria and the smooth endoplasmic reticulum. Certain regions of the Schwann-cell cytoplasm and myelin sheath of myelinated nerve fibres are also reactive.Energy dispersive X-ray microanalysis (EMMA-4) shows that the zinc-iodide-osmium stain is composed mainly of osmium with possibly a trace amount of zinc. Iodine is not shown to be present.