Myelin embedded in polymerized glutaraldehyde-urea

Ultrastructural localization of hyaluronan in myelin sheaths of the rat central and rat and human peripheral nervous systems using hyaluronan-binding protein-gold and link protein-gold

Neural tissue of central (rat spinal cord) and peripheral origin (rat sciatic nerve, nerve fascicles of rat skin and iris and of human conjunctiva) was processed by osmium tetroxide/microwave fixation and embedded in epoxy resin. Hyaluronan-binding proteins and link proteins coupled to 15-20-nm gold particles were used as markers in a one-step post-embedding procedure for identifying hyaluronan (hyaluronic acid) at the ultrastructural level. All myelin sheaths in both rat and human material were found to be intensely labelled. The specificity of the hyaluronan-binding probes was demonstrated by the total loss of labelling following treatment of sections with hyaluronidase or by preincubating either the probes with hyaluronan oligosaccharides or the sections with unlabelled hyaluronan-binding protein. The identified hyaluronan appears to be located extracellularly, but is precise role here remains to be elucidated.

Cytoskeletal organelles and myelin structure of beaded nerve fibers

Freshly removed cat peripheral nerves and lumbar spinal cord roots were prepared by freeze-substitution to study the form changes, cytoskeletal alterations and myelin structure in beaded nerve fibers. Fibers of unstretched nerves so prepared were close to cylindrical. When lightly stretched with tensions of 2-10 g before being rapidly frozen, beading appeared as a series of constrictions between the more normally expanded regions of the internodes with the paranodal regions spared. Beading also was seen in the fibers of sciatic and radial nerves fast-frozen in situ with the limbs placed in full extension to cause stretching. The cross-sectional area of the axon in the constrictions of beaded fibers was reduced by as much as 95%. The compaction of the microtubules and neurofilaments in the constructions was accounted for by the movement of axoplasmic fluid from the constrictions axially into the nearby regions where the axon and fiber diameters are close to normal. The electron-lucid area approximately 5 nm thick around the microtubules appeared to hinder their close approach in the constrictions although some microtubules touch. The neurofilaments are generally separated at a mean distance of 8-10 nm and approach to a mean distance of 4 nm in the constrictions. Neither the beading nor the reversal of beading, which occurs on relaxation from stretch, was blocked by periods of anoxia lasting several hours. Deletion of calcium from the incubating medium initiated some small amount of beading and additionally greatly augmented the beading on stretch. Beading also was present in some of the myelinated fibers of the dorsal columns of the spinal cord where stretch would not be present. These findings suggest that beading is due to a contractile process in the axon initiated by stretch and by other changed states of the fiber. Concomitantly with the contraction of the axon in the beading constrictions, the myelin sheath in that region was greatly reduced in circumference, to as much as 1/3 to 1/5 of normal. The decrease of the sheath diameter was not accompanied by a change in its thickness or in its lamellar fine structure. A repeat distance of the dense lines of 14 nm was measured in both the constricted and nonconstricted regions. To account for these findings lipid, and most likely other components of the myelin lamellar membranes, must move longitudinally from the constrictions in the plane of the lamellar membranes, and do this within 5-10 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct imaging of paracrystalline phospholipid structure in the electron microscope

A long chain amphiphilic molecule--the phospholipid 1,2-dihexadecyl sn glycerophosphoethanolamine--has been crystallized epitaxially so that the interlamellar molecular periodicity is parallel to the substrate and hence normal to the electron beam in the electron microscope. This has permitted the direct resolution of the 55.6 A lamellae in unstained crystals at room temperature. The lattice images have shown the presence of line dislocations and lenticular cracks in the crystals. Of significance to their biological properties is that the lattice is undulating with a periodicity of 0.1-0.5 micron. This would also account for the difficulties encountered by X-ray and electron diffraction techniques when examining these crystals.

A comparison of negatively stained electron micrographs and projections obtained from single crystal X-ray studies: the lipovitellin complex from lamprey

The analysis of single crystals of the lipovitellin complex from lamprey has made it possible to compare electron density projections derived from X-ray diffraction and density modification methods with previously published electron micrographs. The close correlation between the images obtained by the two methods demonstrates that the fidelity of images obtained by electron microscopy is excellent despite the loss of specimen order. These correlations also attest to the ability of density modification methods, currently used in macromolecular crystallography, to estimate the phases of small angle X-ray reflections.

Localizations of ruthenium red positive material in rabbit peripheral nerves

The penetration and distribution of ruthenium red in the axon-myelin-Schwann cell complex of developing rabbit peripheral nerve fibers are investigated. Ruthenium red positive material is established in the axoplasm, axolemma, periaxonal space, major dense lines and intraperiod lines of the compact myelin, mesaxons, split peripheral myelin lamellae, Schmidt-Lanterman and longitudinal incisures, paranodal loops and axo-glial contacts, Schwann cell cytoplasm and basal lamina, nodal extracellular matrix, desmosome-like structures, endoneural collagen. Some features of the distribution of the contrast material in the developing myelin sheath are described. Regional differences of the axolemma and of the Schwann cell cytoplasm and plasmalemma are established. The prevalence of glycoproteins or glycolipids in the ruthenium red stained material in its different localizations is discussed on the basis of trypsin and hyaluronidase digestion performed.

Electron microscopic demonstration of polysaccharides in central and peripheral myelin by thiosemicarbazide-protein-silver staining

Thin sections of glutaraldehyde-fixed central and peripheral nerve myelin were stained with thiosemicarbazide and protein-silver after oxidation with periodic acid on thin sections. In compact CNS myelin, staining was observed exclusively on intraperiod lines. In peripheral myelin, both intraperiod and major dense lines were stained. In addition, dense staining was observed on plasma membranes of oligodendrocytes and Schwann cells, especially periaxonally on tongue processes and in Schmidt-Lanterman incisures. The observed staining was most prominent on glycogen granules in unfixed and freeze-substituted tissues. Therefore, the results strongly suggest that polysaccharides of glycoproteins and glycolipids are visualized in both CNS and PNS compact myelin as well as on surface membranes of oligodendrocytes and Schwann cells.

Isolation and sequence of a cDNA encoding the major structural protein of peripheral myelin

The myelin sheath is a multilayered membrane, unique to the nervous system, which functions as an insulator to increase greatly the velocity of axonal impulse conduction. We have used the techniques of differential screening and hybrid selection to identify a cDNA clone encoding the Schwann cell glycoprotein P0, the major structural protein of the peripheral myelin sheath. The sequence of this protein, deduced from the nucleotide sequence of the cloned cDNA, indicates that P0 is an integral membrane protein containing a single membrane-spanning region, a large hydrophobic extracellular domain, and a smaller basic intracellular domain. The structure of the protein suggests that each of these domains plays an essential role in generating the highly ordered structure of the myelin sheath. Furthermore, we find that the induction of P0 mRNA coincides with the initiation of myelin formation, and we propose a model in which the glycoprotein serves as a molecular guidepost for this process.

Incorporation of P0 protein into liposomes: demonstration of a two-domain structure by immunochemical and PAGE analysis

The amphiphilic nature of P0, the major glycoprotein of peripheral nerve myelin, has been suggested previously. In the present study, purified P0 from human peripheral nerve myelin was incorporated into an artificial lipid bilayer consisting of dimyristoyl lecithin and cholesterol. The liposomes were fractionated on a sucrose gradient. The continued expression of P0 antigenicity by the liposomes was shown by specific complement consumption with a multivalent antiserum against P0 or with an IgM monoclonal antibody. Both antibodies recognized P0 expressed on the surface of peripheral nerve myelin and the P0 liposomes. P0 liposomes and peripheral nerve myelin treated with trypsin lost the surface determinant that reacted with the monoclonal antibody. Analysis of the trypsin-treated liposomes and peripheral nerve myelin by polyacrylamide gel electrophoresis revealed molecular weights for this protein of 19,500 and 20,500, respectively. Similar treatment of the P0 in the fluid phase resulted in many smaller fragments. These results indicate that P0 consists of two domains, a hydrophilic domain accessible to trypsin digestion and a hydrophobic domain, which is potentially trypsin-sensitive, but shielded by the lipid bilayer. Binding studies with an anti-P0 monoclonal antibody and polyacrylamide gel analysis of the lipid-shielded P0 fragment in liposomes and peripheral nerve myelin suggest that the orientation of the protein in the liposome is similar to that in peripheral nerve myelin.

Aminoplastic standards for quantitative X-ray microanalysis of thin sections of plastic-embedded biological material

The preparation of a set of aminoplastic standards for quantitation of sodium, magnesium, phosphorus, sulfur, chlorine, potassium, and calcium in ultrathin plastic sections of soft tissue is described. The standards are low in cost and easy to prepare and handle. They cover concentrations up to 750 mmoles/kg dry weight, and display chemical and physical properties similar to those plastic-embedded samples. The standards can be used to convert peak-to-continuum ratios obtained from the specimen to elemental concentrations. An application of these standards is shown using rat exocrine pancreas as a model. The standards can also be used to advantage for the calibration of the detector efficiency.

Supramolecular aggregation and organization in peripheral nerve myelin

Under certain preparative conditions the lipid bilayers of glutaraldehyde-fixed, PNS myelin demonstrate a marked compartmentalization, which can be augmented by lipid extraction following sectioning. The results are interpreted as indicating a supramolecular domain pattern of arrangement centered upon the transmembrane protein (P0) molecules. The latter are thought to be surrounded by annuli of substantially immobilized phospholipids. In the lamellar planes particular lipids are considered to have a nonrandom distribution. The visualization of bilayer compartmentalization was seen only in negatively stained sections obtained from unembedded or glutaraldehyde-urea-embedded myelin. Lipids were unextracted in the basic preparations except in so far as some unfixed, amphipathic molecules escaped at the trough-fluid interface at the time of sectioning, an observed phenomenon which probably aided in the visualization of the compartmentalization. Visualization was also augmented by surface tension expanding section fragments as they floated on the trough fluid. All stages of transition between well-ordered myelin and dispersed globular units were commonly to be found. Deliberately delipidated myelin exposed more sharply defined and smaller globular units in bilayer regions, but even these are regarded as being supramolecular aggregates including residual lipid annuli around the transmembrane proteins. The addition of cadmium ions as a "fixative" for lecithin seemed to improve the preservation of glutaraldehyde-urea-embedded myelin but was not strictly necessary to reveal its domain structure. A secondary tannic acid fixation was required to process unembedded myelin so as to reveal the fundamental compartmentalization of its lipid bilayers.

Immunocytochemical localization of basic protein in major dense line regions of central and peripheral myelin

To localize basic protein (BP) in the lamellar structure of central and peripheral myelin, we perfused newborn and 7-11-day rat pups with a phosphate-buffered fixative that contained 4% paraformaldehyde and 0.05 or 0.2% glutaraldehyde. Teased, longitudinally split or "brush" preparations of optic and trigeminal nerves were made by gently teasing apart groups of myelinated fibers with fine forceps or needles. Some of these preparations were immunostained without pretreatment in phosphate-buffered antiserum to BP according to the peroxidase-antiperoxidase method. Others were pretreated in ethanol before immunostaining. Then, all of them were dehydrated, embedded in Epon, and sectioned for electron microscopic study. In optic and trigeminal nerves that were not pretreated, myelin, glial cells, and their organelles were well preserved. BP immunostaining was present on cytoplasmic faces of oligodendroglial and Schwann cell membranes that formed mesaxons and loose myelin spirals. In compact central and peripheral myelin, reaction product was located in major dense line regions, and the myelin periodicity was the same as that observed in unstained control myelin that had been treated with preimmune serum. In ethanol-pretreated tissue, the myelin periodicity was reduced but dense line staining still was present. Our immunocytochemical demonstration of dense line localization of BP in both CNS and PNS myelin that was not disrupted or pretreated with solvents is important because of conflicting evidence in earlier immunostaining studies. Our results also support biochemical and histochemical evidence suggesting that BP exists in vivo as a membrane protein interacting with lipids on the cytoplasmic side of the bilayer in the spirally wrapped compact myelin membrane.

Presence of the myelin-associated glycoprotein correlates with alterations in the periodicity of peripheral myelin

The myelin-associated glycoprotein (MAG) is an integral membrane protein (congruent to 100,000 mol wt) which is a minor component of purified peripheral nervus system (PNS) myelin. In the present study, MAG was localized immunocytochemically in 1-micrometer thick Epon sections of 7-d and adult rat peripheral nerves, and its localization was compared to that of the major structural protein (Po) of PNS myelin. To determine more precisely the localization of MAG, immunostained areas in 1 micrometer sections were traced on electron micrographs of identical areas from adjacently cut thin sections.l MAG was localized in periaxonal membranes. Schmidt-Lantermann incisures, paranodal membranes, and the outer mesaxon of PNS myelin sheaths. Compact regions of PNS myelin did not react with MAG antiserum. The results demonstrate MAG's presence in "'semi-compact" Schwann cell or myelin membranes that have a gap of 12-14 nm between extracellular leaflets and a spacing of 5 nm or more between cytoplasmic leaflets. In compact regions of the myelin sheath which do not contain MAG, the cytoplasmic leaflets are "fused" and form the major dense line, whereas the extracellular leaflets are separated by a 2.0 nm gap appearing as paired minor dense lines. Thus, it is proposed that MAG plays a role in maintaining the periaxonal space, Schmidt-Lantermann incisures, paranodal myelin loops, and outer mesaxon by preventing "complete" compaction of Schwann cell and myelin membranes. The presence of MAG in these locations also suggests that MAG may serve a function in regulating myelination in the PNS.

The lipid solubility of fixative, staining and embedding media, and the introduction of LX-112 and poly/bed-812 as dehydrants for epoxy resin embedment

The manufacture of Shell, Epon-812 (E-812) resin has recently been discontinued. E-812 and two newly introduced E-812 substitutes, and Ladd-112 (LX-112) and the Polysciences, Poly/bed-812 (PB-812) resins, were studied biochemically and morphologically for their effectiveness as polar dehydrants. Their technical properties as general E-812 replacements were also explored. In the biochemical studies, acetone was more effective in retaining lung phospholipid components than ethanol, the resin dehydration was more effective than either acetone or ethanol. There was no appreciable difference in lipid solubility among the three resins. Acetone and uranyl magnesium acetate each had a loosening effect on previously fixed phospholipids. The PB-812 and E-812 resin dehydrated blocks of dense animal tissues, demonstrated serious technical difficulties during sectioning. The L-112 resin substitute, due to its low viscosity and improved infiltration, was found to be technically as effective a dehydrant as ethanol or acetone. None of the three resins was successful as dehydrating agents for the plant tissue. With organic solvent dehydration, both epoxy resin substitutes demonstrated excellent embedment properties with both animal and plant tissues.

Immunocytochemical localization of P0 protein in Golgi complex membranes and myelin of developing rat Schwann cells

P0 protein, the dominant protein in peripheral nervous system myelin, was studied immunocytochemically in both developing and mature Schwann cells. Trigeminal and sciatic nerves from newborn, 7-d, and adult rats were processed for transmission electron microscopy. Alternating 1-micrometer-thick Epon sections were stained with paraphenylenediamine (PD) or with P0 antiserum according to the peroxidase-antiperoxidase method. To localize P0 in Schwann cell cytoplasm and myelin membranes, the distribution of immunostaining observed in 1-micrometer sections was mapped on electron micrographs of identical areas found in adjacent thin sections. The first P0 staining was observed around axons and/or in cytoplasm of Schwann cells that had established a 1:1 relationship with axons. In newborn nerves, staining of newly formed myelin sheaths was detected more readily with P0 antiserum than with PD. Myelin sheaths with as few as three lamellae could be identified with the light microscope. Very thin sheaths often stained less intensely and part of their circumference frequently was unstained. Schmidt-Lanterman clefts found in more mature sheaths also were unstained. As myelination progressed, intensely stained myelin rings became much more numerous and, in adult nerves, all sheaths were intensely and uniformly stained. Particulate P0 staining also was observed in juxtanuclear areas of Schwann cell cytoplasm. It was most prominent during development, then decreased, but still was detected in adult nerves. The cytoplasmic areas stained by P0 antiserum were rich in Golgi complex membranes.

Highly conserved lipoprotein assembly in teleost and amphibian yolk-platelet crystals

Ohlendorf et al. have reported that yolk-platelet crystals in the frog, Xenopus laevis, were orthorhombic (space group P21221) and consisted of elongated asymmetric units (approximately 5.5 x 11.5 x 25 nm). The presence of an orthorhombic lattice in amphibian yolk platelets fulfilling the symmetry requirements of crystals had been missed in earlier structural interpretations. A comparative study of teleost, urodele and anuran yolk-platelet crystals performed in our laboratory has shown the widespread occurrence among equatic vertebrates of an orthorhombic lattice with unit-cell sides similar to those of Xenopus. However, we report here that the space group (P212121) and certain features of the macromolecular complex are at variance with those reported for Xenopus.

Immunocytochemical localization of rat peripheral nervous system myelin proteins: P2 protein is not a component of all peripheral nervous system myelin sheaths

Specific antibodies have been developed against P1, P2, and P0 myelin proteins and were used to study the localization of these proteins in the rat peripheral nervous system. Both peripheral and central nervous system myelin sheaths contain P1 protein. P0 and P2 proteins are found exclusively in peripheral nervous system myelin sheaths. Antisera to P1 and P0 proteins stain all peripheral nervous system myelin sheaths uniformly. P2 protein is not a component of all peripheral nervous system myelin sheaths. In sheaths that do contain P2 protein, it is concentrated in the area of the Schmidt-Lanterman incisures.

A fine structural analysis of the myelin sheath in rat spinal roots

The present study is concerned with the fine structure of the myelin sheath in rat and monkey spinal roots. Pictures are obtained that show that the intraperiod gap of the myelin sheath is continuous and opens into both the internal and external mesaxons, which in turn open into the periaxonal and endoneurial extracellular spaces. These results are compatible with the idea that the intraperiod gap is a cleft that traverses the thickness of the myelin sheath and links the periaxonal and endoneurial extracellular spaces.

Morphological localization of PNS myelin proteins

The localization of PNS myelin protein was studied using two methods: (1) lactoperoxidase catalysed iodination of intraperiod band material, (2) solubilization of basic proteins with ammonium acetate--Triton X-100 solutions. When myelin was swollen in the presence of lactoperoxidase and subsequently submitted to lactoperoxidase catalysed iodination P0 and what appeared to be the X protein labeled with 125I. In specimens which were disrupted in 50% ethyl alcohol, the basic proteins P1 and P2 were also iodinated. When the lactoperoxidase was omitted, there was no labeling of proteins. Ammonium acetate--Triton X-100 solutions solubilized basic proteins from both whole nerve and purified myelin preparations. Electron microscopic changes which accompanied this modification included swelling and splitting of the main period band. These data indicate that the P0 and X proteins are available for iodination in the intraperiod band of swollen PNS myelin and that basic proteins are localized in the main period band.

The development of a zonula occludens in peripheral myelin of the chick embryo. A freeze-fracture study

Sciatic nerves of chick embryos, 12 to 18 days incubation, were examined in freeze-fracture replicas with special emphasis placed on the development of tight junctional contacts in the myelin sheaths. In stages of beginning myelination short isolated particulate chains (focal tight junctions) appear in fracture faces of the adjacent membranes in the outer myelin lamellae, i.e., the outer mesaxon. In stages of progressing myelination these tight junctional elements elongate and become more numerous. They can also be found in the membranes of the inner mesaxons, the paranodal loops and the intramyelinic cytoplasmic inclusions. In fibers of advanced myelinogenesis a fusion of these isolated tight junctions--either end-to-end or at an angle--gives rise to continuous zonulae occludentes. This contact zone extends in the mesaxonal membranes along the direction of the fiber, whereas in paranodal myelin it acquires a helical course joining the membranes of the paranodal loops. It is proposed that this zonula occludens, which seals the cytoplasmic border of the Schwann cell, separates an intramyelinic from an extramyelinic, extracellular space already during the developmental stages of myelinogenesis.

Electron microscopy of cutaneous nerves and receptors

Recent ultrastructural observations on the connective tissue sheaths of nerves, Schwann cell-axonal relations, and nerve terminals and receptors are reviewed. It seems likely that endoneurial collagen is formed by perineurial cells during development and postnatally. New observations on "collagen pockets" are presented. Attention is drawn to freeze-fracture studies of peripheral nerve, particularly in relation to junctional complexes associated with compact myelin, and further application of the technique is considered. Current views on Merkel cells, encapsulated endings, and free nerve terminals are discussed.

Freeze-fracture properties of central myelin in the bullfrog

The freeze-fractured membrane of the central myelin sheath has three classes of particulate components: (i) Particles inherent to the compact myelin lamellae. These are distributed at random and cleave predominantly with the P (protoplasmic) face. (ii) Particles which comprise the intramyelinic tight junctions. These are arranged in strands and are located at the inner and outer mesaxon, the paranodal loops, the cytoplasmic incisures, and occasionally within the compact regions of the myelin sheath. (iii) Particles localized exclusively at the portion of the paranodal loop membrane involved in the septate-like junction with the axolemma. These are regularly spaced and are organized in parallel rows. In the central myelin sheaths of bullfrogs fixed by perfusion with aldehydes and cryoprotected in 30% glycerol, the randomly distributed particles differ in size and shape from those of the axolemma. They possess a reasonably well defined bimodal distribution with respect to particle shape--most can be described either as globules or as ellipsoids. The globular particles range in diameter from 60 to 150 A. The ellipsoidal particles are 100-200 A long and 15-50 A wide. The total number of particles per square micron on the P face is approximately 1500. About half of these are of the globular type and half of the ellipsoidal type. In poorly fixed specimens, loss of interlamellar adhesion and loss of randomly distributed particles seem to coincide. Evidence is presented against the hypothesis that the tight junctions between compact myelin lamellae represent the radial component of the myelin. The possible relation between the types of particulate components seen in freeze-fracture and the classes of protein isolated from central myelin fractions is briefly discussed.

Morphological evidence of alteration in myelinstructure with maturation

The periodicity of the myelin sheath in glutaraldehyde carbohydrazide (GACH) embedded tissues from three 10-day-old and two 43-day-old mice was investigated with the electron microscope. The mean repeat period and number of repeats per fiber was measured in 31-89 fibers in the trapezoid bodies, olfactory tract and sciatic nerves. The mean period of the myelin sheaths in both the peripheral and the central nervous systems of the 10-day-old mice (15.1 +/- 0.07 nm (S.E.M.) and 13.4 +/- 0.09 nm respectively) are smaller (rho less than 0.001) than those of the adult mice (15.9 +/- 0.08 nm and 14.7 +/- 0.06 nm respectively). These differences may be related to the known differences in chemical composition of immature and mature myelin.

Lipid losses during processing of cardiac muscle for electron microscopy

The lipid retaining properties of several methods of processing tissue for electron microscopy (EM) have been assessed quantitatively. Guinea-pig hearts were perfused in vitro at 37 degrees C with 3H-oleic acid bound to albumin. The hearts were fixed by perfusion with 4% glutaraldehyde in 0-1 M cacodylate buffer. Pieces of left ventricle and interventricular septum were removed, weighed and processed for EM. The fluids used at each stage of processing were monitored for loss of radioactive lipid by scintillation counting. Lipids were extracted from the processed tissue immediately before the embedding stage using a mixture of chloroform:methanol (2:1 v/v). Counts from processed tissue were compared with counts from tissue extracted directly after perfusion fixation in order to monitor subsequent losses during processing. A modified version of Epon processing, omitting 100% ethanol, acetone or propylene oxide, gave a lipid retention of only 20-6%. The addition of paraphenylenediamine to the procedure did not improve the retention although this has been shown to be a useful stain for intracellular lipid. Water soluble Durcupan which does not involve ethanol or acetone dehydration has an average retention of 63% with 100% recovery while the lesser known polymer GACH, a mixture of glutaraldehyde and carbohydrazide used both for dehydration and embedding showed a lipid retention of 82% of the counts recovered although recovery was only 69%. An attempt was made to determine which classes of lipids were present in the tissue after perfusion fixation using thin layer chromatography. It was found that the presence of any of the processing fluids affected the polarity of the lipids and their rates of migration on thin layer plates.

The investigation of vinylcyclohexane dioxide as a polar dehydrant for the improved retention of lipids in the Spurr embedment

The use of vinylcyclohexane dioxide (VCD) as a polar dehydrant with subsequent embedment in Spurr was studied. The utilization of Epon 812 resin (E 812), hydroxyethyl methacrylate (HEM) and hydroxypropyl methacrylate (HPM) as polar dehydrants for Epon embedment were re-examined, and a polar Epon mix was introduced. The most effective dehydration sequence was: first 90%, then 95% VCD in water for 5 min. followed by two 20 min changes of 100% VCD. After 1 hr in equal quantities of VCD and Spurr mix, tissues were infiltrated with Spurr embedment (two 1 hr changes and overnight) and finally embedded in Spurr and polymerized at 60 degrees C for 16 hr. The most utilizable polar Epon mix was determined to be Epon 812 = 50 ml, NMA=42 ml, DMP-30=1-2 ml. It was somewhat brittle but cut well with both glass and diamond knives. All four polar dehydrants were found to retain lipids and carbohydrates equally well in thin section in striated and cardiac muscle, liver, kidney and brain from the rat. The E 812 was the only dehydrant that retained lung multilamellar bodies. The possible carcinogenic effects of VCD were considered and the probably metabolism and excretion of VCD were discussed.

Substructure in rod photoreceptor membranes

Frog retinae, fixed only in buffered glutaraldehyde, were embedded for sectioning in glutaraldehyde polymerized with urea. In suitably thin sections globular substructures were seen in negative contrast after ionic staining with uranyl acetate and lead citrate, or after staining with neutralized phosphotungstic acid. Efforts to extract at least some of the lipid from sections before ionic staining enhanced the visualization of the "globules". Exposure to KMnO4 solution, used as an oxidative section stain, also outlined globular substructure in negative contrast, but with the additional feature that positively stained surface "leaflets" associated with the aqueous compartment were well defined. Staining sections with OSO4 vapor resulted in positively stained membranes, but without any evident substructure. However, when sections which previously had been exposed to OSO4 vapor were secondarily stained with uranyl acetate and/or lead citrate, positively stained globular substructures then were revealed. The globular substructures always were centered in the hydrophobic core region of the disc membranes, and symmetrically spanned the full thickness of this layer. The diameter of individual particles approximated 50-55 A. Reasons are presented for the supposition that the evident globules incorporate at least hydrophobic components of rhodopsin molecules. Findings are discussed in relation to various models of disc membrane organization that have been proposed in recent years.

Zonulae occludentes of the myelin lamellae in the nerve fibre layer of the retina and in the optic nerve of the rabbit: a demonstration by the freeze-fracture method

Ridges and grooves composing extensive zonulae occludentes are revealed by the freeze-fracture method on split myelin lamellae in the nerve fibre layer of the retina and in the optic nerve of the rabbit. The junctions are located immediately internal to the outer loop of the myelin sheath and in corresponding areas of deeper myelin layers. They follow a straight or gently undulating course along the axis of the fibres. Only at the paranodal region of nodes of Ranvier do they deviate and assume a transverse course, The strands of these zonulae occludentes probably represent the radial thickenings of the intraperiod line described in thin sections.

Electron microscopy of trypsin-digested peripheral nerve myelin

Sciatic nerves from mice were removed and soaked in either PBS (phosphate buffered saline) or PBS plus I% trypsin (Sigma Type III) for various periods of time. Specimens were soaked at either room temperature or 37-degrees C at pH's ranging from 7.5 to 8.0. The epineural and perineural sheaths were split to allow the trypsin to penetrate the nerve. Tissue was prepared for electron microscopy by fixation in cacodylate buffered formaldehyde-glutaraldehyde solutions, post-fixed in OSO4 and embedded in Epon 812 or in glutaraldehyde-urea resin without osmication. After four h incubation at 37-degrees C or eight h at room temperature, the basement membranes of the Schwann cells became fragmented and detached and the myelin intraperiod band lost some density. After 18 h, myelin with swollen intraperiod bands displaying a loss of electron density and split main period bands was noted adjacent to normal myelin. Other areas had been transformed into vesicles indicating that the membranes of these vesicles appeared to have been derived from the detachment of both the intraperiod and main period bands within the myelin. Evidence is presented for the presence of trypsin digestable proteins in both the main period and intraperiod bands of peripheral nervouse system myelin.

The ultrastructure of lipid-depleted rod photoreceptor membranes

The structure of lipid-depleted retinal rod photoreceptor membranes was studied by means of electron microscopy. Aldehyde-fixed retinas were exhaustively extracted with acetone, chloroform-methanol, and acidified chloroform-methanol. The effect of prefixation on the extractability of lipids was evaluated by means of thin-layer chromatography and fatty acid analysis. Prefixation with glutaraldehyde rendered 38% of the phospholipids unextractable, while only 7% were unextractable after formaldehyde fixation. Embedding the retina in a lipid-retaining, polymerizable glutaraldehyde-urea mixture allows a comparison of the interaction of OsO(4) with lipid-depleted membranes and rod disk membranes which contain all their lipids. A decrease in electron density and a deterioration of membrane fine structure in lipid-depleted tissue are correlated with the extent of lipid extraction. These observations are indicative of the role of the lipid bilayer in the ultrastructural visualization of membrane structure with OsO(4). Negatively stained thin sections of extracted tissue reveal substructures in the lipid-depleted rod membranes. These substructures are probably the opsin molecules which are the major protein component of retinal rod photoreceptor membranes.