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The air-blood barrier was studied in replicas of freeze-fractured lung biopsies collected from healthy human subjects. Adjacent pneumocytes display a belt-like network composed of 3-7 superimposed ridges ('fibrils') on the P face and complementary grooves on the E face, i.e., a structure corresponding to a 'tight' junction. On the other hand, adjacent capillary endothelial cells show a continuous system of 2-4 membrane foldings. These appear mainly as smooth surfaced crests on the P face; on the E face furrows are seen, at the bottom of which a row of particles is situated. This arrangement suggests a 'leaky' type of junction. Discontinuous occluding junctions are located in the pericytic venular segment of the alveolar vessels. The present findings are in agreement with previous physiological and ultrastructural tracer studies locating the main part of the diffusion barrier for small polar solutes and proteins in the alveolar epithelium. Communicating junctions are demonstrated between type I and type II pneumocytes, indicating intercellular cooperation between these cells of common embryonic origin, but which fulfill different functions in the adult. In the endothelium of the non-muscular alveolar vessels communicating junctions are lacking. Desmosomes occur in the epithelium between type I and type II pneumocytes; square arrays of particles characterize the plasma membrane of type I pneumocytes. [ABSTRACT FROM AUTHOR]

The peritoneal mesothelium of mouse embryos (12 to 18 day of gestation) was studied by freeze-fracture and in sections in order to reveal the initial formation of the tight junctions. Freeze-fracture observations showed three types of tight junctions. Type I consists of belt-like meshworks of elevations on the P face and of shallow grooves on the E face. No tight junctional particle can be seen either on the elevations or in the grooves. Type II shows rows of discontinuous particles on the elevations on the P face. Type III consists of strands forming ridges on the P face. On the E face, the grooves of Type II and III appear to be narrower and sharper than those of Type I. Quantitatively, Type I junctions are most numerous during the early stages (day 12-13) of embryonic development, while Type III junctions become more common in the later stages, and are the only type seen by day 18. Observations on sections, however, fail to distinguish between the three types. The results suggest that an initial sign of tight junction formation is close apposition of the two cell membranes in the junctional domain, without tight junctional particles. Later, the particles appear to be incorporated in the tight junctions and the strands form by fusion of the particles. [ABSTRACT FROM AUTHOR]

Thyroid glands of 7 to 21 day-old chick embryos were examined by electron microscopy using freeze-fracture and thin-section preparations. The primitive follicle lumen first appears between two adjacent epithelial cells in 8 day-old embryos, and is formed in the region of a focal tight junction (macula occludens). The focal tight junction develops into the zonula occludens when the primitive follicle lumen first forms. The zonula occludens is, at first, composed of 4.6 ± 2.45 strands, but with increasing embryonic age the number of strands increases to 5.9 ± 1.41 in 13 day-old, and 8.0 ± 1.75 in 19 day-old embryos. Thyroglobulin stored within the embryonic gland lumina is isolated from the mesenchymal tissue even at the first appearance of these follicle cavities. Well developed gap junctions already occur in the thyroid gland of the 7 day-old embryo, so that an intimate relationship and communication between these cells already exists at the time of their functional differentiation. [ABSTRACT FROM AUTHOR]

Exocytosis was studied in acetylcholine-stimulated bovine adrenal medulla. During a pre-exocytotic stage, chromaffin granules are found in juxtaposition to the plasma membrane and separated from it by an electron dense space 25-27 Å in width. Freeze-fracture studies show this stage to be characterized by connections between the granules and the plasma membrane. These connections are apparently cytoplasmic but bridge both membranes; they are presumably proteinaceous, but their exact nature remains to be elucidated. Later stages of exocytosis were also studied by the freeze-fracture technique; a typical feature is the lack of intramembrane particles around the fusion site. Both connections and membrane particle movement are discussed in terms of recent biochemical findings. [ABSTRACT FROM AUTHOR]

Colchicine, an agent which disrupts microtubules, inhibits the vasopressin (VP)-induced increase in water permeability as well as intramembranous particle (IMP) aggregation in the luminal plasma membrane of granular cells of toad urinary bladder. However, the hydroosmotic response induced by serosal hypertonicity is not affected by colchicine. The present investigation was initiated to establish whether serosal hypertonicity is associated with IMP aggregation and whether the aggregation, if present, is altered by colchicine. The experimental half of paired hemibladders from the toad, Bufo marinus, treated with 0.1 mM colchicine for 4 h prior to exposure to serosal mannitol (240 mM) demonstrated no significant difference in osmotic water How (Jv) (1.03 × 0.18 vs. 1.13 ± 0.22μl · min · cm; p>0.20) when compared with control hemibladders. Similarly, comparison of control and colchicine-treated bladders revealed no difference in the number of IMP aggregation sites per area of membrane (17.8 ± 2.0 vs. 24.7 ± 3.5/100μ; p>0.10), the relative area of membrane occupied by these sites (0.30 ± 0.06 vs. 0.39 ± 0.07%; p>0.10) or the mean size of the aggregates (17.0 ± 1.4 vs. 15.8 ± 1.0 × 10 μm; p > 0.20). These results indicate that in toad bladder the increase in J induced by serosal hypertonicity is associated with IMP aggregation. Secondly, an intact microtubule system is not required to induce the hydroosmotic or the aggregation responses. If, as has been proposed, the cellular actions of VP and serosal hypertonicity share a common pathway to bring about an increase in osmotic water permeability and cause IMP aggregation in the luminal membrane of the granular cell, the present results suggest that the pathway begins at a step subsequent not only to the generation of cAMP, but also beyond the involvement of the microtubule system. [ABSTRACT FROM AUTHOR]

Two kinds of occluding junctions are found between ileal epithelial cells of suckling rats: apical zonulae occludentes ( ZO) and fasciae occludentes ( FO) which are associated with the lateral plasma membranes of many epithelial cells. In unfixed preparations, glycerol treatment induces the further proliferation of extensive fasciae occludentes. Both kinds of junction have identical structural elements when visualized in freeze fracture replicas, although the arrangement of these elements differs. Zonulae occludentes consist of networks of branching and anastomosing linear ridges or rows of ∼10 nm particles with 20-30 nm spaces between the rows which form narrow belt-like structures around the apical region of adjacent cells. Fasciae occludentes, on the other hand, consist of similar linear ridges or rows of particles but the junction strands are often discontinuous, open ended and only occasionally intersect with each other. Several different fracture planes through the plasma membrane in the region of the occluding junctions have been observed and these provide further evidence that two components, one from each membrane, fused at the level of the extracellular space, form the junction sealing element. Furthermore, we present evidence which indicates a staggered rather than an in-register arrangement of these two components. [ABSTRACT FROM AUTHOR]

Nerve terminals in the palisade zone of the rat median eminence were investigated with freeze-fracture electron microscopy. Fracture face P of the specific terminals showed two populations of intramembranous particles (IMP): a large and a small variety. The large IMP-s often formed small irregular clusters. In nerve terminals the total number of both populations of IMP-s was considerably less than that found on P membrane faces of ependymal feet. On fracture face E of the nerve terminals, the number of IMP-s was about a quarter of that seen on fracture face P. On both fracture faces of most terminals a few small round impressions (or elevations respectively) were found which may be interpreted as broken necks of either exo- or endocytotic vesicles. Neither gap nor tight junctions occurred at lateral membranes of the specific axon terminals. Similarly, no membrane specializations were observed with freeze-fracturing on membrane areas adjacent to the basement membrane. The findings are discussed in relation to a possible exocytosis mechanism of the hypothalamic releasing and inhibiting hormones. [ABSTRACT FROM AUTHOR]

Germ cells and Sertoli cells in embryonic mouse testes (day 14 to 20 of gestation) were examined by sectioning and freeze-fracture. Intercellular cytoplasmic bridges between the germ cells are observed in day 14 and older embryos. Membrane specializations with dense fuzzy material similar to the socalled 'desmosome-like structures' are found between Sertoli cells and germ cells. A cell contact area with dense opposed membranes is also found between adjacent germ cells. Asymmetrical dense fuzzy lining of both Sertoli and germ cell membranes is noted. Pinocytotic pits or caveolae are frequently found in the Sertoli cell membrane. Between adjacent Sertoli cells, gap junctions of various sizes and focal meshworks of the occluding junctions are found. Most of the occluding junctional particles are located in the center of the grooves in the E face, and are similar to those in postnatal and adult Sertoli cell junctions. In addition, on both fractured faces there are ridges and grooves devoid of particles which are continuous with occluding junctions with particles, suggesting an initial stage in the formation of occluding junctions of the Sertoli cells. Particles gathered at the site of desmosome-like structures are present on the P face of the Sertoli cell. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Freeze-fractured preparations of septate junctions between epidermal cells of annelids ( Lumbricus terrestris and Tubifex spec.) have been investigated. In Lumbricus the protoplasmic face (PF) of the plasma membrane is characterized by variously arranged rows of particles. Apically the rows take an undulating course and often are separated by wide distances. In the basal part of the junction the rows run closely together and more or less in parallel. The diameter of the particles measures 80-120 Å, the distance between two particles (centre to centre) is 150-250 Å. Additionally striking rows of large particles (long diameter 150-200 Å). Are to be observed mainly near the basal part of the junction. In Tubifex both faces of the plasma membrane could be studied in detail. The protoplasmic face (PF) contains rows of distinct individual particles (mean diameter 100-150 Å, centre to centre distance approx. 250 Å) whereas the particles of the extracellular face (EF, mean diameter 200-250 Å) usually form continuous strands in which the individual particles seem to fuse. The density of arrangement of the strands varies considerably. Additionally ladder-shaped membrane structures have been observed in plasma membranes of this species. [ABSTRACT FROM AUTHOR]

Freeze-fracture replicas of the rat corpus callosum revealed prominent junctional strands in fractured cell membranes of the somata of oligodendrocytes. The junctional strands were characterized by an elaborate system of straight or slightly undulating rows of linear aggregates of particles or ridges in the P face and furrows in the E face. [ABSTRACT FROM AUTHOR]

Electron microscopy of both thin sections and freeze-fracture replicas has demonstrated the occurrence of gap junctions (nexuses) in the cardiac muscle cells of the lamprey. These gap junctions are identical in basic structure with those found in the mammalian heart. However, they are much smaller (less than 0.5 μ in diameter), and more irregularly distributed than the typical gap junction in the mammalian heart. These small gap junctions seem to provide a structural basis for the electrical coupling between cardiac muscle cells in the lower vertebrates. In addition, the well developed sarcoplasmic reticulum and subsurface cisternae, which contain an electron dense spheroidal cast, are frequently observed in the cardiac muscles of the lamprey. [ABSTRACT FROM AUTHOR]

'Tight' or occluding intercellular junctions occur between adjacent glial processes in normal and regenerating crayfish motor nerve sheaths. Although infrequent, these junctions possess the ridge and groove configuration characteristic of freeze-cleaved occluding junctions. When present, nerve sheath tight junctions consist of a single, or at most a few, parallel intramembrane ridges situated on the EF membrane face of the glial plasma membrane. Consequently, such contacts are rarely recognized in thin sections of plasticembedded nerve sheaths. Crayfish nerve sheath tight junctions are of the fascia occludens type and, therefore, do not impede solute flow across the nerve sheath. Fasciae occludentes of regenerating nerve sheaths occur in close proximity to discoid plaque-like aggregates of particles assumed to represent maculae adhaerentes. This relationship, which was not observed in normal nerve sheaths, suggests a functional association between the two types of junctions, perhaps developmental transformation of one junction type into the other. Although ridges and grooves of tight junctions occur next to crossfractured trans-glial channels, no functional significance is proposed for this relationship. This study is the first report of tight intercellular junctions in crustacean glial nerve sheaths. [ABSTRACT FROM AUTHOR]

The rat brain capillary was studied with freeze-fracture technique. The attached plasmalemmal vesicles were quite few in number on the luminal front and sometimes numerous on the contraluminal side. The fracture appearance of some tight junctions showed interconnecting ridges on face A and complementary furrows devoid of particles on face B, comparable to the common tight junction in the normal epithelia. Other tight junctions revealed a preferential disposition of quasicontinuous rows of particles on shallow furrows of face B, resembling the tight junctional strands of capillary endothelium in non-cerebral tissues. Either behavior is probably due to the difference in the fracture plane around the single fibril. In addition, the tight junctional strand could surround the perimeter of the endothelial cell completely although the exposed strand of tight junction was limited in length. [ABSTRACT FROM AUTHOR]

Labyrinth and nephridial canal cells of the crayfish ( Orconectes virilis) antennal gland possess two types of intercellular junctions revealed by freeze-fracture studies. Apical margins of the cells are connected by long septate junctions. In replicas, these junctions consist of many parallel rows of 80-140 Å intramembrane particles situated on the PF membrane face (EF and PF fracture faces of Branton et al., 1975). Rows of pits are found on the EF fracture face and are deemed complementary to the rows of particles. Moreover, lateral margins of basal regions of the epithelial cells are attached by many intercellular junctions. These contacts are characterized in thin plastic sections by a narrow dense cytoplasmic plaque located subjacent to the plasma membrane at sites of adjoined cells, and 5 to 12 fine strands of dense material that extend across the intercellular gap between adjoined cells. In freeze-fracture replicas, EF intramembrane faces basal to the region of the plasma membrane containing septate junctions exhibit numerous discoid clusters of particles. The particle aggregates, assumed to represent freeze-cleave images of adhering junctions, range from 900 to 3,700 Å in diameter, with individual particles about 185 Å in diameter. These junctions appear to connect epithelial cell processes formed by basal infoldings of the plasma-lemma, and occur between adjacent cells as well as adjacent processes of a single cell. The discrete aggregates of particles resemble replicated desmosomes (Shienvold and Kelly, 1974) and hemi-desmosomes (Shivers, 1976); therefore, they probably do not constitute a basis for electrical coupling between antennal gland epithelial cells. [ABSTRACT FROM AUTHOR]

Hemidesmosomes along the dermal-epidermal junction of larval and post-metamorphic newt skin have been examined in freeze-fracture replica images correlated with electron micrographs of sectioned material. Larval hemidesmosomal sites are characterized by large (200-300 Å) intramembranous granules arranged into clusters, each of which is aligned with a cytoplasmic hemidesmosomal plaque. In unfixed epidermis the granules remain attached to the A-face, while after glutaraldehyde fixation they are found on both A- and B-faces. Following metamorphosis the clusters are less distinct and localized. Replicas of unfixed B-faces and nearby cytoplasm display elongate, filamentous profiles which traverse the cytoplasmic leaflet and extend onto the B-face. The possibility that these components constitute a filamentous network serving to link tonofilaments, hemidesmosomal plaque, and basal plasmalemma is considered in view of the evidence to date. Hemidesmosomal fine structure as revealed by these studies is compared to features of desmosomes as detailed in the following report. [ABSTRACT FROM AUTHOR]

Desmosomes of larval and post-metamorphic newt epidermis have been studied by freeze-fracture replication both with and without prior glutaraldehyde fixation. Characteristic particles of a diameter (70-130 Å) similar to that of typical membrane associated particles are found clustered on the exposed internal faces of adherent desmosomal membranes. They remain attached to the B-face in unfixed material, but occupy the desmosomal A-face after fixation. Membrane associated particles of nondesmosomal surfaces are found predominantly on the A-face in both fixed and unfixed epidermis. Suitably oriented replicas of unfixed desmosomes reveal profiles of apparent fine filaments extending from the region of tonofilament loops through the desmosomal plaque to traverse the cytoplasmic leaflet of the plasmalemma. They can be traced onto the B-face. Their position correlates to fine linear profiles noted in tannic acid/glutaraldehyde-fixed and sectioned desmosomes. The possibility that these represent a mechanism for anchorage of tonofilaments to the plaque and to the membrane is discussed. These and other fine structural features are compared and contrasted to the properties of hemidesmosomes described in the preceding report. [ABSTRACT FROM AUTHOR]

Normal and feminized human testes were examined by means of freeze-fracture. In both cases, the junctional complexes between adjacent Sertoli cells show a unique characteristic feature. Many parallel linear occluding junctions are located circumferentially around the cell surface at the level of the nucleus. They are more than forty in number over one Sertoli cell surface. In the A face, the particles are not prominent on the ridge of the junctions. Instead, they are clearly seen in the center of the groove of the junctions on the B face. Gap junctions consisting of particle aggregation are not found between the Sertoli cells. The gap junctions between the Leydig cells are frequently observed in both normal and feminized testes. They are round or elliptic in contour and about several micrometers in diameter. The particles, about 8 nm in diameter, are closely packed to form a quasi-hexagonal pattern. The pits are found on the B face corresponding to the particle aggregation. [ABSTRACT FROM AUTHOR]

Human proximal tubule cell cultures exposed to doses of cadmium chloride (CdCl) between 0.05 μg/ml and 0.5 μg/ml exhibited alterations in cell membrane structure and transport function. At these Cd concentrations, cell numbers were not significantly altered from control values in either nonreplicating confluent, or actively replicating subconfluent cultures. Transmission electron microscopy revealed few alterations in cultures treated with 0.05 μg/ml Cd. Tight junctions were intact; organelles and myeloid body formation appeared normal. Freeze fracture analysis confirmed the integrity of the tight junctions as well as increased numbers of vesicles or pits along the lateral cell membrane, indicating increased endocytotic activity. Cells exposed to 0.1 μg/ml Cd were characterized by decreased numbers of microvilli and inhibited myeloid body formation. Cd doses of 0.5 μg/ml elicited nuclear chromatin condensation, fragmented sealing strands in 5 to 10% of the tight junction profiles, sparse microvilli, and inhibited myeloid body formation. Electrophysiologic assessments of transport function by Ussing chamber analysis revealed decreases in transepithelial potentials for all three concentrations, with significant differences at Cd concentrations of 0.5 to 0.1 μg/ml. Cells treated with 0.5 μg/ml Cd also exhibited slight decreases in electrical resistance, consistent with the minimal fragmentation of sealing strands observed in freeze fracture replicas. Resistance in cultures treated with 0.1 or 0.05 μg/ml Cd remained within control values and indicated that drops in potential difference and short circuit current in these cells reflected true alterations in ion transport. [ABSTRACT FROM AUTHOR]

In 5 patients with chronic megakaryocytic-granulocytic myelosis (CMGM) bone marrow specimens were studied by electron microscopy to investigate possible abnormalities of the granulocytic cell lineage. Thin sections were compared with freeze-fracture replicas to elucidate further aspects of leucocyte cytology. The atypia exhibited in these cells (eosinophils, basophils and neutrophil granulocytes) consisted mostly of a disorganization of granulopoiesis with hyper- and hypogranulation, a pathological increase in the number of nuclear blebs and a maturation asynchrony sometimes leading to Pelger-like cell forms. Moreover, a presumptive stem cell was demonstrated in the erythopoietic and granulocytic cell lines resembling CFU cells. In conclusion, granulopoiesis in CMGM exhibited abnormalities as generally observed in chronic myelogenous leukaemia. When considered with our previous finding of malignancy in megakaryopoiesis, CMGM has to be classified as a myelosis of mixed cellularity. [ABSTRACT FROM AUTHOR]

We have reconstituted pigment-protein complexes isolated from Rhodopseudomonas palustris photosynthetic membranes into phospholipid liposomes. The various complexes were tested for their ability to promote adhesion of the liposome membrane in the presence and absence of Mg ions. Samples containing a reaction center (RC)/light-harvesting I (LHI) complex appeared to stack in a manner resembling control thylakoids in 2 and 5 mM Mg. We also tested for the effects of Mg on detergent extractablity of pigment-protein complexes from intact membranes. Mg sharply reduced the amount of LHI solubilized from membranes, while having little effect on the extractability of the light harvesting II complex (LHII) and the RC. Based on these results we suggest that LHI is the principal adhesion factor of R. palustris thylakoids. [ABSTRACT FROM AUTHOR]

Chlorella fusca, strain 211-15, cells degreened in a nitrogen-deficient mineral growth medium in the light for 4-6 weeks were regreened for up to 24 hrs in a nitrogen rich medium that leads to synchronous cell division at 24-26 hrs. Structural changes in the plastid membranes during the regreening period were observed by thin section and freeze-fracture electron microscopy. Nitrogen-deficient plastids were found to have non-appressed lamellae, prolamellar body-like membrane aggregations, and only 2 types of freeze-fracture face. At this time no photosynthetic oxygen evolution could be demonstrated. After 6 hrs regreening the plastid lamellae had fused to form bands of appressed lamellae and the four types of freeze-fracture face, described previously, were visible. At this time photosynthetic oxygen evolution could be demonstrated. After 24 hrs regreening the plastids had an appearance typical of normally grown Chlorella and had commenced to divide. Supporting evidence for these developmental stages is presented from isolated chloroplast particle fractions. An unusual type of cell wall proliferation was observed in the nitrogen-deficient Chlorella cells that resulted in the laying down of several walls, each with a trilaminar component. [ABSTRACT FROM AUTHOR]

According to previous studies, a process of endothelial activation seems to be occurring in the chick embryo between days 7 and 18. Also, endothelial cells respond to collagen as a substratum between 12 and 18 days, and this response diminishes until it almost disappears after birth. In the present study, aortas from chick embryos (days 7 to 21), and from chicks (14 days posthatching) were used. The results obtained by the freeze-fracturing technique, showed that between days 12 and 14 the intramembranous particles were aggregated into linear or clustered arrays in the fracture P-face of endothelial cells. This could signify that some kind of gap junction-like coupling may occur between adjacent endothelial cells. Our results also indicate that in advanced stages (21-day-old chick embryos and 14-day-old chicks) the growth of small aggregates into larger aggregates or plaques could occur. In addition to gap junctions, the presence of macular and linear tight junctions, reported as focal tight junctions (day 14 of development) macular and linear tight junctions with free-ending strands orientated parallel to one another (21 days) and smooth contoured ridges (14 days posthatching) were observed. This sequence of changes may represent a development from linear to macular, to a more occluding arrangement, and may also reflect an endothelial cell polarization. Histochemical study of proteoglycans was done by using cuprolinic blue according to the critical electrolyte concentration method. Cuprolinic blue-positive granular, elongated and microfibrillar materials were found in the subendothelial region, forming a meshwork that occupies the extracellular space. Qualitative and quantitative changes were observed both in proteoglycans and in other extracellular matrix components throughout development, suggesting an increase in extracellular matrix complexity. These results lead us to suggest that the assembly of a more complex extracellular matrix, concomitantly with the formation of intercellular junctions during development, might influence the polarization of endothelium in the aorta of the chick embryo. [ABSTRACT FROM AUTHOR]

In the kidney of two elasmobranch fish, the little skate ( Raja erinacea) and the spiny dogfish ( Squalus acanthias), each tubular bundle is wrapped by a continuous sheath of extremely flattened cells which are ordered in several closely superimposed layers. Thin sections and freeze-fracture replicas demonstrate that extensive tight functions exist between the cells of this peritubular sheath. The sheath cells lie on a discontinuous basement membrane which suggests that they do not belong to the connective tissue. Conceivably, each peritubular sheath segregates the milieu inside the sheath (surrounding the bundle of 5 tubules bules and capillaries which form the countercurrent system) from the milieu outside the sheath (connective tissue matrix in which the bundles are embedded). [ABSTRACT FROM AUTHOR]

The formation of gap junctions was studied in pancreatic exocrine cells of rats and mice during late embryonic and neonatal development by the freeze-fracture replica method. Small gap junctions were present in association with tight junctional strands near the cell apex during embryonic development. Independently of tight junctions, small gap junctions were sometimes seen more basally on day 13 to 15 of gestation. The gap junctions increased in number and were rapidly enlarged by day 18 to 20 of gestation. Large particles 12-13 nm in diameter were frequently associated with the gap junction, which consisted of 10 nm particles. The large particles were either irregularly distributed or arranged in hexagonal patterns. The number of large particles decreased with time, so that they sparsely rimmed the gap junction in postnatal animals. This suggests that large particles are precursors of typical gap junctional particles, and that they participate in rapid growth of the gap junction during late embryonic development. It may be also possible that large particles represent functionally different gap junctions. [ABSTRACT FROM AUTHOR]

Correlated thin-section, freeze-fracture and tracer examinations were used to examine the blood-nerve barrier of the Vater-Pacini corpuscles in cat mesentery. A laminar inner core and a multilayered outer core enfolded the terminal nerve fiber of the corpuscle. The lamellar cells of both cores were characterized by numerous vesicular membrane invaginations. Freeze-fracture images and tracer experiments employing lanthanum nitrate proved that these invaginations are static structures mediating in neither active pinocytosis nor the transcellular transport of metabolites. In both inner and outer cores, lamellar cells were connected to one another by tight junctions of either the zonula or the fascia type, that occurred between lamellar-cell processes within the lamella and between the cells of adjacent lamellae. Intravascularly applied lanthanum lay at the out-ermost regions of the corpuscles without entering their internal zones, apparently because lamellar-cell tight junctions hindered further penetration. The results of our investigations suggest strongly that the Vater-Pacini corpuscle lamellae enfolding the nerve terminal form an effective diffusion barrier against the permeation of tissue fluids, thus preserving the corpuscle internal circumference. [ABSTRACT FROM AUTHOR]

Freeze-fracture replicas of the human reflected and placental amnion and chorion laeve at term were studied in order to give a systematic survey of the nature and extension of the intercellular junctions in the fetal membranes. No differences could be detected between the reflected and placental amniotic epithelium. In both the replicas never displayed plasma membrane differentiations typical of occluding junctions, while communicating junctions were occasionally and desmosomes frequently seen. In the chorionic trophoblast maculae occludentes, communicating junctions and desmosomes were regularly encountered. It is assumed that the maculae occludentes are remnants of occluding junctions which early in gestation possibly seal off the chorionic cavity; it appears improbable that they contribute significantly to the permeability properties of the chorionic trophoblast, since it is known from previous ultrastructural studies that large open intercellular channels cross the chorionic trophoblast. Thus the absence of occluding junctions, which could act as effective permeability barriers, in both epithelial components of the fetal membranes suggests that the factors able to influence the amniotic fluid turnover or the paraplacental protein exchange are the geometrical relationships and physico-chemical properties of the intercellular channels in the amniotic epithelium and chorionic trophoblast. In addition, communicating junctions were present between fibroblasts in the chorion laeve but not in the amnion, possibly indicating differences in the functional state of these cells and/or their extracellular microenvironments. [ABSTRACT FROM AUTHOR]

The differentiation and distribution of intercellular junctions especially during the early developmental stages of the rabbit nephron was studied by freeze-fracture electron microscopy. Metanephrogenic cells were found to be connected by sporadic focal tight junctions. During the formation of the renal vesicle similar tight junctions occurred on the periphery as well as near the developing lumen. These focal tight junctions increased in size and coalesced to broad zonulae occludentes lining the vesicular lumen at a later stage. Broad occluding junctions were also observed in the different nephron segments of the S-shaped stage. Ultrastructurally, these early maculae and zonulae occludentes consisted of beaded rows of particles. As development progressed, continuous tight junctions formed, whereas the number of strands decreased with the exception of the distal tubule. In contrast to the parietal glomerular epithelium, the initial occluding zonules of the visceral glomerular cells were gradually reduced to maculae occludentes, and finally disappeared. These results suggest that zonulae occludentes appear synchronously with the establishing lumen; the ultrastructural differentiation of tight junction strands seems to be completed with the onset of glomerular filtration. [ABSTRACT FROM AUTHOR]

Thin section as well as freeze-fracture images of lactating mouse mammary glands were examined with the electron microscope. The well-developed tight junction, consisting of 5-8 strands, is present at the apical part of the lateral plasma membranes of the adjacent glandular cells. The secretory products in this cell are classified into two types, milk fat globules without cores and proteinous granules having dense cores. Some proteinous granules fuse with each other to become larger structures. Membrane particles of the limiting membranes of both adjacent granules are cleared off from the initial site of the fusion. A large apocrine process including a large fat droplet, a few small proteinous granules, and little cytoplasmic matrix is sometimes seen. In addition, some proteinous granules are released by exocytosis. At exocytosis, the membrane particles are cleared off from the fusion site of the plasma membrane and the granule limiting membrane just before the fusion occurs. [ABSTRACT FROM AUTHOR]

Freeze-fracture replicas of visceral yolk sac from rats, mice and hamsters in late stages of gestation were studied by electron microscopy. Special attention was directed toward determining the types of junctional specializations that exist between the columnar endoderm cells of this placental membrane. In all three species, well-developed, zonular tight (occluding) junctions were found on the contiguous lateral surfaces of the endoderm cells. The tight junctional network, located in an immediate subluminal position, was from 0.2-0.5 μm in depth and consisted at any point of 2-5, interconnecting, ∼9 ηm wide, strands (P-face) or shallow furrows (E-face). Patch-like aggregations of irregular intramembrane particles, characteristic of desmosomes (maculae adherentes), also were observed at scattered sites below the tight junctions. However, no evidence of gap (communicating) junctions was encountered. The endoderm cells of the rodent visceral yolk sac have been shown to play a central role in the selective transport of macromolecular substances from the maternal to the fetal system. Tight junctions may be vital to this endodermal cell function by preventing random paracellular fluxes of macromolecules. [ABSTRACT FROM AUTHOR]

The ultrastructure of promastigotes of Leptomonas samueli was studied by the freeze-fracture technique. Both the P and E faces of the plasma membrane of L. samueli have intramembranous particles. However, the particle density was higher in the E than in the P face. The P face of the flagellar membrane had few particles whereas abundant flat membrane particles were evident on the E face. An arrangement of particles indicating membrane specializations was seen on the flagellar membrane and on the membrane lining the flagellar pocket. Cellular structures such as the nucleus, peroxisomes, the mitochondrion, and microtubules were also studied. [ABSTRACT FROM AUTHOR]

Symbiodinium microadriaticum Freudenthal is widely regarded to represent one pandemic species of endosymbiotic dinoflagellates. Thin-sectioned and freeze-fractured chloroplasts of symbionts derived from different hosts reveal the envelope to be composed of three membraneous layers, the middle one featuring an uncommon cleavage pattern. Unusual thylakoid arrangement and inclusions indicate intrinsic differences in the chloroplasts among these algae. The results are discussed in the light of evolutionary differentiation within Symbiodinium. [ABSTRACT FROM AUTHOR]

Sequential changes in epithelial cells of collagenase-dissociated rat ventral prostate were studied by thin-section and freeze-fracture electron microscopy. Epithelial cells did not attach to the substrate for 48 h. Pelleted cells obtained 1, 24, and 48 h after dissociation were assigned to three categories depending on morphology and cellular associations. (a) Solitary epithelial cells degenerated as determined by extensive vacuolization in the cytoplasm and aggregation of intramembranous particles (IMP). (b) Epithelial clusters consisted of a homogeneous population of well-maintained, closely packed cells. Aggregation of IMP was minimal. Tight junctions that formed between cells at the periphery of the clusters appeared normal and provided an effective permeability barrier demonstrated by the exclusion of ruthenium red tracer. (c) Tissue fragments were comprised of varying combinations of epithelial, endothelial, and smooth muscle cells as well as fibroblasts and erythrocytes. Maintenance of tissue fragments was variable. Plasma membranes often displayed aggregated IMP and proliferated tight junctional strands. An effective permeability barrier was absent. After the 48 h 'latent period', epithelial cells in the clusters lost interdependence, disassociated from one another, and attached to the substrate. These isolated cells, which did not display aggregated IMP, retained the ability to form an effective permeability barrier upon reaching confluency. During the first 48 h, epithelial cells did not tolerate solitary existence, yet as participants in clusters they were well maintained. After this interval, they no longer required interactions with neighbors in order to survive. These results indicate that under our experimental conditions, an adaptation period is required by prostatic epithelial cells. The enhanced quality of maintenance associated with epithelial clusters suggests that control over the internal microenvironment, provided by a tight junctional barrier, may be important during the initial period of adaptation in vitro. [ABSTRACT FROM AUTHOR]

Congenital dyserythropoietic anemia type I was diagnosed in a Japanese patient. Electron microscopic examination of thin sections revealed the features of erythroblasts peculiar to CDA type I; namely 'spongy appearance' of nucleus and 'internuclear chromatin bridge' etc. However, the widening of nuclear pores, which has been reported as another peculiar feature of CDA type I erythroblasts, were not confirmed. Aberrations of erythroblast nuclear membrane which have not been hitherto noticed were as follows: The nuclear evelopes of erythroblasts frequently lacked heterochromatin application in wide areas where they often showed a marked invagination or evagination. This is associated with the invasion of cytoplasmic organelles into the nuclear territory and occasionally with the extrusion of the nucleolus into the cytoplasm. There were no nuclear pores on the invaginated or the evaginated portions of the nuclear envelope where the perinuclear cistern was compressed to lose the cisternal space, or widened to contain some membrane debris. The findings with thin sectionings were confirmed with freeze-fracturing. Both findings suggest that an aberration may reside in the erythroblast nuclear membrane itself, but not in nuclear pores. [ABSTRACT FROM AUTHOR]

In the post-partum rat endometrium, ultrastructural distinction could be made between stromal cells (fibroblast-like cells) and macrophages, especially by the freeze-fracture technic. The stromal cells were characterized by a well-developed rough-surfaced endoplasmic reticulum (RER) and intercellular junctions, while the macrophages had many vacuoles and vesicles, but no intercellular contact with each other. The freeze-fracture image showed that the stromal cells had many low linear elevations and gap junctions on the cleaved plane of the cell membranes, while the macrophages had no linear elevations or intercellular junctions. The cell membranes of the stromal cells had more intramembranous particles (IMP) ( P-face 697 ± 63/μm, E-face 303 ± 52/μm) than those of the macrophages ( P-face 467 +- 50/μm, E-face 217 ± 35/μm). It was confirmed that these two types of cell phagocytosed collagen fibrils. [ABSTRACT FROM AUTHOR]

The gap and tight junctions in normal human liver and in cholestasis following bile duct obstruction have been studied by freeze-fracture to analyze quantitatively the changes in these membrane specializations. The length of tight junctions/1 μm of bile canaliculus lies within a wide range both in normal (from 4 to 14 μm:mean 7.4 μm) and cholestatic livers (from 2 to 20 μm:mean 6.5 μm; = −12%). The mean width of the zonulae occludentes is 0.32 μm in the normal liver, consisting on the average of 5.4 strands. In cholestatic livers they exhibit a mean width of 0.26 μm (= −19%) and consist of 4.4 strands (= −19%). 0.5% of all measurements at every 0.25 μm of zonula occludens in the cholestatic liver revealed no junctions at all, i.e., an interruption of the permeability barrier. The packing density of gap junctions is 3.6÷100 μm of plasmalemma in normal livers and 3.1÷100 μm (= −14%) in cholestatic livers. They occupy approximately 1% and 0.3% (= −60%) of the cell surface in normal and cholestatic livers, respectively. The main effects of cholestasis was to a cause a decrease in gap junction area and, to a lesser extent, in the number of gap junctions. Measurements of the center-to-center spacing of gap junctional particles lay in a wide range of values (6-15 nm in the normal liver, 4-23 nm in the cholestatic liver). This distribution probably indicates open and closed configurations of gap junctional particles. Furthermore, this distribution shows that the diminution in gap junctional area in the cholestatic liver may depend partly on either a dispersion or clumping of particles. The structural and quantitative parameters of these liver cell junctions have been correlated with known or proposed physiological functions under normal and pathological conditions. [ABSTRACT FROM AUTHOR]

The technique of freeze-fracture electron microscopy was used to examine the alterations in the rat liver after chronic administration of Thioacetamide (TAA) after two and ten weeks. The results of these studies indicate that TAA altered the morphological appearance of the bile canaliculi and induced a progressive disorganization of the zonulae occludentes. During the treatment the bile canaliculi dilatated, showing a loss of microvilli, irregularities, and side branches extending into the hepatocytes. The tight junctions limited to a narrow band of hepatocytic membrane on each side of the bile canaliculus in the normal adult liver proliferated and anastomosed frequently. The junctional meshwork loosened and the predominantly parallel strands positioned themselves perpendicularly to the lumen of the bile canaliculus. Sometimes the complexity of the branching pattern and the number of strands varied extensively at very short intervals along the zonula occludens. Only one or two parallel strands were observed, whereas in other instances up to ten rows of interconnecting strands were present in one fracture face. In the ablumenal side of the tight junction, extended loops and loose ends were frequently observed. Although most strands appeared as continuous ridges in the protoplasmic face, regions in which strands had a fragmented or even a particulate appearance were common. As a consequence, the lateral and the lumenal regions of the plasmalemma become directly continuous. After five weeks of treatment individual strands were occasionally observed on the lateral surface of the plasmalemma away from their usual pericanalicular location. After ten weeks, junctional strands had proliferated in the lateral plasmalemma yielding large arrays with a very complex appearance and complicated configuration. The results of this investigation are discussed in relation to those obtained in other experimental systems. [ABSTRACT FROM AUTHOR]

Changes in the number and sizes of membrane-associated particles have been quantitated in the protoplasmic (P) and exoplasmic (E) fracture faces of the outer membrane of nuclei isolated from the inner cortex following renal ischemia and reflow in the rat. No changes were observed in the inner nuclear membrane. After 20-min ischemia, the number of particles in both fracture faces decreased. With reflow, the total number of particles decreased after both 20- and 60-min ischemia. The partition coefficient ( K = C/C) increased from 10 to 11 and 17 at 20- and 60-min ischemia then fell below control values to a K of 7 after 120 min. After reflow, K steadily decreased except after 20-min ischemia followed by 240-min reflow when K began to rise. The sizes of particles were predominantly 60 Å in the P face of control outer membranes but became larger after ischemia. After 20- and 60-min ischemia with reflow, the size distribution became more normal. The shifts in particle numbers and sizes seem to indicate modifications within the membrane resulting from ischemia. [ABSTRACT FROM AUTHOR]

Freeze-fracture and thin sections were performed on human bone marrow of chronic megakaryocytic-granulocytic myelosis (CMGM) to study the three-dimensional fine structure and maturation of normal and atypical megakaryocytes and thrombocytes. In the many normally maturing megakaryocytes the development of the demarcation membrane system (DMS) was best investigated by comparison of thin sections with freeze-fracture replicas. The DMS shows no connections with the Golgi apparatus or rough-surfaced endoplasmic reticulum, but originates from tubular infoldings of the plasma membrane. These infoldings are always in continuity with the extracellular space and form an intracellular membranous pool by branching and coalescing of flattened tubules from which finally the perforated cisternae of the DMS arise. Freeze-fracture of the normal thrombocytes confirms earlier findings. The abnormal giant platelets seen in CMGM display extensive areas of smooth membranes of a spongy structure consisting of dense tubules surrounded by the labyrinth of the surface-connected system. Their physiological significance in these atypical platelets remains unsolved. [ABSTRACT FROM AUTHOR]

In the adult cardiac myocyte the functional and structural integrity of the sarcolemma depends on the presence of calcium ions in the extracellular fluid. Ultrastructural damage to the myocardial cell after Ca is repleted following brief periods of Ca depletion includes (1) sarcolemmal disruption, (2) mitochondrial damage and (3) severely contracted myofibrils, Ca depletion is the necessary first step in the generation of the paradox, presumably altering the sarcolemmal permeability to Ca. The structural basis for the altered membrane permeability is not known. In the adult rabbit heart two major sarcolemmal defects occur with Ca depletion: (1) 90% of Ca-depleted cells have separation of the layers of the glycocalyx over 30% of their surface. (2) Within the bilayer the intramembrane particles on the P face appear to be reoriented. Hypothermia and cadmium substitution protect against the functional manifestations of the paradox and prevent separation of the glycocalyx and the intramembrane particle changes as well.While the paradox is found in many species, the adult rat myocardium appears to be functionally more sensitive than the adult rabbit. With Ca depletion the adult rat has 65·9±4·6% of its myocardial cell surface separated while the adult rabbit has 33·4±2·1%. On the other hand, both the rat and rabbit are insensitive to the paradox before 6 days of age. Three-day old rat hearts exposed for 20 min to [Ca]< 10−8 mol l−1 show an intact glycocalyx in 95% of their cells. By 20 days all cells show separation of the glycocalyx over 38·5%±1·6% of their surface.Lanthanum (used as a cytochemical probe of normal membrane permeability) did not penetrate the cytoplasm of 3-day old hearts after both Ca depletion and repletion, whereas adult hearts showed lanthanum deposits throughout the cytoplasm. The fact that changes in both the glycocalyx and the intramembrane particle distribution correlate closely with the development of the Ca paradox suggests a morphological basis to its development. [ABSTRACT FROM PUBLISHER]

Mouse liver was examined following a single intraperitoneal application of 25 mg/kg body weight vincamine. The studies were made using the technique of thinsectioning for electron microscopy as well as the freeze-fracture method. The thin-sectioning technique was useful for observing discrete changes such as an increase in the mitochondria and alterations in the bile canaliculi, e. g. a loss of microvilli and dilatation of the lumen. No other cytoplasmic changes could be observed. It was only with the aid of the freeze-fracture method that alterations in the cell contacts became visible. In contrast to the control animals the tight junctions in the liver tissue of mice treated with vincamin were disorganized and irregularly arranged. The gap junctions showed a very irregular contour as well. The question arises as to the extent to which the freezefracture method should be applied in the testing of pharmaceuticals, so as to exclude the possibility of damage to membranes. [ABSTRACT FROM AUTHOR]

Replicas of the freeze-fracture surfaces of the internodal myelin membranes of large sciatic nerve fibres from normal and diabetic rats were compared by quantitative electron microscopy. The internodal myelin of the diabetics was examined 14 days after streptozotocin (70 mg/kg IV) induced persistent hyperglycaemia, conditions under which sciatic motor nerve conduction velocity (MNCV) is consistently decreased by 20%. The number of intramembranous particles per unit area of both the P-face and the E-face of the internodal myelin membrane was significantly decreased in the diabetics. This alteration in the structure of the internodal myelin membrane was not found in large sciatic nerve fibres from diabetic rats treated with insulin from day 3 through 14, or from diabetic rats fed a diet containing 1% myoinositol; these are conditions under which the development of decreased sciatic MNCV is prevented or ameliorated. An alteration in internodal myelin structure occurs in acute streptozotocin diabetes which may explain the associated decreased sciatic MNCV. [ABSTRACT FROM AUTHOR]

The ultrastructure and the supramolecular organization of the thylakoids of the small green flagellate, Mantoniella squamata, were examined in thin sections and freeze-fracture preparations. The whole chloroplast is tightly packed with thylakoids, which show a pattern of meandering, branching and/or anastomosing membranes. In freeze-fracture preparations only two fracture-faces can be distinguished: the PF- and the EF-face. The PF-face has a much higher particle density than the EF-face (PF: 4086 particles/μm; EF: 865 particles/μm). The EF-face is not as uniform as the PF-face. The areas which are packed with particles probably correspond to closely appressed thylakoid regions or adhesive patches, noticed in thin sections in some areas. The mean particle size on both faces is also different (EF: 10.5 nm; PF: 8.6 nm), but no information about the classification of the particles to special protein complexes is available at this time. [ABSTRACT FROM AUTHOR]

The anterior half of the cell surface of the parasitic flagellate Proteromonas lacertae is corrugated while the posterior half is covered by hair-like appendages, called somatonemes. In the anterior part, the cortical microtubules are lined by a zig-zag shaped microfibril. Here, these two structures seem to be separated from the plasma membrane. In the posterior half of the cell the somatonemes, analogous to the mastigonemes of chrysophytes, are anchored to the cortical microtubules by paired small deposits of dense material. This was clearly demonstrated by Triton X 100 treatment which solubilized the plasma membrane but left the somatonemes attached to the cortical microtubules. Freeze-fracture images revealed the alignment of clustered intramembrane particles on the P-face of the plasma membrane which correspond to the attachment sites of the somatonemes, seen as dots in thin sections. The ER-derived membrane-associated somatonemes are probably linked to the cortical microtubules by anchoring proteins which are part of the plasma membrane. [ABSTRACT FROM AUTHOR]

Unripe avocado fruit ( Persea americana Mill. cv Hass) were held at 6 °C either in air or in an atmosphere with 100 PPM ethylene and were assessed for chilling injury after one and two weeks. Injury did not occur in any fruit after one week. After two weeks, the fruit in air were still uninjured, but the fruit subjected to ethylene exhibited chilling injury. When the uninjured fruit (both air-treated for one and two weeks and ethylene-treated for one week) were allowed to warm to room temperature before freezing for freeze fracture electron microscopy, replicas revealed membranes with a randomly dispersed pattern of intramembranous particles (IMPs). However, when these uninjured fruit were frozen for freeze fracture without warming, particle-free domains were visible in the plasmalemma. The membranes of the ethylene-treated, chilling-injured (2 weeks) fruit, on the other hand, contained particle-depleted regions in the plasmalemma of fruit frozen not only from 6 °C but also in those allowed to warm to room temperature before freezing for freeze fracture. These particle depleted microdomains were not seen in fruit kept continuously at room temperature (20 °C), even in the presence of high levels of endogenous ethylene which is produced during normal ripening. We suggest these particle-depleted microdomains formed in the fruit frozen for freeze fracture from low temperatures and in the chilling-injured fruit to be due to lateral phase separations of the membrane components, possibly due to an increase in the viscosity of some membrane lipids, leading to the formation of microdomains of gel phase lipid in the plane of the membrane. These phase separations appear to be initially reversible by raising the temperature, however, this reversibility is apparently lost after injury has occurred. With regard to the cause of chilling injury in avocados, we suggest that some secondary effect is involved due to the long term presence of gel phase lipids in the membrane. [ABSTRACT FROM AUTHOR]

Germ roots of several higher plants-maize ( Zea mays), mung bean ( Vigna radiata) and cress ( Lepidium sativum)-were freeze-fractured without cryoprotection in order to confirm and extend the informations on frequency and distribution of plasma membrane particle complexes with respect to cellulose formation. In all three objects the PF of developing xylem elements showed rosette accumulations in the regions of wall thickenings. The rosette-distribution pattern ranges from random in a young stage, to more grouped in a probable intermediate stage to strictly localized in later stages. The frequency of rosettes increases from stage to stage. In all three objects the EF of developing xylem elements is relatively poor in particles. Observations of 'terminal globules' were rare and undistinct. This leads to the assumption that rosettes on the PF and terminal globules on the EF are not part of the same complex. A comparison of the number and distribution of microtubules underlying the xylem wall thickenings with rosette frequency and distribution leads to the conclusion that there seem to be no direct connections between these two structures. Microtubules may be involved in grouping of rosettes, thus indirectly orienting microfibril deposition. Calculations based on the observed rosette frequencies and the amount of wall material formed indicate that in xylem development 1,000 nm elementary fibril per rosette per minute may be formed and that the active phase of one rosette may be about 10 minutes. [ABSTRACT FROM AUTHOR]

The photosynthetic pigments of the marine alga Cryptomonas maculata are decreased under energy fluence rates of 4.4 Wm (high light=HL). That this is a result of nitrogen deprivation following an increased cell growth rate triggered by high light in comparison to the control under 1.28 Wm (low light=LL) is evident from combined pigment, growth rate and nutrient analyses. Fine structural studies by electron microscopy revealed that the electron opaque material in the thylakoid lumen of the plastid is lost under high light treatment parallel to the severe loss of almost 90% phycoerythrin-545. The reduction of chlorophyll a and c is accompanied by a reversible disorganization of the thylakoid packing and the thylakoid membranes. In a combined freeze fracture study of HL and LL cells of Cryptomonas maculata it is demonstrated that the exoplasmic fracture face of the thylakoid membranes in HL cells possessed only 10 to 15% of the particles of the LL control; the 12.5 nm particle class was almost lacking, whereas particle populations with main sizes of 10 and 7.5 nm are preserved. The protoplasmic face, on the other hand, was less severely affected with only slight reduction in the particle frequency and a shift of the particle size from two populations with peaks at 10 and 7.5 nm to one class centred around 7.0 nm. [ABSTRACT FROM AUTHOR]

Cryptomonads are unicellular algae with a unique photosynthetic apparatus, both in structure and pigment composition. A cryptomonad, Rhodomonas lens (R. lens), was studied by conventional electron microscopy, freeze-fracture, and freeze-etch in order to determine whether the thylakoids of this alga are everted with respect to those of other plants, as has been postulated ( Gantt et al. 1971, Gantt 1979, 1980) as a means to compensate for the location of the cryptomonad light-harvesting apparatus on the opposite side of the thylakoid membrane from that of related algae. We have characterized the thylakoids of this alga and conclude that they are not everted, but are oriented in the same manner as those of other algae and green plants. Implications for energy transfer are discussed. [ABSTRACT FROM AUTHOR]

Ontogenetic data concerning pollen not only clarifies the mode of deposition of the elaborate walls but has considerable functional and taxonomic relevance. Hitherto such studies have used optical or transmission electron microscopy but here a recently devised preparative technique has enabled pollen development in Cosmos bipinnatus to be studied using the scanning electron microscope. The technique involves freeze-fracturing of osmium fixed, cryoprotected anthers, maceration in dilute osmium tetroxide, critical point drying, sputter coating and examination. The processes of pollen wall development can then be observed in three dimensions, an important aid to understanding the spatial relationships involved in the determination of ornamentation and apertures. Details of the pollen and tapetum are described at various stages between meiosis and anthesis. A close conformity is demonstrated between the results obtained and those of earlier transmission electron microscopic studies of the same and related species although very different interpretations are made. [ABSTRACT FROM AUTHOR]

Numerous plasmodesmata occur in the walls between the secretory cells of Tamarix salt glands. The plasmalemma bounds the plasmodesmata and is continuous from cell to cell. In freeze-fracture, the e-face of the plasmalemma within the plasmodesmata is virtually devoid of intramembranous particles while, in contrast, the p-face is decidedly enriched with particles. The axial components appear to be a tightly curved membrane bilayer, as judged from measurements and their appearance in freeze-fracture, and the e-face of this membrane is also devoid of particles. Observations from both thin sections and freeze-fracture replicas indicate the presence of a circular cluster of six particles around the axial component near the cytoplasmic termini of the plasmodesmata. These particles extend from the p-face of the axial component to the p-face of the plasmalemma. These observations are summarized in a model. [ABSTRACT FROM AUTHOR]