The occurrence of intercellular bridges during oogenesis in the mouse

A fine structural analysis of fetal mouse ovaries reveals the presence of intercellular bridges between developing oocytes. These bridges, which connect two or more oocytes, are most frequently seen prior to the dictyate stage of meiotic prophase. The intercellular connections are limited by a tri-laminar membrane which is continuous with the oocyte plasmalemma. A characteristic feature of all bridges is the presence of an electron-dense material on the cytoplasmic side of the limiting membrane. Since this dense material is a constant and conspicuous component of the entire bridge, identification of these connections is possible in all planes of section. In cross section, the bridges are usually cylindrical, while in longitudinal section, a variety of configurations are observed. Oocytes connected by intercellular bridges exhibit a highly developed Golgi complex which is frequently localized in the region of the cytoplasmic continuities. Vesicular elements, apparently derived from the Golgi, are routinely observed within the boundaries of the bridges. Other cytoplasmic organelles, including rough and smooth endoplasmic reticulum, free ribosomes and mitochondria, are also seen in these bridges. The presence of these vesicles and organelles within intercellular bridges suggests that these connections may provide a means for transfer of organelles and other substances from one oocyte to another. It may be, therefore, that intercellular bridges are important for the nourishment and maturation of certain selected oocytes as well as for the synchronization of meiotic events.     

Neuronal systems immunoreactive with antiserum to lamprey gonadotropin-releasing hormone in the brain of Petromyzon marinus

Summary The wall structure of arteriovenous anastomoses in the rabbit ear was investigated. (1) Clusters of epithelioid smooth muscle cells form 3–4 longitudinally oriented plicae. The channel shows a single, irregularly outlined lumen, and its wall is very thin between adjacent plicae. (2) Endothelial cells covering the plicae protrude into the lumen, thus suggesting active contraction or shortening of the plicae. (3) The tunica adventitia is composed of 4–6 sheaths of flat fibroblasts, which may serve as a barrier to prevent loss of neurotransmitters. Processes of some of the fibroblasts also extend into the tunica media. (4) The tunica media is composed of an outer circular layer of typical smooth muscle cells, and an inner longitudinally running plica of ramified smooth muscle cells. Wide intercellular spaces between these ramified cells are filled with collagen fibrils, microfibrils, amorphous intercellular substances, and fibroblasts. Fibroblasts form close membrane contacts with each other, and with the smooth muscle cells. (5) Fibroblasts and other connective tissue components may function as an elastic support during active motility of the anastomotic channel.     

Intercellular bridges in ovaries of the newborn gerbil

This study describes intercellular bridges in the ovaries of neonatal gerbils. Electron microscopy has revealed the presence of true intercellular bridges, connecting oogonia or oocytes, in ovaries of newborn gerbils. The cytoplasm of the intercellular channels is similar to that of the connected cells, with mitochondria, smooth and rough endoplasmic reticulum, and free ribosomes present. Lysosomes are also occasionally present in the intercellular bridges and they may be involved in early waves of oocyte atresia. An electron-dense substance, 350-500 A thick, is located immediately beneath the unit membrane of the intercellular bridges. Accumulation of electron-dense material increases the thickness of the walls of the intercellular bridges, supporting and maintaining the patency of the channels. It is suggested that the intercellular channels probably allow the interchange of nutrients, organelles, and possibly regulatory materials as well.     

The Occurrence of Intercellular Bridges in Groups of Cells Exhibiting Synchronous Differentiation

A previous electron microscopic study of the cat testis revealed that spermatids derived from the same spermatogonium are joined together by intercellular bridges. The present paper records the observation of similar connections between spermatocytes and between spermatids in Hydra, fruit-fly, opossum, pigeon, rat, hamster, guinea pig, rabbit, monkey, and man. In view of these findings, it is considered likely that a syncytial relationship within groups of developing male germ cells is of general occurrence and is probably responsible for their synchronous differentiation. When clusters of spermatids, freshly isolated from the germinal epithelium are observed by phase contrast microscopy, the constrictions between the cellular units of the syncytium disappear and the whole group coalesces into a spherical multinucleate mass. The significance of this observation in relation to the occurrence of abnormal spermatozoa in semen and the prevalence of multinucleate giant cells in pathological testes is discussed. In the ectoderm of Hydra, the clusters of cnidoblasts that arise from proliferation of interstitial cells are also connected by intercellular bridges. The development of nematocysts within these groups of conjoined cells is precisely synchronized. Both in the testis of vertebrates and the ectoderm of Hydra, a syncytium results from incomplete cytokinesis in the proliferation of relatively undifferentiated cells. The intercellular bridges between daughter cells are formed when the cleavage furrow encounters the spindle remnant and is arrested by it. The subsequent dissolution of the spindle filaments establishes free communication between the cells. The discovery of intercellular bridges in the two unrelated tissues discussed here suggests that a similar syncytial relationship may be found elsewhere in nature where groups of cells of common origin differentiate synchronously.     

The occurrence of intercellular bridges in groups of cells exhibiting synchronous differentiation

A previous electron microscopic study of the cat testis revealed that spermatids derived from the same spermatogonium are joined together by intercellular bridges. The present paper records the observation of similar connections between spermatocytes and between spermatids in Hydra, fruit-fly, opossum, pigeon, rat, hamster, guinea pig, rabbit, monkey, and man. In view of these findings, it is considered likely that a syncytial relationship within groups of developing male germ cells is of general occurrence and is probably responsible for their synchronous differentiation. When clusters of spermatids, freshly isolated from the germinal epithelium are observed by phase contrast microscopy, the constrictions between the cellular units of the syncytium disappear and the whole group coalesces into a spherical multinucleate mass. The significance of this observation in relation to the occurrence of abnormal spermatozoa in semen and the prevalence of multinucleate giant cells in pathological testes is discussed. In the ectoderm of Hydra, the clusters of cnidoblasts that arise from proliferation of interstitial cells are also connected by intercellular bridges. The development of nematocysts within these groups of conjoined cells is precisely synchronized. Both in the testis of vertebrates and the ectoderm of Hydra, a syncytium results from incomplete cytokinesis in the proliferation of relatively undifferentiated cells. The intercellular bridges between daughter cells are formed when the cleavage furrow encounters the spindle remnant and is arrested by it. The subsequent dissolution of the spindle filaments establishes free communication between the cells. The discovery of intercellular bridges in the two unrelated tissues discussed here suggests that a similar syncytial relationship may be found elsewhere in nature where groups of cells of common origin differentiate synchronously.     

THE ULTRASTRUCTURE AND DISPOSITION OF VESICULATED NERVE PROCESSES IN SMOOTH MUSCLE

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

Intercellular Attachment in the Epithelium of Hydra As Revealed by Electron Microscopy

In Hydra adjacent epithelial cells are bound firmly to each other by desmosomes of a type not described in detail hitherto. The most prominent feature of these desmosomes is the presence of a series of parallel lamellae which bridge the intercellular space and connect the two apposed cell surfaces directly. These structures, here termed intercellular attachment lamellae, display two peaks of density about 50 A apart. These dense lines appear in some instances to be continuous with the outer dense components of the plasma unit membranes of the attached cells. The presence of prominent lamellae in intercellular attachments is sufficiently distinctive to deserve special terminology; accordingly, the term septate desmosome is proposed. It is noted that septate desmosomes may have been seen in other animals in instances where published electron micrographs show cross-striations or prominent connections in regions of intercellular attachment. It is suggested that septate desmosomes in Hydra, in addition to binding cells firmly to each other, form barriers to the movement of water into intercellular spaces and thus help to protect the organism's internal environment. Observations on the use of phosphotungstic acid for improving contrast in materials embedded in epoxy resins are also recorded.     

Tubular Bridges for Bronchial Epithelial Cell Migration and Communication

Background

Biological processes from embryogenesis to tumorigenesis rely on the coordinated coalescence of cells and synchronized cell-to-cell communication. Intercellular signaling enables cell masses to communicate through endocrine pathways at a distance or by direct contact over shorter dimensions. Cellular bridges, the longest direct connections between cells, facilitate transfer of cellular signals and components over hundreds of microns in vitro and in vivo.

Methodology/Principal Findings

Using various cellular imaging techniques on human tissue cultures, we identified two types of tubular, bronchial epithelial (EP) connections, up to a millimeter in length, designated EP bridges. Structurally distinct from other cellular connections, the first type of EP bridge may mediate transport of cellular material between cells, while the second type of EP bridge is functionally distinct from all other cellular connections by mediating migration of epithelial cells between EP masses. Morphological and biochemical interactions with other cell types differentially regulated the nuclear factor-κB and cyclooxygenase inflammatory pathways, resulting in increased levels of inflammatory molecules that impeded EP bridge formation. Pharmacologic inhibition of these inflammatory pathways caused increased morphological and mobility changes stimulating the biogenesis of EP bridges, in part through the upregulation of reactive oxygen species pathways.

Conclusions/Significance

EP bridge formation appears to be a normal response of EP physiology in vitro, which is differentially inhibited by inflammatory cellular pathways depending upon the morphological and biochemical interactions between EP cells and other cell types. These tubular EP conduits may represent an ultra long-range form of direct intercellular communication and a completely new mechanism of tissue-mediated cell migration.     

Gonads in Histiostoma mites (Acariformes: Astigmata): Structure and development

The development of male and female gonads in arrhenotokous and thelytokous species of Histiostoma was studied using transmission electron microscopy (TEM). All instars were examined: larvae, protonymphs, facultative heteromorphic deutonymphs (=hypopi), tritonymphs, and adults. In testis primordium, spermatogonia surrounding a testicular central cell (TCC) with a gradually enlarging, branched nucleus are present already at the larval stage. Spermatogonia and the TCC are connected via narrow, tubular intercellular bridges revealing that the TCC is a germline cell. Spermatocytes appear at the protonymphal stage. At the heteromorphic deutonymph stage, the testis primordium is similar to that of the protonymph, but in the tritonymph it is much larger and composed as in the adult: spermatids as well as sperm cells are present. The latter are congregated ventrally in the testis at the entrance of the deferent duct.In the larval ovary, an eccentrically located ovarian nutritive cell (ONC) is surrounded by oogonia which are connected with the ONC via tubular intercellular bridges. In later stages, the ovary grows and oocytes appear in the protonymph. Meiotic synaptonemal complexes in oocytes occur from the tritonymph stage. At about the time of the final molting, tubular intercellular bridges transform into peculiar diaphragm-crossed bridges known only in Histiostoma mites. In the adult female, growing oocytes at the end of previtellogenesis lose intercellular bridges and move ventro-laterally to the ovarian periphery towards the oviduct entrance. Vitellogenesis occurs in oviducts.Germinal cells in both the testis and ovary are embedded in a few somatic stroma cells which may be well discernible already in the larval ovary; in the testis, somatic stroma cells are evident not earlier than the end of the tritonymphal stage. The ovary has a thin wall of flat somatic cells, whereas the testis is covered by a basal lamina only.The obtained results suggest that gonads in Histiostoma and other Astigmata originate from two primordial cells only.     

The Protoplasmic Connection in Volvox

SYNOPSIS. Electron-microscopic observations were performed on 2 species of Volvox , one similar to V. globator , the other to V. aureus. The former has distinct protoplasmic connections in the adult coenobium and specific structures, named "medial bodies," in the connections just at the intersection with the middle lamella. The medial body is disk shaped, about 800 mμ in diameter, and is composed of 3 parts, 2 dense outer layers and an intermediate less dense zone. In the latter species, the connection and medial body were not seen. On the other hand, it was commonly seen in both of them that in younger, dividing gonidia neighboring protoplasts were connected with each other by protoplasmic bridges. The bridges are undoubtedly formed due to incomplete cell separation in the division of a gonidium. The structural difference in the adult coen***bium between the 2 species emerges just after inversion of the coenobium. In the globator type the medial body appears just after inversion, and the connection remains unruptured all thru life. In the aureus type, it seems that the connections are withdrawn or degenerate immediately after inversion. It is discussed whether protoplasmic continuity is really maintained by the connection or not in the freeswimming coenobium of Volvox.     

A freeze fracture and thin section study of intestinal cell membranes and intercellular junctions of a nematode, Ascaris

The microvillar and lumenal plasma membrane P-face of Ascaris intestinal cells is shown to be covered by relatively large (13 nm) particles at a fairly high density (1000/μm²), while the E-face has virtually none. The P-face of the lateral cell membranes, those separating the cells, have fewer and smaller (8 nm) particles. The intestinal cells are also shown to be connected by an apical complex of smooth septate and tricellular junctions similar to those found between some insect midgut cells. A periodic layer of tannic acid staining material is found on the cytoplasmic sides of the smooth septate junction, and when the intercellular space is filled with lanthanum, smoothly curved, 10 nm wide septal walls can be seen. Below the belt of septate junctions are a large number of gap junctions. These have closely packed arrays of particles on the P-face with some particle aggregates adhering to the closely packed pit arrays on the E-face.     

Ultrastructural observations of the tunica muscularis in the small intestine of Xenopus laevis, with special reference to the interstitial cells of Cajal

The distribution and ultrastructure of the interstitial cells of Cajal (ICC) has been examined in the small intestine of the frog Xenopus laevis, as the physiological significance of these cells remains obscure in amphibians and other lower vertebrates. The present study has revealed the existence of a special type of interstitial cell in the tunica muscularis of the small intestine of Xenopus; this cell is characterized by the presence of numerous caveolae, many small mitochondria, and the formation of intercellular connections with the same type of cell. Since these ultrastructural features are shared with mammalian ICC, the cells in the small intestine of Xenopus probably correspond to ICC. These cells also form close contacts with neighboring smooth muscle cells and with nerve varicosities containing accumulations of synaptic vesicles. These cellular networks are likely to be involved in the transmission of nerve impulses to muscle cells, as has been suggested for mammalian tissues. However, true gap junctions have not been detected; they occur neither between the same type of cells nor between the putative ICC and smooth muscle cells. The widespread distribution of ICC or equivalent cells in different groups of vertebrates, together with the conservation of their ultrastructural features, suggests that they differentiated early in vertebrate evolution to play key regulatory roles in gastrointestinal movement.     

Two distinct steps for spontaneous generation of subprotoplasts from a disintegrated bryopsis cell

The unusual nature of protoplasm to generate subprotoplasts spontaneously from disintegrated Bryopsis cells was examined. Protoplasm extruded from algal cells aggregated rapidly in cell sap which was derived mainly from huge central vacuoles of the cells. Electron microscopic observations revealed extensive agglutination of algal cellular membranes in the protoplasmic masses, suggesting that this is of primary importance for the wound-healing ability of the alga. Seawater caused spheration of the resultant protoplasmic aggregates. Gelatinous sheaths were formed temporarily surrounding the spherical protoplasmic masses before reformation of cell membrane. Staining with phosphotungstic and chromic acids suggested that new cell membrane was formed by fusion of the disintegrated original cell membrane with cytoplasmic vesicles on the surfaces of the protoplasmic masses. Both pH and salts were found to be essentially important at the two steps of subprotoplast generation. The newly formed cell membranes were responsible for subsequent notable plasmolysis of the wounded cells in seawater. Thus, it is suggested that unicellular marine algae Bryopsis spp. naturally contain effective materials for agglutinating and fusing particular cellular membranes through the sequential aid of acidic cell sap and alkaline seawater after disintegration of the giant cells.     

Stable intercellular bridges in development: the cytoskeleton lining the tunnel

A wide variety of intercellular junctions that are involved with cell adhesion or signal transduction have been described in recent years. A widespread but less well-characterized type of intercellular junction is the stable intercellular bridge. Several organisms use stable intercellular bridges as cytoplasmic connections, probably to allow rapid transfer of information and organelles between cells. Here, the authors take a detailed look at the assembly of intercellular bridges called ring canals in the Drosophila germline and discuss how examination of mutants that disrupt Drosophila ovarian ring canal assembly indicates that these bridges are required for intercellular transport of cytoplasm.     

Cloning of a gap junctional protein from vascular smooth muscle and expression in two-cell mouse embryos.

Gap junctional proteins (connexins) form aqueous channels that enable direct cell-cell transfer of ions and small molecules. The distribution and conductance of gap junction channels in cardiac muscle determine the pattern and synchrony of cellular activation. However, the capacity for smooth muscle to restrict contractile events temporally and spatially suggests that cell-cell coupling or its regulation may be decidedly different in this tissue. We isolated a cDNA from vascular smooth muscle which encodes a connexin (Mr 43,187) structurally homologous to cardiac connexin43. Vascular smooth muscle connexin43 mRNA was expressed prominently in smooth muscle tissues, cultured vascular myocytes, and arterial endothelial cells. A model for functional expression of connexins was developed in two-cell B6D2 mouse embryos. Microinjection of in vitro transcribed vascular smooth muscle connexin43 mRNA was shown to be sufficient to induce intercellular coupling in previously uncoupled blastomeres. Through the construction of two deletion mutants of connexin43, we also show that the formation of cell-to-cell connections does not depend upon a predicted cytoplasmic region within 98 residues of the carboxyl terminus. Finally, the identification of connexin43 in smooth muscle and endothelial cells provides supporting evidence for the existence of heterocellular coupling between cells of the vascular intima.     

葫芦藓精子发生过程中胞间连丝与胞质桥的超微结构

从超微结构水平上对葫芦藓(Funaria hygrometrica Hedw.)精子发生过程中胞间连接系统的结构及其变化动态进行了研究.结果表明,同一区中的相邻生精细胞由大量胞质桥相连,而不同区的细胞之间则不存在胞质桥.胞间连丝存在于套细胞之间以及套细胞与生精细胞之间,但它在生精细胞间不存在.在精子器发生的后期,当精子细胞壁开始降解时,同一个精子器中所有的精子细胞似乎都由扩大的胞质桥相互连接.胞质桥一直保持到精子分化的后期,最终精子细胞同步分化成精子.胞间连丝与胞质桥具有不同的内部结、分布以及生物发生机制,这表明它们在精子器的发育过程中可能扮演着不同的角色.     

Intercellular bridges in vertebrate gastrulation

The developing zebrafish embryo has been the subject of many studies of regional patterning, stereotypical cell movements and changes in cell shape. To better study the morphological features of cells during gastrulation, we generated mosaic embryos expressing membrane attached Dendra2 to highlight cellular boundaries. We find that intercellular bridges join a significant fraction of epiblast cells in the zebrafish embryo, reaching several cell diameters in length and spanning across different regions of the developing embryos. These intercellular bridges are distinct from the cellular protrusions previously reported as extending from hypoblast cells (1-2 cellular diameters in length) or epiblast cells (which were shorter). Most of the intercellular bridges were formed at pre-gastrula stages by the daughters of a dividing cell maintaining a membrane tether as they move apart after mitosis. These intercellular bridges persist during gastrulation and can mediate the transfer of proteins between distant cells. These findings reveal a surprising feature of the cellular landscape in zebrafish embryos and open new possibilities for cell-cell communication during gastrulation, with implications for modeling, cellular mechanics, and morphogenetic signaling.     

Differentiation of primordial germ cells during embryogenesis of Allacma fusca (L.) (Collembola: Symphypleona)

The youngest primordial germ cells (PGCs) of Allacma fusca (L.) (Collembola: Sminthuridae) can be identified in embryos at the blastoderm stage as scattered in the yolk mass. They are arranged in pairs connected via intercellular bridges and dispersed among the yolk granules over a relatively small area but they never form multicellular clusters. With progressing development, the mesoderm of the germ band differentiates, the PGCs migrate to the abdominal part of the germ band and enter among mesoderm cells making two clusters of cells in the left and right parts of the abdomen. The mesoderm cells neighbouring the PGC cluster differentiate into a one-layered gonad envelope and produce a thin basal lamina separating the gonad from the rest of the mesoderm. The PGCs are still connected in pairs. At the end of the embryonic development, the gonads have regular spherical shapes and are enclosed within the envelope built up by a layer of flat somatic cells. Now, the PGCs do not occur only in pairs, but chains of cells connected with a sequence of intercellular bridges can also be seen.     

Direct evidence for cholesterol crystalline domains in biological membranes: role in human pathobiology

This review will discuss the use of small-angle X-ray diffraction approaches to study the organization of lipids in plasma membranes derived from two distinct mammalian cell types: arterial smooth muscle cells and ocular lens fiber cells. These studies indicate that cholesterol at an elevated concentration can self-associate and form immiscible domains in the plasma membrane, a phenomenon that contributes to both physiologic and pathologic cellular processes, depending on tissue source. In plasma membrane samples isolated from atherosclerotic smooth muscle cells, the formation of sterol-rich domains is associated with loss of normal cell function, including ion transport activity and control of cell replication. Analysis of meridional diffraction patterns from intact and reconstituted plasma membrane samples indicates the presence of an immiscible cholesterol domain with a unit cell periodicity of 34 Å, consistent with a cholesterol monohydrate tail-to-tail bilayer, under disease conditions. These cholesterol domains were observed in smooth muscle cells enriched with cholesterol in vitro as well as from cells obtained ex vivo from an animal model of atherosclerosis. By contrast, well-defined cholesterol domains appear to be essential to the normal physiology of fiber cell plasma membranes of the human ocular lens. The organization of cholesterol into separate domains underlies the role of lens fiber cell plasma membranes in maintaining lens transparency. These domains may also interfere with cataractogenic aggregation of soluble lens proteins at the membrane surface. Taken together, these analyses provide examples of both physiologic and pathologic roles that sterol-rich domains may have in mammalian plasma membranes. These findings support a model of the membrane in which cholesterol aggregates into structurally distinct regions that regulate the function of the cell membrane.