Patterns of organelle distribution in mouse embryos during preimplantation development

We have evaluated the distribution of mitochondria and acidic organelles using, respectively, the specific vital fluorescent dyes rhodamine 123 and acridine orange during preimplantation embryonic development in the mouse. Under conditions used to visualize organelles in living embryos, staining with either dye was found to have no effect on either the rate or extent of in vitro development of five- to eight-cell embryos up to the blastocyst stage. Mitochondria were randomly distributed throughout the cytoplasm and located around nuclei in blastomeres of uncompacted embryos. During compaction, mitochondria initially reorganized to the blastomere cortex; however, these organelles were later confined to the perinuclear region in the trophectoderm (TE) of expanded blastocysts. Acidic organelles were randomly distributed in the cytoplasm of uncompacted embryos, but following compaction, they were concentrated in cortical and perinuclear locations. Moreover, in TE cells of expanded blastocysts, acidic organelles were found exclusively in a tight perinuclear pattern. Microtubules and microfilaments in TE cells were localized in fixed embryos stained with antitubulin antibodies and rhodamine phalloidin, respectively; these structures were found primarily in the cortical cytoplasm at areas of cell-cell contact and secondarily in a perinuclear location. Thus mitochondria and acidic organelles undergo stage-specific redistributions from a diffuse or cortical pattern at the eight-cell stage to a tight perinuclear localization in the TE. We conclude that the polarized distributions of some organelles and cytoskeletal proteins during compaction may not be reliable permanent markers of the mature TE.

Binding and internalization of heparin by vascular smooth muscle cells

Previous work from our laboratory has demonstrated that heparin specifically inhibits the proliferation of vascular smooth muscle cells in vivo and in vitro. In this paper, we examine the binding and mode of internalization of heparin by smooth muscle cells. For these studies, radiolabeled and fluoresceinated (FITC) heparin probes were synthesized that retained their antiproliferative capacity. Binding of 3H-heparin to these cells occurs via specific, high-affinity binding sites (Kd = 10(-9) M, 100,000 binding sites per cell). Approximately 80% of the heparin bound to the cell surface was shed into the culture medium within 2 hr. The heparin that was left on the cell surface was internalized with biphasic kinetics. Approximately 50% of the bound material was internalized within 2 hr. After this initial rapid uptake, the rate slowed substantially, with the remaining heparin requiring 1-2 days to be internalized. Binding and uptake of FITC heparin was monitored using video image intensification fluorescence microscopy. When smooth muscle cells were exposed to FITC heparin at 4 degrees C, a diffuse surface staining pattern was observed. After warming the cells to 37 degrees C, intensely fluorescent vesicles were seen superimposed over the diffuse surface staining within 2 min. After 15 min at 37 degrees C, numerous large punctate vesicles were seen inside the cell. After 2 hr these vesicles had concentrated in the perinuclear region. This pattern of uptake, when considered along with the presence of specific, high-affinity binding sites and the initial rapid uptake of 3H-heparin, suggests that heparin enters smooth muscle cells by both receptor-mediated and other endocytic pathways.

Cytoplasmic motions, rheology, and structure probed by a novel magnetic particle method

The motions of magnetic particles contained within organelles of living cells were followed by measuring magnetic fields generated by the particles. The alignment of particles was sensed magnetometrically and was manipulated by external fields, allowing non-invasive detection of particle motion as well as examination of cytoplasmic viscoelasticity. Motility and rheology data are presented for pulmonary macrophages isolated from lungs of hamsters 1 d after the animals had breathed airborne gamma-Fe2O3 particles. The magnetic directions of particles within phagosomes and secondary lysosomes were aligned, and the weak magnetic field produced by the particles was recorded. For dead cells, this remanent field was constant, but for viable macrophages, the remanent field decreased rapidly so that only 42% of its initial magnitude remained 5 min after alignment. A twisting field was applied perpendicular to the direction of alignment and the rate at which particles reoriented to this new direction was followed. The same twisting was repeated for particles suspended in a series of viscosity standards. Based on this approach, the low-shear apparent intracellular viscosity was estimated to be 1.2-2.7 X 10(3) Pa.s (1.2-2.7 X 10(4) poise). Time-lapse video microscopy confirmed the alignment of ingested particles upon magnetization and showed persistent cellular motility during randomization of alignment. Cytochalasin D and low temperature both reduced cytoplasmic activity and remanent-field decay, but affected rheology differently. Magnetic particles were observed in association with the microtubule organizing center by immunofluorescence microscopy; magnetization did not affect microtubule distribution. However, both vimentin intermediate filaments and f-actin reorganized after magnetization. These data demonstrate that magnetometry of isolated phagocytic cells can probe organelle movements, rheology, and physical properties of the cytoskeleton in living cells.

A time-lapse video image intensification analysis of cytoplasmic organelle movements during endosome translocation

Vital fluorescence staining has been used in conjunction with time-lapse video image intensification microscopy to analyze the distribution and movement of endosomes, lysosomes, and mitochondria in cultured rat ovarian granulosa cells. Exposure of 5-d granulosa cell cultures to pyrene-concanavalin A (P-Con A) or 3,3'-dioctadecylindocarbocyanine-labeled low-density lipoprotein (dil-LDL) at 4 degrees C results in the formation of randomly distributed endosomes 10 min after warming to 37 degrees C that exhibit saltatory motion for 20 min. If granulosa cells are labeled at 4 degrees C with both P-Con A and dil-LDL and warmed to 37 degrees C, both ligands are found within the same endosomes which migrate centripetally to the cell center where label accumulates within phase-dense structures by 60 min. The initial endosome saltations occur over short distances (mean distance = 4.6 micron) with a mean velocity of 0.03 micron/s. Endosome saltations then cease and are followed by a gradual centriptal migration of endosomes to the cell center where they accumulate and fuse with phase-dense structures. The second phase of movement involves a continuous, unidirectional migration of endosomes over distances ranging from 5 to 40 micron at a mean velocity of 0.05 micron/s. Lysosomes were simultaneously visualized as acridine orange-staining, phase-dense structures in control cells and cells exposed to fluorescent ligands. In untreated cells, lysosomes are dispersed throughout the cytoplasm and undergo bidirectional saltations covering a mean distance of 8.7 micron with a mean velocity of 0.3 micron/s. Lysosomes redistribute centripetally to the perinuclear region of the cell by saltatory movement within 20 min of exposure to ligand. Mitochondria were visualized with the fluorescent dye rhodamine 123 in granulosa cells labeled with P-Con A and were found to redistribute to the cell center coincident with endosomes. The microtubule-disrupting agent nocodazole was found to inhibit lysosome saltations and all phases of endosome movement. Taxol, a microtubule-stabilizing agent, partially impaired lysosome movement and led to a redistribution of lysosomes into linear aggregates surrounding the nucleus. Taxol was also found to inhibit endosome movement. The data indicate that (a) endosome movement proceeds initially by saltation and later by a nonsaltatory centripetal migration in association with mitochondria, that (b) lysosomes and endosomes undergo a temporally distinct but spatially similar change in cytoplasmic distribution, and that (c) microtubules are required for the directed translocation of endosomes and lysosomes towards the cell center.

Microtubule regulation of cell surface receptor topography during granulosa cell differentiation

A possible role for cytoplasmic microtubules in modulating lectin binding site topography has been examined during the hormone-directed differentiation of rat ovarian granulosa cells in vitro. Indirect immunofluorescence staining with anti-tubulin antibodies indicates that undifferentiated cultured granulosa cells contain a network of microtubules which radiate from the cell center to the cell periphery. Cultures induced to differentiate by a three day treatment with 1 microgram/ml prolactin exhibit a marginal distribution of microtubules and a centrally-located primary cilium. Prolactin enhances the incidence of granulosa cells containing a primary cilium from 9% in undifferentiated cultures to 53% in hormone-treated cultures. The pattern of lectin binding site redistribution induced by Concanavalin A (Con A) is also modified by prolactin treatment. In contrast to undifferentiated cells, which randomly endocytose fluorescein Con A, granulosa cells exposed to prolactin respond to fluorescein Con A by forming central surface caps to a greater extent (75%) than undifferentiated controls (25%). Double label fluorescence microscopy and transmission electron microscopy on Con A labeled cells show that caps form at central cell surface sites which contain the primary cilium. Disruption of cytoplasmic microtubules by colchicine, in undifferentiated granulosa cells, results in the formation of cell surface caps upon Con A addition. These data suggest that cytoplasmic microtubules modulate the topography of lectin bindings sites which is subject to hormonal control during the in vitro differentiation of ovarian granulosa cells.

Novel morphological approaches for the study of oocyte maturation

Recent advances in light microscopy are discussed with respect to their application for the study of cell surface, cytoskeletal and organellar changes that occur during meiotic maturation in mammalian oocytes. Three techniques are considered: 1) multiple fluorochrome labeling using immunocytochemical or pharmacological probes to analyze the spatial and temporal disposition of components in oocytes fixed at various stages of meiosis, 2) the use of vital fluorescence stains to study the actual movement of organelles in living oocytes, and 3) video image intensification microscopy of living cells to record dynamic cellular changes with enhanced optical capabilities for fluorescence, polarization and differential interference contrast microscopy. A method is described for simultaneously localizing chromosomes, microtubules and f-actin in fixed rodent oocytes using, respectively, Hoechst 33258, antitubulin antibodies and NBD-phallicidin. Acridine orange and the laser dye rhodamine 123 are employed as vital stains to visualize lysosomes and mitochondria, respectively, in rat oocytes undergoing meiotic maturation. Finally, the application of time-lapse video image intensification microscopy for the study of fluorescently labeled cellular components is discussed with special reference to extended monitoring of cellular organelles for the analysis of dynamic movements of oocyte constituents during maturation.

In vivo and in vitro studies on the role of HMW-MAPs in taxol-induced microtubule bundling

The involvement of high molecular weight microtubule-associated proteins (HMW-MAPs) in the process of taxol-induced microtubule bundling has been studied using immunofluorescence and electron microscopy. Immunofluorescence microscopy shows that HMW-MAPs are released from microtubules in granulosa cells which have been extracted in a Triton X-100 microtubule-stabilizing buffer (T-MTSB), unless the cells are pretreated with taxol. 1.0 microM taxol treatment for 48 h results in microtubule bundle formation and the retention of HMW-MAPs in these cells upon extraction with T-MTSB. Electron microscopy demonstrates that microtubules in control cytoskeletons are devoid of surface structures whereas the microtubules in taxol-treated cytoskeletons are decorated by globular particles of a mean diameter of 19.5 nm. The assembly of 3 X cycled whole microtubule protein (tubulin plus associated proteins) in vitro in the presence of 1.0 microM taxol, results in the formation of closely packed microtubules decorated with irregularly spaced globular particles, similar in size to those observed in cytoskeletons of taxol-treated granulosa cells. Microtubules assembled in vitro in the absence of taxol display prominent filamentous extensions from the microtubule surface and center-to-center spacings greater than that observed for microtubules assembled in the presence of taxol. Brain microtubule protein was purified into 6 s and HMW-MAP-enriched fractions, and the effects of taxol on the assembly and morphology of these fractions, separately or in combination, were examined. Microtubules assembled from 6 s tubulin alone or 6 s tubulin plus taxol (without HMW-MAPs) were short, free structures whereas those formed in the presence of taxol from 6 s tubulin and a HMW-MAP-enriched fraction were extensively crosslinked into aggregates. These data suggest that taxol induces microtubule bundling by stabilizing the association of HMW-MAPs with the microtubule surface which promotes lateral aggregation.

Slow internalization of human chorionic gonadotropin by cultured granulosa cells

Kinetic studies were performed on two-day cultures of rat ovarian granulosa cells to follow the fate of surface-bound 125I-labeled human chorionic gonadotropin (125I-hCG). Low pH was used to release hCG from its surface receptor, allowing us to distinguish between surface-bound and internalized hormone. Because our results indicated that hormone is lost from the cell surface by dissociation as well as internalization, equations were derived to determine independent rate constants for each process. We calculate that if hormone binding were irreversible, the t 1/2 for internalization would be 8.5 hour. Morphometric studies on the uptake of horseradish peroxidase indicate that the t 1/2 for internalization of bulk membrane in granulosa cells is 55 to 77 minutes. Thus, the rate of uptake of surface-bound hCG appears to be seven to nine times slower than the rate of uptake of bulk plasma membrane, which suggests that the LH/hCG receptor may be selectively excluded from the endocytic vesicles of granulosa cells.

Ligand-induced rapid redistribution of lysosomes is temporally distinct from endosome translocation

When many ligands bind to cell-surface receptors, ligand-receptor complexes are internalized via clathrin coated pits by a process called receptor-mediated endocytosis. The cytoplasmic fate of ligands internalized within endocytic vesicles or endosomes is variable. For example, maternal immunoglobulins are transported through the cytoplasm of neonatal intestinal epithelial cells and are exocytosed at the basolateral surface. However, other ligands are degraded as a result of their delivery to the lysosomal compartment of cells. Although the translocation of endosomes to the Golgi region in the cell centre seems to be a general phenomenon presumably coupled to ligand degradation by lysosomes and endosomes and lysosomes undergo saltatory movements within the cytoplasm, the spatial control of interaction between the two structures is not understood. To address this problem we have begun to examine the spatial and temporal intracellular distribution of endosomes and lysosomes. Utilizing a new fluorescent microscopic approach, we have now been able simultaneously to visualize endosome and lysosome populations in living cells. Our results suggest that a specific relocation of lysosomes is rapidly induced upon binding of different types of ligands to the cell surface; this migration of lysosomes to the Golgi region of the cell precedes the translocation of endosomes into the same area.

The effects of taxol on the organization of the cytoskeleton in cultured ovarian granulosa cells

Exposure of ovarian granulosa cells to taxol, a potent microtubule assembly promoting and stabilization agent, results in a time and dose-dependent alteration in the organization of cytoplasmic filaments (microtubules, microfilaments, intermediate filaments) and organelles. Transmission electron microscopy and fluorescence microscopy are used in conjunction with various antimitotic drugs to evaluate the action of taxol on suspension and monolayer cultures. Taxol treatment (1.0 microM) of freshly isolated suspended cells for 4 h leads to the formation of multiple bundles of microtubules emanating from a single centrosomal organizing center. Within 4 h after addition to monolayer cultures, 1.0 microM taxol induces a lateral aggregation of microtubules which, by 12 to 24 h of treatment, results in the appearance of dense bundles of tightly-packed microtubules located near the centrosome, close to the outer nuclear envelope, and at peripheral cytoplasmic sites. By electron microscopy, both lateral and end-on associations between bundled microtubules and the nuclear envelope are apparent. During the course of taxol-induced bundling, intermediate filaments are altered from a normally dispersed fibrous network into perinuclear aggregates. Microtubule bundling is also associated with a rearrangement of actin filaments from stress fibers into a marginal distribution, and actin appears to be excluded from sites of bundle formation. When granulosa cells are treated with equivalent concentrations of the microtubule-disrupting drugs, colchicine or nocodazole, either before or during taxol treatment, bundle formation is prevented suggesting that an intact microtubule network is required for taxol-induced bundling. Colchicine, but not nocodazole, is able to reverse the effects of taxol on bundle formation. The data suggest that the cellular distribution of cytoplasmic organelles, intermediate filaments, and microfilaments are regulated by the cytoplasmic microtubule complex.

The intracellular movement of endocytic vesicles in cultured granulosa cells

Ligand binding to cell surface receptors induces rapid internalization of ligand-receptor complexes by receptor mediated endocytosis. We have examined the intracellular movement of endocytic vesicles, induced by the lectin concanavalin A (Con A), in cultured rat ovarian granulosa cells using fluorescence and electron microscopy. Within 20 minutes of ligand treatment at 37 degrees C, numerous Con A-containing endocytic vesicles form, which migrate to the cell center by 60 minutes. Double label fluorescence microscopy, using fluorescein-Con-A and rhodamine immunofluorescent staining of tubulin or vimentin, indicates that during vesicle migration microtubules and 10-nm filaments are altered in their organization. By 30 minutes, vesicles are associated with microtubule bundles, which subsequently collapse around the nucleus. Similarly, 10-nm filaments accumulate around the nucleus in conjunction with the perinuclear aggregation of endocytic vesicles. Electron microscopy of Con A-horseradish peroxidase-labeled cells demonstrates that endocytic vesicles fuse to form large receptosome-like structures during intracellular migration and these structures are associated with cytoplasmic microtubules and 10-nm filaments. Taxol, a drug that stabilizes microtubules, prevents endocytic vesicle translocation to the Golgi region. Nocodazole, which causes microtubule disassembly, results in the collapse of 10-nm filaments and the central aggregation of endocytic vesicles. The data indicate that the cytoskeleton participates in the directed intracellular movement of endocytic vesicles; the possible subcellular basis for this movement is discussed.

Visualization of assembled and disassembled microtubule protein by double label fluorescence microscopy

Indirect immunofluorescence with rhodamine labelled antibodies and fluoresceinated colchicine (FC) are used to simultaneously localize microtubules and soluble tubulin in cultured ovarian granulosa cells. FC labelled tubulin is most concentrated in regions of the cell occupied by antitubulin stained microtubule bundles. Pretreatment of granulosa cells with colchicine results in a central accumulation of FC and antibody labelled tubulin that coincides with the disposition of 10-nm filament cables. In contrast, the microtubule disrupting agent nocodazole produces a diffuse tubulin distribution as detected with both FC and antibody probes. Taxol treatment, which enhances microtubule assembly, results in a striking concentration of microtubule bundles associated with the nucleus that avidly bind FC. These results suggest that disassembled tubulin is preferentially associated with cytoplasmic microtubules and possibly other formed elements of the cytoskeleton.

Isolation and biochemical characterization of ten-nanometer filaments from cultured ovarian granulosa cells

Ten-nm filaments have been isolated from control and colchicine-treated primary cultures of rat ovarian granulosa cells. Negative stain electron microscopy indicates an average filament diameter of 10.3 nm in the isolated fiber bundles, which, upon sodium dodecyl sulfate polyacrylamide gel electrophoresis, are found to contain a major polypeptide with a molecular weight of 57,000 and several minor components including actin. One-dimensional peptide mapping and two-dimensional gel electrophoresis demonstrate similarity between the granulosa cell and baby hamster kidney cell 10-nm filament subunit protein, both of which are distinguishable from keratin, desmin, actin, and tubulin. Quantitative gel densitometry experiments demonstrate little difference in the total amount of the 10-nm filament protein in control cells or cells treated with colchicine, accounting for 12 or 15% of the total cellular protein, respectively. The purification procedure, which involves extraction in Triton X-100 and KCl followed by DNase I treatment, yields 709% of the total granulosa cell intermediate filament protein, and 70% of the newly synthesized 57,000 molecular weight component. Two-dimensional gel electrophoresis of cultures labeled with [32P]phosphate show by autoradiography that the 57,000-dalton polypeptide, actin, and a 130,000-dalton protein are the most readily phosphorylated polypeptides in granulosa cell cultures. These studies identify the major intermediate filament subunit protein of granulosa cells as a 57,000-dalton phosphorylatable polypeptide which comprises a substantial portion of the granulosa cell cytoskeleton.

Impaired microtubule assembly and polymorphonuclear leucocyte function in the Chediak-Higashi syndrome correctable by ascorbic acid

It was previously shown that the abnormal surface characteristics and defective bactericidal function of polymorphonuclear leucocytes (PMN) in the Chediak-Higashi syndrome (CHS) are correlated with impaired microtubule assembly, and in one patient direct electron microscopic evidence for an anomaly in microtubule assembly following surface membrane activation by concanavalin A (Con A). We show that very few microtubules are visible in CHS leucocytes from two additional patients under conditions where normal PMNs contain abundant microtubules, and that both in vivo and in vitro exposure of the CHS leucocytes to ascorbic acid promotes the assembly of microtubules. This agent, which normalizes chemotaxis and degranulation in CHS leucocytes, is shown also to correct granulocyte adherence in these leucocytes. It is suggested that the improved clinical course of patients with CHS following treatment with ascorbic acid is related at least in part to improvement of microtubule assembly and PMN function by the ascorbic acid.

Microtubule and microfilament rearrangements during capping of concanavalin A receptors on cultured ovarian granulosa cells

Thin-section electron microscope analysis of rat and rabbit-cultured granulosa cells treated with concanavalin A (Con A) at 37 degrees C revealed coordinated changes in the cytoplasmic disposition of microfilaments, thick filaments, and microtubules during cap formation and internalization of lectin-receptor complexes. Con A-receptor clustering is accompanied by an accumulation of subplasmalemmal microfilaments which assemble into a loosely woven ring as patches of receptor move centrally on the cell surface. Periodic densities appear in the microfilament ring which becomes reduced in diameter as patches coalesce to form a single central cap. Microtubules and thick filaments emerge associated with the capped membrane. Capping is followed by endocytosis of the con A-receptor complexes. During this process, the microfilament ring is displaced basally into the cytoplasm and endocytic vesicles are transported to the paranuclear Golgi complex along microtubules and thick filaments. Eventually, these vesicles aggregate near the cell center where they are embedded in a dense meshwork of thick filaments. Freeze-fracture analysis of Con A-capped granulosa cells revealed no alteration in the arrangement of peripheral intramembrane particles but large, smooth domains were conspicuous in the capped region of the plasma membrane. The data are discussed with reference to the participation of microtubules and microfilaments in the capping process.

Effects of glutathione-oxidizing agents on microtubule assembly and microtubule-dependent surface properties of human neutrophils

In human peripheral blood polymorphonuclear leukocytes and lymphocytes, GSH-oxidizing agents promote the movement of surface-bound concanavalin A (Con A) into caps and inhibit the assembly of microtubules (MT) that is normally induced by Con A binding. Con A capping and inhibition of MT assembly occur when GSH levels in cell suspensions are decreased by 30-70%, and return to GSH to control levels is accompanied by the appearance of cytoplasmic MT and by inhibition of the capping response with Con A. Oxidation of GSH markedly stimulates the hexose monophosphate shunt, and regeneration of GSH occurs rapidly. The data indicate that MT cannot be assembled or maintained in the face of decreased GSH levels. Thus, GSH homeostasis becomes critical during physiological events such as phagocytosis which simultaneously induce the assembly of MT and the production of agents like H2O2 that can oxidize GSH.

Gap junctions between the oocyte and companion follicle cells in the mammalian ovary

Tracer and freeze-fracture electron microscopy of the ovaries of neonatal rat and adult mouse, rat, rabbit, and primate have revealed the presence of gap junctions between follicle cells and oocytes. The junctional connections are found at the ends of follicle cell projections which traverse the zona pellucida and terminate upon microvilli and evenly contoured nonmicrovillar regions of the oolemma. Gap junctions are often seen associated with a macula adherens type of junction. The gap junctions occasionally consist of minute ovoid plaques, but nore frequently appear as rectilinear single- or multiple-row aggregates of particles on the P-face or pits on the E-face. The functional significance of follicle cell-oocyte gap junctions is discussed with respect to the regulation of meiosis and luteinization.

Cytological observations of the ovarian epithelium in mammals during the reproductive cycle

We have studied the ovarian epithelijm at various stages of the reproductive cycle in a number of mammalian species utilizing light microscopy, scanning microscopy, the freeze-fracture technique, transmission microscopy and by employing specialized tracers that use lanthanum and horseradish peroxidase. We found that the epithelial cells are joined by incomplete tight junctions, gap junctions, and desmosomes. The cytoplasmic matrix contains a large irregularly shaped nucleus, few microtubules, microfilaments, mitochondria, endoplasmic reticulum and a host of coated and non-coated vesicles of varying diameters. The saccules comprising the large Golgi complex and its companion vesicles are associated with a basal body-centriole complex: some of these saccules and affiliated vesicles are acid phosphatase positive. Surface modifications of ovarian epithelial cells include numerous microvilli, some of which have a bulbous tip, and plications of the lateral plasma membrane which are thought to accomodate volume changes of the ovary during follicular development. Many coated and non-coated endocytotic caveolae were found on these cells, particularly in the basal area. These caveolae internalized exogeneously administered horseradish peroxidase. We view the marked endocytotic activity as an efficient transport mechanism for partially removing substances from the interstitium of the ovary and the peritoneum.

Cytochalasin B-induced pseudo-cleavage of mouse oocytes in vitro: asymmetric localization of mitochondria and microvilli associated with a stage-specific response

Mouse oocytes are induced by cytochalasin B to undergo ‘pseudo-cleavage’ in vitro into 2 equally sized and separable compartments. This response to the drug is dependent upon the meiotic state of the oocytes, as well as upon the presence of an intact zona pellucida. The resulting 2 cellular compartments can be completely separated from another and cultured in vitro. Each of the compartments possesses characteristic structural features. The most pronounced structural differences include: (i) the presence of a nucleus (germinal vesicle) and nucleolus in one compartment; (ii) the presence of microvilli on the surface of the anucleate, but not the nucleate, compartment; and (iii) the localization (segregation) of mitochondria at the periphery of the anucleate, but not the nucleate, compartment. The results presented suggest that pseudo-cleavage induced by cytochalasin B arises as a consequence of a limited interaction of the drug with the oocyte surface and/or cortex and that it may represent a topographical dissociation of transporting and non-transporting regions of the membrane. These and other features of mouse oocytes treated with cytochalasin B are of interest in view of the involvement of the oocyte zona pellucida and plasma membrane during meiotic maturation, fertilization, and early embryogenesis.

Membrane-microtubule interactions: concanavalin A capping induced redistribution of cytoplasmic microtubules and colchicine binding proteins

The relationship between microtubules and concanavalin A surface receptors during concanavalin A capping in primary cultures of rabbit ovarian granulosa cells was examined by electron microscopic and fluorescence labeling techniques. Cells treated with concanavalin A and hemocyanin at 4 degree or 37 degree and then incubated at 37 degree for 1 hr formed large juxtanuclear caps that were observed with shadow cast replicas of the cell surface. Thin section analysis of capped cells revealed an abundance of microtubules immediately beneath the cap which were arranged approximately perpendicular to the plane of the membrane. The capping process was unaffected by the antimicrotubule agents colchicine or vinblastine. Further, vinblastine treatment of capped calls resulted in the formation of numerous paracrystals that were confined to the cytoplasm underlying the capped region of the membrane; uncapped cells displayed paracrystals that were randomly dispersed in the cytoplasm. Exposure of fixed cells to fluorescein thiocarbamyl colchicine, which localizes colchicine binding proteins, revealed an intensely fluorescent region that corresponded to the cap; this staining pattern was absent in uncapped cells. These findings indicate that concanavalin A mediated capping modifies the cytoplasmic disposition of microtubules and colchicine binding proteins. Further, it is suggested that the capped region of the plasma membrane is a preferred site of microtubule polymerization.

Structural modifications of lutein cell gap junctions during pregnancy in the rat and the mouse

By use of lanthanum tracer and freeze-fracture procedures it was found that granulosa-lutein cells of the pregnant mouse and rat ovaries are connected by gap junctions and septate-like zones of contact. Lutein cell gap junctions enlarge and become partially internalized by the end of the first week of gestation. Expansion of the gap junction domain appears to be due initially to intercalation of particles along borders of small gap junctions devoid of smaller non-junctional particles. The number of gap junction lined processes appearing at the cell border increases concomitantly with hypertrophy of the lutein cell during the second week of pregnancy. Strands of particulate or grooved membrane emanate from the margin of larger gap junctions undergoing interiorization. Most large gap junctions are intimately associated with elements of the smooth endoplasmic reticulum. Spherical gap junctional profiles assume a deeper location in the lutein cell and may form concentric arrays by term while true surface gap junctions appear to fragment in the post-partum corpus luteum. The modifications observed are interpreted with respect to biogenesis of the gap junction and the hormonal control of lutein cell function.

Morphological variations in gap junctions of ovarian granulosa cells

Granulosa cells in ovarian follicles of rat, mouse, rabbit and hamster were studied by lanthanum tracer and freeze-fracture techniques. Abundant gap junctions exhibited striking intraspecific variation in size and pattern of particle aggregation. The smaller gap junctions showed close packing of the intramembranous A face particles. In large gap junctions, ranging up to 6 mu in diameter, particles were packed in rectilinear arrays separated by a labyrinthine network of particle-free 'aisles'. Small clusters of particles in a particle-poor circumferential zone suggested enlargement of junctions by peripheral accretion. Linear intramembranous structures, resembling those of occluding junctions, occasionally bounded large gap junctions. Spherical intracytoplasmic structures limited by gap junctional membranes were shown by tracer studies to arise by invagination of the cell surface. These were intrepreted as a means of disposal of junctions by interiorization.

The appearance and structure of intercellular connections during the ontogeny of the rabbit ovarian follicle with particular reference to gap junctions

Lanthanum tracer and freeze-fracture electron microscope techniques were used to study junctional complexes between granulosa cells during the differentiation of the rabbit ovarian follicle. For convenience we refer to cells encompassing the oocyte, before antrum and gap junction formation, as follicle cells. After the appearance of an antrum and gap junctions we call the cells granulosa cells. Maculae adherentes are found at the interfaces of oocyte-follicle-granulosa cells throughout folliculogenesis. Gap junctions are first detected in follicles when the antrum appears. In early antral follicles typical large gap junctions are randomly distributed between granulosa cells. In freeze-fracture replicas, they are characterized by polygonally packed 90-A particles arranged in rows separated by nonparticulate A-face membrane. A particle-sparse zone surrounds gap junctions and is frequently occupied by small particle aggregates of closely packed intramembranous particles. The gap junctions of granulosa cells appear to increase in size with further differentiation of the follicle. The granulosa cells of large Graafian follicles are adjoined by small and large gap junctions; annular gap junctions are also present. The large gap junctions are rarely surrounded by a particle-free zone on their A-faces, but are further distinguished by particle rows displaying a higher degree of organization.