Dynein anchors its mRNA cargo after apical transport in the Drosophila blastoderm embryo

Molecular motors actively transport many types of cargo along the cytoskeleton in a wide range of organisms. One class of cargo is localized mRNAs, which are transported by myosin on actin filaments or by kinesin and dynein on microtubules. How the cargo is kept at its final intracellular destination and whether the motors are recycled after completion of transport are poorly understood. Here, we use a new RNA anchoring assay in living Drosophila blastoderm embryos to show that apical anchoring of mRNA after completion of dynein transport does not depend on actin or on continuous active transport by the motor. Instead, apical anchoring of RNA requires microtubules and involves dynein as a static anchor that remains with the cargo at its final destination. We propose a general principle that could also apply to other dynein cargo and to some other molecular motors, whereby cargo transport and anchoring reside in the same molecule.     

Organization of the cytoskeleton in early Drosophila embryos

The cytoskeleton of early, non-cellularized Drosophila embryos has been examined by indirect immunofluorescence techniques, using whole mounts to visualize the cortical cytoplasm and sections to visualize the interior. Before the completion of outward nuclear migration at nuclear cycle 10, both actin filaments and microtubules are concentrated in a uniform surface layer a few micrometers deep, while a network of microtubules surrounds each of the nuclei in the embryo interior. These two filament-rich regions in the early embryo correspond to special regions of cytoplasm that tend to exclude cytoplasmic particles in light micrographs of histological sections. After the nuclei in the interior migrate to the cell surface and form the syncytial blastoderm, each nucleus is seen to be surrounded by its own domain of filament-rich cytoplasm, into which the cytoskeletal proteins of the original surface layer have presumably been incorporated. At interphase, the microtubules seem to be organized from the centrosome directly above each nucleus, extending to a depth of at least 40 microns throughout the cortical region of cytoplasm (the periplasm). During this stage of the cell cycle, there is also an actin "cap" underlying the plasma membrane immediately above each nucleus. As each nucleus enters mitosis, the centrosome splits and the microtubules are rearranged to form a mitotic spindle. The actin underlying the plasma membrane spreads out, and closely spaced adjacent spindles become separated by transient membrane furrows that are associated with a continuous actin filament-rich layer. Thus, each nucleus in the syncytial blastoderm is surrounded by its own individualized region of the cytoplasm, despite the fact that it shares a single cytoplasmic compartment with thousands of other nuclei.     

The actin cytoskeleton is required for maintenance of posterior pole plasm components in the Drosophila embryo.

Localization of mRNAs is one of many aspects of cellular organization that requires the cytoskeleton. In Drosophila, microtubules are known to be required for correct localization of developmentally important mRNAs and proteins during oogenesis; however, the role of the actin cytoskeleton in localization is less clear. Furthermore, it is not known whether either of these cytoskeletal systems are necessary for maintenance of RNA localization in the early embryo. We have examined the contribution of the actin and microtubule cytoskeletons to maintenance of RNA and protein localization in the early Drosophila embryo. We have found that while microtubules are not necessary, the actin cytoskeleton is needed for stable association of nanos, oskar, germ cell-less and cyclin B mRNAs and Oskar and Vasa proteins at the posterior pole in the early embryo. In contrast, bicoid RNA, which is located at the anterior pole, does not require either cytoskeletal system to remain at the anterior.     

Conifer somatic embryogenesis for studies of plant cell biology

Summary Embryogenic cultures have been produced for a wide range of conifers and current methods developed for spruce permit the maturation of high quality embryos that can be desiccated and then germinated to form plantlets. Embryogenic suspensions consisting of immature embryos are an excellent source of regenerable protoplasts. This review considers examples of applications of embryogenic suspension cultures for basic studies in three areas of plant cell biology. a) Immunofluorescence studies of microtubules in mitotic spruce cells reveal focused spindle poles at prophase and anphase, suggesting the presence of microtubule organizing centers (MTOCs). Antibodies known to recognize animal MTOCs do not stain the polar regions but do stain developing kinetochores. b) Embryo-derived protoplasts regenerate directly to somatic embryos. Fluorescence studies of the cytoskeleton in freshly derived protoplasts reveal random cortical microtubules and a fine network of actin filaments. During culture, protoplasts change shape and develop transverse cortical microtubule arrays. Embryonal cells of newly formed embryos possess distinctive arrays of cortical microtubules and networks of fine actin filaments while suspensor cells are characterized by transverse cortical microtubules and longitudinal actin cables. c) Transmission electron microscope studies of endocytosis in spruce protoplasts reveal an endocytotic pathway similar to that described previously for soybean. Uptake results are confirmed using high pressure freeze fixation instead of conventional chemical fixation. Presented in the Session-in-Depth Morphogenesis: Plant Cell and Tissue Differentiation at the 1994 Congress on Cell and Tissue Culture, Research Triangle Park, NC, June 4–7, 1994.     

Actin-binding proteins from Drosophila embryos: a complex network of interacting proteins detected by F-actin affinity chromatography

By using F-actin affinity chromatography columns to select proteins solely by their ability to bind to actin filaments, we have identified and partially purified greater than 40 proteins from early Drosophila embryos. These proteins represent approximately 0.5% of the total protein present in soluble cell extracts, and 2 mg are obtained by chromatography of an extract from 10 g of embryos. As judged by immunofluorescence of fixed embryos, 90% of the proteins that we have detected in F-actin column eluates are actin-associated in vivo (12 of 13 proteins tested). The distributions of antigens observed suggest that groups of these proteins cooperate in generating unique actin structures at different places in the cell. These structures change as cells progress through the cell cycle and as they undergo the specializations that accompany development. The variety of different spatial localizations that we have observed in a small subset of the total actin-binding proteins suggests that the actin cytoskeleton is a very complex network of interacting proteins.     

Fertilization in Torenia fournieri: Actin organization and nuclear behavior in the central cell and primary endosperm

Studies of the living embryo sacs of Torenia fournieri reveal that the actin cytoskeleton undergoes dramatic changes that correlate with nuclear migration within the central cell and the primary endosperm. Before pollination, actin filaments appear as short bundles randomly distributed in the cortex of the central cell. Two days after anthesis, they become organized into a distinct actin network. At this stage the secondary nucleus, which is located in the central region of the central cell, possesses an associated array of short actin filaments. Soon after pollination, the actin filaments become fragmented in the micropylar end and the secondary nucleus is located next to the egg apparatus. After fertilization, the primary endosperm nucleus moves away from the egg cell and actin filaments reorganize into a prominent network in the cytoplasm of the primary endosperm. Disruption of the actin cytoskeleton with latrunculin A and cytochalasin B indicates that actin is involved in the migration of the nucleus in the central cell. Our data also suggest that the dynamics of actin cytoskeleton may be responsible for the reorganization of the central cell and primary endosperm cytoplasm during fertilization.     

Fertilization in Torenia fournieri: Actin organization and nuclear behavior in the central cell and primary endosperm

Studies of the living embryo sacs of Torenia fournieri reveal that the actin cytoskeleton undergoes dramatic changes that correlate with nuclear migration within the central cell and the primary endosperm. Before pollination, actin filaments appear as short bundles randomly distributed in the cortex of the central cell. Two days after anthesis, they become organized into a distinct actin network. At this stage the secondary nucleus, which is located in the central region of the central cell, possesses an associated array of short actin filaments. Soon after pollination, the actin filaments become fragmented in the micropylar end and the secondary nucleus is located next to the egg apparatus. After fertilization, the primary endosperm nucleus moves away from the egg cell and actin filaments reorganize into a prominent network in the cytoplasm of the primary endosperm. Disruption of the actin cytoskeleton with latrunculin A and cytochalasin B indicates that actin is involved in the migration of the nucleus     

Cytoskeletal alterations in interphase cells of the green alga Spirogyra decimina in response to heavy metals exposure: I. The effect of cadmium

Summary. The aim of the study was to elucidate the effect of cadmium ions on the arrangement of the actin and tubulin cytoskeleton, as well as the relationships between cytoskeletal changes and growth processes in the green filamentous alga Spirogyra decimina. Batch cultures of algae were carried out under defined conditions in the presence of various cadmium concentrations. In control cells, the cytoskeleton appeared to be a transversely oriented pattern of both microtubules and actin filaments of various thickness in the cell cortex; colocalization of cortical microtubules and actin filaments was apparent. Microtubules were very sensitive to the presence of cadmium ions. Depending on the cadmium concentration and the time of exposure, microtubules disintegrated into short rod-shaped fragments or they completely disappeared. A steep increase in cell width and a decrease in growth rate accompanied (and probably ensued) a very rapid disintegration of microtubules. Actin filaments were more stable because they were disturbed several hours later than microtubules at any cadmium concentration used. When cadmium ions were washed out, the actin cytoskeleton was rebuilt even in cells in which actin filaments were completely disintegrated at higher cadmium concentrations (40 or 100 μM). The much more sensitive microtubules were regenerated after treatment with lower cadmium concentrations (10 or 15 μM) only. Correspondence and reprints: Centre of Phycology, Institute of Botany, Academy of Sciences of the Czech Republic, Dukelská 135, 379 82 Třeboň, Czech Republic.     

Differences in the organization of actin in the growth cones compared with the neurites of cultured neurons from chick embryos

Sensory neurons from chick embryos were cultured on substrata that support neurite growth, and were fixed and prepared for both cytochemical localization of actin and electron microscopic observation of actin filaments in whole-mounted specimens. Samples of cells were treated with the detergent Triton X-100 before, during, or after fixation with glutaraldehyde to determine the organization of actin in simpler preparations of extracted cytoskeletons. Antibodies to actin and a fluorescent derivative of phallacidin bound strongly to the leading margins of growth cones, but in neurites the binding of these markers for actin was very weak. This was true in all cases of Triton X- 100 treatment, even when cells were extracted for 4 min before fixation. In whole-mounted cytoskeletons there were bundles and networks of 6-7-nm filaments in leading edges of growth cones but very few 6-7-n filaments were present among the microtubules and neurofilaments in the cytoskeletons of neurites. These filaments, which are prominent in growth cones, were identified as actin because they were stabilized against detergent extraction by the presence of phallacidin or the heavy meromyosin and S1 fragments of myosin. In addition, heavy meromyosin and S1 decorated these filaments as expected for binding to F-actin. Microtubules extended into growth cone margins and terminated within the network of actin filaments and bundles. Interactions between microtubule ends and these actin filaments may account for the frequently observed alignment of microtubules with filopodia at the growth cone margins.     

Actin filament distribution in blocked and developing pig embryos

Actin filaments play an important role in cell division. The present study was designed to examine the relationship between actin filament distribution and pig embryo development. When in vivo matured and fertilised pig oocytes were cultured in TCM 199 or NCSU 23, in various proportions, 45-65% of inseminated oocytes developed to the 2- to 4-cell stages but blastocyst development was observed only in NCSU 23 (34%) or NCSU 23 containing 10% TCM 199 (7%). Supplementation of NCSU 23 medium with 20% or more TCM 199 resulted in no blastocyst formation. Examination of actin filaments indicated that microfilaments were distributed in the cortex, at the junction of blastomeres and in the perinuclear area in the embryos cultured in NCSU 23, but perinuclear actin filaments were not observed in embryos cultured in TCM 199. When 2- to 4-cell stage embryos obtained from TCM 199 were transferred to NCSU 23 medium at 36 h after in vivo fertilisation, 57% of the cleaved embryos developed to blastocysts, which was no different from the proportion obtained after culture in NCSU 23 alone (56%). In addition, when 2- to 4-cell stage embryos obtained from TCM 199 were transferred to NCSU 23, most embryos showed perinuclear actin filaments within 6h. The results indicate that the composition of the culture medium plays an important role in the polymerisation of actin filaments, which in turn influences embryo development. It is possible that pig embryo development was blocked by some components in TCM 199 which prevented actin filament polymerisation.     

Qualitative and quantitative differences of cytoskeleton proteins in embryos produced in vitro, in vivo, and by somatic nuclear transfer

The cytoskeleton, consisting of complex and dynamic systems of structural filaments, intermediate filaments and microtubules, is not only a structural element but also contributes to many cellular processes such as functional compartments, transportation, mitosis, secretion, formation of cell extensions, and intercellular communication. Suggestions in rat 2-cell embryos that abnormal distributions of cytoskeletal proteins occurred following the initiations of developmental arrest and our former studies showing reduced intercellular contact zones in cloned bovine embryos prompted us to conduct comparative studies on 8-cell stage bovine embryos from nuclear transfer (NT), in vitro, and in vivo production. Immunohistochemistry and Laser-Scanning-Microscopy facilitated detection of cytoskeleton proteins--alpha-tubulin, F-actin, beta-catenin, and the cell adhesion protein cadherin; image and cluster analysis were subsequently used to study the distribution pattern of the proteins, whereas Western blot was carried out for their qualitative and quantitative analysis. The maximum fluorescence intensity of stained alpha-tubulin was observed in the cloned and the in vitro embryos. A significant higher intensity of staining for F-actin was observed in the in vivo and in vitro embryos. In contrast, Western blot revealed no differences of actin, tubulin, and catenin between the three tested groups whereas a lower abundance of cadherin proteins in the cloned embryos was visible. The distribution of actin filaments in cloned embryos was more centric or one-sided and not peripheral whereas the stained spots of catenin were smaller in comparison to in vivo or in vitro produced embryos. These differences recorded in the distribution patterns may be associated with cell physiological processes related to an influenced actin-catenin-cadherin system. In conclusion, reduced intercellular contacts coupled with abnormal distribution of cytoskeletal proteins seem to play an important role in the developmental arrest encountered normally at the 8-cell stage in bovine cloned embryos.     

Three-dimensional organization and dynamic changes of the actin cytoskeleton in embryo sacs ofZea mays andTorenia fournieri

Summary Actin organization was observed inm-maleimidobenzoic acid N-hydroxysuccinimide ester(MBS)-treated maize embryo sacs by confocal laser scanning microscopy. The results revealed that dynamic changes of actin occur not only in the degenerating synergid, but also in the egg during fertilization. The actin filaments distribute randomly in the chalazal part of the synergid before fertilization; they later become organized into numerous aggregates in the chalazal end after pollination. The accumulation of actin at this region is intensified after the pollen tube discharges its contents. Concurrently, actin patches have also been found in the cytoplasm of the egg cell and later they accumulate in the cortical region. To compare with MBS-treated maize embryo sacs, we have performed phalloidin microinjection to label the actin cytoskeleton in living embryo sacs ofTorenia fournieri. The results have extended the previous observations on the three-dimensional organization of the actin arrays in the cells of the female germ unit and confirm the occurrence of the actin coronas in the embryo sac during fertilization. We have found that there is an actin cap occurring near the filiform apparatus after anthesis. In addition, phalloidin microinjection into the Torenia embryo sac has proved the presence of intercellular actin between the cells of the female germ unit and thus confirms the occurrence of the actin coronas in the embryo sac during fertilization. Moreover, actin dynamic changes also take place in the egg and the central cell, accomplished with the interaction between the male and female gametes. The actin filaments initially organize into a distinct actin network in the cortex of the central cell after anthesis; they become fragmented in the micropylar end of the cell after pollination. Similar to maize, actin patches have also been observed in the egg cortex after pollination. This is the first report of actin dynamics in the living embryo sac. The results suggest that the actin cytoskeleton may play an essential role in the reception of the pollen tube, migration of the male gametes, and even gametic fusion.     

Three-dimensional organization and dynamic changes of the actin cytoskeleton in embryo sacs ofZea mays andTorenia fournieri

Actin organization was observed in m-maleimidobenzoic acid N-hydroxysuccinimide ester(MBS)-treated maize embryo sacs by confocal laser scanning microscopy. The results revealed that dynamic changes of actin occur not only in the degenerating synergid, but also in the egg during fertilization. The actin filaments distribute randomly in the chalazal part of the synergid before fertilization; they later become organized into numerous aggregates in the chalazal end after pollination. The accumulation of actin at this region is intensified after the pollen tube discharges its contents. Concurrently, actin patches have also been found in the cytoplasm of the egg cell and later they accumulate in the cortical region. To compare with MBS-treated maize embryo sacs, we have performed phalloidin microinjection to label the actin cytoskeleton in living embryo sacs of Torenia fournieri. The results have extended the previous observations on the three-dimensional organization of the actin arrays in the cells of the female germ unit and confirm the occurrence of the actin coronas in the embryo sac during fertilization. We have found that there is an actin cap occurring near the filiform apparatus after anthesis. In addition, phalloidin microinjection into the Torenia embryo sac has proved the presence of intercellular actin between the cells of the female germ unit and thus confirms the occurrence of the actin coronas in the embryo sac during fertilization. Moreover, actin dynamic changes also take place in the egg and the central cell, accomplished with the interaction between the male and female gametes. The actin filaments initially organize into a distinct actin network in the cortex of the central cell after anthesis; they become fragmented in the micropylar end of the cell after pollination. Similar to maize, actin patches have also been observed in the egg cortex after pollination. This is the first report of actin dynamics in the living embryo sac. The results suggest that the actin cytoskeleton may play an essential role in the reception of the pollen tube, migration of the male gametes, and even gametic fusion.     

The cytoskeletal involvement in cellularization of theDrosophila melanogaster embryo

Summary The distribution and arrangement of cytoskeletal components in the early embryo ofDrosophila melanogaster were examined by thin-section electron microscopy to elucidate their involvement in the formation of the cellular blastoderm, a process called cellularization. During the final nuclear division in the cortex of the syncytial blastoderm bundles of astral microtubules were closely associated with the surface plasma membrane along the midline where a new gutter was initiated. Thus the new gutter together with the pre-formed ones compartmentalized the embryo surface to reflect underlying individual daughter nuclei. Subsequently such gutters became deeper by further invagination of the plasma membrane between adjacent nuclei to form so-called cleavage furrows. Nuclei simultaneously elongated in the direction perpendicular to the embryo surface and numerous microtubules from the centrosomes ran longitudinally between the nucleus and the cleavage furrow. Microtubules often appeared to be in close association with the nuclear envelope and the cleavage furrow membrane. The plasma membrane at the advancing tip of the furrow was always undercoated with an electron-dense layer, which could be shown to be mainly composed of 5–6 nm microfilaments. These microfilaments were decorated with H-meromyosin to be identified as actin filaments. As cleavage proceeded, each nucleus with its perikaryon became demarcated by the furrow membrane, which then extended laterally to constrict the cytoplasmic connection between each newly forming cell and the central yolk region. The cytoplasmic strand thus formed possessed a prominent circular bundle of microfilaments which were also decorated with H-meromyosin and bidirectionally arranged, similar in structure to the contractile ring in cytokinesis. These observations strongly suggest that both microtubules and actin filaments play a crucial role in cellularization ofDrosophila embryos.     

Actin-dependent membrane association of a Drosophila epithelial APC protein and its effect on junctional Armadillo

BACKGROUND: The adenomatous polyposis coli (APC) protein is an important tumour suppressor in the colon. It promotes the destabilisation of free cytoplasmic beta-catenin (the vertebrate homologue of the Drosophila protein Armadillo), a critical effector of the Wnt signalling pathway. The beta-catenin protein is also a component of adherens junctions, linking these to the actin cytoskeleton. In Drosophila epithelial cells, the ubiquitous form of APC, known as E-APC, is associated with adherens junctions. This association appears to be necessary for E-APC to function in destabilising Armadillo. RESULTS: Using actin-depolymerising drugs, we established that an intact actin cytoskeleton is required for the association of E-APC with adherens junctions in the Drosophila embryo. From an analysis of profilin mutants, whose actin cytoskeleton is disrupted, we found that E-APC also requires actin filaments to associate with adhesive cell membranes in the ovary. Notably, conditions that delocalised E-APC from membranes, including a mutation in E-APC itself, caused partial detachment of Armadillo from adhesive membranes. CONCLUSIONS: Actin filaments are continuously required for E-APC to be associated with junctional membranes. These filaments may serve as tracks for E-APC to reach the adherens junctions. The failure of E-APC to do so appears to affect the integrity of junctional complexes.     

Zebrafish lines expressing UAS‐driven red probes for monitoring cytoskeletal dynamics

Actin filaments and microtubules are principal components of the cytoskeleton that regulate the basic cellular phenomena underlying many fundamental cellular processes. Therefore, analyzing their dynamics in living cells is important for understanding cellular events more precisely. In this article, we report two novel transgenic zebrafish lines expressing red fluorescent proteins tagged with Lifeact or EB1 that interact with actin filaments and microtubule plus ends, respectively, under the control of the GAL4‐UAS system. Using these transgenic lines, we could detect F‐actin and microtubule plus end dynamics in specific tissues of living zebrafish embryos by crossing with GAL4 driver lines. In addition, we could achieve multi‐color imaging using these transgenic lines with GFP‐expressing transgenic lines. Therefore, our transgenic lines that carry UAS‐driven red fluorescent cytoskeletal probes are useful tools for analyzing spatiotemporal changes of the cytoskeletal elements using multicolor live imaging.     

Microtubules and mitotic cycle phase modulate spatiotemporal distributions of F-actin and myosin II in Drosophila syncytial blastoderm embryos

We studied cyclic reorganizations of filamentous actin, myosin II and microtubules in syncytial Drosophila blastoderms using drug treatments, time-lapse movies and laser scanning confocal microscopy of fixed stained embryos (including multiprobe three-dimensional reconstructions). Our observations imply interactions between microtubules and the actomyosin cytoskeleton. They provide evidence that filamentous actin and cytoplasmic myosin II are transported along microtubules towards microtubule plus ends, with actin and myosin exhibiting different affinities for the cell's cortex. Our studies further reveal that cell cycle phase modulates the amounts of both polymerized actin and myosin II associated with the cortex. We analogize pseudocleavage furrow formation in the Drosophila blastoderm with how the mitotic apparatus positions the cleavage furrow for standard cytokinesis, and relate our findings to polar relaxation/global contraction mechanisms for furrow formation.     

Apparent stiffness of vimentin intermediate filaments in living cells and its relation with other cytoskeletal polymers

The cytoskeleton is a complex network of interconnected biopolymers intimately involved in the generation and transmission of forces. Several mechanical properties of microtubules and actin filaments have been extensively explored in cells. In contrast, intermediate filaments (IFs) received comparatively less attention despite their central role in defining cell shape, motility and adhesion during physiological processes as well as in tumor progression. Here, we explored relevant biophysical properties of vimentin IFs in living cells combining confocal microscopy and a filament tracking routine that allows localizing filaments with ~20 nm precision. A Fourier-based analysis showed that IFs curvatures followed a thermal-like behavior characterized by an apparent persistence length (lp*) similar to that measured in aqueous solution. Additionally, we determined that certain perturbations of the cytoskeleton affect lp* and the lateral mobility of IFs as assessed in cells in which either the microtubule dynamic instability was reduced or actin filaments were partially depolymerized. Our results provide relevant clues on how vimentin IFs mechanically couple with microtubules and actin filaments in cells and support a role of this network in the response to mechanical stress.     

Embryogenesis in <Emphasis Type="Italic">Sedum acre</Emphasis> L.: structural and immunocytochemical aspects of suspensor development

The changes in the formation of both the actin and the microtubular cytoskeleton during the differentiation of the embryo-suspensor in Sedum acre were studied in comparison with the development of the embryo-proper. The presence and distribution of the cytoskeletal elements were examined ultrastructurally and with the light microscope using immunolabelling and rhodamine-phalloidin staining. At the globular stage of embryo development extensive array of actin filaments is present in the cytoplasm of basal cell, the microfilament bundles generally run parallel to the long axis of basal cell and pass in close to the nucleus. Microtubules form irregular bundles in the cytoplasm of the basal cell. A strongly fluorescent densely packed microtubules are present in the cytoplasmic layer adjacent to the wall separating the basal cell from the first layer of the chalazal suspensor cells. At the heart-stage of embryo development, in the basal cell, extremely dense arrays of actin materials are located near the micropylar and chalazal end of the cell. At this stage of basal cell formation, numerous actin filaments congregate around the nucleus. In the fully differentiated basal cell and micropylar haustorium, the tubulin cytoskeleton forms a dense prominent network composed of numerous cross-linked filaments. In the distal region of the basal cell, a distinct microtubular cytoskeleton with numerous microtubules is observed in the cytoplasmic layer adjacent to the wall, separating the basal cell from the first layer of the chalazal suspensor cells. The role of cytoskeleton during the development of the suspensor in S. acre is discussed.     

The Mechanical Properties of Early Drosophila Embryos Measured by High-Speed Video Microrheology

In early development, Drosophila melanogaster embryos form a syncytium, i.e., multiplying nuclei are not yet separated by cell membranes, but are interconnected by cytoskeletal polymer networks consisting of actin and microtubules. Between division cycles 9 and 13, nuclei and cytoskeleton form a two-dimensional cortical layer. To probe the mechanical properties and dynamics of this self-organizing pre-tissue, we measured shear moduli in the embryo by high-speed video microrheology. We recorded position fluctuations of injected micron-sized fluorescent beads with kHz sampling frequencies and characterized the viscoelasticity of the embryo in different locations. Thermal fluctuations dominated over nonequilibrium activity for frequencies between 0.3 and 1000 Hz. Between the nuclear layer and the yolk, the cytoplasm was homogeneous and viscously dominated, with a viscosity three orders of magnitude higher than that of water. Within the nuclear layer we found an increase of the elastic and viscous moduli consistent with an increased microtubule density. Drug-interference experiments showed that microtubules contribute to the measured viscoelasticity inside the embryo whereas actin only plays a minor role in the regions outside of the actin caps that are closely associated with the nuclei. Measurements at different stages of the nuclear division cycle showed little variation.