The form and arrangement of microtubules: an historical, primarily morphological, review

Macroscopic stiffening of embryonic tissues via microtubules, RhoGEF and the assembly of contractile bundles of actomyosin

During morphogenesis, forces generated by cells are coordinated and channeled by the viscoelastic properties of the embryo. Microtubules and F-actin are considered to be two of the most important structural elements within living cells accounting for both force production and mechanical stiffness. In this paper, we investigate the contribution of microtubules to the stiffness of converging and extending dorsal tissues in Xenopus laevis embryos using cell biological, biophysical and embryological techniques. Surprisingly, we discovered that depolymerizing microtubules stiffens embryonic tissues by three- to fourfold. We attribute tissue stiffening to Xlfc, a previously identified RhoGEF, which binds microtubules and regulates the actomyosin cytoskeleton. Combining drug treatments and Xlfc activation and knockdown lead us to the conclusion that mechanical properties of tissues such as viscoelasticity can be regulated through RhoGTPase pathways and rule out a direct contribution of microtubules to tissue stiffness in the frog embryo. We can rescue nocodazole-induced stiffening with drugs that reduce actomyosin contractility and can partially rescue morphogenetic defects that affect stiffened embryos. We support these conclusions with a multi-scale analysis of cytoskeletal dynamics, tissue-scale traction and measurements of tissue stiffness to separate the role of microtubules from RhoGEF activation. These findings suggest a re-evaluation of the effects of nocodazole and increased focus on the role of Rho family GTPases as regulators of the mechanical properties of cells and their mechanical interactions with surrounding tissues.

Mesenchyme formation from the trigeminal placodes of the mouse embryo

The trigeminal placode is a thickened region of ectodermal epithelium located along the side of the embryonic head. Mesenchyme escapes from the placode to form neurons of the trigeminal (V) ganglion. To further our knowledge of the morphogenesis of this escape, plastic thick sections were cut from mouse embryos and stained for light microscopy by using a technique which revealed escaping mesenchyme. The escape of trigeminal mesenchyme began at approximately 12 somites of age and was substantially complete by 30 somites. These results provided spatial/temporal orientation for a subsequent electron microscopic study. The first ultrastructural manifestation of escape was the penetration of an otherwise continuous basal lamina by small cell processes. The presence of longitudinally oriented microtubules within these processes suggests that mesenchymal cells escape through the basal lamina by using microtubules to direct/move their contents (e.g., the cell nucleus) into an enlarging process. Nuclei were distorted as they passed into these processes. This distortion suggests that basal lamina, together with a possible contribution from basal microfilaments, forms a rigid obstruction which is disrupted in the region from which a process is formed. In some cases a collar of basal lamina was observed around the necks of processes, but their distal membranes were invariably lamina-free. This lamina-free membrane is possibly that which is newly formed to accommodate the growing process. In later stages of escape, instances were observed in which the lamina was completely absent beneath an escaping cell and partially degraded beneath adjacent cells as well. These instances suggest that enzymatic digestion may play a role in degrading the lamina during mesenchymal escape. Apical desmosomes were often retained beyond the initial stages of escape. Mechanisms involved in their disruption are thus not among those which initiate escape.

The structure of the chorion and associated surface filaments in Oryzias--evidence for the presence of extracellular tubules

The structure of the chorion with its associated surface filaments has been examined in Oryzias latipes using several techniques, including scanning and transmission electron microscopy, enzymatic digestion, and sodium dodecylsulfate-polyacrylamide gel electrophoresis. The chorion of the recently fertilized egg was found to be organized into three zones: an outer, fuzzy electron-lucent zone that was continuous over the surface of filaments, a middle, homogeneous electron-dense zone, and an inner zone of ten to 12 horizontal, fibrous lamellae. Two topographically distinct types of filaments were found on the chorionic surface: nonattaching and attaching. Nonattaching filaments showed a regular spatial distribution over the chorion with an interfilament distance of about 60-70 microns. Attaching filaments originated from a localized portion of the chorion and united with those of neighboring eggs to anchor the egg cluster to the gonoduct of the female. Both nonattaching and attaching filaments were morphologically regionalized into basal and distal segments. Internally, nonattaching and attaching filaments were constructed of unbranched, packed tubules with an average outside diameter of approximately 19.5 and 18.8 nm, respectively. Using the attaching filament for further study, it was determined by rotational analysis (Markham et al., '63) that the wall of each tubule was a cylinder composed of 14 globular subunits. Two structural types of attaching filaments were identified. The type I attaching filament was similar in internal organization to the nonattaching filament and consisted of only tubules. The type II attaching filament, however, showed a highly osmiophilic, electron-dense bar surrounded by packed tubules. Tubules of attaching filaments of the adult were resistant to the action of Triton X-100 and colchicine, but sensitive to a 0.1% protease solution. However, colchicine-treated ovary tissue showed an absence and pattern of disorganization of tubules at the periphery of developing filaments. Solubilized attaching filament samples electrophoresed on 7.5% polyacrylamide-SDS gels were resolved into a pair of Coomassie-blue-positive bands that comigrated with purified porcine brain tubulin. The apparent molecular weight of the attaching filament polypeptide was determined to be approximately 55,000 daltons. These data suggest that the extracellular, tubular components of attaching filaments (as well as nonattaching filaments) are proteinaceous and show properties similar to those of cytoplasmic microtubules. Tubular precursor material was electron-dense and appeared to originate in the cisternae of the rough endoplasmic reticulum of ovarian foll

The activity of 2',3'-cyclic nucleotide 3'-phosphodiesterase in rat tissues

The activity of the myelin-associated enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) was measured in 14 rat tissues and in subcellular fractions of rat liver by a sensitive fluorometric method, using cyclic NADP as substrate. CNP activity in brain (339 mumol/h/mg protein) was fourfold that of the sciatic nerve. The activities in tissues outside the nervous system ranged from a low of 0.42 mumol/h/mg protein in the unwashed red blood cell to a high of 9.96 in the spleen. The activity was highest in tissues containing cells with membranes capable of undergoing transformation and elaboration (spleen and thymus) and low in those in which the cell membranes are morphologically stable (muscle and red cell). The enzyme was found in all major liver subfractions, with the highest activities in the microsomal and nuclear fractions. Despite the large difference in the maximal velocities of CNP in brain and liver, the affinity of the liver enzyme for the substrate (km) was similar to that of brain enzyme. Brain CNP was stable over a 48-h postmortem period.

Actin-dependent cell elongation in teleost retinal rods: requirement for actin filament assembly

Teleost retinal rods elongate when exposed to light. Elongation is mediated by a narrow necklike region called the myoid. In the cichlid Sarotherodon mossambicus, the rod inner segment (composed of the myoid with adjacent ellipsoid) increases in length from 12 micrometers in the dark to 41 micrometers in the light. Long light-adapted myoids contain longitudinally oriented microtubules and bundles of parallel 60-A filaments that we have identified as actin by their ability to bind myosin subfragment 1. In short dark-adapted myoids, only microtubules are recognizable. Colchicine experiments reveal that light-induced rod elongation can occur in the absence of myoid microtubules. Intraocular injections of colchicine at concentrations that disrupt virtually all rod myoid microtubules do not block rod elongation. However, rod elongation is blocked by intraocular injections of cytochalasin B or cytochalasin D. The hierarchy of effectiveness of these drugs is consistent with their effectiveness in inhibiting actin assembly and in disrupting other actin-dependent motile processes. On the basis of ultrastructural observations and the results of these inhibitor studies, we propose that the forces responsible for rod elongation are dependent not on microtubules but on actin filament assembly.

Microtubular aggregates in the rough endoplasmic reticulum of a myxoid chondrosarcoma

A tumor with the light microscopic and histochemical characteristics of myxoid chondrosarcoma is examined ultrastructurally. Instead of the usual features of chondroblasts, the tumor cells exhibited prominent collections of long straight microtubules within the endoplasmic reticulum. This finding is compared with other previously reported tubular aggregates. The differential diagnosis and ultrastructure of myxoid chondrosarcoma are also discussed.

Infectious mononucleosis and acute lymphoblastic leukemia--hand mirror cells: a qualitative and quantitative ultrastructural study

The hand mirror cell (HMC) in infectious mononucleosis (IM) is a T cell which appears at the time of improvement and apparently has an important role in immune surveillance. Fifty electron micrographs of HMC from an unusual untreated patient with ALL-HMC variant were compared to 50 electron micrographs of HMC from five patients beginning to recover from IM. Quantitative differences between the IM-HMC and ALL-HMC were observed in multivariate analysis of variance (p < 0.001), nuclear perimeter (p < 0.0001), nuclear length (p < 0.0001), cytoplasmic length (p < 0.0001), total cell area (p < 0.0001), cytoplasmic area (p < 0.0001) and nuclear cytoplasmic ratio (p < 0.004), and numbers of mitochondria (p < 0.001). The qualitative differences included greater nuclear irregularity in the IM-HMC, and prominent nucleolonema in some of the ALL-HMC. In addition, the IM-HMC showed more polyribosomes, parallel tubular arrays and microtubules, but much less glycogen. Since the IM-HMC apparently enter effectively into immune surveillance, and some cases of ALL have HMC, the ultrastructural similarities and differences above contribute to a better understanding of the importance of this unique cell configuration in benign and malignant disorders.

On the evaluation of photoreceptor properties by micro-fluorimetric measurements of fluorochrome diffusion

By use of the microfluorimetric technique it is possible to study the diffusion of the fluorochrome di-dansylcystine (DDC) within isolated frog rod outer segments (ros) which are immobilysed in agarose gel. For this purpose, by a short hypotonic shock a leak is applied to one end of the ros. By this open end the DDC enters the rod and migrates through the whole outer segment. Following the propagation of the fluorescence boundary with time the cytoplasmatic diffusion constant can be determined if a chromatographic model is used to allow for the considerable binding of DDC to the inner membrane surface. With a binding constant K = 5 . 10(-4) cm the cytoplasmatic diffusion constant was found to be D = 1.3 . 10(-6) cm2/s whereas Dg = 2 . 10(-6 cm2/s and Dr = 3.5 . 10(-6) cm2/s were found in agarose gel or ringer solution, respectively. Using the mobility reduction factor given by D/Dr approximately equal to 0.4 to calculate the cytoplasmatic conductivity an inner resistance per length of 1.7 M omega/mu could be calculated for a frog rod which is in good agreement with corresponding data obtained from electrophysiological measurements.

[Changes in the outer plexiform layer of the rabbit retina after long-term application of vincristine]

Rabbits which had been intravenously injected with Vincristine for six weeks showed changes in the outer plexiform layer of the retina after one year, which suggests the destruction of receptor cells, bipolar ganglion cells, and horizontal cells. Compensated cell damage accompanied by loss of microtubuli in the cytoplasma, autolytic vacuoles, granular material deposits, and membrane changes of the ribbon complex were frequently observed. A comparison between axons and dendrites showed a predominance of damaged dendritis over axons. Because short-term studies have not produced the same results, different regeneration capacities, which might have their origin in the endoplasmatic reticulum, are discussed.

Scanning electron microscopy of developing photoreceptors in the chick retina

Photoreceptor morphogenesis in the sensory retina of chicks of 2 to 20 days incubation age was studied by scanning electron microscopy (SEM) and correlative transmission electron microscopy (TEM). At 9.5 days spherical inner segments extend into the subretinal space (optic ventricle). They are randomly arranged with chiefly smooth surfaces and contain centrioles, polyribosomes and microtubules. Microvilli project primarily from Müller cells. By the twelfth day immature ellipsoid and myoid regions have formed. Microvilli are abundant on the lateral surfaces of inner segments and extend over the entire spherical surface by the fifteenth day. Occasional cilia with surrounding depressions at their bases were also observed. Inner segments are more symmetrically arranged due to close proximity of photoreceptor cells. Inner segments elongate during the sixteenth day; many display a transitional ovoid form. Microvilli become less numerous but some persist as calycal processes. By the eighteenth day, conical shaped outer segments appear. Thereafter, all photoreceptor cells are comparable to those in the mature retina. Abundant microvilli on the external surface of the sensory retina suggest a supportive role in supplying adequate nutrition to the sensory retina during morphogenesis. The establishment and continual development of the ellipsoid and myoid appear to be primarily responsible for the elongation of photoreceptor cells.

Flight muscle differentiation in nymphs of a dragonfly Anax imperator

Flight muscle fibers of Anax imperator nymphs, in different developmental stages are analyzed for several morphological features, such as the arragnement and numerical ratio of actin and myosin filaments, the pattern of the T system and sarcoplasmic reticulum, the number of microtubules and the fractional volume of mitochondria in each fiber. The T system is initially represented by longitudinal grooves on the cell surface, joined with vesicles of the sarcoplasmic reticulum; this pattern rapidly changes and the grooves start to break up into longitudinal segments. The thin to thick filament ratio is at first quite high (about 4-4.5:1) but rapidly falls to the final (3:1) when the myofibrils are well developed at the fiber periphery. Statistical analyses show that the measured values are significantly different in the various stages of development, also indicating a progressive reduction of the ratio variability. The reduction of thin to thick filament ratio and the variance decrease fit quite well with the hypothesis that the synthesis of actin and myosin depends on independently regulated messenger RNA molecules.

Microtubules and control of insect egg shape

This study evidence for tension transmission by microtubules and desmosomes in the follicular epithelium during anisometric growth of certain insect eggs. Most insect oocytes, and the follicles which surround them, grow anisometrically as they assume shapes which approximate to those of long prolate spheroids. Surface growth is most rapid in directions which parallel the polar axis of an oocyte and slowest in circumferential directions at right angles to this axis. The longitudinal axes of microtubule bundles in follicle cells of the gall midge Heteropeza and the cockroach Periplaneta are oriented circumferentially with respect to the surfaces of developing eggs and at right angles to the polar axes of eggs. At cell boundaries, the tubules appear to be attached to spot desmosomes. It is suggested that microtubules and desmosomes form a mechanical continuum throughout a follicular epithelium which transmits tensile forces around the circumference of a growing egg. Follicular resistance to circumferential expansion may be largely responsible for defining the elongate form of insect eggs.

Effect of vinblastine on the brush border of proximal tubule cells of rat kidney

A vinblastine-induced lesion of the brush border of proximal tubule cells of rat kidney was studied by scanning and transmission electron microscopy. Adult rats were given two tail-vein injections of vinblastine sulfate and then sacrificed either three or sixteen hours after administration of the first injection. The proximal tubules of animals treated with the drug for three hours differed from those of saline-treated controls in possessing (1) well defined areas of apical cell surface devoid of microvilli and (2) fewer microcraters in the brush border. In tubules of animals treated with vinblastine for sixteen hours, the areas devoid of microvilli were much more extensive, and microcraters were less frequently seen. Cilia did not appear to be affected by exposure to the drug and were observed both in regions devoid of microvilli and in areas with a well developed brush border. The mechanism of action of vinblastine in eliciting focal loss of microvilli of proximal tubule cells is not known. Several hypotheses to account for this lesion of the brush border are discussed.

Sperm aster in rabbit zygotes: its structure and function

Microscope observations of rabbit zygotes demonstrate that a sperm aster forms in association with the male pronucleus approximately 1 h postinsemination and consists of two regions. One, the centrosphere, contains a dense aggregation of cisternae of smooth endoplasmic reticulum and microtubules. The second consists of fascicles of microtubules which emanate from the centrosphere. Fertilized rabbit eggs were cultured in medium containing colcemid in order to determine its effects on various events of fertilization, such as movements of the male and female pronuclei and DNA synthesis. No evidence was obtained to indicate that a sperm aster is formed in colcemid-treated zygotes. In addition, migration and close apposition of the pronuclei do not take place. Breakdown of the pronuclear envelopes and condensation of the maternally and paternally derived chromosomes occur even though the pronuclei fail to migrate centrad. Autoradiographic analysis of the synthesis of DNA by both pronuclei demonstrates that their migration into close apposition to one another is not required for the incorporation of tritiated thymidine.

Studies on experimental malignant nerve sheath tumors maintained in tissue and organ culture systems. II. Electron microscopy observations

The sequential electron microscopic features of six malignant nerve sheath tumors (three cranial and three spinal) induced in rats by transplacental ethylnitrosourea and maintained in organ culture systems were compared with those of a human acoustic Schwannoma similarly cultured. After 4 weeks in vitro, the malignant tumor cells often showed progressive elongation of their processes, with the development of an interdigitating pattern resembling that seen in well-differentiated Schwannomas. This was accompanied by an increase of microtubules. Basal lamina formation, less well-developed and less complete than in the benign Schwannoma in this study, was maintained in culture. Some explants demonstrated an increase in number and width of collagen fibrils, accompanied by a relative concomitant decrease of inter-cellular basement membrane material. The malignant tumor cells also showed numerous micropinocytotic vesicles and various junctional complexes, which are characteristic of perineurial cells. Since the origin of the experimental tumors from adult Schwann cells seems well established, this apparent contradiction is best resolved by the concept that Schwann cells and perineurial fibroblasts are functional variants of the same cell type.

Microtubules and actin filaments in teleost visual cone elongation and contraction

Teleost retinal cones contract in light and elongate in darkness. This paper describes the disposition of microtubules and cytoplasmic filaments in cone cells of 2 species of fish (Haemulon sciurus and Lutjanus griseus). In Haemulon, the neck-like "myoid" region of the cone changes in length from 5 mu to 75 mu. Maximal observed rates of elongation and contraction are comparable to that of chromosome movement in mitosis (2-3 mu/min). Microtubules presumably participate in cone elongation, since numerous longitudinal microtubules are present in the myoid region, and colchicine blocks dark-induced elongation. Myoid shortening, on the other hand, appears to be an active contractile process. Disruption of microtubules in dark-adapted cones does not produce myoid shortening in the absence of light, and light-induced myoid shortening is blocked by cytochalasin-B. Cone cells possess longitudinally-oriented thin filaments which bind myosin subfragment-1 to form arrowhead complexes typical of muscle actin. Myoid thin filaments are clearly observed in negatively stained preparations of isolated cones which have been disrupted with detergent after attachment to grids. These myoid filaments are not, however, generally preserved by conventional fixation, though bundles of thin filaments are preserved in other regions of the cell. Thus, actin filaments are poorly retained by fixation in precisely the region of the cone cell where contraction occurs. Cone cells also possess longitudinally-oriented thick filaments 130-160 A in diameter. That these thick filaments may be myosin is suggested by the presence of side-arms with approximately 150 A periodicity. The linear organization of the contractile apparatus of the retinal cone cell makes this cell a promising model for morphological characterization of the disposition of actin and myosin filaments during contraction in a nonmuscle cell.