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J R McIntosh

Publications and source records attributed to J R McIntosh.

At least 127 records · Page 7Linked to original sources
PubMed1974

A functional mitotic spindle prepared from mammalian cells in culture.

MITOTIC CELLS LYSED INTO SOLUTIONS OF POLYMERIZABLE MICROTUBULE PROTEIN CONTAIN A SPINDLE WHICH IS SIMILAR TO THE LIVING SPINDLE IN TWO RESPECTS: it will lose and gain birefringence when cooled and warmed, and it will move anaphase chromosomes to the opposite ends of the cell. Early anaphase cells lysed into buffers containing high molecular weight polyethylene glycol and nucleotide triphosphates will continue chromosome motion and spindle elongation in the absence of exogenous spindle subunits. These results suggest that while spindle growth requires microtubule polymerization, anaphase motions do not.

Animals↗
PubMed1974

Bridges between microtubules.

Bridges between microtubules have been studied with the electron microscope in the axostyle of Saccinobaculus and in various tubule systems of chicken testis, including the helix of tubules surrounding the elongating spermatid nucleus and the flagellum of the sperm tail. In addition to the previously described periodic bridges, evidence is presented that nonperiodic bridges exist between certain tubules. An analysis of axial spacing between adjacent nonperiodic bridges suggests that these structures are attached to periodic binding sites on the microtubule wall, but that not all the binding sites are filled. The bridges appear nonperiodic as a result of random occupancy of some fraction of the periodic sites. The distribution of these binding sites is related to the substructure of the microtubule wall as seen with negative staining and optical diffraction.

Animals↗
PubMed1973

The axostyle of Saccinobaculus. I. Structure of the organism and its microtubule bundle.

The axostyle of the flagellate Saccinobaculus is a motile ribbon composed of microtubules, cross-bridged to form interconnected rows. We find a centriole-related row of dark-staining tubules near the nucleus at the anterior end of the axostyle. Other tubule rows bind parallel to this primary row, acquire ordered relationships, and become the tubules of the axostyle proper. The number of tubule rows is constant in Saccinobaculus lata from the region near the nucleus to within a few micrometers of the posterior tip of the cell. In Saccinobaculus ambloaxostylus a few tubule rows are added to the axostyle posterior to the nucleus, giving this axostyle a leaf spring construction. The tubules of S. lata are held in rows by links with a 140 A periodicity along the tubule axis; bridges between rows of tubules are also seen but are not apparently periodic. Each tubule in S. ambloaxostylus shows an axial periodicity of 150 A due to pairs of arms, one of which is always part of the intrarow link. Interrow bridges in this species run either from tubule to tubule or from tubule to the free arm, but as in S. lata they do not display an obvious axial periodicity. An average unit cell is presented for the axostyle of each species, and the relation of the intertubule links to the microtubule substructure is discussed.

Animals↗
PubMed1973

The axostyle of Saccinobaculus. II. Motion of the microtubule bundle and a structural comparison of straight and bent axostyles.

Undulations of the flagellate Saccinobaculus result from motility in its axostyle, a bundle consisting of thousands of cross-bridged microtubules. In its resting state, the axostyle is a helix of large pitch and slowly varying radius. The active state as seen by light microscopy involves first a bending of the anterior end of the axostyle to a radius of about 8 microm with a circular arc ranging from 60 degrees to 180 degrees , and then the propagation of this bend without damping to the posterior end of the organism at speeds up to 100 microm/s. The cross section of an unbent axostyle is crescent shaped. This crescent flattens as the bend arrives and reappears as the bend passes by. Intertubule bridges impart to the axostyle tubules an axial periodicity of about 150 A which can serve as a marker for the investigation of tubule sliding or contraction associated with bend formation. Optical diffraction measurements on electron micrographs of the bend demonstrate that the axostyle tubules slide over one another and that the tubules on the inside of a bend usually contract, sometimes by as much as 25%. Possible relationships between the contraction and sliding of the tubules are discussed.

Animals↗
PubMed1967

Microtubules in the spermatids of the domestic fowl.

Spermiogenesis in chicken has been examined in order to see whether the radical changes observed in cell shape can be related to the presence of cytoplasmic microtubules. A highly ordered array of tubules has been found which surrounds the nucleus as it elongates from a sphere to a slender cylinder. The structure of the array has been determined by following the tubules through 12-14 adjacent serial sections, and it is a left-handed double helix. Faint cross-bridges connect consecutive turns of the two helices. After the change in nuclear shape is complete, the helical system of microtubules disappears and is replaced by a set of almost straight tubules which run parallel to the long axis of the nucleus. These tubules remain while the spermatid nucleus condenses isotropically to its final size. We suggest that the helix is the agent which effects nuclear elongation and that the subsequent system of paraxial tubules determines the curvature of the final sperm head. Evidence for these suggestions is found in the form of spermatids which have failed to develop properly. In an appendix we consider the kinematics of single and multiple helix systems and discuss the revelance of these models to the morphogenesis of chicken spermatids.

Animals↗
PubMedDate not supplied

High voltage cryomicroscopy of human blood platelets.

We have analyzed the ultrastructure of human blood platelets using ultrarapid freezing techniques followed by either freeze-substitution fixation or direct imaging of frozen hydrated specimens in a high voltage electron microscope. Freeze substitution preserved a regularity in the arrangement of microtubules when compared with chemically fixed specimens, as identified by a preferred center-to-center distance of 30 nm. Platelets imaged in the frozen hydrated state during different stages of adhesion and spreading were rich in cytoplasmic detail and revealed novel features not previously reported. Different granule populations were distinguished in the unstained samples, and the morphology of the cells was dramatically different from that of dried cells. Membrane channels formed as the cells spread, increasing in size and number as the number of granules in the cell decreased, suggesting a fusion of granule membranes with one another or an additional membrane system to release their contents. The data support the idea that direct cryoimaging is an optimum method for characterizing rapid structural changes of labile organelles in whole cells.

Blood Platelets↗
PubMedDate not supplied

Computer visualization of three-dimensional image data using IMOD.

We have developed a computer software package, IMOD, as a tool for analyzing and viewing three-dimensional biological image data. IMOD is useful for studying and modeling data from tomographic, serial section, and optical section reconstructions. The software allows image data to be visualized by several different methods. Models of the image data can be visualized by volume or contour surface rendering and can yield quantitative information.

Animals↗
PubMedDate not supplied

Identification of kinesin in sea urchin eggs, and evidence for its localization in the mitotic spindle.

To understand the molecular basis of microtubule-associated motility during mitosis, the mechanochemical factors that generate the relevant motile force must be identified. Myosin, the ATPase that interacts with actin to produce the force for muscle contraction and other forms of cell motility, is believed to be involved in cytokinesis but not in mitosis. Dynein, the mechanochemical enzyme that drives microtubule sliding in eukaryotic cilia and flagella, has been identified in the cytoplasm of sea urchin eggs, but the evidence that it is involved in cytoplasmic microtubule-based motility (rather than serving as a precursor for embryonic cilia) is equivocal. Microtubule-associated ATPases have been prepared from other tissues, but their role in cytoplasmic motility is also unknown. Recent work on axoplasmic transport, however, has led to the identification of a novel mechanochemical protein called kinesin, which is thought to generate the force for moving vesicles along axonal microtubules. These results suggest that kinesin may also be a mechanochemical factor for non-axoplasmic forms of microtubule-based motility, such as mitosis. We describe here the identification and isolation of a kinesin-like protein from the cytoplasm of sea urchin eggs. We present evidence that this protein is localized in the mitotic spindle, and propose that it may be a mechanochemical factor for some form of motility associated with the mitotic spindle.

Adenosine Triphosphatases↗