File:Analysis of development (1955) (18169707295).jpg

Title: Analysis of development
Identifier: analysisofdevelo00will (find matches)
Year: 1955 (1950s)
Authors: Willier, Benjamin H. (Benjamin Harrison), b. 1890
Subjects: Embryology; Embryology
Publisher: Philadelphia, Saunders
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Cell Division 103 regard to its long axis (Schmidt, '39; Swann, '51; Inoue, '52). This indicates that it consists of elongated svibmicroscopic units, macro- molecules or micelles, that are oriented paral- lel to the spindle axis (Figs. 17A and B). The similarity of the spindle to the tactoids formed in suspensions of elongated macro- molecules (for instance, tobacco mosaic virus) has suggested that the spindle also is a tactoid (Freundlich; Bernal; see Swann, '52). There are, however, fvmdaraental differences between tactoids and the spindle and they were rightly emphasized by Swann ('52). In a tactoid the particles are oriented and held together by long-range ionic forces and the antagonizing action of these with surface tension causes their spindle shape. In the mitotic spindle, however, the micelles must be held together also by chemical bonds or else the spindle could not be fixed or iso- lated intact from the living cell. The pres- ence of S—S linkages is suggested by the observations of Mazia and Dan ('52). Ferry ('48) has recently reviewed various types of protein gels and the forces involved in their formation. Perhaps the spindle has properties in common with both tactoids and gels of denatured proteins. Electrostatic forces would be mainly involved in the orientation of the micelle into a bipolar structure, while chemical bonding at cer- tain points would give it the observed rigid- ity. The appearance of the spindle in the elec- tron microscope depends on fixation (Rosza and Wyckoff, '51; Beams et al., '50a,b; Sedar and Wilson, '51). After Formalin fixation the spindle looks quite homogeneous, but if acid fixatives are used definite fibers be- come visible. This suggests that the struc- tural units in the spindle are svibmicroscopic and less than a few hundred A thick, but that they have the property to bunch to- gether, possibly depending on the degree of hydration, and thus form fibers that are visible in the light microscope. The behavior of the spindle under in- creased hydrostatic pressure indicates that it is similar to other protoplasmic gels and myosin, with endothermic gelation reaction and increase in volume upon gelation. It is destroyed by a short exposure to hydro- static pressure of 5000 to 6000 lbs. per square inch (Pease, '41, '46; Marsland, '51). Rather little is known about the chemical composition of the spindle. The most prom- ising advance is the recent development of methods to isolate large cleavage spindles in quantity for chemical study (Mazia and Dan, '52). The bulk of the isolated cleavage spindles and asters of sea urchin eggs was found to consist of a protein that formed a single boundary in the analytical ultracen- trifuge. The molecular weight of the particle was calculated to be approximately 45,000. In addition to protein, cytochemical studies indicate the presence of PNA (Brachet, '42; Pollister and Ris, '47; Stich, '51a) and vari-
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Fig. 18. Cleavage spindle of Steatococcus, showing diffuse kinetochore. One of the two chromosomes has been broken in two by x-rays. (After Hughes- Schrader and Ris, '41.) able amounts of polysaccharides (Monne and Slautterback, '50; Stich, '51b) in some but not all spindles. The finding of Brachet that nuclear sap, spindle and aster of amphibian eggs and insect testes contain proteins rich in —SH groups is of special interest in view of Rapkine's theory on the role of —SH groups and reversible denaturization of pro- teins in the formation of gel structures dur- ing mitosis (reviewed in Brachet, '50). KINETOCHORE (contromere, spindle attach- ment). Chromosomes do not move in the spindle imless they are attached to it by chromosomal fibers (traction fibers). In most organisms these fibers originate in connec- tion with a specialized region of the chromo- some, the localized kinetochore. In certain animals and plants chromosomal fibers at- tach along the entire length of the chromo- some (diffuse kinetochore, see Fig. 18). Where the kinetochore is localized, chromo- some fragments lacking this organelle fail

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