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. 17 A 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- 



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 



