CHROM-OSMIC-ACETIC 



75 



CHROMOSOMES 



Chrom-Osmic-Acetic fixative, see Lillie's. 



Chromosomes. — Written by A. R. Gopal- 

 Ayengar, Barnard Free Skin and Cancer 

 Hospital, St. Louis, Sept. 10, 1946— 

 (Now Pata Memorial Hospital, Bom- 

 bay) . These are discrete bodies usually 

 constant in number in the cells of a 

 given species and frequently having 

 distinctive structure into which the 

 chromatin material of a nucleus re- 

 solves itself during the mitotic process 

 (see Mitosis). From a chemical stand- 

 point the chromosome is a protein fiber 

 like silk or hair, presumably depending 

 on a peptide chain linkage — C — C — N — . 

 On this structural framework the per- 

 manent hereditary units, the genes 

 (which may be considered as the atoms 

 of heredity), are located at definite 

 loci. In a sense, therefore, the chromo- 

 some may be considered a giant mole- 

 cule (Darlington, C. D., Nature, 1942, 

 149, 66-69, Astbury, W. T., Proc. 7th 

 Int. Genet. Congress., 1939 (Camb.), 

 1940, 49-51). It is generally stated 

 that the chromosomes of sperm cells 

 consist of basic proteins, such as pro- 

 tamines or histones, in combination 

 with highly polymerized desoxyribose 

 nucleic acid (Mirsky, A. E., Advances 

 in Enzymology, 1943, 3, 1-34). 



Smear-Squash technique. The rapid 

 and spectacular advances in our knowl- 

 edge of cytology and cytochemistry 

 during the last fifteen years have 

 greatly altered our ideas of chromo- 

 some structure and behavior. Progress 

 in this direction has been possible 

 through the introduction of newer tech- 

 niques, such as smears and squashes, 

 which have largely supplanted the 

 classical methods of paraffin sectioning. 

 In order to obtain the most satisfactory 

 results, it is imperative to employ cyto- 

 logical techniques calculated to bring 

 out fine structures and details as 

 quickly and accurately as possible. 

 Most of the smear-squash methods, de- 

 spite their extreme simplicity, give 

 superior results because of instantane- 

 ous fixation and rapidity of staining and 

 finishing schedules. Moreover, it is 

 possible to examine thin layers of cells 

 or even isolated cells and their parts. 

 They are especially useful in the anal- 

 ysis of chromosome complexes and 

 associations and in the exact establish- 

 ment of numerical relations. Squash 

 preparations have special advantages. 

 Individual chromosomes can be recog- 

 nized more readily and easily, inter- 

 relations between chromosomes become 

 clearer, the preparations are almost 

 two-dimensional, which is of undoubted 

 value in chromosome measurement 

 since it obviates errors of measurements 



due to differential focussing of three 

 dimensional structures. However, there 

 are a few disadvantages resulting 

 from the disturbances of the natural 

 relationships of the chromosomes. 

 These are more than offset by the merits 

 of the method. 



Of course a uniform application of the 

 same technique is not likely to prove 

 suitable for every kind of material, but 

 the general principles are valid, requir- 

 ing a few modifications depending upon 

 the particular type of material. No 

 single method can be recommended 

 which would prove adequate for all 

 stages of development. The methods 

 that have proved particularly satisfac- 

 tory and have given the best results are 

 the Feulgen-smear-squash technique of 

 Coleman, L. C, Am. J. Bot., 1940, 27, 

 887-895; Coleman, L. C, Genetics, 

 1943, 28, 2-8; Hillary, B.B., Bot. Gaz., 



1939, 101, 276-300; Hillary, B.B., Ibid, 



1940, 102, 225-235; Heitz, E., Ber. d. 

 biol. Ges., 1936, 53, 870-878; Darlington, 

 C. D., and LaCour, L. F., The Handling 

 of Chromosomes, 1942. New York: 

 The Macmillan Company; and McClin- 

 tock, B., Stain Tech., 1929, 4, 53-56. 



In my experience of making prepara- 

 tions for the study of somatic and meio- 

 tic chromosomes, the method given in 

 the schedule here has proven most satis- 

 factory. It is essentially a modifica- 

 tion of the technique developed by Dr. 

 Coleman and his associates of the Uni- 

 versity of Toronto and may be applied 

 to a wide variety of materials, both 

 plant and animal. The tissues are 

 fixed in Carnoy, one of the Navashin 

 modifications or Flemming-Heitz. One 

 of the best fixatives for small mam- 

 malian tissues is that given by LaCour, 

 L. F., Proc. Roy. Soc. Edin. B., 1944, 

 62, 73-85. It is a mixture of methyl 

 alcohol (15 parts), 5% acetic acid (5 

 parts), formalin (1 part), and water 

 (5 parts). The proportions may need 

 to be varied according to the material. 

 For a study of the morphology of mouse 

 chromosomes the liver tissue of a new- 

 born mouse is particularly good. 



Chromosome structure. The nuclear 

 cycle, whether in plants or animals, is an 

 alternation between two stable states — 

 spiralization and despiralization. The 

 metaphase chromosomes usually repre- 

 sent a state of maximum spiralization 

 and the chromosomes in the resting 

 nucleus, the maximum of despiraliza- 

 tion or minimum of spiralization. For 

 demonstration of the spiral structure 

 of the chromosomes and unravelling of 

 the coils, fixation should be preceded 

 by some form of pretreatment. Differ- 

 ent methods such as hydration, dessi- 



