Fixation of Nuclei in Locust Testis 



109 





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Figs. 1-2. Electron micrograph (2) of a thin section of an 

 osmium fixed nucleus in locust testis with an inset of an 

 ultra-violet micrograph (1) of the same nucleus in an adja- 

 cent thicker section. Two clusters of mitochondria (w) are 

 indicated. They possess considerable ultra-violet absorption. 



solve this discrepancy, we have examined adjacent 

 thick. (| /<) and thin (about 200 A) sections of osmium 

 fixed testis in ultra-violet and electron microscopes 

 respectively. Low magnification micrographs permit 

 the identification of identical nuclei in the two sec- 

 tions. A comparison between two sections of the 

 same nucleus is shown in figs. 1-2. Fig. I is an ultra- 

 violet micrograph of the thick section and shows 



that the inhomogeneous distribution of chromatin 

 in the live nucleus is at least partly preserved by 

 osmium fixation. Fig. 2 is an electron micrograph 

 c'f the ihin sectiiMi of the same nucleus, it shows the 

 homogenecHis appearance typical of osmium fixed 

 nuclei in this material and little trace of the inhomo- 

 geneities of chromatin existing in the section is dis- 

 cernible. This homogeneous appearance of nuclei in 

 electron micrographs must, therefore, be ascribed 

 not to the failure of osmium fixation to preserve 

 chromatin distribution, but to its failure to produce 

 appreciable contrast between chromatin and nuclear 

 sap. We believe that this lack of contrast is evidence 

 that osmium tetroxide does nt)l react with desoxyri- 

 bonucleic acid (2). 



The general appearance of nuclei after osmium 

 fixation is shown in fig. 2. The nucleus shows a 

 rather homogeneous fine-grained structure with little 

 trace of organisation. When examined at high magni- 

 fication occasional traces of organised structure are 

 seen appearing as groups. 



The general appearance of nuclei after formalde- 

 hyde fixation is shown in fig. 3. The distribution of 

 chromatin within the nucleus is easily visible because 

 the relative contrast between chromatin and nuclear 

 sap is higher than in osmium fixed nuclei. The pairing 

 of the chromosomes during prophase is indicated but 

 the chromosomes still show no apparently highly 

 organised structure. As regards contracted chromo- 

 somes, the contrast between chromatin and cyto- 

 plasm is higher than in osmium lixed material but 

 there is a similar apparent lack of organisation. The 

 chromosomes may appear to possess a split down the 

 centre but the significance of this is hard to interpret 

 because of the difficulty of identifying the stage of 

 meiosis from a thin section. 



One of the classic fixatives used in light micro- 

 scope studies of chromosomes is 45 ",, acetic acid 

 and we have considered it worth while to examine 

 material fixed in this way in the electron micro- 

 scope. In sections which are not too thin it is possible 



Fig. 3. Electron micrograph of locust testis fixed in 5 % for- 

 maldehyde. Most of the field is occupied by a primary sper- 

 matocyte nucleus in which the pairing of the chromatids is 

 indicated by the arrow. 



Fig. 4. Electron micrograph of locust testis fixed in 45 °o 

 acetic acid. The pairing of the chromatids is indicated in 

 transverse section (M) and longitudinal section (N;. 



