110 



I. R. GIBBONS AND J. R. G. BRADFIELD 



Fig. 5. Electron micrograph of a longitudinal section of a 

 locust immature sperm head fixed in 1 % osmium tetroxide. 



to distinguish such gross characteristics as pairing of 

 chromatids (fig. 4) both in transverse section (M) and 

 longitudinal section (N). When thinner sections are 

 used in order to obtain higher resolution the poor 

 preservation makes interpretation almost impossible. 

 Sperm and spermatid heads of the locust have 

 been shown by birefringence studies and ultra-violet 

 dichroism studies (1) to contain nucleic acid mole- 

 cules orientated along the axis of the head. It is, 

 therefore, interesting to examine this material in the 

 electron microscope after various fixatives in order to 

 obtain some idea of the value of these fixatives in 

 preserving chromatin fine structure. While we have 

 not as yet been able to demonstrate structure in 

 mature sperm heads, we have observed well defined 

 and characteristic structure in the heads of sperma- 

 tids. This structure in the most highly developed 

 form we have been able to observe is shown in 

 longitudinal section in fig. 5 where it appears as a 

 large number of parallel lines of thickness about 70 A 

 which are orientated along the axis of the head. The 

 corresponding transverse section — which is from the 

 same cyst in the follicle and so must represent the 

 same stage of development — is shown in fig. 6 

 to be a large number (about 270 in this case) of 

 tightly packed polygons. We interpret these sections 

 to mean that the head contains a large number of 

 parallel tubes orientated along its axis (3). At this 

 stage of development the head is shrinking rapidly 

 and at later stages this structure seems to become 

 too tightly packed to be easily visible and the head 

 appears structureless. At earlier stages of develop- 

 ment we have observed that material first seems to 

 aggregate into sheets and that these sheets then 

 appear to wrap around each other to form the tubed 

 structure described above. 



Fig. 6. Electron micrograph of a transverse section of a 

 locust immature sperm head in the same cyst in the follicle 

 as that shown in fig. 5 and hence at the same stage of de- 

 velopment. 



We have observed this structure after fixation in 

 1 "o buff'ered osmium tetroxide and after 5 % buffered 

 formaldehyde there being little obvious difference 

 in the quality of preservation. We have also been 

 able to observe the structure, though less well- 

 preserved, after fixation in 45 °o acetic acid — a 

 classic but brutal chromatin fixative. 



It must be admitted that the results of the appli- 

 cation of electron microscopy to the study of nuclei 

 have been somewhat disappointing. In particular no 

 further insight has been gained into the processes 

 of mitosis and meiosis which at the level of the 

 light microscope appear so mysterious and dramatic. 

 It is not yet possible to say whether this is due to 

 poor preservation by the existing fixatives, or to the 

 presence of a type of organisation very hard to 

 analyse in thin sections or to an actual lack of orga- 

 nisation within the region of size that can be observed 

 with this technique. Our opinion is that a combina- 

 tion of the first two reasons given above is perhaps 

 the most likely explanation. In this connection we 

 feel it is pertinent to point out the possible use of 

 high magnification stereo-photographs of fairly thin 

 sections (from which removal of embedding material 

 is not necessary) in elucidating the complex fine 

 structure of chromatin. 



References 



1. Caspersson, T., Chromosoma 1, 605 (1940). 



2. Gibbons, I. R. and Bradfield, J. R. G., Biochhu. et Bio- 



pliys. Acta (1957, in press). 



3. — /. Biophys. Biochem. Cytol. (1957, in press). 



4. Palade, G. E., J. E.xptl. Med. 95, 285 (1952). 



5. Porter, K. R. and Kallmann, F., Exptl. Cell Research 



4. 127 (1953). 



