BIOLOGY OF SPERMATOZOA 



753 



cleus, instead of occupying only the pos- 

 terior portion, seems rather to extend the 

 entire length of the head (cf. Bishop and 

 Austin, 19571. During differentiation, the 

 nuclear cln-omatin condenses into a homo- 

 geneous, electron-dense mass, but Yasu- 

 zumi, Fujinmra, Tanaka, Ishida and Ma- 

 suda (19561 demonstrated in enzymatically 

 treated bull sperm helical strands which 

 may correspond to distinct chromosomes. 

 During spermiogenesis in the guinea pig the 

 four spermatids resulting from meiosis re- 

 main attached by intercellular bridges until 

 late in the development of the gametes 

 (Fawcett, 1959). Such connections may al- 

 low for significant interchange of materials 

 and for mutual interaction among the mem- 

 bers of the tetrad. 



Electron microscopy has confirmed the 

 traditional view that there are two cen- 

 trioles present in the neck region of the 

 sperm which are directly or indirectly as- 

 sociated with the axillary bundle extending 

 into the flagellum (Fawcett, 1958). The 

 homology of the centriolar body with the 

 basal granule (blepharoplast) is assumed. 



The spiral body, typical of the middle 

 piece of the sperm, is made up principally of 

 the mitochondrial elements, arranged spi- 

 rally but not in a continuous helix. The dis- 

 tribution of the mitochondria, constituting 

 in large measure the "power plant" of the 

 cell by reason of their oxidative and phos- 

 phorylative activities, is in close association 

 with the flagellar apparatus, particularly 

 the fibrillar elements of the tail. The mito- 

 chondrial system is derived from or related 

 to the Nebenkern, a prominent cell inclusion 

 in spermatids of lower forms. In some in- 

 sect sperm, in the absence of a true mid- 

 piece, the mitochondria extend far down into 

 the flagellum (Rothschild, 1955). What had 

 been considered the helical covering of the 

 sperm tail might better be regarded as a 

 "fibrous sheath" since the structure is nei- 

 ther continuous nor constituted of uniform 

 successive gyres (Fawcett, 1958). The outer 

 membrane, probably the true physiologic 

 surface of the cell, is a continuous envelope 

 and is apparently derived from the sper- 

 matid cell membrane. 



Emanating from electron micrographic in- 

 vestigations, a universal fibrillar pattern in 



flagella and cilia is generally acknowledgeu. 

 ]\lodifications exist but incontestable evi- 

 dence indicates that the basic arrangement, 

 as seen in transverse sections, is the now 

 familiar 2 X 9 -t- 2 array. Surrounding 2 

 central filaments is a ring of 9 double fibrils 

 (Figs. 13.16, 13.17^1, B), all of which seem 

 to extend, uninterrupted, from proximal to 

 distal tip of the flagellum. On extensive, but 

 nevertheless largely circumstantial evidence, 

 the outer filaments are generally regarded 

 as the motile organelles. Inoue (1959), how- 

 ever, in summarizing the evidence pertinent 

 to ciliary movement, suggests that the outer 

 fibrils may actually be conductile elements, 

 whereas the two central filaments take a 

 more active part in motility. Certain other 

 features of the sperm tail, including the 



Fig. 13.16. Electron micrographs of fowl sperm 

 flagella. Of the 11 major filaments, two (M fibrils) 

 are differentiated from the remainder and consti- 

 tute the central pair. Sperm were exposed to dis- 

 tilled water, fixed in formalin, and shadow-cast 

 with platinum. (From G. W. Grigg and A. J. 

 Hodge, Australian J. Scient. Res., ser. B, 2, 271-286, 

 1949.) 



