100 



Cellular Structure and Activity 



some, centriole and aster), spindle, kineto- 

 chore, and chromosomal fibers. 



CENTRosoME. The cells of animals and 

 lower plants contain a self-duplicating cyto- 

 plasmic structure, the centrosome. During 

 mitosis it organizes the aster and plays a part 

 in the polarization of the spindle. In certain 

 cells it also acts as blepharoplast organizing 

 the flagellum of flagellated cells and the axial 



N.sp. 



Fig. 15. Fertilization in Nereis. Amphiaster and 

 spindle of sperm and second maturation spindle of 

 egg. Note the difference between the central spin- 

 dles (C.sp.) and the nuclear spindles (N.sp.). (After 

 Liilie, '12.) 



filament of spermatozoa. It is probably re- 

 lated to the basal body of cilia in ciliated 

 cells (Renyi, '24) and the kinetosomes of the 

 Ciliata (Lwoff, '50). 



The centrosome usually consists of a small 

 granule (centriole) that is either round, rod- 

 shaped or V-shaped (for examples see John- 

 son, '31) and is surrounded by a spherical, 

 homogeneous or finely granular area of cyto- 

 plasm. Its history during the mitotic cycle is 

 illustrated in Figure 14, which represents 

 the nuclear division during early cleavage in 

 Drosophila (Huettner, '33). The centriole 

 divides during mitosis and persists during 

 interphase as a double body (diplosome) 

 (for examples in amphibian tissues see Pol- 

 lister, '33). The centrosome divides in pro- 

 phase. Each half contains a centriole and 

 forms an aster. Under favorable conditions 

 the centrosomes and centrioles are visible in 

 the living cell both during mitosis (Fig. 12c) 

 and in the non-dividing cell. The size of 

 centrioles is usually near the limit of resolu- 

 tion of the light microscope (about 0.2 mi- 

 cron) but the rod-shaped centrioles of some 

 insects may be more than 1 micron long (see 

 for instance Johnson, '31). Changes in size 

 and stainability during the mitotic cycle (for 



instance Jorgensen, '13; Johnson, '31; Chick- 

 ering, '27) in tissue culture and in ttunor 

 cells (Ludford, '25) have been reported. The 

 chemical composition of the centriole is un- 

 known. During prophase the aster develops 

 around the centriole. The aster rays are posi- 

 tively birefringent gel fibers (cf. Inoue and 

 Dan, '51) and are anchored in the cortical 

 gel. They can be moved around and bent 

 with the microdissection needle (Chambers, 

 '17). Often they are visible in the living cell, 

 especially where the cytoplasm contains 

 many granular inclusions that contrast with 

 the clear aster rays, or where filamentous 

 mitochondria become oriented between the 

 aster rays (Fell and Hughes, '49). According 

 to Chambers, aster rays are hollow canals 

 but this has not been confirmed by other in- 

 vestigators. More likely the observed flow of 

 less viscous cytoplasm occurs between the 

 aster rays. The area around the centriole is 

 usually free of cytoplasmic inclusions and 

 increases in size from prophase to anaphase. 

 Aster-like structures are sometimes present 

 also in the cytoplasm of non-dividing cells 

 such as leukocytes, mesenchyme cells and 

 other cells in which the centriole is near the 

 center of the cell. Accumulation of hyalo- 

 plasm around the centriole in such cells was 

 observed by Lewis ('20). 



Apparently the centriole influences struc- 

 ture and orientation in the cytoplasm not 

 only during division but also in the resting 

 cell, possibly by setting up diffusion currents 

 as Pollister ('41) has suggested. 



The rays between two asters usually con- 

 nect so that a spindle-like structure is formed 

 (amphiaster). This purely cytoplasmic struc- 

 tvire has been called the "central spindle," 

 but is entirely different from the real mitotic 

 spindle in which chromosome movement 

 takes place. The "central spindle" is a system 

 of aster rays, and cytoplasmic inclusions are 

 free to move through it between the rays 

 (Fig. 15). 



Chemically the asters contain, in addition 

 to protein, pentose nucleic acids (Brachet, 

 '42; Pollister and Ris, '47; Stich, '51a) and 

 polysaccharides (Monne and Slautterback, 

 '50; Stich, '51b). 



Supernumerary asters (cytasters) appear 

 in the cytoplasm of some invertebrate eggs 

 after certain experimental treatments (cf. 

 Wilson, '28). This fact has sometimes been 

 used as evidence for a de novo origin of 

 centrioles. However, it has not been estab- 

 lished that cytasters contain centrioles and 

 the possibility thus exists that asters may 

 arise independently of real centrioles. 



