326 



PHYSIOLOGY OF GONADS 



SCHEMAT I C REPRESENT AT I ON 



PROGENY or ONE A CELL 



Of STAGE VIII 



A In B R 



FINAL HYPOTHESIS 



|b [bIb [bIb [bIb [bIb [bIb 



R Ir'rIrirIrirIriR JRiR jfilR iR|RlR!R"|RiRlRiR IrJrIr 



0-12 

 12 



12 0-24 

 24 



Fig. 5.9. Diagrammatic representation of the most probable pattern for the development 

 of spermatogonia (or "stem cell renewal theory"). The Roman numerals on either side 

 of the diagram indicate the stages of the cycle. A, type A spermatogonia; Ad, dormant 

 type A spermatogonia ; In, intermediate type of spermatogonia ; B, type B spermatogonia ; 

 R, resting spermatocyte. In this hypothesis, the two daughter cells of the stage IX mitosis 

 do not divide simultaneously. One of the granddaughter cells becomes a new dormant type 

 A cell (Ad), ensuring the renewal of the spermatogonial population at the subsequent 

 cycle, whereas the three other daughter type A cells divide again to produce intermediate 

 tvpe cells, which in turn produce type B cells, which in turn produce spermatocytes. (From 

 Y. Clermont and C. P. Leblond, Am. J. Anat., 93, 475. 1953.) 



mouse (Fig. 5.9). Type A spermatogonia 

 give rise to either intermediate spermato- 

 gonia or to dormant type A spermatogonia. 

 The intermediate type of spermatogonia 

 gives rise to the type B forms, which pro- 

 duce spermatocytes. The dormant type A 

 spermatogonia are so designated because 

 they do not divide for 8 stages. At the 9th 

 stage, the dormant type A spermatogonium 

 forms 4 large type A spermatogonia. In the 

 next cycle, one of these 4 type A spermato- 

 gonia becomes another dormant type A 

 spermatogonium; the others form 6 of the 

 intermediate types of spermatogonia and 

 eventually 24 spermatocytes. The cytologic 

 details and the alterations in numbers of the 

 three types of spermatogonia are illustrated 

 in Figure 5.10. Full information can only 

 be obtained by consulting the original pa- 

 pers. 



Considerable degeneration of the primary 

 germ cells occurs during development of the 

 testis in the mouse and the rat (Allen and 

 Altland, 1952). Degeneration usually ceases 

 on the ninth day of age in the rat. Over the 

 next 4 days, however, considerable multipli- 

 cation occurs, but from day 14 to day 48 

 degenerating cells also may be seen in many 

 tubules. Six different types of degeneration 

 are evident — loss of cells in layers (exfoli- 

 ation or shedding) , necrosis, loss of individ- 



ual cells, pyknosis, degeneration of lepto- 

 tene forms, and abnormal mitosis in stem 

 cells and spermatocytes. 



The degeneration of the germ cells, or 

 gonocytes, soon after birth had given the 

 impression that the spermatogonia arise 

 from the small supporting cells that also 

 form the Sertoli cells in the adult. Gono- 

 cytes have a large, light, spherical nucleus, 

 fine chromatin, and a sharp nuclear mem- 

 brane. The supporting cells have smaller 

 nuclei and coarse chromatin. The fourth 

 day of life in the rat the supporting cells in- 

 crease in number and form a palisaded layer 

 along the basement membrane. The gono- 

 cytes swell and begin to degenerate; how- 

 ever, some of them look like type A sper- 

 matogonia. By day 6, most of the 

 spermatogonia are tyi)e A but a few inter- 

 mediate spermatogonia and type B forms 

 appear. By days 9 to 12, gonocytes are 

 no longer present. Primary spermatocytes 

 appear for the first time in resting leptotene 

 stages. By days 15 to 18, two generations of 

 germ cells are present. By days 23 to 26, 

 the spermatocytes are in meiotic prophase 

 and some spermatids are being formed. By 

 days 33 to 50, the Sertoli cells have matured. 

 Because the supporting cells do not divide 

 after day 15, type B spermatogonia can- 



