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H Chiarini-Garcia and LD Russell

Characteristics of the various type A, intermediate (In) and B spermatogonia were determined in C57BL/6J mice using transmission electron microscopy. Spermatogonia were photographed at all stages of the cycle of the seminiferous epithelium. Over 450 images were taken. Spermatogonia could be classified into As, Apr, Aal, A1 cells, A2 cells, A3 cells, A4 cells, intermediate type and type B cells primarily on the basis of nuclear and nucleolar characteristics. The most primitive spermatogonia (As, Apr, Aal) had mottled chromatin; A1 cells contained homogeneously finely granular chromatin throughout the nucleus; A2, A3, A4 and intermediate type spermatogonia had progressively increasing amounts of chromatin encrusting the nuclear envelope; type B spermatogonia had less heterochromatin along the nuclear envelope, although the particles were more dense and rounded than in intermediate type spermatogonia. Mitochondrial size and position of Golgi complexes varied in different types of spermatogonia. These data show that types of spermatogonia can be differentiated such that these characteristics can be used in functional studies.

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H Chiarini-Garcia, AM Raymer, and LD Russell

The relationships and distribution of spermatogonia were studied as a function of the stage of the seminiferous epithelium cycle in rats. Primitive spermatogonia in the mouse are located along regions of the basal lamina that face the interstitium. Before studying the distribution of spermatogonia in rats, it was necessary to characterize the various types of spermatogonia, as recently performed for mice. The Strauss' linear index (Li) selectivity method was then used and spermatogonia of the A(single) (A(s)) to A(aligned) (A(al)) lineage were preferentially found to be located in regions opposing the interstitium at stages V, VII and IX of the spermatogenic cycle. Because relatively little tubule-to-tubule contact occurs in rats, the aim of this study was to determine whether tubule-to-tubule contact or tubule proximity (or alternatively, the amount of interstitium) was an important factor in spermatogonial position. In this regard, another method (tubule proximity) was devised to determine spermatogonial position that accounted for the presence of adjacent tubules. This method showed that the position of tubules, rather than tubule contact, was more accurate than the Li method in determining the location of spermatogonia in the rat. The results also showed a non-random distribution of spermatogonia resembling that of the mouse, and that tubule-to-tubule contact is not essential for the positioning of spermatogonia. In conclusion, the results of this study strongly indicate that the most primitive type A spermatogonia (A(s), A(paired) and A(al)) in rats are present in niches located in those areas of the seminiferous tubules that border the interstitial tissue.

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DS Johnston, LD Russell, and MD Griswold

Spermatogonial stem cell transplantation was first reported by Ralph Brinster's laboratory in 1994. It has proven to be a technological breakthrough in the study of both stem cells and Sertoli cell-germ cell interactions. This technique can be used to transfer testicular stem cells successfully from one animal to another of the same species (referred to as syngeneic transplants) and sometimes to an animal of a different species (xenogeneic transplants). This transfer technique, combined with developments in cryopreservation, long-term culture, and the enrichment of stem cell populations makes more significant breakthroughs likely in the near future. Ultimately, the application of spermatogonial stem cell transfer will allow transplantation of cultured stem cells manipulated genetically in vitro to give rise to functional male gametes with an altered genotype. This achievement will have applications in basic science, human medicine, and domestic and wild animal reproduction. Although progress toward this goal has been swift, potentially significant barriers, such as the stable incorporation of genetic material into stem cells and immunological responses to the introduced germ cells, remain to be overcome. This article is a review of the scientific advances made since the initial report of successful transplantation in 1994.