Spermatogonial multiplication in the Chinese hamster. III. Labelling indices of undifferentiated spermatogonia throughout the cycle of the seminiferous epithelium.

Radioautography of whole mounts of seminiferous tubules of the Chinese hamster was performed after injection of [3H]thymidine. The labelling index (LI) of undifferentiated spermatogonia was determined throughout the whole cycle of the seminiferous epithelium. The LI of these cells was found to be highest between stages XI and IV. The LI for Apr and Aal spermatogonia decreases around stage III but remains at a relatively high level for As spermatogonia until stage VII. From these data, which agree well with previously recorded cell counts and mitotic indices (Lok, Jansen & Rooij, 1982a; Lok, Weenk & Rooij, 1982b), and from the known cell cycle properties (Lok et al., 1982a), the growth fractions of the various types of undifferentiated spermatogonia during the epithelial cycle could be calculated. Furthermore, it could also be calculated from the labelling indices that the proliferative activity of the As and Apr spermatogonia is sufficient to yield the necessary number of Aal/A1 spermatogonia for each cycle of the seminiferous epithelium. An interesting phenomenon is described; many more labelled Apr spermatogonia were found compared with the number of labelled As spermatogonia than one would expect in a steady state kinetic system. It is proposed that some of the labelled Apr spermatogonia are false pairs and still lose their cytoplasmic bridges at mitosis, thus yielding new As and/or Apr spermatogonia.