INHIBITION OF DEOXYRIBONUCLEIC-ACID SYNTHESIS DURING PREMEIOTIC STAGES OF SPERMATOGENESIS BY A FACTOR FROM TESTIS-ASSOCIATED LYMPHOMYELOID TISSUE IN THE DOGFISH SHARK (SQUALUS-ACANTHIAS)

To investigate factors controlling spermatogonial proliferation, we us ed premeiotic (PrM) spermatocysts (stage-synchronized germ cell/Sertol i cell clones) derived from the testis of the dogfish shark (Squalus a canthias) as an in vitro test system to estimate DNA synthesis by [H-3 ]thymidine incorporation. Coculture of PrM spermatocysts with spermato cysts of the same (PrM) or more advanced stages (M, meiotic; PoM, post meiotic) revealed the presence of an increasing gradient of inhibitory bioactivity from immature to mature stages (PoM>M>PrM). An even more potent and effective inhibition was detected when PrM spermatocysts we re cocultured with equivalent amounts of epigonal organ, a lymphomyelo id tissue encapsulating the testis adjacent to the mature (PoM) region and immediately upstream in the vascular pathway. Inhibitory bioactiv ity also was present in spent media from cultured epigonal fragments a nd in cytosolic subfractions of epigonal homogenates but was undetecta ble in epididymis, muscle, serum, and red blood cells. Lower but signi ficant inhibition was obtained with the white blood cell fraction of p eripheral blood in one of two experiments. Effects of the epigonal gro wth-inhibitory factor (EGIF) were dose- and time-dependent, had a shor t response latency (3 h), were completely reversible (< 24 h after was hout), and counteracted but did not block the stimulatory effects of i nsulin on [H-3]thymidine incorporation by PrM spermatocysts. EGIF was equally inhibitory when tested on each of five PrM substages (stem cel ls -> preleptotene), Analysis of the entire series of experiments show ed that testis-derived inhibitory activity varied seasonally, with max imum effectiveness correlated with periods of spermatogenic inactivity . By contrast, EGIF was effective throughout the year, Results indicat e that a spermatogonial growth regulatory factor emanating from a test is-associated immune tissue enters the testis proper via vascular path ways to form a gradient of inhibitory bioactivity countercurrent to th e spermatogenic progression. This gradient may be par? of a mechanism accounting for the orderly diametric arrangement of maturing spermatoc ysts and the seasonal onset and arrest of spermatogenesis characterist ic of this model, The strict temporal and spatial arrangement of succe ssive germ cell stages in the testis of all vertebrates suggests that a mechanism of this type may have general relevance.