The aim of the present study was to assess whether the whole meiotic process of spermatogenic cells is able to take place in vitro. Fragments of seminiferous tubules from 20-to 22-or 28-day-old rats were seeded in medium containing 0.2% fetal calf serum in bicameral chambers and then cultured for 4 weeks in a chemically defined medium. The differentiation of meiotic germinal cells was followed by four criteria: (i) ultramicroscopic examination of the different types of germ cells present in the cell layer throughout the culture period; (ii) determination of the changes in DNA content per nucleus of the cell population seeded with time in culture; (iii) assessment of the ability of germinal cells to transcribe genes expressed after completion of meiosis; and (iv) monitoring the fate of BrdU-labeled leptotene spermatocytes. The ultrastructural study showed that the overall organization of the cells in the culture well recalls that of the seminiferous epithelium throughout the culture period. Moreover the identification of young round spermatids 21 days after seeding suggested that these spermatids had been formed very recently in culture. Determination of DNA content per nucleus showed that a 1C cell population could be observed after several days of cultures reaching 6 to 10% of total cells. An exponential-like increase in the amounts of the mRNAs encoding for TP1 or TP2 occurred from the time when 1C cells appeared in the culture until the end of the experiment. Finally, BrdU-labeled leptotene spermatocytes differentiated into pachytene spermatocytes and then into secondary spermatocytes, and BdrU-labeled round spermatids were observed from Day 21 of culture onward. Taken together these results indicate that the whole meiotic process from leptotene spermatocyte to round spermatid can indeed occur in vitro under the present culture conditions.

The present study was aimed at examining, by reverse transcription polymerase chain reaction, the expression of germ cell-specific genes in cocultures of Sertoli cells with either pachytene spermatocytes (PS) or round spermatids (RS). In situ hybridization studies showed that the mRNAs encoding phosphoprotein p19 and the testis-specific histone TH2B were specifically expressed in PS whereas those encoding the transition proteins TP1 and TP2 were specific to RS. This resulted in p19:TP1 and TH2B:TP2 ratios that were much higher in PS fractions than in RS fractions prepared by elutriation. When PS or RS were seeded on Sertoli cell monolayers in bicameral chambers, both the number and the viability of the cells decreased during the coculture. However, both parameters were equal to, or higher than, 60% after 2 wk. In PS-Sertoli cell cocultures, the ratios of p19:TP1 and TH2B:TP2 decreased dramatically during the second week of culture; this was due not only to a decrease in the levels of p19 and TH2B mRNAs but also to an enhancement in the relative amounts of TP1 and TP2 as compared to the amounts present on the first day of the coculture. Conversely, both ratios remained low in RS-Sertoli cell cocultures; this was due to a decrease in the levels of the four mRNAs studied during the coculture period. DNA flow cytometry studies showed the occurrence of a haploid cell population (1C) in PS-Sertoli cell cocultures from Day 2 onward, together with a decrease in the tetraploid cell population (4C). No such changes were observed in Sertoli cell-only cultures. By contrast, the haploid population decreased 3-fold during the first week in RS-Sertoli cell cocultures. Immunocytochemical studies demonstrated further that 5-bromo-2'-deoxyuridine-labeled PS of stages V-VIII were able to differentiate into RS under the present coculture conditions. Hence, although clearly imperfect, the present coculture system should help to clarify the local regulations governing spermatogenesis and should allow easier study of spermatogenic cell gene expression.

The aim of the present study was to set up a culture system allowing most of the meiotic phase of rat spermatogenesis to occur in vitro. For that purpose, the differentiation of spermatogenic cells was monitored by three criteria: 1) examination of expression of genes specifically expressed at a high level in pachytene spermatocytes (the phosphoprotein p19 [p19] and the testis-specific histone TH2B) or in round spermatids (transition protein 1 [TP1] and transition protein 2 [TP2]) by reverse transcription-polymerase chain reaction (RT-PCR); 2) ploidy analysis; and 3) cytological and immunocytochemical study of the germ cells. In the first trial, we determined the changes in the ratios of p19:TP1 and TH2B:TP2 mRNA-related PCR products in the whole testis of rats between 18 and 60 days postpartum and related those results to the sequential appearance of the various types of spermatogenic cells during that period. In the second trial, our aim was to reproduce, in a culture system using seminiferous tubules from 23- to 25-day-old rats, the changes observed in vivo. The p19:TP1 and TH2B:TP2 ratios decreased dramatically in testicular extracts of rats between 32 and 40 days postpartum, i.e., at the time period during which round spermatids become more and more numerous in the testis. When seminiferous tubules were seeded in bicameral chambers, cell viability remained close to 70% of total cells throughout the 3-wk culture period. Both p19:TP1 and TH2B:TP2 ratios decreased during the first week of culture. This was attributable to a decrease in the levels of p19 and TH2B mRNAs and also to an enhancement in the relative amounts of TP1 and TP2. These changes were correlated with the appearance of a 1C cell population in the culture. Histological examination of the culture demonstrated that under the conditions of the present study, 5-bromo-2'-deoxyuridine-labeled pachytene spermatocytes of stages IV-VI were able to differentiate into secondary spermatocytes, then into round spermatids.