Spermatocyte

With regard to the potential function of Raf-1 in mammalian germ cells, the testis is a site of abundant expression of c-ra/-l mRNAs. Northern blot and in situ hybridization analysis has shown that although c-ra/-l is expressed most abundantly in early pachytene spermatocytes, some transcripts were also detected in germ cells from type A and B spermatogonia through to the round spermatid stage (Wolfes et al., 1989 Wadewitz et al., 1993). 

The highly restricted tissue-specific transcription of c-mos poses an interesting problem in gene regulation during germ cell development. Moreover, c-mos is transcribed from different promoters in mouse spermatocytes and oocytes (Fig. 4) (Propst et al., 1987). In spermatocytes, transcription initiates approximately 280 nucleotides upstream of the c-mos ATG (Propst et al., 1987), whereas the transcription start site in oocytes has been mapped to 53 base pairs upstream of the ATG (Pal et al., 1991). Neither the spermatocyte nor oocyte promoter regions are... 

Figure 4. Transcriptional regulatory sequences of the mouse c-mos gene. Transcription in oocytes and spermatocytes initiates 53 and approximately 280 base pairs upstream of the c-mos ATG, respectively. Efficient transcription in oocytes requires an initiator (Inr)-I ike sequence located downstream of the transcription start site. A negative regulatory element (NRE) located upstream of the spermatocyte promoter acts to suppress c-mos transcription in somatic cells.

Figure 6. The c-mos negative regulatory element (NRE). Nucleotide positions of the NRE are shown relative to the spermatocyte transcription start site, taken as 280 base pairs upstream of the c-mos ATG (see Fig. 4). The endpoints of the NRE are defined by deletions that allow c-mos expression in NIH 3T3 and other somatic cells. Mutations of the sequences designated by boxes 1,2, and 3 also allow c-mos transcription in NIH 3T3 cells, indicating that these sequences represent functional elements within the NRE. Boxes 1 and 2 are similar to sequences upstream of the protamine (Prot) promoter that inhibit in vitro transcription in HeLa cell extracts. A sequence just upstream of box 2 is also similar to a putative repressor-binding site in the regulatory region of Pgk2.

It should be noted that the NRE defined in these experiments is distinct from the previously described c-mos UMS sequence (Blair et al., 1984 Wood et al., 1984). The UMS is located approximately 1.4 kb upstream of the c-mos spermatocyte promoter and was identified because it blocked activation of c-mos transforming potential by insertion of retroviral promoters. It is thought to act as a transcriptional terminator, blocking transcription of c-mos initiated at upstream sequences. However, both the spermatocyte and oocyte transcription initiation sites are substantially downstream of the UMS. Moreover, the presence or absence of the UMS does not affect c-mos expression in either microin-jected oocytes (Pal et al., 1991) or transfected NIH 3T3 cells (Zinkel et al., 1992). It thus appears unlikely that the UMS functions as a negative regulator of c-mos transcription from either the spermatocyte or oocyte promoters in somatic cells. 

Also in need of further study is the function of Mos in male germ cells. It appears that Mos is expressed prior to meiosis of spermatocytes, consistent with the possibility that it acts in the initiation of meiosis and/or during progression from meiosis I to meiosis II. However, c-mos expression continues in postmeiotic spermatids, where it does not induce metaphase II arrest. This may be due to the absence of other components of cytostatic factor, but the role of c-mos expression in postmeiotic male germ cells remains unclear. 

Hurtt ME, Bentley KS, Working PK. 1987. Effects of acrylamide and acrylonitrile on unscheduled DNA synthesis (UDS) in rat spermatocytes. Environ Mutagen 9 49-50. 

Adult males given 500 mg Zn/kg ration, as ZnS04, for 6 weeks After 3 weeks, spermatogenesis was arrested at the primary spermatocyte stage. After 4 weeks, food consumption declined, forelimb lameness, and swelling in cervical lymph nodes. At 6 weeks, testes showed enlarged lumen and abnormal germinal epithelium. 20... 

After exposure for 12 days, sexually mature females had significant declines in plasma levels of reproductive and metabolic hormones, and sexually mature males had decreased spermatocytes and selective cell loss in pituitary gland 34, 35... 

Rat (Sprague- Dawley) 24 hr 5000 M (spermatid and spermatocyte degeneration, exfoliation) De Martino et al. 1987... 

Reproductive Effects. Reproductive effects have not been examined in humans after exposure to -hexane. A dominant-lethal test in mice showed no effect on male fertility (Litton Bionetics 1980). No effects were seen on reproductive tissues in male rats after intermediate-duration inhalation exposure at 500 ppm (IRDC 1981) or in either sex of mice after intermediate-duration inhalation exposure to up to 10,000 ppm -hexane (Dunnick et al. 1989 NTP 1991). However, inhalation exposure in male rats to higher concentrations of -hexane showed effects after acute-duration exposure to 5,000 ppm (spermatid and spermatocyte degeneration and exfoliation) and atrophy of testicular germinal epithelium after intermediate-duration exposure to 1,000 ppm (De Martino et al. 1987 Nylen et al. 1989). Testicular atrophy in rats was also noted after intermediate-duration oral exposure at 4,000 mg/kg/day (Krasavage et al. 1980). Similar to -hexane neurotoxicity after inhalation exposure, effects on the testes in rats can be reproduced by oral administration of the w-hexane metabolite 2,5-hexanedione (Chapin et al. 1982 ... 

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