Genetic differences in follicular DNA synthesis between two strains of hamsters

This study was designed to compare our previous results on ovarian follicular DNA synthesis by hamsters obtained from Sasco Laboratories with a different breeding colony: Harlan. Follicles from proestrous Harlan hamsters required twice as much [3H] thymidine and a minimum of 4 hr of in vitro exposure to 100 ng of ovine follicle-stimulating hormone (FSH) before a significant increase in DNA synthesis was elicited compared with 30-120 min for the Sasco breed. Peak responsiveness to FSH was observed at 8-hr incubation for the Harlan strain with significant increases in DNA per follicle at 8-12 hr. Both strains increased DNA synthesis with as little as 25 ng of ovine FSH and the response was elicited in all growing follicles, from preantral stages with one to four layers of granulosa cells, lacking theca (Stages 1-4) to mature antral follicles (Stages 8-10). A recombinant bovine FSH, devoid of luteinizing hormone activity, was not as effective as ovine FSH (which has 4% luteinizing hormone contamination) in stimulating DNA synthesis by large preantral and antral follicles. In vitro responsiveness to ovine FSH was abolished in the absence of Ca2+ in the culture medium and 0.05 mM Ca2+ was the optimal amount. For both strains of hamsters, the highest rate of DNA synthesis in response to endogenous gonadotropins was on the morning of estrus--when the second surge of FSH was in progress--and Harlan follicles in vitro also showed maximal stimulation by FSH on this day. Where the two strains differed was that the Harlan strain did not show an increase in follicular DNA synthesis on the afternoon of proestrus--when the preovulatory increase in gonadotropins commenced. When expressed as DNA per follicle, DNA approximately doubled from Stages 1 to 5 and then entered a new growth phase at Stage 6 (large preantral follicles) with a steeper increase. Collectively, these experiments show that strain characteristics can alter the latency and degree of follicular DNA replication in response to endogenous or exogenous FSH.