Date of Award
12-1-2010
Degree Name
Master of Science
Department
Molecular Cellular and Systemic Physiology
First Advisor
Narayan, Dr. Prema
Abstract
Luteinizing hormone (LH), one of the two gonadotropin hormones released from the anterior pituitary gland, binds to its receptor (LHR) in the gonads to stimulate steroid hormone production, in addition to ovulation and gametogenesis. Mutations of the receptors amino acid sequence have the ability to either constitutively activate or inactivate it. All activating mutations result in male-limited precocious puberty. Males with this condition undergo puberty around 4 years of age, and have a premature elevation in testosterone levels and premature skeletal development. In order to understand how chronic ligand-mediated activation of the LHR affects gonadal development and function, a mouse model expressing a yoked hormone-receptor (YHR) complex, engineered by covalently linking the hormone human chorionic gonadotropin to the rat LHR, has been studied. YHR+ males have prepubertally elevated testosterone and decreased gonadotropin levels, smaller testis, and smaller average seminiferous tubule diameters when compared to wild type (WT) animals. In a preliminary breeding study it was shown that YHR+ males were sub-fertile. Based the phenotype exhibited by the YHR+ mice, it was hypothesized that increased levels of testosterone in addition to decreased gonadotropin hormone levels in neonatal and prepubertal mice that results from premature activation of the luteinizing hormone receptor causes spermatogenesis to be impaired. The first objective of this study was to determine if there was a difference in the testicular germ cell and Sertoli cell populations in the WT and YHR+ animals using flow cytometry and systematic Sertoli cell counting, respectively. There was no difference in the Sertoli cell population between YHR+ animals and WT controls, but there were significantly fewer total germ cells in YHR+ animals at 10 days and from 4 weeks through adulthood. The second objective was to calculate the daily sperm production in the testis and epididymis of WT and YHR+ animals in order to determine if there is a further decrease in the total sperm count due to an epididymal dysfunction. Interestingly, there were significantly fewer sperm calculated in the caput/corpus region of the epididymis in YHR+ males at 12 weeks of age, but not in the testis and cauda epididymis. Furthermore, the daily sperm production in WT and YHR+ mice at 16 weeks of age were not significantly different. The final objective was to determine if the decrease in germ cells observed in YHR+ animals is the result of decreased proliferation or an increase in either germ cell or Sertoli cell apoptosis. Quantification of germ cell and Sertoli cell proliferation revealed no significant difference between the WT and YHR+ animals. Similar findings were found after quantification of apoptotic germ cell and Sertoli cells. Taken together, these data suggest that premature elevation in testosterone and persistently lower levels of circulating follicle stimulating hormone (FSH) are affecting Sertoli cell function, which is causing a reduced germ cell to Sertoli cell ratio in the YHR+ mice. These data suggest that the decrease in testis weight and seminiferous tubule diameter in YHR+ mine is due to a decrease in germ cell rather than Sertoli cell number.
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