We have shown here that cells extracted from the BM of normal syngeneic mice grafted to a preclinical FSHR knock-out mouse model (FORKO) stimulate follicular maturation to the antral stage, increase E2 production, decrease FSH secretion, and express FSHR mRNA in the ovary. Several investigators have evaluated bone marrow stem cell transplantation for the treatment of different disorders, although the mechanisms of action remain unknown and controversial. Some investigators claim that these cells differentiate, while others claim that the cells may fuse with the existing native cells, improving function of these cells by contributing their own genetic and cellular materials. Our experiments were not designed to address differentiation vs. fusion. Further experiments are clearly necessary to elucidate the exact mechanisms by which stem cells improve organ function. Our study supports the existence of stem cells in adult mammalian female's bone marrow that can influence ovarian function, even though BM transplantations in our experiment did not yield mature eggs following natural or induced ovulation. Regular ovulatory menstrual cycles come about via the coordination between the hypothalamus, pituitary, ovary, and lower reproductive tract. Normal female reproductive function requires an intact hypothalamic-pituitary unit, functional ovaries and a normally responsive uterus 
. The murine estrous cycle generally lasts 4–5 days and can be divided into four stages called proestrus, estrus, metestrus, and diestrus. Characteristic features of specific phases, as described by Rugh et. al. 
, were as follows: a smear consisting almost exclusively of leukocytes depicted diestrus; a thin smear of equal numbers of leukocytes and elongated nucleated epithelium indicated proestrus; large cornified epithelial cells were exclusively found in estrus; and metestrus was marked by a thick smear composed of equal numbers of nucleated epithelial cells and leukocytes. Mice enter the estrus stage in the early hours of the night. Circulating levels of E2 peak prior to ovulation, which occurs at estrus, while P4 levels rise during metestrus and diestrus, and then decline from proestrus to estrus 
. Our animal model carries a mutation in FSHR gene, which lacks any response of granulosa cells to FSH. The stem cells engrafted in our experimental model made granulosa cells responsive to FSH. However, in vitro formation of granulosa cells from embryonic and skin stem cells has been reported in two different studies 
. In these studies, the authors provided evidence of estradiol biosynthesis and expression of FSHR in response to FSH in the follicles derived from skin stem cells. In a more recent study 
, the expression of FSHR in Sertoli cells by bone marrow transplantation in male mice with testicular failure has been reported. Another study has shown that premature ovarian failure and infertility observed in chemotherapy-conditioned adult female mice can be rescued by the transplantation of BM from non-treated adult female donors 
. The functional unit within the ovary is the follicle, which consists of germ cells that become ova, and granulosa and theca cells, which produce steroid hormones. Due to the structural and functional interdependence within the follicle between the sex hormone-producing cells and the oocyte, insults that damage the endocrine cells lead to germ cell failure and infertility 
. Mutations in the FSHR gene have been reported as a cause for primary ovarian failure (POF). Primary ovarian failure, also known as resistance ovarian syndrome (ROS), is a heterogeneous disorder characterized by amenorrhea and infertility in a normal karyotype female with an elevated serum level of follicle-stimulating hormone (FSH) and a decrease level of estrogen. We and other investigators have shown that bone marrow stem cells (BMSCs) are promising grafts for the treatment of a variety of diseases, including reproductive dysfunction 
. The phenotype of FSHR (−/−) mouse, FORKO, is a good model to study human ROS. Female (−/−) mice have elevated serum levels of FSH, decreased E2 levels and are sterile, because of the absence of folliculogenesis at the primary follicle stage. These mice display thin uteri and small nonfunctional ovaries. The objective of our study was to determine the effects of bone marrow transplantation (BMT) on reproductive physiology of this animal model. Interestingly, the treated animals showed estroidogenic changes in the daily vaginal smears, 24 hrs after BMT, while vaginal smears in all control animals remained unchanged. Reproductive organs which are highly modulated by estrogen, both structurally and functionally, showed remarkable increase in weight at all time points of the experiment. Significant increase in both the maturation and the total number of the follicles in treated animals compared to control animals was observed. The peak follicles/ovary was observed at 3 and 6 weeks post BMT. In the study by Balla et.al. 
regarding the dynamics of ovarian development in the FORKO mice, the authors studied ovaries of FORKO mice at different ages and concluded a postnatal and perinatal deficit consequent to FSH receptor ablation. They provided data that showed the total number of follicles was significantly lesser (P
<0.05) in 2-day-old FORKO ovary compared to normal wild type counterpart. At 10 days of age, the total number of follicles was significantly reduced in the FORKOs compared with the wild types (P<
0.05). These differences were spread across different developing follicle stages. They concluded that there was an age dependent change in the number of follicles in the FORKO ovary relative to the normally developing follicles in the wild type. Thus it would be conceivable that reintroducing a functioning FSHR would support an increase in size as well as number of ovarian follicles and prevent reduction the number of follicles in ovary. FSH level dropped to 40–50% and serum level of estrogen increased 4–5.5 times in treated animals compared to control animals. We detected the expression of FSHR mRNA in the ovaries of treated animals, while expression was not observed in other organs.
Our results prove that intravenously injected bone marrow cells stimulate the ovaries of female FSHR (−/−) FORKO mice to express FHSR gene, restore follicular maturation and steroid hormone production. Even though treated females did not ovulate and, even after mating with age-matched males, did not conceive, the ultimate goal of the treatment of infertility, our experiment has set the stage for subsequent efforts in the treatment of this type of infertility in humans, which currently has no effective treatment. Further work is ongoing in our laboratory to better characterize this disease with the long-term goal of devising effective treatment modalities.