Jonathan L. Tilly

Germline stem cells and follicular renewal in the postnatal mammalian ovary.

A basic doctrine of reproductive biology is that most mammalian females lose the capacity for germ-cell renewal during fetal life, such that a fixed reserve of germ cells (oocytes) enclosed within follicles is endowed at birth. Here we show that juvenile and adult mouse ovaries possess mitotically active germ cells that, based on rates of oocyte degeneration (atresia) and clearance, are needed to continuously replenish the follicle pool. Consistent with this, treatment of prepubertal female mice with the mitotic germ-cell toxicant busulphan eliminates the primordial follicle reserve by early adulthood without inducing atresia. Furthermore, we demonstrate cells expressing the meiotic entry marker synaptonemal complex protein 3 in juvenile and adult mouse ovaries. Wild-type ovaries grafted into transgenic female mice with ubiquitous expression of green fluorescent protein (GFP) become infiltrated with GFP-positive germ cells that form follicles. Collectively, these data establish the existence of proliferative germ cells that sustain oocyte and follicle production in the postnatal mammalian ovary.A basic doctrine of reproductive biology is that most mammalian females lose the capacity for germ-cell renewal during fetal life, such that a fixed reserve of germ cells (oocytes) enclosed within follicles is endowed at birth. Here we show that juvenile and adult mouse ovaries possess mitotically active germ cells that, based on rates of oocyte degeneration (atresia) and clearance, are needed to continuously replenish the follicle pool. Consistent with this, treatment of prepubertal female mice with the mitotic germ-cell toxicant busulphan eliminates the primordial follicle reserve by early adulthood without inducing atresia. Furthermore, we demonstrate cells expressing the meiotic entry marker synaptonemal complex protein 3 in juvenile and adult mouse ovaries. Wild-type ovaries grafted into transgenic female mice with ubiquitous expression of green fluorescent protein (GFP) become infiltrated with GFP-positive germ cells that form follicles. Collectively, these data establish the existence of proliferative germ cells that sustain oocyte and follicle production in the postnatal mammalian ovary.

Oocyte Generation in Adult Mammalian Ovaries by Putative Germ Cells in Bone Marrow and Peripheral Blood

It has been suggested that germline stem cells maintain oogenesis in postnatal mouse ovaries. Here we show that adult mouse ovaries rapidly generate hundreds of oocytes, despite a small premeiotic germ cell pool. In considering the possibility of an extragonadal source of germ cells, we show expression of germline markers in bone marrow (BM). Further, BM transplantation restores oocyte production in wild-type mice sterilized by chemotherapy, as well as in ataxia telangiectasia-mutated gene-deficient mice, which are otherwise incapable of making oocytes. Donor-derived oocytes are also observed in female mice following peripheral blood transplantation. Although the fertilizability and developmental competency of the BM and peripheral blood-derived oocytes remain to be established, their morphology, enclosure within follicles, and expression of germ-cell- and oocyte-specific markers collectively support that these cells are bona fide oocytes. These results identify BM as a potential source of germ cells that could sustain oocyte production in adulthood.

Oocyte apoptosis is suppressed by disruption of the acid sphingomyelinase gene or by sphingosine-1-phosphate therapy.

The time at which ovarian failure (menopause) occurs in females is determined by the size of the oocyte reserve provided at birth, as well as by the rate at which this endowment is depleted throughout post-natal life. Here we show that disruption of the gene for acid sphingomyelinase in female mice suppressed the normal apoptotic deletion of fetal oocytes, leading to neonatal ovarian hyperplasia. Ex vivo, oocytes lacking the gene for acid sphingomyelinase or wild-type oocytes treated with sphingosine-1-phosphate resisted developmental apoptosis and apoptosis induced by anti-cancer therapy, confirming cell autonomy of the death defect. Moreover, radiation-induced oocyte loss in adult wild-type female mice, the event that drives premature ovarian failure and infertility in female cancer patients, was completely prevented by in vivo therapy with sphingosine-1-phosphate. Thus, the sphingomyelin pathway regulates developmental death of oocytes, and sphingosine-1-phosphate provides a new approach to preserve ovarian function in vivo.

Aromatic hydrocarbon receptor-driven Bax gene expression is required for premature ovarian failure caused by biohazardous environmental chemicals

Polycyclic aromatic hydrocarbons (PAHs) are toxic chemicals released into the environment by fossil fuel combustion. Moreover, a primary route of human exposure to PAHs is tobacco smoke. Oocyte destruction and ovarian failure occur in PAH-treated mice, and cigarette smoking causes early menopause in women. In many cells, PAHs activate the aromatic hydrocarbon receptor (Ahr), a member of the Per-Arnt-Sim family of transcription factors. The Ahr is also activated by dioxin, one of the most intensively studied environmental contaminants. Here we show that an exposure of mice to PAHs induces the expression of Bax in oocytes, followed by apoptosis. Ovarian damage caused by PAHs is prevented by Ahr or Bax inactivation. Oocytes microinjected with a Bax promoter–reporter construct show Ahr-dependent transcriptional activation after PAH, but not dioxin, treatment, consistent with findings that dioxin is not cytotoxic to oocytes. This difference in the action of PAHs versus dioxin is conveyed by a single base pair flanking each Ahr response element in the Bax promoter. Oocytes in human ovarian biopsies grafted into immunodeficient mice also accumulate Bax and undergo apoptosis after PAH exposure in vivo. Thus, Ahr-driven Bax transcription is a novel and evolutionarily conserved cell-death signaling pathway responsible for environmental toxicant-induced ovarian failure.

Oocyte formation by mitotically active germ cells purified from ovaries of reproductive-age women

Germline stem cells that produce oocytes in vitro and fertilization-competent eggs in vivo have been identified in and isolated from adult mouse ovaries. Here we describe and validate a fluorescence-activated cell sorting-based protocol that can be used with adult mouse ovaries and human ovarian cortical tissue to purify rare mitotically active cells that have a gene expression profile that is consistent with primitive germ cells. Once established in vitro, these cells can be expanded for months and can spontaneously generate 35- to 50-μm oocytes, as determined by morphology, gene expression and haploid (1n) status. Injection of the human germline cells, engineered to stably express GFP, into human ovarian cortical biopsies leads to formation of follicles containing GFP-positive oocytes 1–2 weeks after xenotransplantation into immunodeficient female mice. Thus, ovaries of reproductive-age women, similar to adult mice, possess rare mitotically active germ cells that can be propagated in vitro as well as generate oocytes in vitro and in vivo.

Prolongation of ovarian lifespan into advanced chronological age by Bax -deficiency

Female mammals are endowed with a finite number of oocytes at birth, each enclosed by a single layer of somatic (granulosa) cells in a primordial follicle. The fate of most follicles is atretic degeneration, a process that culminates in near exhaustion of the oocyte reserve at approximately the fifth decade of life in women, leading to menopause. Apoptosis has a fundamental role in follicular atresia, and recent studies have shown that Bax, which is expressed in both granulosa cells and oocytes, may be central to ovarian cell death. Here we show that young adult female Bax–/– mice possess threefold more primordial follicles in their ovarian reserve than their wild–type sisters, and this surfeit of follicles is maintained in advanced chronological age, such that 20–22–month–old female Bax–/– mice possess hundreds of follicles at all developmental stages and exhibit ovarian steroid–driven uterine hypertrophy. These observations contrast with the ovarian and uterine atrophy seen in aged wild–type female mice. Aged female Bax–/– mice fail to become pregnant when housed with young adult males; however, metaphase II oocytes can be retrieved from, and corpora lutea form in, ovaries of aged Bax–/– females following superovulation with exogenous gonadotropins, and some oocytes are competent for in vitro fertilization and early embryogenesis. Therefore, ovarian lifespan can be extended by selectively disrupting Bax function, but other aspects of normal reproductive performance remain defective in aged Bax–/– female mice.

Commuting the death sentence: how oocytes strive to survive

Programmed cell death claims up to 99.9% of the cells in the mammalian female germ line, which eventually drives irreversible infertility and ovarian failure — the menopause in humans. New insights into the mechanisms that underlie germ-cell apoptosis have been provided by the study of oocyte death in lower organisms and in genetically manipulated mice that lack apoptosis-regulatory proteins. With new therapeutic tools to control fertility, oocyte quality and ovarian lifespan on the horizon, understanding how and why the female body creates, only to delete, so many germ cells is imperative.

Bone Marrow Transplantation Generates Immature Oocytes and Rescues Long-Term Fertility in a Preclinical Mouse Model of Chemotherapy-Induced Premature Ovarian Failure

PURPOSE Although early menopause frequently occurs in female cancer patients after chemotherapy (CTx), bone marrow (BM) transplantation (BMT) has been linked to an unexplained return of ovarian function and fertility in some survivors. Studies modeling this in mice have shown that BMT generates donor-derived oocytes in CTx-treated recipients. However, a subsequent report claimed that ovulated eggs are not derived from BM and that BM-derived oocytes reported previously are misidentified immune cells. This study was conducted to further clarify the impact of BMT on female reproductive function after CTx using a preclinical mouse model. METHODS Female mice were administered CTx followed by BMT using coat color-mismatched female donors. After housing with males, the number of pregnancies and offspring genotype were recorded. For cell tracking, BM from germline-specific green fluorescent protein-transgenic mice was transplanted into CTx-treated wild-type recipients. Immune cells were sorted from blood and analyzed for germline markers. RESULTS BMT rescued long-term fertility in CTx-treated females, but all offspring were derived from the recipient germline. Cell tracking showed that donor-derived oocytes were generated in ovaries of recipients after BMT, and two lines of evidence dispelled the claim that these oocytes are misidentified immune cells. CONCLUSION These data from a preclinical mouse model validate a testable clinical strategy for preserving or resurrecting ovarian function and fertility in female cancer patients after CTx, thus aligning with recommendations of the 2005 National Cancer Institute Breast Cancer Progress Review Group and Presidents Cancer Panel to prioritize research efforts aimed at improving the quality of life in cancer survivors.

Sphingosine 1-phosphate preserves fertility in irradiated female mice without propagating genomic damage in offspring

Sphingosine 1-phosphate preserves fertility in irradiated female mice without propagating genomic damage in offspring

A program of cell death and extracellular matrix degradation is activated in the amnion before the onset of labor.

Fetal membranes usually rupture during the process of labor. Premature fetal membrane rupture occurs not infrequently and is associated with significant fetal and maternal morbidity. The mechanisms of normal and pathologic fetal membrane rupture are not well understood. We have examined structural and biochemical changes in the rat amnion as labor approaches in order to characterize this process in normal pregnancy. Here we report that before the onset of active labor the amnion epithelial cells undergo apoptotic cell death which encompasses degradation of 28S ribosomal subunit RNA and associated P proteins and fragmentation of nuclear DNA. Concurrent with these cellular changes, the amnion type I collagen matrix is degraded with the accumulation of three-quarter length type I collagen fragments in extraembryonic fluid, characteristic of the cleavage of fibrillar collagen by interstitial collagenase. Western blot and immunohistochemical analyses confirmed that interstitial collagenase protein appears in association with the loss of amnion type I collagen. We conclude that amnion epithelial cells undergo a process of programmed cell death associated with orchestrated extracellular matrix degradation which begins before the onset of active labor. Thus, fetal membrane rupture is likely to be the result of biochemical changes as well as physical forces.