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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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2

State of the Science

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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The first day of the workshop focused on the state of the science for in vitro gametogenesis (IVG). Experts discussed research achievements with humans and other mammalian model systems, roadblocks for fully reconstituting human gametogenesis in vitro, and near-term prospects for overcoming the remaining scientific and technical barriers. The presentations and discussions were moderated by Amander Clark (University of California, Los Angeles) and Kotaro Sasaki (University of Pennsylvania).

CONTEXT AND BACKGROUND

Azim Surani of the Gurdon Institute at the University of Cambridge oriented participants to subsequent scientific conversations by providing a high-level overview of germ cell development and animal model systems used in germ cell biology research.

Early Germ Cell Development in Humans

Surani outlined the initial stages of human embryonic germ cell development (see Figure 2-1). Gamete precursors are first specified as primordial germ cells (PGCs) from pluripotent epiblast cells in the gastrula stage embryo about 2 weeks postfertilization. PGCs migrate to the

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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Image
FIGURE 2-1 Human germ cell development.
SOURCE: Presented by Surani, April 19, 2023.

developing gonads, reaching their destination at around 6 weeks after fertilization. While germ cells are migrating, they go through extensive epigenetic reprogramming and genomic imprinting that Surani noted is vital for their ability to give rise to the next generation. Upon colonizing the gonads, bipotential PGCs are exposed to somatic cell signals that induce their differentiation into prospermatogonia in a testicular environment and oogonia in an ovarian environment.

Given the lack of availability of newly specified human PGCs, Surani said that in vitro reconstitution of PGC-like cells (PGCLCs) from pluripotent stem cells (PSCs) (i.e., embryonic stem cells [ESCs]1 or induced pluripotent stem cells [iPSCs]2) has been used to reveal key cell fate decisions during PGC specification. Experiments using PGCLCs identified the three key transcriptional regulators controlling the human germ cell fate: SOX17, TFAP2C, and PRDM1 (Tang et al., 2016). In addition to in vitro systems, Surani also asserted the value of examining PGCs isolated from human embryos in high molecular detail at different developmental

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1 Pluripotent epiblast cells found in the inner cell mass of embryos at the blastocyst stage are used to derive ESCs capable of giving rise to any cell type.

2 iPSCs are diverse body cells that have been reprogrammed into a pluripotent epiblast embryonic-like state.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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stages. Surani posited that this information, although descriptive, could be used to evaluate the authenticity of in vitro–derived gametes if developed.

Animal Models of Germ Cell Development

Many of the foundational IVG experiments were performed using rodent model systems. Despite several key conserved regulatory features (i.e., the role of proteins such as TFAP2C and PRDM1), early embryonic germ cell fate specification differs between rodents and humans (Tang et al., 2016). Surani noted that PRDM14 appears to be a key transcriptional regulator of PGC specification in rodents (Tang et al., 2016; Yamaji et al., 2008), whereas SOX17 regulates this in humans and other mammals (e.g., rabbits, pigs, nonhuman primates) (Alberio et al., 2021; Irie et al., 2015; Kobayashi et al., 2017, 2021; Kojima et al., 2017). Structural differences in the postimplantation embryo may contribute to these variations. Embryos in humans and most other mammals develop as bilaminar discs, whereas rodent embryos develop as egg cylinders (Tang et al., 2016). Surani suggested that rabbits could serve as an important surrogate model of human germ cell development because (1) PGC development more closely resembles that of humans, (2) genetic experiments and direct imaging are possible, and (3) rabbits have a short gestation period (Kobayashi et al., 2021). Investigations on such species could “accelerate the gain of vital knowledge” toward authentically reconstituting human gametogenesis, he said.

LESSONS LEARNED FROM IVG IN RODENTS

Pioneering research in mice has sparked many advancements in human reproductive technologies, including in vitro fertilization, cryopreservation, ESCs, and iPSCs. Katsuhiko Hayashi of Osaka University discussed the foundational research in mice that has led many to question whether IVG will be possible in humans.

Key Elements for IVG

In a landmark achievement for the field, Hayashi successfully reconstituted the totality of oogenesis in vitro from mouse PSCs; Mitinori Saitou achieved the same for spermatogenesis. Based on lessons learned from this research, Hayashi identified “three essential elements” for reconstituting gametogenesis in culture (see Figure 2-2):

  1. Generating PGCLCs from PSCs;
  2. Producing gonadal somatic cell–like cells from PSCs; and
Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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FIGURE 2-2 Three essential elements for reconstituting gametogenesis in culture.
NOTE: Hayashi identified (1) PGCs, the precursor of the gametes; (2) gonadal somatic cells, cells that provide an environment for gametogenesis; and (3) optimal culture conditions for gamete maturation.
SOURCE: Presented by Hayashi, April 19, 2023.
  1. Coculturing PGCLCs and gonadal somatic cell–like cells for functional gamete maturation.

As Surani explained, PGCs rely on signals from gonadal somatic cells to differentiate into sperm in a testicular environment or eggs in an ovarian environment. Therefore, fully reconstituting gametogenesis in vitro requires coculturing PGCLCs and gonadal somatic cell–like cells under optimal conditions (Hayashi et al., 2017; Hikabe et al., 2016; Yoshino et al., 2021). Hayashi described how his laboratory fulfilled these three requirements and reconstituted functional eggs from murine ESCs without using embryonic tissues. After fertilizing in vitro–derived eggs with in vivo sperm, embryos were transplanted into surrogate mothers at the two-cell stage, leading to the birth of pups (Yoshino et al., 2021).

Sex-Converted Gametes

Next, Hayashi touched on recent work in his laboratory generating “sex-converted gametes”: in vitro–derived eggs from male mice (Murakami et al., 2023). To reconstitute eggs from XY iPSCs, Hayashi and his colleagues converted the XY chromosome set to XX, which relied on cells spontaneously losing their Y chromosome in culture and then duplicating the X chromosome. Next, sex-converted XX iPSCs were cultured to

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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generate PGCLCs, which were aggregated and cultured with embryonic female gonadal somatic cells to generate functional eggs. These sex-converted, in vitro–derived eggs were fertilized and yielded pups, albeit at a low efficiency.

Quality of IVG-Derived Eggs

Although scientists can make functional eggs from iPSCs, Hayashi said that they cannot yet faithfully reconstitute egg quality. Mature in vitro–derived oocytes can have abnormal gene expression, low mitochondrial DNA copy number, mispaired chromosomes in meiotic prophase, high rates of aneuploidy, fragile cytoplasmic membranes, and low developmental potential: only 5.2 percent of embryos created using an in vitro–derived egg gave rise to pups compared to 61.7 percent of embryos created using an in vivo–derived egg (Yoshino et al., 2021). For sex-converted eggs, the efficiency is even lower; only 1 percent of embryos yielded pups (Murakami et al., 2023). Therefore, Hayashi emphasized the need to refine IVG culture conditions and develop methods to select high-quality eggs.

THE “UNFINISHED AGENDA”3 IN HUMANS AND NONHUMAN PRIMATES

Scientists at the forefront of IVG research, Mitinori Saitou of Kyoto University and Kotaro Sasaki of the University of Pennsylvania, addressed efforts to reconstitute gametes from PSCs in nonhuman primates and humans. Neither oogenesis nor spermatogenesis has been reconstructed in full. Saitou and Sasaki outlined the landscape of the field, recent advancements, and roadblocks to success.

PGCs

Oogenesis and spermatogenesis are complicated processes that ultimately begin with the same foundational cells: PGCs. Therefore, Sasaki said, the first step in recapitulating gametogenesis in vitro is producing these cells from PSCs. In a foundational achievement for the field, Saitou and Sasaki established a robust method to produce human PGCLCs (hPGCLCs) from iPSCs (Sasaki et al., 2015). First, iPSCs are differentiated into incipient mesoderm-like cells (iMeLCs). Then, iMeLCs are stimulated with key cytokines (i.e., BMP4, LIF, SCF, EGF) to generate hPGCLCs. Based on transcriptomic analyses, Saitou and Sasaki concluded

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3 A quote from Adashi’s opening remarks at the workshop referencing what has yet to be completed in human and nonhuman primates.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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that hPGCLCs closely resemble PGCs in their premigratory stage (Sasaki et al., 2016).

Oogenesis

PGCs begin to differentiate into oogonia upon reaching the developing ovary. At the onset of oogenesis, oogonia respond to signals from somatic cells in the fetal ovary by differentiating into primary oocytes and entering meiotic prophase. The oocytes create primordial follicles by forming tight complexes with granulosa cells. Oocytes advance into meiotic prophase, and development is paused until puberty, when hormonal cues signal follicles to resume growth and develop into fully matured antral follicles. Upon ovulation and fertilization with spermatozoa, the antral follicles complete their first and second meiotic divisions, respectively (Saitou and Hayashi, 2021). Reconstructing oogenesis in vitro requires authentic recapitulation of each of these phases (see Figure 2-3).

Saitou discussed efforts in his laboratory to differentiate hPGCLCs into oogonia, the initial differentiating step in oogenesis, using a xenogeneic4 culture system (Yamashiro et al., 2018). Saitou and his colleagues combined hPGCLCs with mouse fetal ovarian somatic cells to form reconstituted ovarian organoids.5 In this culture system, hPGCLCs spontaneously acquire transcriptomic and epigenetic properties similar to those of human oogonia, notably including genome-wide DNA methylation. Therefore, Saitou concluded that oogonia-like cells can be reconstituted under these culture conditions.

Saitou then described research aimed at reconstituting later stages of oogenesis using PGCLCs derived from cynomolgus monkey ESCs (Gyobu-Motani et al., 2023). By culturing cynomolgus monkey PGCLCs in reconstituted ovaries as described, Saitou and his colleagues successfully reconstituted meiotic oocytes in the zygotene stage at a low efficiency. However, transcriptomic analyses revealed that these cells progressed into meiosis through a pathway distinct from in vivo cynomolgus monkey fetal oocytes. Based on these results, Saitou concluded that meiosis is impaired in xenogeneic culture systems, and syngeneic6 systems need to be developed instead to reconstitute later stages of gametogenesis more authentically.

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4 Xenogeneic culture systems involve cells belonging to different species: in this case, PGCLCs derived from humans and fetal ovarian somatic cells derived from mice.

5 Organoids are three-dimensional, organ-like structures made by growing stem cells under specialized culture conditions.

6 Syngeneic culture systems use cells all belonging to the same species.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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Image
FIGURE 2-3 Schematic of human in vitro gametogenesis.
NOTE: GV = germinal vesicle; hPGCLCs = human primordial germ cell–like cells; hPSCs = human pluripotent stem cells; ICSI = intracytoplasmic sperm injection; IVF = in vitro fertilization; IVM = in vitro meiosis; MII = metaphase II.
SOURCES: Presented by Saitou, April 19, 2023; Ishii and Saitou, 2017.

By developing syngeneic culture systems, Saitou and his colleagues successfully reconstituted human and cynomolgus monkey fetal oocyte development (Mizuta et al., 2022). Human and cynomolgus monkey fetal ovaries, in which oogonia are the predominant germ cell population, were dissociated into single cells and reaggregated to form reconstituted ovaries that were cultured under floating conditions. In this syngeneic culture system, human and cynomolgus monkey oogonia entered and completed the first meiotic prophase to differentiate into diplotene oocytes that formed primordial follicles. The next step, Saitou said, is reconstructing the process with PGCLCs and somatic cell–like cells produced from human or cynomolgus monkey iPSCs.

Saitou closed by identifying key challenges for human IVG. The syngeneic experiments described relied on ex vivo reconstituted ovaries. To fully reconstitute gametogenesis, human ovarian somatic cell–like cells must be produced from iPSCs and optimum coculture conditions with hPGCLCs must be identified. Furthermore, culture conditions must support appropriate ovarian follicle growth. Finally, Saitou echoed Hayashi on the impor-

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

tance of understanding and controlling factors that affect the efficiency of the development processes and critically evaluating the genetic and epigenetic properties of in vitro–derived oocytes to determine their quality.

Spermatogenesis

To orient participants to discussions on reconstituting human spermatogenesis, Sasaki provided a brief overview of male germ cell development. Bipotential PGCs are specified early in development and then migrate to the developing testis, where they differentiate into M prospermatogonia and begin to proliferate. At the beginning of the second trimester, M prospermatogonia gradually transition into T1 prospermatogonia and become quiescent. After birth, T1 prospermatogonia remain quiescent, but differentiate into undifferentiated spermatogonia. Spermatogenesis resumes at the onset of puberty, when these cells undergo numerous transitions to eventually yield spermatids. To reconstruct this process in vitro, Sasaki noted, each step must be carefully recapitulated (see Figure 2-4).

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FIGURE 2-4 Schematic of mouse and human spermatogenesis.
NOTE: E = embryonic day; P = postnatal day; PGC = primordial germ cell; wpf = weeks postfertilization; yo = years old.
SOURCES: Presented by Sasaki, April 19, 2023; Sasaki and Sangrithi, 2023.
Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

Sasaki presented his laboratory’s achievements producing prospermatogonia from hPGCLCs, the initial differentiating step in spermatogenesis (Hwang et al., 2020). These attempts to reconstruct spermatogenesis relied on culturing hPGCLCs with mouse fetal testes cells; these xenogeneic reconstituted testis organoids form seminiferous tubules with PGCLCs incorporated along the basement membrane. Single-cell sequencing transcriptomic analyses indicated that this culture system successfully reconstitutes M prospermatogonia–like cells and T1 prospermatogonia–like cells, although efficiency remains to be more fully understood and optimized.

To reconstruct later stages of spermatogenesis, Sasaki and his colleagues transplanted these reconstituted testes under the kidney capsule of immunodeficient mice. After culturing for 6 months, they compared single-cell sequencing transcriptomic analyses from human iPSC–derived spermatogonia to a comprehensive human spermatogenesis atlas. Sasaki concluded that this culture system can reconstruct several stages of spermatogenesis, including meiotic spermatocytes through the leptotene/zygotene stages. Neither spermatids nor spermatocytes in later stages of meiosis have been recapitulated in vitro this way.

Sasaki acknowledged the difficulty of validating the functionality of in vitro–derived human gametes due to ethical and legal constraints. He suggested that experiments in nonhuman primate models will be critical to fill this gap. PGCLCs have been established for marmosets (Seita et al., 2023), and Sasaki and his colleagues are now performing the described experiments using reconstituted testes under the kidney capsule of immunodeficient mice. Sasaki hopes that “parallel and complementary” research approaches in humans and nonhuman primates will help complete the reconstruction of human spermatogenesis in vitro.

ASSAYING THE FUNCTIONAL POTENTIAL OF IVG-DERIVED GAMETES

If gametogenesis is successfully reconstituted, then these in vitro–derived cells need to be able to perform all the functions of a healthy gamete. As an expert in spermatogonial stem cell (SSC) transplantation, Kyle Orwig of the University of Pittsburgh School of Medicine discussed pathways to assay the functional potential of in vitro–derived gametes.

SSC Transplantation

Spermatogenesis occurs in the seminiferous tubules of the testis, all of which begin and end in the rete testis. Experiments in several animal systems, including pioneering experiments by Brinster in mice (see Fig-

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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Image
FIGURE 2-5 Spermatogonial stem cell transplantation.
SOURCES: Presented by Orwig, April 19, 2023; Brinster, 2002.

ure 2-5), have shown that SSCs can be transplanted into the seminiferous tubules through the rete testis to regenerate spermatogenesis (Brinster, 2002; Kanatsu-Shinohara et al., 2003; Kubota et al., 2004; Ogawa et al., 2000; Tran et al., 2022). If SSCs are produced in vitro that are capable of recapitulating spermatogenesis, Orwig said, those cells should engraft into the basement membrane of the seminiferous tubules and initiate spermatogenesis. This system could be used to validate the function of in vitro–derived gametes from nonhuman primates.

Xenotransplant Assays

Considering that SSC transplantation cannot be used as a routine bioassay for every species, other functional assays are needed to evaluate in vitro–derived gametes from nonhuman primates and humans, Orwig said. Xenotransplant assays may be a valuable functional tool. When rat SSCs are transplanted into the mouse testis, they populate the seminiferous tubule, ultimately producing rat sperm that can be used successfully to create progeny (Clouthier et al., 1996; Orwig et al., 2002; Shinohara et al., 2006). Taken a step further, nonhuman primate and human SSCs have also been xenotransplanted into mouse testes. SSCs of both species recapitulate the early stages of gametogenesis but fail to complete spermatogenesis (Nagano et al., 2001, 2002). Even so, xenotransplantation can still serve as a useful bioassay, Orwig stated, to test for whether transplanted cells

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

(1) migrate and engraft the basement membrane of seminiferous tubules, (2) proliferate to produce characteristic chains and clusters of spermatogonia (frequently connected by intercytoplasmic bridges), and (3) persist for a long time. Therefore, xenotransplant assays allow scientists to determine whether the transplanted cells are behaving like germ cells, Orwig concluded.

Building a Testis-Physiomimetic System

Given the limitations of the xenotransplant assay, Orwig urged that in vitro methods need to be developed that can support every stage of primate gametogenesis. He then discussed progress and identified some roadblocks for developing such methods. Research in mice has shown that the heterogenous cells of the testis can self-organize into seminiferous tubules when grafted ectopically.7 However, organoids from human testis cells have had highly variable outcomes. Sometimes seminiferous tubules fail to form, and when they do, they often fail to orient correctly; Orwig posited that structural information may be needed to direct them to organize appropriately. Organoids are now being studied on bioprinted three-dimensional constructs that have structures the size and shape of human seminiferous tubules. However, based on the self-organizing properties of the testis, Orwig can imagine a future where mixing in vitro–derived gametes with either somatic cells or in vitro–derived somatic cells could regenerate a seminiferous epithelium complete with spermatogenesis.

Translation to Human Systems and Potentially to the Clinic

Orwig stated that primate models are “critical to establish safety and feasibility that will justify translation to the clinic,” particularly for indicating whether in vitro–derived gametes have fertilization potential and can produce heathy offspring. Studies on human cells are necessary, but the tools to study the function of these cells are still in development. Orwig closed by acknowledging that the “human patient is the ultimate animal model,” so it is possible scientists would learn additional information from clinical use of IVG in humans that could not have been predicted from the preclinical work in human cells or with other animals.

ROADBLOCKS FOR ACHIEVING IVG

Germ cells are distinct from somatic cells in many ways, and these differences pose some of the greatest barriers to IVG. Three experts in

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7 These cells were transplanted to a region of the body outside of the testis.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

their respective fields, Paula Cohen (Cornell University), Petra Hajkova (Imperial College), and Anne Goriely (MRC Weatherall Institute of Molecular Medicine at the University of Oxford), discussed features and processes that are unique to germ cells and their implications for successfully achieving IVG.

Meiosis—A Biological Hurdle

Meiosis is a special type of cell division during which haploid gametes are produced from diploid cells.8 Paula Cohen reflected on why meiosis is “one of the biggest roadblocks” for IVG development and what would need to be considered to move the technology forward in both males and females.

Meiotic Prophase I

Paula Cohen began by offering a brief overview of meiosis prophase I, “the business end of meiosis where all the bottlenecks are really happening,” when the synaptonemal complex assembles to physically tether chromosomes together through protein–protein interactions. Simultaneously, hundreds of double strand breaks (DSBs) are created to achieve pairing and recombination between maternal and paternal chromosomes. These crossover events, marked by the protein MLH1, yield a set number of chiasmata, which are physical structures that ensure accurate segregation of chromosomes at the first meiotic division. Because most DSBs do not lead to crossover events, the rest must be repaired to avoid meiotic catastrophe. Ultimately, Paula Cohen emphasized that “all of these features must be recapitulated in an in vitro or ex vivo system” to yield functional gametes.

Sex Differences in Meiosis

Male and female germ cells progress through meiotic prophase differently. Female germ cells enter prophase I during embryogenesis, but male germ cells do not do so until puberty. Substantial differences in meiotic error rates also exist between male and female germ cells (Hua and Liu, 2021). The checkpoints that regulate prophase I are less stringent in female cells. In humans, >30 percent of female germ cells have non-disjunction events during meiosis versus only 1–3 percent in male germ cells. Faithfully reconstituting oogenesis and spermatogenesis will neces-

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8 Haploid cells contain a single set of unpaired chromosomes; diploid cells contain two complete sets of chromosomes, one from each parent.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

sitate a deeper understanding of the differences in timing and phenotypic errors between male and female meiosis, Paula Cohen concluded.

Implications for IVG

Although meiosis can proceed in mouse PGCLCs in vitro to yield gametes that produce viable pups, its efficiency is limited (Zhou et al., 2016). Furthermore, Paula Cohen emphasized, the intricacies of in vitro meiosis have not been fully examined, including DSB repair and MLH1 loading as a proxy for crossover events (Lei et al., 2020; Zhou et al., 2016), so it remains unclear whether meiosis is proceeding normally and/or chromosomal imbalances have occurred. Efforts in her laboratory are aimed at developing culture conditions for in vivo–derived spermatogonia that allow for more robust prophase I progression, including three-dimensional (3-D) culture systems. These efforts will be extended to female meiosis. Such systems are necessary to recapitulate gametogenesis in vitro.

Paula Cohen closed by sharing “gold standards to apply and red herrings to avoid” for in vitro meiosis (Handel et al., 2014). To ensure prophase I of meiosis proceeds normally, she suggested that scientists would need to track certain “gold standards” of meiosis: synapsis (chromosome pairing), sex body formation, transcriptional activity, and the frequency of DSBs and crossovers; the loss of even one or two crossover events can result in cell death. Scientists would also need to consider that in vivo errors in female germ cells often go undetected and result in aneuploidy and early pregnancy loss. Such errors in in vitro–derived gametes may serve as a “red herring” for poor gamete quality. Further research could ensure that in vitro–derived cells can nonetheless authentically proceed through the meiotic program to ensure correct ploidy of the gametes.

The Importance of the Epigenetic Program

Epigenetic resetting and genomic imprinting are processes unique to germ cells that are “fundamental for normal embryonic development and healthy physiological postnatal development”; Hajkova discussed these critical processes and the problems they pose to authentically reconstituting gametogenesis in vitro.

Epigenetic Programming

After their specification early in development, PGCs undergo global DNA demethylation and erasure of genomic imprints via “epigenetic resetting” (Figure 2-6). Germline demethylation affects the whole genome,

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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FIGURE 2-6 DNA methylation across germ cell development in mice.
NOTE: E = embryonic day; PGC = primordial germ cell.
SOURCES: Presented by Hajkova, April 21, 2023. Schematic designed by Deborah Bourc’his.

including promoters, genes, intergenic regions, enhancers, CpG islands, and imprinting control regions. Only highly repetitive sequences in the genome are demethylated to a lesser extent. When erasure of these imprints is complete upon reaching the gonad, Hajkova stated, this is the “most epigenetically naïve state” in development with the “lowest amount of [epigenetic] information” available. Epigenetic resetting also impacts chromatin and nuclear structure; whereas somatic cells at this stage of development have clusters of highly compacted chromatin, PGCs have a euchromatic and open chromatin structure.

After epigenetic resetting, “the developmental and epigenetic routes diverge” for male and female germ cells, Hajkova said. Male germ cells quickly accumulate DNA methylation, complete the final rounds of proliferation, and then enter mitotic arrest until after birth. Female germ cells remain unmethylated and enter meiotic prophase; they only reaccumulate methylation marks during the final stages of follicle maturation. As male and female germ cells reaccumulate methylation marks, they also reestablish genomic imprints. Genomic imprints are reestablished in a sex-specific way and carry parent-of-origin specific marks that they ultimately contribute to an embryo. The hallmarks of epigenetic resetting and genomic imprinting are conserved from mice to humans (Gruhn et al., 2023; Tang et al., 2015).

Purpose of the Epigenetic Ground State

Hajkova said that reaching an epigenetic ground state through epigenetic resetting may be critical for germ cells to enter and complete meiosis correctly. When resetting removes repressive marks, the

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

expected widespread transcriptional activation in germ cells does not take place; the only group of genes that are transcriptionally responsive to the removal of these repressive marks is the meiotic program, indicating that “germline demethylation enables meiotic progression” (Hill et al., 2018).

A Barrier to IVG

Scientists face a problem of how to reconstitute the erasure and reacquisition of imprinting in a sex-specific manner to reconstruct both eggs and sperm in vitro. Although scientists can produce PGCLCs that resemble the earlier stages of PGC development in mammals, these cells do not authentically progress through the full epigenetic resetting that occurs in vivo. Hajkova said that the gonadal somatic environment enables germ cells to go through the epigenic program appropriately and, therefore, reconstituting the niche is a major barrier to creating an environment in which in vitro–derived gametes can undergo epigenetic resetting. By continuing to study the gonadal somatic environment and epigenetic resetting in vivo, Hajkova concluded, scientists may learn how to faithfully reconstruct this process in vitro.

Germline Mutations and Genomic Variation

Unlike mutations in the genomes of somatic cells, germline mutations can be transmitted across generations. Goriely discussed research on DNA mutation rates and clonality in germ cells and how these factors could be considered for gametogenesis in vitro.

Germline Mutation Rates

The human genomic mutation rate in germ cells is much lower and more tightly regulated than that of somatic cells, Goriely said, resulting in only ~70 de novo point mutations (DNMs) per offspring (Goriely, 2016). Male and female germ cells take different approaches to maintaining mutation rates, both of which are not fully understood, said Goriely. SSCs actively divide into old age, accumulating point mutations through multiple replication errors over time (see Figure 2-7); 80 percent of DNMs are paternally inherited (Goriely, 2016). Alternatively, female germ cells do not divide after birth, and instead, face maternal age effects from aneuploidy rather than DNMs (Goldmann et al., 2018).

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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FIGURE 2-7 Opportunities for de novo mutations are increased in male versus female germ cell development.
SOURCES: Presented by Goriely, April 19, 2023; Goriely, 2016.

The “Selfishness” of the Male Germline

Within the testis, a class of pathogenic DNMs that are up to 1,000 times more frequent than other DNMs are clonally selected because they provide an advantage to mutant SSCs. Goriely said, these mutations are “good for the testis”—increasing sperm production by colonizing seminiferous tubules at the expense of other spermatogonia—“but nasty for the next generation.” These DNMs cause gain-of-function mutations that lead to debilitating disorders, including Apert Syndrome, achondroplasia, and Noonan syndrome. The DNMs causing these disorders are exclusively of paternal origin. These mutations are found in the sperm of all men as they age, Goriely said, such that “the aging testis” acts as a “reservoir” of functional, pathogenic DNMs (Maher et al., 2016, 2018).

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

Considerations for IVG

IVG would involve taking a few cells out of the body, converting them to iPSCs, and expanding them in a culture dish. Culturing iPSCs is not overly mutagenic under the appropriate conditions (Hasaart et al., 2022; Kuijk et al., 2020; Thompson et al., 2020), Goriely said, but only mutation rates observed in somatic cells have been used for comparison, and these are several orders of magnitude higher than germline mutation rates. Moreover, both mutation rates and clonality must be considered in reconstituting gametogenesis. Given that both are likely controlled by the homeostatic properties of the gonad rather than genetic determinants, Goriely said, scientists must consider the consequences of culturing these cells outside of their normal environment.

HUMAN EMBRYOLOGY

Although animal models provide important foundational information, to understand human development, “we have to address the situation directly in humans,” said Ali Brivanlou of Rockefeller University. Brivanlou shared his expertise in human embryology and ESCs.

Through a collaborative relationship with theoretical physics groups, Brivanlou and his colleagues have created a system to model early human embryonic development. ESCs are seeded on microchips that impose a certain architecture and allow for single-cell quantification using artificial intelligence (Warmflash et al., 2014). Brivanlou said that these approaches have revealed new facets of early human embryo organization and reaffirmed findings from other mammalian model systems that embryonic patterning is controlled from the edge of the embryo (Martyn et al., 2018).

In addition to studying early embryonic development, these chips can be used to make self-organizing synthetic human organs, including gastruloids, cerebroids, neuroloids, ovoids, and lungoids. By culturing ESCs on these micropatterned chips and exposing them to BMP4 among other signals, Brivanlou and his laboratory suggested that self-organizing ovoids can be produced that resemble fetal ovaries and seem to contain granulosa-like cells surrounding a cell that he speculated could be an oocyte.

Ultimately, the putative germ cells generated in the ovoid need to be functionally validated, Brivanlou explained, but this has many ethical limitations. Brivanlou and Clark discussed recently updated International Society for Stem Cell Research (ISSCR) Guidelines9 concerning in vitro–

___________________

9 The ISSCR Guidelines can be reviewed at https://static1.squarespace.com/static/611faaa8fee682525ee16489/t/62ed69b184e2ed258e6eb7e4/1659726257773/isscr-guidelines-for-stem-cell-research-and-clinical-translation-2021.pdf/ (accessed August 8, 2023).

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

derived gametes: they could be fertilized, but the embryos produced could only be cultured in the laboratory for 14 days. Brivanlou expressed plans to functionally validate the cells they have observed through such fertilization and other assays.

PANEL DISCUSSIONS

Overcoming Technical Barriers

Several participants’ questions revolved around technical barriers to reconstructing gametogenesis in vitro. Speakers’ and participants’ answers often revealed their considerations and near-term ideas for overcoming these roadblocks.

Considering Developmental Timing

Hayashi asked Surani how to consider timing when developing culture systems for IVG. Considering that gametogenesis takes years in humans, he questioned whether it would be necessary for culture conditions to recapitulate normal developmental timing or if certain stages could be skipped or accelerated. Being aware of developmental timers will likely be important for culture optimization, Surani noted, adding that research is needed to determine whether skipping or accelerating certain stages would allow for generating functional gametes. Surani suggested that gathering high-quality, descriptive information from in vivo-derived gametes will be key for determining whether in vitro–derived gametes are progressing appropriately, despite potentially altered developmental timing. Paula Cohen added that timing in culture systems needs to account for how germ cells behave differently ex vivo or in vitro; for instance, human fetal ovaries grafted into mice performed oogenesis but at an accelerated time line. Surani noted how helpful it would be to compare these cells to in vivo–derived gametes to determine if they are authentically recapitulating gametogenesis.

Selecting a Source Cell for IVG

A virtual participant asked the experts to identify the best source cell for iPSC generation and ultimately IVG. Skin cells are a common source but often carry UV mutations, Clark said, noting that many question whether tissue cells ought to be used instead. Sasaki suggested that tissue from the organ with the least mutations ought to be used to make iPSCs, and then iPSCs sequenced before IVG. If an iPSC carried a deleterious mutation, Sasaki posited, CRISPR/Cas9 could be used to correct it before

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

IVG. Hajkova agreed that sequencing iPSCs would be an important part of selecting the best source cell but suggested that CRISPR/Cas9 genome editing ought to be avoided. Clark echoed Hajkova and Sasaki, saying that “sequencing is a really good first step,” particularly considering that the cell culture passages and tools needed to ensure iPSCs are competent for IVG can introduce anomalies.

Generating Offspring

Clark asked Hayashi about the limited efficiency of two-cell stage embryos derived from an in vitro egg to produce mouse pups. She wondered whether allowing these embryos to proceed to the blastocyst stage would provide an opportunity for selection before implantation to establish a pregnancy. Hayashi responded that implanting blastocysts does not lead to an increase in live births, speculating that blastocysts may fare worse due to an extended culture period.

Optimizing Culture Conditions

Hannah Landecker of the University of California, Los Angeles, inquired whether the challenge of optimizing culture conditions was primarily biochemical or mechanical. Saitou replied that both aspects, including proteins, cytokines, mechanical tension, and even oxygen concentration, need to be addressed to create an ideal environment for in vitro gamete maturation.

Using Ex Vivo Systems

Katherine Kraschel (Yale Law School) said that before the presentations, she had understood IVG as taking place entirely within a culture dish. However, speakers outlined many experiments involving ex vivo tissue coculture or transplantation in vivo. Given that, Kraschel questioned whether a clinical pathway might involve maturing partially in vitro–derived gametes by transplanting into ovaries or testes to complete the process rather than doing so fully in culture. Transferring the ovarian cortex10 is already performed clinically, Hayashi noted, paving a potential path for IVG. Hayashi added that transplantation would minimize the

___________________

10 The ovarian cortex is the outer portion of the ovary that contains the ovarian follicles. Cryopreserving the ovarian cortex is used clinically to preserve fertility mainly in prepubertal patients, where oocyte collection is not possible. Over 100 live births have been reported. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6558345/ (accessed August 8, 2023).

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

length of IVG culture, reducing the risk of abnormalities. Sasaki mentioned clinically relevant models for males (SSC transplantation).

Considerations for Reconstituting Meiosis In Vitro

Several participants suggested that determining how to reconstruct meiosis in vitro is one of the central roadblocks to achieving IVG. The panel discussions yielded valuable conversations about technical barriers and ideas for how to move forward, including developing optimized culture conditions and protocols for monitoring meiosis.

Timing of Errors

In Hayashi’s experiments fertilizing in vitro–derived gametes from mice, the embryos had high rates of aneuploidy and resulted in few live births. Clark asked Hayashi and Paula Cohen to discuss when the aneuploidy is likely arising, during meiosis prophase I or meiosis II (MII). Hayashi said that the in vitro–derived gametes he produced have increased rates of mispaired chromosomes in meiosis prophase I, so the aneuploidy in the MII stage is likely due to these upstream errors. Hayashi added that optimized culture conditions are needed to allow proper chromosome pairing. He was impressed with Cohen’s 3-D culture and hoped that these might improve outcomes. Paula Cohen concurred: “not to make it all about prophase I—but it is about prophase I.”

Monitoring Checkpoints

A participant inquired whether issues with meiosis come down to cell cycle checkpoints, and particularly for females, their “leaky[ness]” during meiosis. Paula Cohen began by stating that not all the checkpoints have been defined very well, and some may not yet have been discovered. If no stringently held checkpoints can be identified in females, the key may be monitoring meiotic progression, she suggested. Finding markers indicative of meiotic success could allow scientists to impose checkpoints through gamete selection rather than relying on the cell itself.

Errant Progression

Sasaki asked whether Saitou understood the underlying defect behind why oogonia-like cells in his xenogeneic transplant experiments progressed to zygotene stage without first completing leptotene. Saitou speculated that certain signaling pathways may be hyperactivated, and inhibiting these might more efficiently allow cells to go through the

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

appropriate meiotic stages. To recapitulate meiosis and the oogonia to oocyte transition in vitro, careful in vivo studies are needed to understand the necessary signals more deeply, added Saitou.

A Universal Gamete

Heather Youngs of Open Philanthropy asked the panelists if they could envision a world where it would be possible to make a generic haploid gamete, rather than creating specialized gametes (eggs and sperm). Paula Cohen mused that it could be possible to make an XX “universal gamete,” but introducing the Y chromosome would likely cause several problems. During meiosis, the X and Y chromosomes do not synapse completely and are instead packaged into a heterochromatic structure known as the “sex body.” Until scientists understand how the sex body contributes to male gametogenesis, figuring out how to impose a Y chromosome into a universal gamete would be difficult, she concluded.

The Next Generation

“Proof in the Progeny”11

A number of workshop participants emphasized that ensuring health and safety of offspring generated by IVG would be of critical importance if this technology were ever to be used clinically. David Cyranoski (Kyoto University) asked whether Hayashi observed gross phenotypic changes in the mouse pups generated from an in vitro–derived egg. The pups appear normal, Hayashi said. The most serious problem that occurs is aneuploidy at MII, but these meiotic failures result in embryonic death. Similarly, a virtual participant inquired about the fertility of pups generated via IVG and whether Hayashi had observed any transgenerational effects in the ensuing generations. Their fertility and fecundity are normal, but transgenerational effects have not yet been assessed, said Hayashi. He speculated that any epigenetic defects may be reprogrammed in successive generations during epigenetic reprogramming.

“Proof in the Petri Dish”12

Panelists had a discussion in response to a participant’s question concerning how to assess the health of gametes generated by IVG. When considering using IVG as a reproductive technology, how much quality

___________________

11 A quote from Crockin’s remarks closing the first day of the workshop.

12 A quote from Crockin’s remarks closing the first day of the workshop.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

control is needed has yet to be determined, said Sasaki, but it will likely be quite extensive. Sasaki and Hajkova both noted the importance of developing quality control assays to determine the fitness of gametes without destroying them. Hajkova said that by identifying key biomarkers or indicators to select healthy gametes, clinicians may be able to avoid implanting embryos that will miscarry. Sasaki said that transcriptomic and epigenetic profiling can be used to assess whether in vitro–derived gametes are progressing appropriately, but it would be difficult to fully screen all elements. Developing a standardized set of assays would be helpful, said Sasaki.

Goriely warned that “having babies is risky,” reminding participants that even with normal gametogenesis, 1 in 300 births have severe developmental disorders due to de novo genetic mutations. Were IVG ever to be used clinically, Dr. Goriely said that it is “unavoidable” that some babies would be born with disorders even if everything went perfectly from a technological perspective. Discussions need to be had concerning the psychological and cultural implications of such events, she said.

Orwig concluded by affirming that scientists cannot fully understand the fitness of gametes produced by IVG without testing their ability to produce embryos—ethical, legal, and funding issues notwithstanding. Although scientists cannot use in vitro–derived human gametes to produce offspring, once developed, they could generate an embryo that can be observed for 14 days.13 These experiments would provide valuable information about the safety and function of these gametes, said Orwig.

Time Line for Human Use

The question of a time line for accomplishing IVG in humans was raised by Ubaka Ogbogu (University of Alberta). He asked Hayashi whether he sees this as a technology that is on the horizon for human use. Hayashi indicated that others may be better suited to answer, particularly from social, legal, and regulatory perspectives, before repeating that foundational research in mice has led to many reproductive technologies for humans. Because IVG has already been accomplished in mice, it is in principle achievable in humans, but it will take time, Hayashi concluded.

__________________

13 The ISSCR Guidelines for Stem Cell Research and Clinical Translation indicate that with the proper oversight, in vitro culture of human embryos for research can proceed until the formation of the primitive streak or 14 days from fertilization, whichever occurs first.

Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×

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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
Page 24
Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
Page 25
Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
Page 26
Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Suggested Citation:"2 State of the Science." National Academies of Sciences, Engineering, and Medicine. 2023. In Vitro–Derived Human Gametes as a Reproductive Technology: Scientific, Ethical, and Regulatory Implications: Proceedings of a Workshop. Washington, DC: The National Academies Press. doi: 10.17226/27259.
×
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Next: 3 Potential Clinical Implications of IVG »
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Current assisted reproductive technologies such as in vitro fertilization (IVF) do not enable all prospective parents to have genetically related children. The National Academies Board on Health Sciences Policy hosted a workshop in April 2023 to explore the development of in vitro-derived human eggs and sperm from pluripotent stem cells through a process known as in vitro gametogenesis (IVG). Speakers emphasized the impacts of the potential biotechnology on research and reproductive medicine should clinical IVG ever be approved, along with the many social, ethical, legal, and technical considerations its development raises. This proceedings document summarizes workshop discussions.

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