Making gametes from pluripotent stem cells – a promising role for very small embryonic-like stem cells

Abstract

The urge to have one’s own biological child supersedes any desire in life. Several options have been used to obtain gametes including pluripotent stem cells (embryonic ES and induced pluripotent iPS stem cells); gonadal stem cells (spermatogonial SSCs, ovarian OSCs stem cells), bone marrow, mesenchymal cells and fetal skin. However, the field poses a huge challenge including inefficient existing protocols for differentiation, epigenetic and genetic changes associated with extensive in vitro manipulation and also ethical/regulatory constraints. A tremendous leap in the field occurred using mouse ES and iPS cells wherein they were first differentiated into epiblast-like cells and then primordial germ cell-like cells. These on further development produced sperm, oocytes and live offspring (had associated genetic problems). Evidently differentiating pluripotent stem cells into primordial germ cells (PGCs) remains a major bottleneck. Against this backdrop, we propose that a novel population of pluripotent stem cells termed very small embryonic-like stem cells (VSELs) may serve as an alternative, potential source of autologus gametes, keeping in mind that they are indeed PGCs surviving in adult mammalian ovaries and testes. Both VSELs and PGCs are pluripotent, relatively quiescent because of epigenetic modifications of parentally imprinted genes loci like Igf2-H19 and KCNQ1p57, share several markers like Stella, Fragilis, Mvh, Dppa2, Dppa4, Sall4, Blimp1 and functional receptors. VSELs are localized in the basement membrane of seminiferous tubules in testis and in the ovary surface epithelium. Ovarian stem cells from mouse, rabbit, sheep, marmoset and humans (menopausal women and those with premature ovarian failure) spontaneously differentiate into oocyte-like structures in vitro with no additional requirement of growth factors. Thus a more pragmatic option to obtain autologus gametes may be the pluripotent VSELs and if we could manipulate them in vivo – existing ethical and epigenetic/genetic concerns associated with in vitro culture may also be minimized. The field of oncofertility may undergo a sea-change and existing strategies of cryopreservation of gametes and gonadal tissue for fertility preservation in cancer patients will necessitate a revision. However, first the scientific community needs to arrive at a consensus about VSELs in the gonads and then work towards exploiting their potential.

Mouse Ovarian Very Small Embryonic-Like Stem Cells Resist Chemotherapy and Retain Ability to Initiate Oocyte-Specific Differentiation

“…77 Pluripotent VSELs with a distinct epigenetic profile may provide a better option to obtain gametes, because they are indeed the primordial germ cells that survive in adult ovaries and testes. 78 Authors' Note NIRRH Accession number for this article RA/36/10-2013.…”

A new possibility in fertility preservation: The artificial ovary

“…It improved neovascularisation when added to a transplantation material (Ko et al, 2012). Autologous mesenchymal stem cell transplantation to the ovary was reported to recover ovarian function in a patient with POF and resulted in a pregnancy (Bhartiya, Hinduja, Patel, & Bhilawadikar, 2014;Edessy et al, 2016). As in angiogenesis, effective methods to compare the degradation rates and mechanical strength for different biomaterials should be evaluated.…”

“…Oogonial stem cells and very small embryonic-like stem cells in the adult mammalian ovary present as other potential sources for oocytes (Bhartiya et al, 2014;White et al, 2012). However, the use of oogonial stem cells and very small embryonic-like stem cells is still being debated upon, and more reproducible studies have to be conducted to assess their usefulness and efficacy in this regard.…”

“…However, the use of oogonial stem cells and very small embryonic-like stem cells is still being debated upon, and more reproducible studies have to be conducted to assess their usefulness and efficacy in this regard. Table 5 summarises the advantages and disadvantages of different sources for gametes used for transplantable artificial ovary (R. Beauchamp, Evers, & Mattox, 2016;Bhartiya et al, 2014;Hanna & Hennebold, 2014). Despite the risk of transmitting remaining or undiagnosed cancer cells, patients' own follicles from cryopreserved ovarian tissues can be the source for gametes in the near future because no immunological, mutational, and ethical complications are associated with them in contrast to other sources.…”

Making gametes from pluripotent stem cells: embryonic stem cells or very small embryonic-like stem cells?

“…We had earlier discussed that rather than ES cells [obtained from the inner cell mass (ICM) of blastocyststage embryo], very small embryonic-like stem cells (VSELs) in adult gonads, which are equivalent to PGCs (primordial germ cells obtained from epiblaststage embryo), may be better stem cell candidates to make gametes (11). Based on their ontogeny, VSELs are relatively more mature developmentally as well as epigenetically compared to ES cells and thus spontaneously differentiate into sperm (12) and eggs (13)(14)(15) in vitro.…”