Trends in Biotechnology
Volume 33, Issue 12, December 2015, Pages 735-746
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Review
Possibilities in Germ Cell Research: An Engineering Insight

https://doi.org/10.1016/j.tibtech.2015.09.004Get rights and content

Trends

Advanced therapies for infertility require developing systems for the efficient production of germ cells from stem cells in the laboratory.

We discuss recent advances and challenges of germ cell research.

Here, we propose engineering approaches for designing an artificial niche for germ cell development in vitro.

Novel platforms are demonstrated for the epigenetic analysis of germ cells.

Innovative approaches are detailed for the efficient production of germ cells from pluripotent stem cells.

Germ cells (GCs) are responsible for fertility and disruptions in their development or function cause infertility. However, current knowledge about the diverse mechanisms involved in GC development and function is still in its infancy. This is mainly because there are low numbers of GCs, especially during embryonic development. A deeper understanding of GCs would enhance our ability to produce them from stem cells. In addition, such information would enable the production of healthy gametes for infertile couples. In this regard, pluripotent stem cells (PSCs) demonstrated a promising ability to produce GCs in vitro. In this review, we highlight recent advances in the field of tissue engineering that suggest novel strategies to enhance GC research.

Section snippets

Call for Engineering Approaches in Germ Cell Research

It is now well understood that GCs can generate a new body. However, current knowledge of the diverse mechanisms involved in GC development is still in its infancy. This is mainly because there are low numbers of GCs, especially during embryonic development, and the analysis of these cells is tricky. Even minor damage to GCs during their developmental stage can cause infertility, which is a major medical problem that affects 10–15% of couples worldwide [1]. Accordingly, differentiation of PSCs

Male Germ Line Development

How are GCs created in the testis? The answer lies in embryonic development. Mammalian development commences with fertilization that results in the formation of zygote. Zygotes have the ability to build a body by sequential cell fate decisions. In the first cell fate resolution, the inner cell mass (ICM), which produces the future body, becomes set apart from the trophectoderm (TE), developing into extra embryonic tissues. During the second cell fate decision, the epiblast is separated from

Spermatogenesis Niche

In the spermatogenesis niche (Figure 2), seminiferous tubules comprising Sertoli cells (SCs) serve as somatic cells that support GCs during their developmental journey from spermatogonial stem cells (SSCs) to the spermatids. GC–SC contacts support spermatogenesis and enable spermatogonia to attach to the SCs and receive signals necessary for their survival, proliferation, translocation, and differentiation. The number and type of cell junctions are dynamic during this development: occluding

Artificial Niche to Support In Vitro Spermatogenesis

There are dynamic contacts between SSCs and/or PGCLCs in SCs during spermatogenesis. Cell–cell contacts in testes are involved in both cellular localization in the niche and signaling events, which provide adhesion for withstanding mechanical forces 38, 39. Of note, cytoskeleton dynamics and homeostatic pressure exert mechanical forces on the cell–cell contacts [40]. In turn, such a mechanical microenvironment affects important aspects of cell behavior, including migration, proliferation, and

Gradient Pattern of GDNF during Spermatogenesis

A growth factor immobilization strategy can extend the availability of these agents, allowing for spatial control and reducing the amount of growth factor required, resulting in decreased cost and increased efficiency [42]. In addition, surface-immobilized growth factors have a higher bioactivity, which will result in more cell and tissue responses. Last but not least, the multivalency of immobilized growth factors provides a high local concentration of these factors on the biomaterial surface

Screen the Response of PGCLCs to Different Microenvironments

In essence, micropatterned platforms are robotically synthesized growth factor microarrays on a cell-repellent surface. Micropatterning technologies allow for investigating the spatiotemporal aspects of a microenvironment for the regulation of cell behavior [46]. We propose microcontact printing as a way to generate large numbers of islands on a surface where each island represents a microenvironment with different combinations of growth factors (GDNF, FGF, and SCF) and CAMs. Subsequent seeding

Mimicking the Dynamic Process of Spermatogenesis

Microfluidics provides a platform for the sophisticated high-throughput manipulation of small volumes of fluids on a submillimetre scale [50]. This multidisciplinary technology provides a way to fabricate miniaturized and inexpensive [51] devices. This technology can be exploited for investigating high-throughput intercellular communications on a chip [50] and for improving biological research in general [52]. Microfluidic platforms are particularly useful when time-dependent processes are to

Inducing Meiosis in PGCLCs Using Nanostructured Carriers

Low water solubility and a short half-life might hinder the delivery of biomolecules to the target cells [58]. To overcome these challenges, nanoparticles are practical options for intracellular transport and the timely controlled release of biomolecules (Figure 4A). Intracellular delivery of biomolecules can both reduce adverse effects and enhance the amount of necessary biomolecules [59]. Researchers used nanoparticles to deliver anticancer drugs [60] and immunomodulatory compounds 61, 62

Efficient Germ Cell Induction

Currently, in the most efficient protocol [3], PGCLCs are produced in an aggregate-based system. Thus, a 3D differentiation method may provide a more reliable environment for the generation of PGCLCs. However, the 3D approach represents challenges for the homogenous delivery of growth factors by cells in the aggregates [65]. Scanning electron microscopy analysis has shown that secretion of ECM by cells produces a time-dependent shell around the embryonic bodies that significantly reduces the

Concluding Remarks and Future Perspectives

Despite decades of research in the GC area, there are still several controversial challenges in the field (see Outstanding Questions). By contrast, advanced therapies for infertility require the production of GCs and the provision of healthy gametes in the laboratory. To that end, cell- and tissue-engineering approaches have demonstrated promising candidates. Figure 5 illustrates recent fabricated platforms (environments) that could be utilized to advance GC studies. Coupling these approaches

Acknowledgments

This work was supported by a grant provided from Royan Institute, Iranian Council of Stem Cell Research and Technology, the Iran National Science Foundation (INSF).

Glossary

Epigenetic
heritable changes in gene expression that that are not related to DNA sequences. It can be result from changes in DNA methylation or histone modifications (e.g., methylation or demethylation of lysine amino acids, K, of histones). Epigenetic changes cause alterations in phenotype without alterations of the genotype.
Extracellular matrix (ECM)
a noncellular structure that is secreted by cells in a tissue. The ECM functions as scaffold and is also involved in signaling events.
Gastrulation

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