Insights into the germline cycle

By Alice de Bernardy

Following the fertilisation of an egg by a spermatozoon, from a single totipotent cell develops an entire organism and extraembryonic cells that support its development. Some of these cells later differentiate into gametes, with fertilisation causing yet another round of embryonic development to take place. This is called the germline cycle.¹Due to the ethics surrounding the study of human embryonic development, most of the knowledge we have comes from mouse studies. Nevertheless, an increasing number of in vitro models are now starting to enable us to recreate specific times in development and to better understand this process.

The progressive differentiation of stem cells into specific tissues is a complex and tightly regulated process. A few days following the zygote stage, a group of embryonic cells called the blastocyst is formed and made of two cell types: the trophectoderm, which gives rise to the placenta, and the Inner Cell Mass. The latter then differentiates into the primitive endoderm, making up most of the yolk sac, and the epiblast, which ultimately contributes to the different cells of an adult organism.²

Fourteen days after fertilization, gastrulation subdivides the epiblast into the 3 germ layers: the ectoderm, which differentiates into the skin and neural components of the body; the mesoderm, which develops into the cardiovascular and skeletomuscular systems; and the ectoderm, later maturing into cells forming the gut.²

Most cells in an organism make up the soma and are not passed on throughout the generations. Germ cells, however, are responsible for producing gametes – spermatozoa in males and oocytes in females – and so perpetuate the germline cycle via fertilization.¹ This makes them particularly interesting to investigate.

Primordial Germ Cells (PGCs) have been identified in embryos soon after the gastrulation stage. PGCs migrate from the yolk sac to the gonadal ridge by crossing the primitive streak, which dictates tissue specification by secreting different molecular factors. Some of those factors are thought to guide PGC migration by chemotaxis, with Stem Cell Factor (SCF), for instance, binding the receptor c-Kit on the cell surface of PGCs to trigger directional motility. It has also been hypothesised that PGCs migrate along peripheral nervous system fibres, allowing them to move along the midline of the body.³

In principle, PGC migration should be a tightly regulated process due to the highly proliferative nature of these cells. In reality, however, many PGCs get lost during migration and are found in the midline of the body, further the genital ridges and ultimately in the developing brain. These anomalies can sometimes result in germ cell tumours when their new niche provides sustained resources, enabling proliferation. Fortunately, germ cell tumours are relatively rare. This is because PGCs depend on specific combinations of chemotaxis factors and usually die of apoptosis when outside of their regular environment that provides the right pro-survival signals. In the event that PGCs do proliferate in ectopic sites, interesting observations have been made. For example, PGCs have previously been observed to accumulate abnormally in the adrenal glands of developing mice, at which point ‘lost’ PGCs had the ability to advance in development and mature into oocytes in both females and males.^4,5

Once inside the genital ridge, PGCs undergo epigenetic reprogramming and exhibit very low levels of DNA methylation. This process is followed by sex specification, leading to the maturation of PGCs into germ cells. If the cells have a Y chromosome expressing the Sry gene, germ cells will then proliferate and enter mitotic arrest as spermatogonia (the precursors to spermatozoa) arranged in primitive tubules in the testis cord. Meanwhile, germ cells without a Y chromosome will divide and start meiosis before entering meiotic arrest as oogonia, whereby they are arranged in primary follicles.⁶

As the germ cells go through meiosis, they regain epigenetic imprinting in a sex-dependent manner. This decreases the expression of certain genes, with spermatozoa and oogonia compensating for each other’s imprinting to enable normal development. Researchers in Philadelphia and Cambridge have previously shown that the contribution from both gametes is not equivalent. In one study, they used the genetic material from two gametes of the same sex to maintain one normal gamete and one hybrid, – made, for example, by injecting an enucleated egg with the genetic material of a spermatozoa. They then fertilized a hybrid egg with a normal sperm cell and vice versa. Fascinatingly, both experiments led to abnormal embryo development due to the incompatibility of epigenetic imprinting and gene expression in each gamete.^7,8

Altogether, after germ cells mature into gametes, fertilization of an egg by a sperm cell enables a new round of the germline cycle, wherein blastocyst formation and gastrulation lead all the way to germ cell maturation again. And so the cycle continues.

Early development and germ cells are tricky to study in humans due to ethical regulations, but, with the advances of in vitro and in vivo models, researchers have an increasing number of available models to study the phenomenon. These range from animals to in vitro blastocysts and induced pluripotent stem cells differentiated into Primordial Germ Cell-like cells, each of which plays a role in our comprehension of this crucial biological process. Future work may provide insight not only in how healthy germ cell development occurs, but also in how it can instigate diseases like infertility.^9,10

References

1.  Leitch HG, Smith A. The mammalian germline as a pluripotency cycle. Development [Internet]. 2013 Jun 15;140(12):2495–501. Available from: https://doi.org/10.1242/dev.091603 
2.  Rossant J, Tam PPL. Early human embryonic development: Blastocyst formation to gastrulation. Dev Cell. 2022 Jan;57(2):152–65.  
3.  Mollgard K, Jespersen A, Lutterodt MC, Yding Andersen C, Hoyer PE, Byskov AG. Human primordial germ cells migrate along nerve fibers and Schwann cells from the dorsal hind gut mesentery to the gonadal ridge. Mol Hum Reprod. 2010 Sep 1;16(9):621–31.  
4.  Oosterhuis JW, Looijenga LHJ. Human germ cell tumours from a developmental perspective. Nat Rev Cancer. 2019 Sep 14;19(9):522–37.  
5.  Zamboni L, Upadhyay S. Germ cell differentiation in mouse adrenal glands. Journal of Experimental Zoology. 1983 Nov;228(2):173–93. 
6.  Val P, Swain A. Mechanisms of Disease: normal and abnormal gonadal development and sex determination in mammals. Nat Clin Pract Urol. 2005 Dec;2(12):616–27.  
7.  McGrath J, Solter D. Completion of mouse embryogenesis requires both the maternal and paternal genomes. Cell. 1984 May;37(1):179–83. 
8.  Surani MAH, Barton SC, Norris ML. Development of reconstituted mouse eggs suggests imprinting of the genome during gametogenesis. Nature. 1984 Apr;308(5959):548–50.  
9.  Sasaki K, Yokobayashi S, Nakamura T, Okamoto I, Yabuta Y, Kurimoto K, et al. Robust In Vitro Induction of Human Germ Cell Fate from Pluripotent Stem Cells. Cell Stem Cell. 2015 Aug 6;17(2):178–94.  
10.  Yu L, Wei Y, Duan J, Schmitz DA, Sakurai M, Wang L, et al. Author Correction: Blastocyst-like structures generated from human pluripotent stem cells. Nature. 2021 Aug 19;596(7872):E5–E5.  

---