Oogenesis and Polar Bodies: What and Why?
During Oogenesis in humans — the process in females in which the primary oocyte becomes a mature ovum — two meiotic divisions occur.
(1) The first occurs when the primary oocyte splits into a secondary oocyte and the first polar body.
(2) The second occurs when the secondary oocyte splits into an ootid and second polar body. The ootid then goes on to differentiate and become a mature ovum (Klug et al., 2019).
In both of the above meioses, the secondary oocyte and the ootid contain the majority of the passed on cytoplasm, while the first and second polar bodies, respectively, are left with a void of genetic material.
So, why is this?
Polar bodies were first discovered in gastropods in 1824, but it wasn’t until 1877 that they were slowly being understood. They were first thought to be “egg fragments” or “expelled yolk masses” but later seen as indicators of egg maturation (Schmerler & Wessel, 2011).
What’s surprising is that polar bodies are still largely complex cells, with a nucleus, ribosomes, mitochondria, and other organelles; they even maintain the potential to be fertilized and result in an embryoid (embryonic organism grown in vitro).
So surely, their complexity and potential supersedes that of mere apoptosis?
You may be familiar with the more rudimentary role of the polar bodies:
That of providing an outlet for half of the diploid chromosome resulting from meiotic division, thus resulting in a haploid cell and gamete creation.
But, in the field of genetics, burgeoning studies are actually utilizing polar bodies in humans for clinical assessment of disease and of embryonic potential; more specifically, polar bodies are being used as a valuable source of DNA that accurately mirrors maturing oocytes (Schmerler & Wessel, 2011).
Accordingly, this allows for genetic testing by what’s called qPCR, not too dissimilar from the PCR testing used today with COVID-19 testing.
The ability to use polar bodies as alternative, non-invasive forms of genetic screening is perhaps one of the most insightful tools in the arsenal of assessing healthy pregnancies that we have today.
Adding to the capabilities above, cytogenetic analysis using what’s called fluorescent in-situ hybridization (FISH), as well as chromosol painting, allow us to prognosticate partial — or even full — chromosomal status in the maturing egg (Gitlin et al., 2003).
This is useful to identify maternally inherited gene translocations and defects (something we learned about in Chapter 6) as well as mitochondrial mutations.
Polar bodies are not merely partially empty “baggage” left behind for apoptosis, but rather, they are a complex and invaluable resource that we are still trying to better understand.
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References:
Klug, W. S., Cummings, M. R., Spencer, C. A., Killian, D., & Palladino, M. A. (2019). Essentials of Genetics, Loose-Leaf Edition. Pearson Education.
Schmerler, S., & Wessel, G. M. (2011). Polar bodies — more a lack of understanding than a lack of respect. Molecular reproduction and development, 78(1), 3–8. https://doi.org/10.1002/mrd.21266.
Gitlin, S., Gibbons, W., & Gosden, R. (2003). Oocyte biology and genetics revelations from polar bodies. Reproductive BioMedicine Online, 6(4), 403–409. https://doi.org/10.1016/s1472-6483(10)62158-x.
