Ovum — Egg cell

Every living being on the planet today began as a single, tiny cell called an egg, a silent powerhouse that contains the code for life. For human development to begin, a sperm cell must fertilize this egg, fusing the genetic material of both parents. The complex process of human development, which starts with this single cell and covers everything from the formation of our brain to the first heartbeat, begins here. So, what is this cell, and how does it work in our body?

The female reproductive cell is called the egg cell or ovum. The egg cell is released from the ovaries in the female reproductive system. It is produced through the process of oogenesis and has the potential to develop into a new organism if fertilized by a sperm cell [1]. Egg cells are haploid, which means they have half the number of chromosomes of somatic cells (non-reproductive cells). Among countless cells that exist, it is one of the few that has the ability to create entirely new life [2].

Structure of an Egg Cell

With a diameter of 100 μm, the ovum is one of the largest cells in the human body, which can be seen with the unaided eye without the use of a microscope or other magnifying tool [3]. The egg is not just a cell; it’s a work of art, a little world clothed in layers of purpose and protection. The nucleus is at the centre of the cell. It is a quiet vault that holds half of the genetic code needed to make a human being. This nucleus is like a scroll of old instructions handed down through generations, waiting for its partner, the sperm, to finish the story. The cytoplasm, also known as cell plasma or yolk, surrounds the nucleus and contains nutrients, stores energy, and molecular machinery [4,5]. This is the egg’s life-support system, which provides everything an embryo requires in its first few days before connecting to a mother’s body. You may see it as packed luggage for a long trip, filled with supplies for the road ahead.

The egg cell is surrounded by three basic exterior layers. The outermost layer, the corona radiata, is composed of follicular cells that provide nourishment and protection. Beneath it lies the zona pellucida, a thick, translucent glycoprotein layer that maintains the egg’s structure and plays a crucial role in fertilization by enabling sperm binding and preventing polyspermy (fertilization by more than one sperm) [6,7]. The innermost layer, the vitelline membrane, is a delicate wall encasing the egg’s contents, offering an additional barrier against potential damage.

The Life Story of an Ovum — Oogenesis

Multiplication phase

The ovaries of a baby girl start working long before she is born. Oogonia, which are tiny cells, begin to form around the 6th to 7th week of pregnancy. They proliferate quickly by a process known as mitosis — a type of cell division that produces exact duplicates of a cell, somewhat like in a biological copying machine. This serves to multiply their numbers in a short time. However, although thousands are produced, the majority of them will gradually degenerate, and only a few fortunate ones will progress to the next level towards becoming a mature egg [8].

Growth phase

The oogonia that do not degenerate differentiate into a primary oocyte. The growth is linked with both nuclear and cytoplasmic growth. The nuclear growth is caused by the accumulation of large quantities of nuclear sap and is referred to as a germinal vesicle. The cytoplasmic growth is linked to an increase in the number of mitochondria, endoplasmic reticulum, the Golgi complex and accumulation of reserve food material. Each primary oocyte is shielded by a tiny bubble of support cells known as a follicle [9].

Maturation phase

During puberty, hormones instruct a few follicles to wake up every month. Most of them don’t make it, but one egg usually gets old enough to keep going. The selected primary oocyte completes the first half of meiosis and turns into a secondary oocyte, with a smaller byproduct, a polar body (which does not develop into an egg). The secondary oocyte begins the second half of meiosis, but once again halts, this time at a stage known as metaphase II, and awaits fertilization [10].

Fertilization

The egg completes meiosis rapidly if a sperm makes it to the secondary oocyte and fertilizes it successfully. The outcome? Creation of the first human cell, called a zygote. A zygote is the first stage of life and has 46 chromosomes, 23 from each parent. In the absence of fertilization, the secondary oocyte simply decomposes, and the body initiates a new menstrual cycle [11].

The Ovum’s Big Responsibilities

Each egg cell has 23 chromosomes — that’s half of the genetic information necessary to create a human being. The other half is contributed by the sperm. Together, they decide traits such as eye color, hair type, and even the risk for certain inherited diseases [12].

When a healthy sperm encounters a mature egg, fertilization can take place. The sperm and egg unite their DNA to create a brand new cell known as a zygote. The outer shell of the egg is smart too — it hinders any additional sperm after one enters, to ensure only one fertilization occurs. This protection system is precisely timed. Fetuin-B is a protein that regulates the hardening of the egg’s shell, keeping it flexible enough to enable sperm access at the correct time but then switching to block all others after fertilization occurs. In mouse experiments, the absence of Fetuin-B causes the shell to harden too early, limiting fertilization entirely, whereas too much can delay hardening and risk access by sperms [13]. This delicate harmony demonstrates how well the egg maintains the open and closed signs on its surface.

The egg’s cytoplasm, or jelly-like interior, contains nutrients that nourish the early-stage embryo (known as a blastocyst) at the very beginning, before the embryo even reaches the uterus. These nutrients enable it to grow and live until it implants in the uterus and becomes attached to the mother’s bloodstream, eventually turning into the placenta. Apart from their role in nutrition, the cytoplasm also contains molecular gear that guarantees the process of egg division is proper. To split chromosomes equally, a microscopic structure known as the spindle must be precisely positioned. Because egg cells don’t have the structures that other cells possess, they create mild currents in their cytoplasm that hold the spindle in place. These currents are driven by motor proteins and actin filaments [14].

Once a sperm passes through the egg’s protective coat, it reaches the egg’s outer skin, known as the oolemma. A very distinctive handshake occurs here: IZUMO1, a protein on the sperm, binds effectively with JUNO, its matching companion on the egg [15]. Without these two molecules, the two cells cannot join, and fertilization ceases before it begins. Scientists discovered that if either IZUMO1 or JUNO is missing, no embryo can form, demonstrating how important this moment of recognition is in beginning a new life [16].

Growing Eggs Outside the Body: A Glimpse into the Future

Recently, scientists have accomplished something that previously seemed impossible: they have successfully grown human egg cells from the earliest stage to full maturity outside of the body [17,18]. Eggs take many years to develop and mature inside a woman’s ovary. But in this new technique, scientists were able to duplicate that complicated process entirely in the lab. What makes this breakthrough so remarkable? These cells have resisted artificial cultivation for more than 30 years because they are extremely sensitive to their external environment. But now? The spell has been broken. The lab turns into a warm milieu of potential, a surrogate ovary. This is a reproductive biology revolution rather than merely a lab success [19]. This milestone is a ray of hope that becoming a mother won’t be prevented by age, diseases, or other circumstances. It may eventually aid in the development of new infertility treatments and regenerative medicine therapies [20,21].

Recognizing and appreciating the labs working in this space

• Dr. Carlos Telleria’s lab, Experimental Pathology Unit, Department of Pathology, McGill University, Montreal, QC H3A 2B4, Canada. https://www.mcgill.ca/pathology/carlos-telleria
• Schindler lab, Department of Genetics at Rutgers University, New Jersey. https://sites.rutgers.edu/schindler-lab/research/
• Dr. Elvan Böke’s lab, Centre for Genomic Regulation, Barcelona, Spain. https://www.crg.eu/en/elvan-boke#block-block-2, X: @LabBoke
• Prof. A. Gutierrez-Adan, Departamento de Reproducción Animal, INIA-CSIC, Avenida Puerta de Hierro 12. Local 10, 28040 Madrid, Spain. https://laborepro.wixsite.com/gutierrez-adan
• Elnur Babayev, Department of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA. https://www.feinberg.northwestern.edu/faculty-profiles/az/profile.html?xid=55663
• Eyelyn Telfer, Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, UK. https://edwebprofiles.ed.ac.uk/profile/evelyn-telfe, X: @EdinUni_CRH
• Christopher Thomas, The Marseille Developmental Biology Institute (IBDM), Marseille, France. https://www.ibdm.univ-amu.fr/team/control-and-dynamics-of-the-ovarian-cycle/ X: @LastChrisThomas
• Schuh lab, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany. https://www.mpinat.mpg.de/639000/Research, X: @SchuhLab
• Dr Ali Abbara, Department of Metabolism, Digestion and Reproduction — Faculty of Medicine. https://profiles.imperial.ac.uk/ali.abbara/publications
• Dr. Mark Nachtigal, Max Rady College of Medicine, Biochemistry and Medical Genetics, University of Manitoba, Winnipeg, Canada. https://research.cancercare.mb.ca/research-profile/mark-w-nachtigal/

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References

1. Sánchez, F., & Smitz, J. (2012). Molecular control of oogenesis. Biochimica et Biophysica Acta (BBA) — Molecular Basis of Disease, 1822(12), 1896–1912. https://doi.org/10.1016/j.bbadis.2012.05.013
2. Britannica. Ovum: structure, function, and fertilization. URL: https://www.britannica.com/science/ovum.
3. Milo R, Phillips R, Orme N. 2016. Cell Biology by the Numbers New York: Garland Sci.
4. Hadek, Robert. “The structure of the mammalian egg.” International review of cytology 18 (1965): 29–71.
5. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Eggs. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26842/
6. Familiari G, Relucenti M, Heyn R, Micara G, Correr S. Three-dimensional structure of the zona pellucida at ovulation. Microsc Res Tech. 2006 Jun;69(6):415–26. doi: 10.1002/jemt.20301. PMID: 16703610.
7. Sathananthan, A. H. (1997). Ultrastructure of the human egg. Human Cell, 10(1), 21–38. PMID: 9234062
8. Krajnik, K.; Mietkiewska, K.; Skowronska, A.; Kordowitzki, P.; Skowronski, M.T. Oogenesis in Women: From Molecular Regulatory Pathways and Maternal Age to Stem Cells. Int. J. Mol. Sci. 2023, 24, 6837. https://doi.org/10.3390/ijms24076837
9. Holesh, J. E., Bass, A. N., & Lord, M. (2023). Physiology, ovulation. In StatPearls. StatPearls Publishing. PMID: 28723025
10. Jamnongjit, M., & Hammes, S. R. (2005). Oocyte maturation: The coming of age of a germ cell. Seminars in Reproductive Medicine, 23(3), 234–241. https://doi.org/10.1055/s-2005-872451
11. Oliver R, Basit H. Embryology, Fertilization. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542186/
12. Jackson, M., Marks, L., May, G. H. W., & Wilson, J. B. (2018). The genetic basis of disease. Essays in Biochemistry, 62(5), 643–723. https://doi.org/10.1042/EBC20170053
13. Dietzel, E., J. Floehr, and W. Jahnen-Dechent. “The biological role of fetuin-B in female reproduction.” Ann Reprod Med Treat1.1 (2016): 1003.
14. Yi, K., Rubinstein, B., & Li, R. (2013). Symmetry breaking and polarity establishment during mouse oocyte maturation. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1629), 20130002. https://doi.org/10.1098/rstb.2013.0002
15. Bianchi, E.; Doe, B.; Goulding, D.; Wright, G.J. Juno is the egg Izumo receptor and is essential for mammalian fertilization. Nature 2014, 508, 483–487.
16. Aydin, Halil, et al. “Molecular architecture of the human sperm IZUMO1 and egg JUNO fertilization complex.” Nature534.7608 (2016): 562–565.
17. Telfer, E. E., & Andersen, C. Y. (2021). In vitro growth and maturation of primordial follicles and immature oocytes. Fertility and Sterility, 115(5), 1116–1125. https://doi.org/10.1016/j.fertnstert.2021.03.004
18. Vitale, F., & Dolmans, M. M. (2024). Comprehensive review of in vitro human follicle development for fertility restoration: Recent achievements, current challenges, and future optimization strategies. Journal of Clinical Medicine, 13(6), 1791. https://doi.org/10.3390/jcm13061791
19. Reuters. (2018). Scientists grow human eggs to full maturity in a lab Reuters.https://www.reuters.com/article/us-health-human-eggs/scientists-grow-human-eggs-to-full-maturity-in-a-lab-idUSKBN1FT00P/
20. Matzuk, M., Lamb, D. The biology of infertility: research advances and clinical challenges. Nat Med 14, 1197–1213 (2008). https://doi.org/10.1038/nm.f.1895
21. Sittadjody, S., Criswell, T., Jackson, J.D. et al. Regenerative Medicine Approaches in Bioengineering Female Reproductive Tissues. Reprod. Sci. 28, 1573–1595 (2021). https://doi.org/10.1007/s43032-021-00548-9

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About the author:

Dr. Akanksha Gandhi

Content Editor The League of Extraordinary Cell Types, Sci-Illustrate Stories

Dr. Akanksha Gandhi earned her PhD in Molecular Biology from the Max Planck Institute for Chemical Ecology in Jena, Germany, in 2024. Prior to that, she completed her Master’s degree in Biology at the University of Texas Rio Grande Valley, USA. Her academic journey has been shaped by a deep curiosity about the natural world and a passion for scientific discovery. Outside the lab, Akanksha enjoys cooking, watching movies, and spending time outdoors, especially going for walks and hiking. She also has a growing interest in science communication and is currently exploring ways to make complex scientific ideas more accessible and engaging to broader audiences.

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My passion for art and love for medicine led me to the field of medical illustration, a profession in which I have been dedicated for many years. Through my work, I have the privilege of meeting and collaborating with remarkable individuals — doctors and scientists who are at the forefront of global scientific advancements. Their dedication and discoveries continue to inspire me. As a medical illustrator at a medical communications agency, my primary role is to transform complex processes and concepts into visually appealing and easily understandable images that become part of an animation or publication. Additionally, my works have been featured in numerous scientific magazines and books. Now I live and work in Poland.

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Dr. Petretto received his Ph.D. in Biochemistry at the University of Fribourg, Switzerland, focusing on the behavior of matter at nanoscopic scales and the stability of colloidal systems. Using molecular dynamics simulations, he explored the delicate interaction among particles, interfaces, and solvents.

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The League of Extraordinary Cell types

The team at Sci-Illustrate and Endosymbiont bring to you an exciting series where we dive deep into the wondrous cell types in our body, that make our hearts tick ❤.