Ovaries are the sexual organs (gonads) in most female bodies that produce eggs, scientifically called ‘oocytes’. There is one to the left and one to the right of the womb. The ovaries are similar in purpose to the testicles.
This article is pending medical review.
Contributors
Written by Darina Obukhova and Juliëtte Gossens
Reviewed by Sophie Oppelt
Edited by Juliëtte Gossens
Human oocytes (eggs) are large cells that develop within the ovaries before you’re even born. They are the cells that develop into a pregnancy when they’re fertilized by sperm.
The oocytes develop inside structures called follicles. Each follicle consists of an oocyte and several supportive cells called granulosa cells. By 6 months of gestation, a female fetus has around 7 million oocytes (1). However, most of them will degenerate fast along the course of life, including before birth. At birth, there are about 2 million oocytes left (2). On average, there are around 400,000 follicles left by the time puberty comes around (1). Only about 0.1% of those (about 400 follicles) will fully mature into oocytes that can take part in ovulation and that can be fertilized.
The total number of healthy eggs in your ovaries at any time is called the ovarian reserve. These are all the eggs that could become available for fertilization by sperm cells. The size of your ovarian reserve will grow smaller as you age, as egg cells die off. You can read more about the ovarian reserve here.
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How oocytes and follicles develop
All oocytes (egg cells) are inside their own follicle, which is a protective and nourishing structure. These follicles develop into mature follicles over time. It takes an egg cell about a year to fully mature (2, 3), and then it’s ready for ovulation.
As a follicle develops, it goes through four different stages (3, 4). This happens under the influence of hormones. First, these hormones come from the egg cell’s local environment (other cells in the ovary). Later in its development, the egg cell also responds to hormones from the brain (2).
We explain the different stages an egg cell goes through below.
Stage 1: Primordial follicle
The primordial follicles are the follicles that are present in your ovaries when you’re still a fetus. These are very small structures, with just one layer of supportive cells (granulosa cells) covering the egg. Most of these follicles will either degenerate and die off throughout your life, but some of them develop further.
The oocytes in primordial follicles start to divide to become true sex cells (meiosis). At this point, they will only go through part of the division process. The division will be completed at a later stage. For now, the oocytes are called primary oocytes (5).
Stage 2: Primary follicle
As you go into puberty, the layer of supportive cells grows a bit thicker, and a layer of protein forms around the egg cell to separate it from the supportive cells and further protect it. This layer is called the zona pellucida. The follicle is now called a primary follicle.
Stage 3 and 4: Secondary and tertiary (antral) follicle
In the next stage of development, the outermost layer of cells splits into two layers with blood vessels in between. These layers are called the theca externa (the outermost layer) and the theca interna (the inner layer). The word “theca” means case, like an encasing around a structure. The blood vessels are important to bring nutrients and hormones to the follicle, to support its growth.
Then, a cavity forms right next to the egg cell in the follicle. This cavity is filled with fluid. The cavity is called the antrum, and this stage of follicle is visible on ultrasound. That’s important for fertility treatments, for example, as you can read here. This last stage is also when the egg cell goes a little further in its cell division (it is now a secondary oocyte), but still doesn’t finish quite yet. Completion of cell division only happens if the egg cell is fertilized by a sperm cell (5, 6).
Not all scientists agree on when a follicle becomes a tertiary follicle, but most put the cut-off at the formation of the antrum. This means the terms tertiary follicle and antral follicle usually mean the same thing: both refer to follicles in the latest stage of their development that have a fluid-filled cavity (the antrum). Within this stage, there are several sub-stages and classes, but that’s too detailed and complex for the purpose of this article. If you’re interested, there’s more detail in reference 3.
Each menstrual cycle, one of the tertiary follicles in the ovary grows the largest (3). This is the dominant follicle. Only the dominant follicle will take part in ovulation. The rest of the tertiary follicles shrink and degrade. This is called atresia. It’s a form of apoptosis, which is the death of a cell according to a neat and orderly process. This process in which one follicle becomes dominant and the rest dies off, repeats every menstrual cycle.
References
Ruth K, Perry J, Henley W et al. Events in Early Life are Associated with Female Reproductive Ageing: A UK Biobank Study. Scientific Reports. 2016;6:24710. DOI: 10.1038/srep24710
Cox E, Takov V. Embryology, Ovarian Follicle Development. In: StatPearls. Treasure Island (USA); StatPearls Publishing; 2022.
Williams CJ, Erickson GF. Morphology and Physiology of the Ovary. In: Feingold KR, Anawalt B, Boyce A et al (eds.). Endotext. http://www.endotext.org/; 2012.
Coelho Neto MA, Ludwin A, Borrell A, Benacerraf B, Dewailly D, Da Silva Costa F et al. Counting ovarian antral follicles by ultrasound: a practical guide. Ultrasound in Obstetrics & Gynecology. 2017;51(1):10-20. DOI: 10.1002/uog.18945
Peckham M. The Histology Guide. Available from: https://www.histology.leeds.ac.uk/female/FRS_ova.php [Accessed January 15th, 2023]
He M, Zhang T, Yang Y, Wang C. Mechanisms of Oocyte Maturation and Related Epigenetic Regulation. Frontiers in Cell and Developmental Biology. 2021;9:654028. DOI: 10.3389/fcell.2021.654028
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