The Human Egg Cell, Explained by an Embryologist

How many eggs are there in the ovary?

The number of eggs in the ovary vary depending on the age. Females are born with approximately 1-2 million potential eggs or follicles in each ovary. 

Unlike sperm, females cannot produce new eggs and this number goes on declining due to a process known as cell death or atresia, where some follicles die and are absorbed back into the body. Due to this, the number decreases to 400,000 by puberty. 

In the 40 years or so between puberty and menopause, ovulation and menstrual cycles occur. During this, every month, a group of follicles are recruited, with only a single follicle releasing the mature egg. The rest? They go through the cell death process, or atresia. 

In the average span of 40 years, only about 400-500 eggs get released.

By the time we reach menopause, very few follicles remain, which either cannot mature or are not viable due to the hormonal changes that happen during this phase.

What is the structure of an egg cell?

The structure of an egg cell, from the outermost part, consists of the corona radiata, the zona pellucida, the cytoplasm and the nucleus of the egg cell. Here’s a breakdown of what each part does:

1. The outermost layer of cells, the corona radiata

The corona radiata surrounds the egg cell and consists of two to three layers of cell from the follicle. It is right next to the zona pellucida and works as a communicative bridge between the follicle and the growing egg. 

These cells form junctions that transfer signals and nutrition as the egg grows from immature to mature stages.

2. The zona pellucida, the outer membrane

The zona pellucida is the harder, outer membrane of the egg. It plays a role in allowing sperm inside the egg to fertilise it.  

From the vast majority of sperm that reach the egg, the zona pellucida and its receptors allow a single sperm to enter and fertilise. It then hardens, preventing any other sperm from entering. 

“The zona pellucida also hardens with age”, says Navya Muralidhar. “It also hardens during the egg freezing process, which is why ICSI (Intra cytoplasmic sperm injection) is preferred for the fertilisation process in the lab”.

3. The cytoplasm

The cytoplasm is a gel-like substance that holds the cell's organelles (internal structures) and takes up the space between the nucleus and the zona pellucida. There are many cell organelles responsible for the cell’s function. Of these, the major one is mitochondria (yep, the powerhouse of the cell), which majorly provides energy to the egg cell. 

4. The nucleus 

The nucleus is at the heart of the cell, containing the genetic blueprint or DNA, packed in the form of chromosomes. Unlike other cells in our body, sex cells such as the egg cell or sperm cell only contain half the required number of chromosomes, that is, 23. So once the egg and sperm fertilize, the chromosomes combine to the required number, that is, 46. 

What does an actual human female egg look like?

Here is a real-time image from the lab on how the human egg cell looks like:

Human egg cells, in a droplet, on a glass dish on the left. Magnified, on the right. © Navya Muralidhar

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On the left, we have the egg cell surrounded by the corona radiata. “This is how we see it on the day of egg retrieval,” says Navya Muralidhar. “Once we identify these in the follicular fluid, we keep them aside, and then remove the outer cells to assess whether the egg cell is mature or immature”. 

Also read: What to expect on egg retrieval day

By what process are eggs formed?

The formation of an egg cell starts in the ovaries. They are formed as small cells within follicles and go through a growth phase known as oogenesis, within the follicles. 

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Every month, a bunch of small follicles are selected to grow, in the race for the “egg of the month”. The follicles grow in response to hormones such as FSH and LH from the baby phase (primordial) to the teenage and adult phases (pre-antral and antral), and at each growth spurt, some follicles go ahead, while some degenerate. 

However, in the end, only one or two mature follicles remain. There is a surge of LH or luteinizing hormone during this stage of the cycle, and the mature egg is released.

What is the function of an egg cell?

The function of the egg cell once mature and released from the ovary is to position itself in the fallopian tube till it is viable or live. This is also the ideal time for sperm to swim up to the fallopian tubes and for one of them to penetrate the egg leading to fertilisation.

The egg cell then turns into a zygote, once fertilised. From then onwards, one cell multiplies to two, two to four and four to eight as the growing cell mass travels the length of the fallopian tube.

By the time it reaches the top of the uterus and is nearing the fundus (where it implants), a single egg cell would have transformed into a mass of 60-180 cells, called a blastocyst or an embryo.

And, if fertilisation does not occur, then the egg cell degenerates and is passed out from the body during the menstrual cycle. 

What happens in an egg-freezing cycle?

In an egg-freezing cycle, we start stimulating the ovaries at the stage where they’ve selected the follicles of the month. Rather than the natural race of follicles to release one mature egg, in an egg-freezing cycle, we stimulate most of the recruited follicles to grow. 

The medication in the cycle is to target these follicles to grow to the desired size and release mature eggs. Unlike a natural cycle where we get one mature egg a month, with an egg freezing cycle we aim to retrieve multiple mature eggs in a single cycle.

However, the final number of mature eggs may not directly correlate with the number of follicles. Hypothetically, if you have 8-10 follicles growing, you could expect 6-8 to grow to the desired size and about 4-5 mature eggs.

Wondering why this happens? Read all about follicle and egg yield to know more!

Affordable Egg Freezing and Fertility Care in the UK, via Amilis

About 751,000 women are waiting for gynaecological care in the UK, of which 30,000 women have been waiting more than a year for an appointment.

Be it egg freezing, getting a hormone panel done, or even an ultrasound appointment.

So at Amilis, we decided: This narrative, no more.

That’s why we are working to make reproductive care accessible and affordable in the UK.

Here’s how:

• AMH testing or a full hormone panel test via Randox with minimal to no wait times. Get tested at a nearby Randox clinic, at your schedule
• A personalised, free AMH report 
• Free consultations with top fertility clinics, at your convenience
• Match with fertility specialists and expert doctors with no waiting times

And throughout, you’ll have Amilis experts supporting you on your journey.

Figuring out where to get started? Book a free call, or take our personalised fertility quiz to know more!

We’re bridging the healthcare gap for millions of women in the UK, one day at a time 💪🏻