What is the difference between primordial follicles and primary follicles?

When we talk about the female reproductive system, we tend to say that a woman is born with all the eggs she will ever have. This is essentially true, but the fact is that, at the time a girl is born, and even before birth while she is still in the womb, the eggs in her ovaries are not exactly eggs. They are more precisely described as potential eggs. This is the primordial follicle and it is certainly accurate to say that at birth, a girl’s ovaries contain all the primordial follicles that she will ever have. These need to go through various stages of development, starting with the primary follicle stage and going on to others before finally becoming an egg capable of fertilisation.

This distinction is important because the number of follicles, or eggs, is critical when it comes to the normal age-related decline in natural fertility. It is also important for assisted fertility treatments such as IUI and IVF.

What is a primordial follicle?

From their beginnings in the developing foetus, primordial follicles numbering around a million are present inside the ovaries of baby girls when they are born.

• The primordial follicle is the starting point. It is the ‘sleeping’ or ‘dormant’ stage. These follicles are so small that they are invisible to the naked eye.

From this point some have many stages of development to go through; others never develop and are naturally lost. This process of depletion starts even before puberty. By the time a girl starts her periods, approximately 300,000 primordial follicles are left. After the onset of puberty, around 1,000 follicles can be activated each month to prepare for ovulation. When one or occasionally two of these take the lead in the race for maturity, the others drop back, cease to develop and are reabsorbed by the body.

Those that do make it to maturity go through a number of stages. Although this, not all of them will develop through each stage. These are the stages for those few that reach full maturity:

• Primary follicles are the next stage. A few of the original primordial follicles progress into the primary follicle stage every day. The process starts at puberty and continues each day, regardless of other factors, regardless of whether you are pregnant or even ovulating at all, right through until menopause.
• Secondary follicles represent the next stage, in which a layer of cells outside the follicle becomes evident. These theca cells contribute to the production of oestrogens.
• Tertiary follicles are also called antral follicles. These contain a cavity known as the antrum filled with fluid. When follicles reach this stage of development, they are very much larger, and can be seen on an ultrasound scan.
• Graafian follicles are what we call the mature egg when it is ready to ovulate. These are the one or occasionally two of the tertiary follicles that actually make it to full maturity.
• The corpus luteum is what’s left after the egg has been released during ovulation. However, its role in fertility is not yet over. The open follicle closes off to become the corpus luteum which is responsible for producing progesterone, stimulating the lining of the uterus to thicken even further in order to prepare for the implantation of an embryo originated by a fertilised egg.

In the absence of a fertilised egg to implant in the uterus, the body sheds the thickened lining with the monthly menstrual bleed and the whole cycle begins again. The high rate of depletion as the eggs progress from primordial follicle, primary follicle and right through to the ovulation of a mature egg means that at some point the supply of viable eggs will run out. For any woman concerned about her fertility, it can be vitally important to know how many follicles actually remain.

How can the follicles be counted?

If you could know how many follicles are left in your ovaries, it would give you a good idea of how many eggs are remaining and thus your fertility prospects. Unfortunately, however, primordial follicles, primary follicles and all those that fail to mature, at least partially, are too small to be seen. Only when they reach the tertiary stage (Antral Follicle), just before developing into fully mature eggs, they are large enough to be counted.

• The Antral Follicle Test can be carried out by an intravaginal ultrasound scan. This is frequently performed between day 2 and day 5 of your menstrual cycle.
• During the test, the technician can visualise the antral follicles on the screen, and count those that have reached a size of between 2 mm and 10 mm. This can give your doctor a fairly accurate picture of your fertility status in relation to your age, and whether or not you have the expected number for your age, or more or fewer than expected.

How many follicles do you need for fertility treatment?

Commonly used fertility treatments include intrauterine insemination (IUI), also known as artificial insemination, and in vitro fertilisation (IVF). Both make use of hormonal medication to stimulate the ovaries to produce a number of eggs, but the numbers required are very different.

• For IUI, ideally only one or two follicles will be stimulated to mature. Any more would carry a risk of multiple pregnancy with all its attendant dangers for both mother and children. This means that, when undergoing this type of fertility treatment, if your ovaries respond too readily and produce too many eggs, your doctor may need to cancel the cycle.
• IVF on the other hand needs as many eggs, or oocytes, as your ovaries are able to produce in response to the ovarian stimulation. Any number between 8 and 15 oocytes is considered to be a good response. Do bear in mind that not all of these will necessarily be of good quality and so the number of eggs available for fertilisation could end up being fewer than the number of follicles that were counted.

Contact IVI to find out more

We hope that knowing a bit more about the follicles in your ovaries and their various stages of development will help you to consider your fertility options. If you would like to know more, or discuss any aspect of your fertility with us at IVI, do get in touch. Just fill in our online contact form and we’ll get straight back to you.