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Wednesday, 15 January 2014

Role of ovary in Conception: Back to Basics

The process of conception is intricate and a series of events must happen in the right order and at right time for success. Ovary is where it all starts.  

Egg Reserve
The eggs start to develop in the ovaries while we are still in our mother's womb. The egg cells multiply to the tune of 6-7 million Unfortunately, our natural egg bank (ovary) is not very good at saving too many eggs for future use!
From midpregnancy onward the reserve of eggs starts depleting rapidly such that by the time we are born only 1-2 million eggs are left in the ovaries – a loss of 80%! The depletion continues through childhood and by the time puberty arrives, the figure is between 300,000 and 500,000. To put things in perspective, only about 300 to 500 eggs are released in a woman's lifetime. So nature is not being too inefficient either.
As the number decreases, there comes a point when only a few hundred eggs are left in the ovaries and this is when menopause happens. The speed at which the egg count diminishes varies and that is why menopause occurs variably. 
Ovarian reserve is a reflection of the number of resting eggs in the ovary.What is sad is that the best eggs start getting used up first and as we come towards the end of our reproductive careers, it is the ones that are not good that are left with us!

What are Antral follicles?
Once the eggs are formed they are surrounded by flat cells and this structure is called a primordial or a very early follicle. The eggs remain in a state of sleep or dormancy and in this stage cannot be ‘stimulated’ to grow. An internal signal causes a small number of these sleeping follicles to become active. The flat cells change shape and start multiplying. The follicle develops a fluid filled cavity within. This fluid makes these follicles visible on ultrasound. When you hear your doctors talking about antral follicles at the time of ultrasound scans, they are talking about these ones. What is special about the antral follicles is that at this stage these follicles can be ‘stimulated’ into growing.
The growth of follicles from when it ‘wakes up’ to when it is ready to be ‘stimulated takes place over a period of 70 days and if at this stage there are no hormones to make them grow further, the follicles die. Only those antral follicles that are ‘ready to go’ at the beginning of the menstrual cycle are the ones which have a chance at ovulation.

FSH and LH
The two hormones that play a role in making the ovary work are Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH). These hormones are released by a small organ in the brain called Pituitary Gland (which in turn is stimulated by another part of brain called Hypothalamus). Just as pituitary gland releases FSH and LH to control the ovary, the follicles that are growing in the ovary release the hormones Estrogen and Progesterone. Estrogen and progesterone have a role in cyclical development of the lining of the uterus in preparation of an anticipated pregnancy. Once estrogen starts increasing, the brain responds by decreasing the FSH and LH that are released.

Follicular development and Ovulation
The hormone FSH and to a certain extent LH rise at the beginning of the menstrual cycle and cause the growth of a number of follicles (this is termed as recruitment of follicles). One of these follicles is more efficient in taking up the FSH hormone - the food for growth and as such takes the lead. The others start lagging behind. As all the ‘recruited’ follicles grow, they release estrogen hormone into the blood stream.  The rising levels of estrogen sends a signal to the pituitary to decrease the amount of FSH and LH. With the increasingly hard competition in getting FSH (without which growth cannot continue), the follicles that are lagging behind slowly wither away while the dominant follicle continues to grow. Levels of oestrogen released from the growing follicle reaches a threshold which ‘triggers’ a response from the pituitary gland (a small organ in the brain) in the form of release of a large amount of LH hormone. This release is called LH surge and is crucial for triggering ovulation. It leads to the final maturation of the egg in the follicle which is destined to ovulate and also triggers a chain of processes that culminate in the expulsion of the egg from the dominant follicle.

Role of tubes and formation of embryo in the next post.....

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