Ovaries and Egg Production

The gonads – the testes in males and the ovaries in females – are the basic organs involved in the sexual reproduction of humans.  

The ovaries in humans are the two oval-shaped organs measuring approximately 3 cm x 1.5 cm x 1.5 cm located in a region called the ovarian fossa in the lateral wall of the pelvis. A female is born with about 400,000 eggs, but these numbers drop as she ages.

Production of eggs prior to conception 

Prior to and during menstruation, estrogen levels rise and induce the thickening of the uterine wall. At the same time, maturation of the egg in one of the ovaries takes place. This developing egg is encased in the Graafian follicle – a sac that produces the hormone estrogen. 

At the middle of the uterine cycle, this sac ruptures and the ovum leaves the ovary. This process is what's known as ovulation. For women with regular periods, it is estimated that ovulation happens around the 14th or 15th day of the cycle. Once the egg is expelled, the sac produces the hormone progesterone. At this stage, the sac remains in the ovary and becomes what is called the corpus luteum.  

After the egg leaves the ovary, it passes through the fallopian tube where it can then be fertilized by sperm. 

Note:  

Unlike males whose spermatid production stages happen continuously during the person's lifespan, the females' egg production stages only take place during certain periods in their lifetimes (Fig. 2). Even as the female is still an embryo, about 7,000,000 immature germ cells are already developing in her embryonic ovary. Many of these cells eventually die while roughly half a million would proceed to the initial meiotic stage (meiosis is the method of replication undergone by reproductive cells). These surviving cells become the primary oocytes before the female is born. 

Until puberty, they remain to be precursory egg cells, and only one to a few would resume undergoing the meiotic process each month. These that do are contained in a follicle and go on to the 1st and 2nd reduction divisions, only to stop the process of development once again. 

The last meiotic stage is still incomplete when one of these secondary oocytes is released by the ovary into the fallopian tube. Only when the sperm penetrates the ovum, giving rise to various chemical changes, can the meiotic process be considered complete. 

The significance of these numbers 

Although there are about 7 million primary oocytes in the embryonic ovary, only about 1.2 million remain at birth. At puberty, another two-thirds of this figure dies, and only about 40,000 are left to be released throughout a woman's life. These numbers continue to dwindle as a woman ages. Simply put, there are about 2000 eggs lost for every egg that is successfully ovulated. 

It is important to note, too, that an average woman is expected to have 11-14 ovulations each year over 33 to 36 years. This means that a mere 500 or so secondary oocytes are produced, and this is even more aggravated by several factors that lead to decreased egg production such as psychological stress, nutrition, pathological conditions, hormonal factors, and physical activity or the lack of it.