How Eggs Are Produced
Reproduction is effected and controlled by natural chemicals in the body called hormones. Hormones are the body’s chemical messengers that are produced by glands ,tissues and organs,and flow through the blood to regulate most of the body’s activities such as cell growth, tissue development, sexual function and reproductive processes.
Production and release of an egg (ovulation [an egg is called ovum]) from the ovary is the result of a complicated series of events involving parts of the brain (called hypothalamus and pituitary gland), and the ovaries. If that egg is fertilized by a sperm and implants in the lining of the womb (endometrium), an intrauterine pregnancy has started.
All of a female's eggs will already be present when she is born. During fetal life ( the unborn baby) the ovaries contain about 7 million eggs(follicles). At birth the eggs are reduced to 1 to 2 million and at puberty the number has further reduced to 400,000. Of these only some 470 will ovulate during the reproductive years. Thus most of them will be wasted, but it is calculated that a woman aged 50 still has about 1000 of her eggs left which will not develop to ovulate.
A woman's ovulation or menstrual cycle begins with a hormonal signal from the hypothalamus which releases a hormone called gonadotrophin releasing hormone which causes another structure at the base of the brain called the pituitary gland to produce other hormones. The pituitary releases a hormone called follicle stimulating hormone (FSH) [so called because it causes growth and development of the follicles that contain the eggs.). FSH stimulates the ovaries and causes a group of about 20 follicles on the surface of the ovary to grow. Within the follicles are the developing eggs. In natural cycle usually only one follicle called the dominant follicle will grow to maturity. This is why most pregnancies result in only a single baby. Maturation of more than one follicle resulting in the release of more than one egg leads to multiple pregnancy. Another hormone from the pituitary called luteinizing hormone (LH) causes the follicle to mature and release the egg, a process and an event called ovulation. The follicles in turn produce two hormones-estrogens and progesterone. Increasing levels of estrogens together with FSH is responsible for the selection, growth and development of the dominant follicle..
After ovulation, the empty follicle forms a structure called corpus luteum which produces more progesterone. Levels of progesterone rise after ovulation and prepare the womb to receive a fertilized egg (embryo). Fertilization by a sperm, if this occurs, takes place in the Fallopian tube, which then moves the early embryo along towards the womb itself. If the egg is not fertilized, or the embryo does not develop, the progesterone levels fall and menstruation starts. The whole cycle then begins again.