The thick and spongy uterine wall breaks when the egg is not fertilized by sperm. The uterine wall, also known as the endometrium, is the inner lining of the uterus. It undergoes changes throughout the menstrual cycle to prepare for the implantation of a fertilized egg. When fertilization does not occur, the uterine wall sheds during a process called menstruation.

Menstruation:
During the menstrual cycle, the uterus prepares for a potential pregnancy by thickening its lining. This thickened lining is rich in blood vessels and nutrients, providing a suitable environment for a fertilized egg to implant and develop. If fertilization does not occur, the hormone levels in the body change, causing the uterine wall to break down and shed.

Hormonal changes:
In a typical menstrual cycle, the hormone levels fluctuate. The levels of estrogen and progesterone rise to prepare the uterine lining for pregnancy. However, when fertilization does not occur, the levels of these hormones decrease. This decrease in hormone levels triggers the breakdown of the uterine wall.

Release of prostaglandins:
When the uterine wall breaks down, it releases chemicals called prostaglandins. These prostaglandins cause the blood vessels in the uterine lining to constrict, reducing blood flow to the area. This constriction leads to the shedding of the uterine wall, resulting in menstrual bleeding.

Menstrual bleeding:
During menstruation, the broken down uterine wall, along with blood and tissue, is expelled from the body through the cervix and vagina. This process typically lasts for a few days and is characterized by menstrual bleeding.

Importance of menstruation:
Menstruation is a normal physiological process that occurs in females of reproductive age who have not become pregnant. It is a sign that the body is not pregnant and is preparing for a new menstrual cycle. Menstruation also helps to remove any potentially damaged or unused eggs, as well as the uterine lining that is no longer needed.

In conclusion, the thick and spongy uterine wall breaks when the egg is not fertilized by sperm. This triggers hormonal changes, the release of prostaglandins, and the shedding of the uterine lining, resulting in menstrual bleeding.