Ovarian and Uterine Cycles

Key Terms –

Ovarian cycle: the normal cycle of reproductive function and development in the female ovary, which includes development of an ovarian follicle, rupture of the follicle, discharge of the egg, and formation and regression of a corpus luteum (yellow tissue formed in the ovary after the release of the egg).

Uterine cycle: also known as the menstrual cycle, in which the endometrial lining of the uterus prepares for pregnancy. If pregnancy does not occur, the lining is shed at menstruation.  The average menstrual cycle is 28 days.

GnRH: a hormone which is produced by the hypothalamus in the brain. It stimulates the pituitary gland to produce and release both LH and FSH.

LH: a hormone produced during the menstrual cycle. The luteinizing hormone causes an egg to be released from the ovaries.

FSH: (follicle-stimulating hormone) promotes follicle development within the ovary, thus allowing certain eggs to mature and the follicle cells surrounding each egg to produce estrogen in preparation for fertilization.

Estrogen: female steroid sex hormones. They are secreted by the ovary and responsible for typical female sexual characteristics.

Progesterone: a steroid hormone produced in the ovary, which prepares and maintains the uterus for pregnancy.

hCG: (human chorionic gonadotropin) hormone produced early in pregnancy by the placenta. Detection of the hormone in the urine is the basis for one type of pregnancy test.

Positive feedback loops: hormone levels increase after the release of an egg into the ovary because more hormones are needed to ensure proper development of the egg to a fetus.

Negative feedback loops: a decrease in the hormone levels within the body leads to a decrease of the production of GnRH.

Menstruation vs. Pregnancy: hormone levels are much lower during menstruation than during pregnancy because there is no egg for fertilization.

Fetal Development: is the process in which an embryo or fetus gestates during pregnancy, from fertilization until birth.

1^st Trimester – the first twelve weeks of pregnancy: the zygote is turning into a fetus and the embryo is made up of three layers, ectoderm, endoderm and mesoderm. These are the first stages of organ development.

2^nd Trimester – the second twelve weeks of pregnancy: fetus continues to grow and organs are developing. The ectoderm forms skin tissue, the endoderm forms the digestive and respiratory systems, and the mesoderm forms muscles, cartilage, bone and blood.

3^rd Trimester – the third and final twelve weeks of pregnancy: the fetus is growing rapidly. It will fully develop during this period of time.