module 12-16

• The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days.
• The first half of the ovarian cycle is the follicular phase.
• Slowly rising levels of FSH and LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation.
• As the follicles grow, they begin releasing estrogens and a low level of progesterone.
• Progesterone maintains the endometrium to help ensure pregnancy.
• The trip through the fallopian tube takes about seven days. At this stage of development, called the morula, there are 30-60 cells.
• After about five days, estrogen levels rise and the menstrual cycle enters the proliferative phase. The endometrium begins to regrow, replacing the blood vessels and glands that deteriorated during the end of the last cycle.
• The ovarian and menstrual cycles of female reproduction are regulated by hormones produced by the hypothalamus, pituitary, and ovaries.
• the high level of estrogen causes FSH and especially LH to rise rapidly, then fall.
• The spike in LH causes ovulation.
• The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.
• Following ovulation, the ovarian cycle enters its luteal phase, the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28.
• The luteal and secretory phases refer to changes in the ruptured follicle. The cells in the follicle undergo physical changes and produce a structure called a corpus luteum.
• The corpus luteum produces estrogen and progesterone.
• The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus is being prepared to accept a fertilized egg, should it occur during this cycle. The inhibition of FSH and LH prevents any further eggs and follicles from developing, while the progesterone is elevated.

  The level of estrogen produced by the corpus luteum increases to a steady level for the next few days.
• The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again
• As women approach their mid-40s to mid-50s, their ovaries begin to lose their sensitivity to FSH and LH. Menstrual periods become less frequent and finally cease; this is menopause
• Estrogen is involved in calcium metabolism and, without it, blood levels of calcium decrease.
• Supplementation of estrogen in the form of hormone replacement therapy (HRT) can prevent bone loss, but the therapy can have negative side effects.
• Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization.
• Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can occur either inside (internal fertilization) or outside (external fertilization) the body of the female.
• Fertilization, is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes.
• In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place.
• Fertilization is the process in which sperm and egg fuse to form a zygote.
• Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus.
• To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. If this mechanism fails, multiple sperm can fuse with the egg, resulting in polyspermy. The resulting embryo is not genetically viable and dies within a few days.
• External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water
• Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time.
• Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. There are three ways that offspring are produced following internal fertilization
• In oviparity, fertilized eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a part of the egg.
• In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched.
• In viviparity the young develop within the female, receiving nourishment from the mother’s blood through a placenta.
• Some insects have a specialized sac, called a spermatheca, which stores sperm for later use, sometimes up to a year.
• Non-mammals, such as birds and reptiles, have a common body opening, called a cloaca, for the digestive, excretory and reproductive systems.