Ovarian and Uterine Cycles

http://www.bioteach.ubc.ca/legal-rights-and-the-maternal-fetal-conflict/

The ovarian cycle lasts approximately 28 days, and begins with menstruation, which takes up the first week of the cycle. During that time, 6-12 follicles begin to develop. The follicle is a primary oocyte surrounded by follicle cells, and as it develops, the follicle cells rapidly divide and feed nutrients into the oocyte through gap junctions. All but one of these oocytes will eventually degenerate, with the remainder completing Meiosis I, in which the diploid oocyte divides into two haploid cells. One half, the polar body, contains little cytoplasm while the other half, the secondary oocyte, enters Meiosis II and prepares for fertilization. Around day 14 of the cycle, the oocyte bursts through the wall of the ovary in a process called ovulation, which ends the follicular phase. Cilia lining the oviduct propel the egg forward in case a sperm cell is ready to fertilize it. Meanwhile, during this luteal phase, the remaining follicle tissue in the uterus forms the corpus luteum, an endocrine structure that secretes the hormones estrogen and progesterone, hormones that stimulate the maintenance and development of the uterine lining. In the event that the woman does not become pregnant, the corpus luteum disintegrates.

The uterine cycle is closely tied to the ovarian cycle. It also begins with menstruation when the previous uterine lining, the endometrium, leaves the vagina. The new lining of the uterus prepares for an embryo to be embedded. The period between menstruation and ovulation (days 7-14) is known as proliferation, in which the uterine lining continues to grow. Capillaries in the lining supply the uterus with plenty of nutrients to prepare for a pregnancy, while estrogen and progesterone keep the lining growing. However, if at the end of the secretory phase of the cycle an oocyte has not been fertilized, then no embryo will be created and nothing will be implanted within the endometrium. The lining dies and menstruation occurs, restarting the cycle. Because the woman was not impregnated, the levels of estrogen and progesterone decline until the uterine lining is removed, but these hormones will continue to be produced if an embryo is implanted.

http://www.tutorvista.com/content/biology/biology-iv

Summary of hormones:


LH (luteinizing hormone) - Produced from the anterior pituitary gland shortly before the cycle begins; stimulates oocytes to develop until they rupture the ovarian wall; maintains corpus luteum, and in doing so increases levels of estrogen and progesterone


FSH (follicle-stimulating hormone) - Produced from the anterior pituitary gland shortly before the cycle begins; stimulates oocytes to develop until they rupture the ovarian wall; stimulates the production of estrogen and progesterone


GnRH (gonadotropin-releasing hormone) - Released from the hypothalamus; triggers the anterior pituitary gland to release LH and FSH


Estrogen - Stimulates development of uterine lining; inhibits LH, FSH and GnRH except for days 12-14, when it suddenly has a positive feedback effect that causes a surge of LH and FSH for ovulation; maintains uterine lining


Progesterone - Stimulates development of uterine lining; inhibits LH, FSH and GnRH except for days 12-14, when it suddenly has a positive feedback effect that causes a surge of LH and FSH for ovulation; maintains uterine lining


hCG (human chorionic gonadotropin) - Secreted by a layer of cells surrounding the blastocyst (the multicellular embryo); can cause surges of LH and FSH, but can also maintain the corpus luteum in the event of fertilization, which inhibits the release of LH, FSH and GnRH


Hormonal changes are important for the body to either prepare for a pregnancy or repeat the ovarian and uterine cycles. The aforementioned hormones do not work independently, however; instead, they use positive and negative feedback loops to either promote or hinder their production, respectively. Positive feedback loops mean that as a substance "A" is created, it encourages the production of another substance "B". In a negative feedback loop, "A"'s production inhibits the synthesis of "B." For example, at the beginning of the cycles, low levels of LH and FSH steadily increase the levels of estrogen and progesterone, which in turn inhibit LH and FSH, but not enough to stop the development of an oocyte. Then, around day 12, estrogen and progesterone suddenly encourage a surge of these hormones so that the oocyte quickly grows and breaks through the wall of the ovary. After day 14, estrogen and progesterone created by the corpus luteum return to inhibiting LH and FSH if fertilization has not occurred so that the corpus luteum dissolves and the endometrium can be shed. During ovluation, many hormones are at their peak levels, which may explain the emotional changes that occur in women to cause PMS (premenstrual syndrome). However, if fertilization has occurred, the embryo will release hCG in order to trigger another surge of LH and FSH to keep the corpus luteum producing estrogen and progesterone to prevent menstruation, which is why pregnant woman don't have periods.

Pregnancy is an exciting but critical period in a young mother's life. Intercourse has to be performed at the right time during the ovarian and uterine cycles in order for an oocyte to be fertilized. While hundreds of thousands of sperm are released through ejaculation during intercourse, only one sperm cell will be able to survive the acidic and protective environment of the vagina. Once the woman's reproductive system allows for a sperm cell to fuse with the zona (a protein shield around the oocyte), a zygote is formed. The female and male chromosomes are spread out, providing the genetic material that will determine all the characteristic of the child. The zygote undergoes mitotic divisions called cleavage, formign a ball of cells called a blastocyst. Rarely, the dividing cells split into two groups, beginning life for identical twins. During all these divisions, the blastocyst is being pushed along the fallopian tube into the uterus. It breaks out of the zona and implants itself into the uterine lining.

http://doctorstevenpark.com/what-all-pregnant-women-must-know

Two weeks after fertilization, the blastocyst's cells begin to separate into specific layers in a process known as gastrulation, which I discussed in my blog post on Embryology. Immediately, the embryo begins its nine-month-long journey into becoming a functioning infant. It does so by turning certain genes on and off within the specialized cells to form tissues and organs. The nervous system begins to develop during the first few weeks as a primitive brain is trying to make connections with the environment. Blood vessels, the heart, the spinal cord, and the limbs form within four and a half weeks. Human embryos even form a tail, which will disappear within the first trimester. During this time, chemical messages between cells continue to turn certain genes on and off to create the proteins that help a specific type of cell perform its functions.

The chromosomes can identify sex: girls have two X chromosomes, while boys have an X and a Y chromosome. Around the sixth week of development, genitalia is determined by whether certain genes get turned on by a Y chromosome or not. However, it isn't until the eighteenth week when this genitalia will be viewable by an ultrasound so the parents can identify the sex of their baby. When the embryo is two months old, enough cells have been specialized to resemble the shape of a human infant. At this stage, we call the developing child a fetus, rapidly growing within the amniotic sac. Its organs are in place, but not working yet. The fetus continues to grow in size and its heart beats twice as fast as an adult heart. The other organs gain function because of nutrients delivered through the umbilical cord. The umbilical cord connects the fetus to the placenta, an organ which connects the fetus to the uterine lining. Villi, tiny projections on the placenta, take oxygen and nutrients from the mother's blood and send it to the fetus. It is essential for a mother to properly nourish her child by eating the proper foods and avoiding bad habits such as smoking and drinking.

http://www.charlottetufts.com/ultrasound/

During the second trimester, the organs continue to develop. Ears are formed, and eyes begin to sense light. Soft cartilage gives the fetus support, eventually forming bone as early as the fourth month. The fetus gains hearing in the fifth month. The womb is relatively busy during the first few months of pregnancy, and by the end of the second trimester, all the organ systems have been developed. The third trimester gives the fetus time to grow, as well as to form fat deposits around the body and in the brain to enhance brain power. The mother is also undergoing major changes during this time, as her stomach gets bigger to accomodate more blood flow and room for the child. By the ninth month, the baby can realize that the mother is unable to keep up with his or her needs, so the baby is ready to be born. The uterus begins to contract to dilate the cervix, and as these contractions increase in frequency, it is a sign that the birth will happen soon. Water breaking from the rupture of the amniotic sac is another sign. There is a possibility that the proportions of the baby will cause it to be unable to exit the vagina. If this happens, doctors can perform a Cesarian section (C-section) to take out the baby through the mother's abdomen.

http://cjkennedy.wordpress.com/2007/06/27/

Nowadays, science allows us to fertilize eggs outside of the body through IVF (in vitro fertilization). The first "test tube baby," Louise Brown, was born on July 25th, 1978, and the doctor who created IVF treatment, Robert G. Edwards, was awarded the Nobel Prize in 2010. Technology that alters the natural course of human reproduction is highly controversial, but no matter what the method is, there is nothing more beautiful than the miracle of life which keeps the human race going strong for generations to come. Plus, babies are undeniably cute.

http://blog.bioethics.net/2009/03/