Female Reproductive System: Oogenesis and Folliculogenesis...Ovulation and Fertilization.
An ectopic pregnancy is a rare situation that occurs if the fertilized ovum implants in a place other than the uterus, such as in the fallopian tube. Because the fallopian tube cannot expand to accommodate a growing baby, it could rupture leading to a life-threatening situation for the mother. While the cause for a tubal pregnancy is not always known, there is an increased risk for an ectopic pregnancy if there is any damage to the fallopian tubes caused by pelvic inflammatory disease; lesions due to chlamydia infection or endometriosis; use of intrauterine devices or IUD for birth control; abdominal surgeries or intrauterine surgeries, including abortions or tubal ligation; or use of assisted reproductive technology, like IVF.
*
Beginning at puberty and continuing throughout the reproduction years of the female, each menstrual cycle some oocytes will complete meiosis 1 to become secondary oocytes. These secondary oocytes begin meiosis 2 but again stop, this time at metaphase 2. Typically, only one oocyte will be ovulated each month and then, if it is fertilized, it will complete the second meiotic division. If it is not fertilized, it does not complete meiosis 2 and degenerates within about 24 hours after ovulation.
*
Beginning at puberty, primordial follicles may be recruited to develop further. It is also not known what regulates or determines which primordial follicles will begin further development, but it is known that several hormones are required for the process: the gonadotropins of the anterior pituitary, FSH and LH, as well as the ovarian hormone estrogen.
*
During the time of ovulation, high estrogen levels cause the cilia to beat more strongly toward the uterus. High estrogen levels also create regular contractions of the smooth muscle along the length of the fallopian tube resulting in a sweeping movement slowly moving the ovum toward the uterus.
*
Even though there are millions of primordial follicles in the fetus, by the onset of puberty, a female will have only about 400,000 primordial follicles. This is due to the continual process of atresia (apoptosis).
*
Fertilization typically takes place in the ampulla (distal end) of the fallopian tube. In order to penetrate the ovum, the sperm will have to negotiate their way through the corona radiata cells and the zona pellucida. Many sperm cells will die trying to do this and only one will ultimately be successful. Penetration of the sperm triggers completion of the 2nd meiotic division. The union of the sperm and the ovum forms a zygote.
*
If the oocyte is not fertilized, the corpus luteum survives about 10-12 days and then begins to degenerate becoming the corpus albicans. If the oocyte is fertilized, the corpus luteum is maintained throughout the first part of pregnancy, but later, the placenta will take over the job of producing estrogen and progesterone. After ovulation, the oocyte, along with its cornona radiata, will enter the fallopian tubes.
*
Implantation generally occurs on the endometrium at the superior end of the uterus, typically 6-7 days after fertilization.
*
In the Graafian follicle, the oocyte is located on one side of the antrum and is surrounded by several layers of granulosa cells, the cumulus oophorus. The graafian follicle is now almost ready for ovulation. The final event, occurring several hours prior to ovulation, is completion of meiosis 1 to produce a secondary oocyte which immediately starts meiosis 2. Once again, the oocyte arrests this time at metaphase 2 where it will remain until it is fertilized by a sperm.
*
In the fetal ovary, primary oocytes become surrounded by flat cells that will later become granulosa cells. Granulosa cells are analogous to the Sertoli cells that surround and support the spermatogonia.
*
Progesterone is a hormone critical for establishing and maintaining pregnancy. The progesterone levels secreted by the corpus luteum during pregnancy send negative feedback to the hypothalamus and pituitary gland to keep GnRH, LH and FSH secretions low so that no other dominant follicles develop while a woman is pregnant.
*
Recall that the two meiotic divisions of the spermatocytes result in the production of four spermatids. However, when the oocytes undergo meiosis the divisions produce only one daughter cell and one polar body. It's essential that the ovum, when ovulated, have enough stored energy to keep the developing embryo alive until it implants in the uterine wall. Consequently, the meiotic divisions are unequal. The daughter cell retains all of the stored nutrients and cellular organelles while the polar body contains only the chromosomes from the nuclear division.
*
The Graafian follicle moves to the surface of the ovary and appears like a small blister on the ovarian wall. Ovulation is achieved when this blister bursts and the oocyte, the follicular fluid, and the cumulus layer of granulosa cells is expelled from the follicle.
*
The change of the follicle to a corpus luteum is called luteinization and is brought about by the increase of luteinizing hormone (LH). The corpus luteum becomes the endocrine portion of the ovary and secretes estrogen and progesterone.
*
While still in the fallopian tube, the zygote begins the process of cell division, mitosis, and becomes an embryo. The embryo continues to grow by cell division as it travels toward the uterus. The fallopian tubes contain mucus secreting cells and ciliated cells.
*
by puberty there are only around 400,000 primary oocytes remaining, of which only 400 will be ovulated.
*
by the end of the fourth month of fetal development there are roughly 7 million oogonia in the ovaries. Oogonia are analogous to spermatogonia and are the cells that will develop to become the egg or oocyte (ovum).
*
the process of follicular development (folliculogenesis) begins even before the birth of the woman.
*
unlike spermatogonia that remain quiescent until puberty, the oogonia begin meiosis 1 even before birth. However, the future ova at this point called primary oocytes, arrest in prophase 1 where they will remain until puberty. may of them degenerate and by birth only about 2 million of the original 7 million remain.
*
As the follicle continues to develop, the thecal cells mutliply forming several layers on the outside of the follicle. Additionally, the granulosa cells continue to increase in number and begin to secrete fluid (follicular fluid) resulting in the formation of fluid filled spaces among the granulosa cells. With the aid of the granulosa cells the primary oocyte continues to enlarge. Eventually all of the fluid filled spaces will coalesce into one large cavity called the antrum. The follicle is now called a mature _______ follicle.
Graafian
The cumulus cells surrounding the oocyte at ovulation are called the corona radiata. The remaining granulosa cells and thecal cells of the follicle become the ______ _______.
corpus luteum.
Beginning at puberty small groups of primordial follicles start developing. The pre-granulosa cells that sorround the oocyte increase in size and become cuboidal in shape. At this point, we refer to these surrounding cells as _____ cells. At the same time, the primary oocyte inside this layer of cells begins to increase in size and secrete proteins. Also, the granulosa cells nearest the oocyte secrete mucopolysaccharides. Together the proteins from the oocyte and the mucopolysaccharides from the granulosa cells form the ____ ______, a clear layer between the oocyte and the granulosa cells. It's this barrier the sperm will have to penetrate in order to fertilize the ovum. The follicle is now considered a ______ follicle.
granulosa zona pellucida primary follicle.
Ovulation is the event that releases the oocyte from the follicle. This event is under the control of LH. just prior to ovulation there is a sharp increase in the levels of LH. This increase is referred to as the LH surge and is the apparent trigger for the events of _________.
ovulation
Once the primary oocyte becomes surrounded by these pre-granulosa cells, we call the group of cells, along with the basement membrane that surrounds it, a ________ follicle.
primordial
the surrounding granulosa cells begin to divide by mitosis and form multiple layers. Simulataneously, stromal cells (connective tissue cells) are recruited by the follicle to form a layer of ________ cells just outside of the basement membrane. Once these processes are complete we call the follicle a secondary follicle.
theca
