Understanding Oogenesis - Biology

Down's syndrome is caused by an extra copy of chromosome 21. This trisomy is a result of nondisjunction, which can occur during either meiosis I or meiosis II. Nondisjunction most often occurs during anaphase I of meiosis. Note that most other trisomies and monosomies are lethal.

Oogonia are the primordial oocytes formed inside females either during or shortly after birth. At this time, the formation of primary oocytes occurs during dictyate, which lasts until the surge of luteinizing hormone (LH) just before the onset of puberty. After menarche, a few of these cells will further develop each period into secondary oocytes, which are halted in metaphase II until fertilization. At the end of meiosis II, both polar bodies created to discard extra haploid sets of chromosomes disintegrate, leaving behind the oocyte which can then mature into an ovum. Thus, the correct order is the following order: oogonium, primary oocyte, secondary oocyte, and ovum.

Follicular atresia is a hormone-controlled, apoptotic (cell-suicide) process by which immature follicles degenerate and are resorbed into the main body of the ovary, leaving one out of typically 20 primary follicles standing as a secondary follicle. This process, moderated by follicle stimulating hormone and tumor necrosis factor alpha (TNF), assists the body in forming the corpus luteum out of the remaining follicle following ovulation, as the body would otherwise not be able to generate enough progesterone to continue the process.

The female gamete is called the ovum (ova, plural). A zygote is the cell that results from fertilization occurs between two gametes. An ovary is the female reproductive organ that produces ova. Sperm is the male gamete. The uterus is the organ where the fetus develops.

After completing meiosis II, ova are haploid cells containing one chromosome. Haploid cells have half the number of chromosomes (n) as a diploid cell. Haploid cells like ova and sperm will merge during fertilization and form a diploid cell with two complete sets of chromosomes (2n).

An oocyte develops in the ovaries during female gametogenesis. If the oocyte eventually becomes an ovum and is fertilized in the fallopian tubes, the resulting gamete will be implanted in the uterus. The placenta is an organ that connects the developing fetus to the uterine wall, and the cervix is the lower portion of the uterus that separates the vagina from the uterus.

The release of the secondary oocyte from the ovaries is ovulation. This occurs after the follicles surrounding the oocyte mature and rupture, releasing the cell to be available for fertilization. Menstruation occurs after ovulation - if the ovum fails to be fertilized, the uterus will shed its lining. Oogenesis, the creation of an ovum, is the female form of gametogenesis (creation of a gamete). Meiosis is the process of cell division in which the number of chromosomes is halved.

Primary oocytes enter meiosis I and replicate their genomes, but they do not make their first meiotic division. They remain in prophase I until a female begins her first menstrual cycle. Then, each month, one primary oocyte resumes the process of meiotic division.

A primary oocyte becomes a secondary oocyte when the follicle grows and matures and the primary oocyte completes its first meiotic division. Shortly after, the follicle will rupture and release the secondary oocyte to be fertilized. Later, after the oocyte develops into an ovum and is fertilized, it becomes a diploid zygote, which develops into an embryo through the process of mitosis. The ovary is the reproductive organ where the ova are produced, and a gamete is the haploid cell (ovum or sperm) that is fertilized and forms a zygote.

During the menstrual cycle, typically only one follicle matures and is fertilized. The follicles that do not mature are called polar bodies and degenerate. However, sometimes multiple follicles remain and are available to be fertilized, resulting in genetically distinct embryos, known as fraternal twins.

Typically, each menstrual cycle produces three primary oocytes. Two of these primary oocytes are inevitably smaller than the third and usually disintegrate before maturation and fertilization. These two cells are known as polar bodies. Usually, meiosis results in four daughter cells that are all haploid. However, after meiosis I of oogenesis, there are two daughter cells, one primary oocyte, and a polar body. After meiosis II, the primary oocyte divides into a secondary oocyte and another polar body. However, the polar body that was a result of the first meiotic division usually does not complete meiosis, and thus does not divide again.

At the beginning of the menstrual cycle, elevated follicle-stimulating hormone (FSH) stimulates primary oocytes to resume meiosis. A spike in luteinizing hormone (LH) triggers ovulation. Thyroid-stimulating hormone (TSH) and human chorionic gonadotropin (hCG) are involved in thyroid hormone secretion, and in maintenance of the corpus luteum during pregnancy, respectively. Estrogen is a female sex hormone, and has an indirect regulatory role in meiotic divisions of ova.

Gametes are formed during the process of meiosis. Oogenesis is the process by which the female games are produced, which occurs in the ovary. The product of oogenesis is one mature egg from one primary oocyte; this occurs about once every four weeks in humans.

The secondary oocyte completes meiosis II only when fertilized by a spermatozoan. After fertilization is initiated, the secondary oocyte begins its second meiotic division, resulting in the formation of a mature ovum and another polar body. At this point, the ovum is ready to fuse with the spermatozoan.

Normally, one ovum is released every 28 days in human females. These 28 days constitute the menstrual cycle, which may vary in length, but on average, the cycle takes 28 days. A female is born with all the eggs she will ever have, and after the initiation of menstruation, one egg is released every 28 days until menopause.

Fertilization typically occurs in the fallopian tube. After fertilization, the diploid zygote will begin the process of implantation into the uterus, where it undergoes mitotic divisions. The resulting multicellular organism is called the embryo, which further develops in the uterus into a fetus.

When gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes. The type of cell division that produces gametes with half the normal chromosomes is called meiosis.

During oogenesis—at the end of meiosis and cytokinesis—an oogonium divides into one mature ovum capable of being fertilized and three polar bodies that are reabsorbed, while a spermatogonium divides into four viable spermatozoa capable of fertilization.

When a primary oogonium undergoes meiosis, it will only result in one viable germ cell, or egg. The other smaller cells are called polar bodies and typically disappear following division. Spermatogenesis will result in four separate sperm cells, each capable of producing offspring.

Both eggs and sperm are haploid, and both processes can involve crossing over during meiosis.