Function
An oocyte is a female germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. It is one of the largest cells in the body (approx. 110µm in diameter) and develops in the ovarian follicle, a specialized unit of the ovary, during the process of oogenesis/folliculogenesis in the cortex.
Oogonia are formed during fetal development, and divide via mitosis, much like spermatogonia in the testis. In other words, primary oocytes reach their maximum development at 20 weeks of gestational age, when approximately seven million primary oocytes have been created; however, at birth, this number has already been reduced to approximately 1-2 million. The process of oogenesis/folliculogenesis is highly regulated by hormones and other substances.
Oocytes are commonly divided into:
• Primary oocytes (GV + MI) - dictyate in prophase I for up to 50 years. The oocyte maturation inhibitor (OMI) is an inhibitory factor created by follicular cells during a primary oocyte maturation. It is believed to be the reason why the oocyte remains for so long in the immature dictyate state of meiosis. During the fist dictyate in prophase I (pro-MI) the nucleus takes a special form of a germinal vesicle (GV) (Pic. 1). The breakdown of the germinal vesicle (GVBD, equivalent to nuclear envelope breakdown in somatic cells) indicates a resumption of meiosis and the extrusion of the first polar body (1 PB) indicates completion of the first meiotic division in human MI oocytes (Pic. 2). Then, the oocytes immediately enter the second meiotic division.
• Secondary oocytes (MII) - halted in metaphase II until fertilization (Pic.3). Meiosis of a secondary oocyte is completed only if a sperm succeeds in penetrating its barriers. Meiosis II then resumes, producing one haploid ovum that, at the instant of fertilization by a (haploid) sperm, becomes the first diploid cell of the new offspring (a zygote) (Pic.4) Thus, the ovum can be thought of as a brief, transitional, haploid stage between the diploid oocyte and diploid zygote.
It has become a current social trend for women to delay childbearing. However, the quality of oocytes from older females is compromised and the pregnancy rate of older women is lower. With the increased rate of delayed childbearing, it is becoming more and more crucial to understand the mechanisms underlying the compromised quality of oocytes from older women, including mitochondrial dysfunctions, aneuploidy and epigenetic changes (Pic.5) . Establishing proper epigenetic modifications during oogenesis and early embryo development is an important aspect in reproduction. The reprogramming process may be influenced by external and internal factors that result in improper epigenetic changes in germ cells. Furthermore, germ cell epigenetic changes might be inherited by the next generations.
Studies suggest that age-related effects, including epigenetic changes, on oocytes might be prevented by diets, medicine or other methods. However, until now we still cannot effectively prevent the age-related deleterious effects on oocytes.
An eating disorder characterized by the maintenance of a body weight below average, fear of gaining weight, and a distorted body image.
A condition in which a woman has an imbalance of female sex hormones. This may lead to changes in the menstrual cycle, cysts in the ovaries, trouble g
Failure of the ovaries to release an oocyte over a period of time generally exceeding 3 months.
An abnormal condition in a woman's menstrual cycle.
An obstruction prevents the egg or sperm from traveling down the tube, thus making fertilization impossible.
A state in which pieces of the tissue alike to the lining of the uterus (endometrium) grow in other parts of the body.
Turner syndrome is a genetic disorder in which a female is partly or completely missing one X chromosome that results in ovarian dysgenesis.
A genetic condition where the primary symptom is a failure to start puberty or a failure to fully complete puberty.
Hematosalpinx is a medical condition involving bleeding into the fallopian tube.
A type of female genital malformation resulting from an abnormal development of the Müllerian duct(s) during embryogenesis.
The loss of function of the ovaries before age 40.
A medical term which describes a diminished functional activity of the gonads – the testes and ovaries.
The time in most women's lives when menstrual periods stop permanently, and the woman is no longer able to have children.
A hydrosalpinx is an abnormal pouch containing liquid in a fallopian tube.
A distally blocked Fallopian tube filled with pus.
Infection of the upper part of the female reproductive system and a common complication of some sexually transmitted diseases.
The absence of a menstrual period in women of reproductive age.
The most common benign smooth muscle tumors of the uterus encountered in women of reproductive age.
A type of cancer in which abnormal cells begin to grow in one or both of a woman's ovaries.
Light or infrequent menstrual flow at intervals of 39 days to 6 months or 5–7 cycles in a year.
A permanent form of female sterilization, in which the fallopian tubes are severed and sealed or "pinched shut", in order to prevent fertilization.
Surgical removal of one or both ovaries.
A condition of low fertility characterized by low numbers of remaining oocytes in the ovaries or possibly impaired oocyte development or recruitment.
The presence of abnormally high levels of prolactin in the blood.
The luteinisation of ovulatory follicle without a release of an oocyte.
The type of blockage that affects the part of the fallopian tube end towards the ovary.
The absence of any developmental stage of sperm cell in the testes.
A surgery performed to remove a woman's uterus.
The ovum-producing organs of the internal female reproductive system
The fusion of an ovum with a sperm to initiate the development of a new individual organism.
