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Definition of Oogenesis ‒ compendium of concepts and meanings
1. Definition of OogenesisThe oogenesis is female gametogenesis, i.e. the development and differentiation of the female gamete or ovum through a meiotic division. This process occurs from a diploid cell and a cell are formed as products functional haploid (the egg), and three non-functional haploid cells (Polar bodies).
Oogonia are formed from the primordial germ cells or CGP's. they originate in the epiblast from the second week and migrate through the primitive gut to the undifferentiated gonad area around the fifth week of gestation. Once in the ovary, undergo mitosis until the 20th week, moment in which the number of oogonia has reached a maximum of 7 million this figure drops to 40000 and only 400 will be ovuladas from puberty until menopause around age 50. Since week octave, up to 6 months after birth, oogonia differentiate into primary oocytes that come in the prophase of meiosis and begins to form the follicle, initially called primordial follicle. The process of meiosis is arrested in prophase through inhibitory hormones until sexual maturation.
The oogenesis is the growth, maturation, processing and release of gametes in puberty. The differentiation of the egg makes it developed a cytoplasm quite complex. The female gamete provides the future embryo, in addition to a haploid nucleus, reservations of Monocistronic, organelles, enzymes and metabolic substrates. Some species produce thousands or millions eggs throughout its lifecycle (such as sea urchins and frogs), while others produce only some few (mammals). In the former, there are stem cells called oogonia that persist throughout the life of the organism, replicating and autorrenovando are. Species with a limited number of gametes, the oogonium divides during early embryonic States to generate all the endowment of eggs from the female. For example, in humans comes to have approximately 7 million oogonia towards the seventh month of gestation, time from which this number decreases dramatically. Oogonia that survive this process become primary oocytes and enter a phase of meiosis. These cross Prophase I until the diplotene phase and enter into a stadium called dictioteno and its development is stopped by the action of the factor of inhibition of meiosis. Only until the female matures sexually will continue meiosis, so some primary oocytes are kept in the dictioteno for more than 50 years. Approximately 400 of the original primary oocytes mature in the life time of a normal woman. 24 to 48 hours prior to ovulation a peak of luteinizing hormone that initiates meiosis II occurs and this becomes to stop the second arrest meiotic (metaphase II) 3 hours before be ovulation by the factor p37mos and not resumed until a sperm fertilize to the egg. During telophase, when primary oocytes continue meiosis, with one of the descendant cells virtually no contains cytoplasm, while the other descendant, has almost all of the cellular constituents. This first cell is known as polar body and the other as oocyte. Upon entering the secondary oocyte meiosis II, again takes place a distribution of the uneven cytoplasm in which cell that receives a little more than a haploid nucleus will form another polar body and which receives most of the cytoplasmic components will form the mature female egg.
2. Concept of OogenesisThe process of creation of the OVA is called oogenesis. Its development implies the meiosis of a cell diploid, forming a haploid cell of a functional nature (the female gamete, i.e. the ovum) and three others that are not functional.
Oogenesis, therefore, is a kind of gametogenesis: the creation of gametes after meiosis. This type of procedure allows to reduce the number of chromosomes found in the cells from diploid to haploid. In the case of the oogenesis occurs in the ovaries.
In women, the germ cells derived in lasovogonias, giving rise to oocytes, and finally (after ripening), to the eggs. You can say, therefore, that the oogenesis consists of the development, maturation and loosening of eggs from puberty.
With the mature female gamete, the woman is able to become pregnant and give birth. So this can be carried out, the egg must be fertilized by elespermatozoide (the sexual male gamete).
Menstruation is the name given to the female cycle that involves maturity and detachment of the eggs. Develops each twenty-eight days from puberty until menopause. Three internal cycles can be recognized in the own menstruation: ovarian elciclo (with the maturation of the oocyte), the uterine cycle (the conditioning of the environment) and cervical cycle (which allows the arrival of sperm into the egg).
3. Meaning of OogenesisThe oogenesis is the process of formation of ova or female gametes which takes place in the ovaries of females.
The diploid germ cells generated by mitosis, called oogonia, are located in the ovarian follicles, grow and have modifications, by what are called primary oocytes. They carried out the first meiotic division, giving rise a bulky cell or secondary oocyte that contains most of the original cytoplasm and another small cell or first polar body.
These two cells made the second meiotic division; the secondary oocyte other two cells are formed: a large, containing most of the original cytoplasm, and another small or second polar body. Polar bodies disintegrate quickly, while another cell develops into a mature egg haploid.
Some recent research found that approximately 400 thousand eggs are generated in each ovary. It is believed that all of them already exist in the ovary of the newborn, even if they are inactive from birth up to the influence of hormones in puberty.
In humans, the female fetus begins to form oogonia, but stops the process of meiosis in the secondary oocyte stage until since puberty and hormonal effects, an oocyte is apparent in every menstrual cycle; the second meiotic division occurs after performing the penetration of sperm. In males, meiosis begins when the individual reaches sexual maturity.
Where is it done?The primary sexual organs or gonads are ovaries in women. The gonads function as mixed glands where they produce hormones and gametes. The secondary sex organs are those that mature at puberty and structures which are essential in the care and transportation of gametes. Secondary sex characteristics are traits that are considered sexual attraction.
The ovaries are two bodies with almond-shaped, from 4 to 5 centimeters in diameter, located in the upper part of the pelvic cavity, in a depression of the side wall of the abdomen, sustained by several ligaments. The outer region of each ovary has tiny mass of cells called primary follicles; each of these contains an immature egg. Not less than 20 follicles begin to develop at the beginning of the ovarian cycle of 28 days; However, usually only one follicle reaches its full development and others degenerate. The main function of the ovaries is because the oogenesis is the development and release of an ovum or female haploid gamete. Addition, the ovaries produce several hormones steroidal at different stages of the menstrual cycle: estrogens and progesterone.
Function of sexual hormonesDuring a woman's reproductive life, menstrual cycles are interrupted upon the occurrence of pregnancy.
The activities of the ovary and the uterus are regulated by the interaction of various hormones.
The pituitary gland or pituitary gland which produces the hormone follicle stimulating (FSH), which has action on the ovaries, stimulating a primary oocyte to complete meiosis I and form the secondary oocyte is located at the base of the brain. At the same time, FSH stimulates the ovaries to produce hormones, estrogen, causing thickening of the walls of the uterus. These changes will last about ten days.
At the end of the production of FSH hormone makes its appearance luteinizing (LH), which produces the ovulation by breaking of the follicle and release of the egg around the 14th day of the menstrual cycle.
Broken follicle becomes the corpus luteum, that by the action of LH and hormone luteotropica (LTH) secreted by the pituitary gland produces progesterone, which keeps the uterus properly prepared for pregnancy. The LTH also stimulates the production of milk from the mammary glands after giving birth.
4. What is OogenesisThe oogenesis is the production of ova and occurs in the ovaries. The eggs are forming part of the ovarian follicles. This process is regulated by the pituitary hormones: follicle stimulating hormone and luteinizing hormone.
In mammals, it is very common to speak of foliculogenesis, to indicate the different phases that through the follicle in its development. The foliculogenesis is a dynamic and complex process through which the follicle passes through various stages of development.
Early in embryonic development (from week eight to twelve of gestation) the early oocyte surrounded by a layer of flattened follicular cells can be seen in the bark of the ovary. The set of oocyte and its companion cells receives the name of primordial follicle (Fig.1A). The primordial follicles are small and enclose an oocyte, (arrested in prophase of meiosis I), of approximately 35 µm in diameter. During a woman's reproductive life, more than 90% of the follicles are paramount, constituting the fixed follicular reserve of follicles at rest, will gradually be consuming in each menstrual cycle and when they are exhausted, the ovarian activity ceases and menopause ensues.
When girls reach puberty, ovaries begin a phase of activity characterized by the crecimieno and cyclic small groups of follicles maturing. Usually in each cycle, only one follicle reaches full maturity and the oocyte is released from the ovary. So from 400,000 oocytes present at birth, only 400 come to complete their maturation during the fertile woman's life.
Progression of the foliculogenesis, i.e. the passage through the different stages of development of the follicle is based on capacity that have follicular cells to divide and differentiate.
The primordial follicles continually leave the reserve to become primary follicles (Fig. 1B and C). This transition involves certain changes. It reaches two to three times greater than the primordial follicles diameter and oocyte increases in size, the single layer of flattened follicular cells initially becomes a layer of cells then cylindrical and cubic. These cells gradually show increased mitotic activity and proliferate by adopting the appearance of a stratified epithelium. As their cytoplasm looks granular, often also called granulosa cells. In this phase the oocyte with a diameter of 50 to 80 µm, is separated from the follicular cells by a transparent and homogeneous, membrane that is called the zona pellucida. As the follicles become larger, ranging by moving gradually towards the inside of the crust.
Continuing with the follicular development and when the granulosa layer reaches a thickness of several rows of cells, between them, appear several small cavities filled with fluid that is secreted by the follicular cells themselves. At this stage follicle is called the secondary or antral follicle (Fig. 1 d) and has a diameter of 10 mm or more. Subsequently these cavities confluirán for form a space in the form of semiluna called follicular antrum. The presence of cavities will run eccentrically displacing the oocyte, also more developed (125 µm).
Fully ripe follicle is called a follicle of De Graaf (Fig. 1E) characterized by the presence of a large follicular antrum. Follicular antrum determines that the oocyte is displaced toward one pole of the follicle and is situated in a small cell accumulation called cluster ooforo.
In this phase, the follicle occupies virtually the entire thickness of the crust and begins to make relief on the surface of the ovary. The impending rupture of the follicle is preceded by a fast-growing: near the surface the follicle wall thins, becomes translucent, avascular and breaks, releasing the oocyte that immediately completes its first meiotic division. Once produced the detachment of the oocyte, the remains of the empty follicle and surrounding tissue are transformed into a glandular structure known as a corpus luteum (Fig. 1F). The corpus luteum is a structure formed by the follicular cells endocrine and thecal remnants of the follicle that released the oocyte, its main function is to secrete progesterone, which has numerous functions, including the preparation of the uterus for the period of pregnancy and the regulation of contractions of the oviduct for the transport of the egg to the uterus.