when do oocytes mature?

Question

hjtis Raza · Answer

The answer is C, but there is a bit of truth to all of the choices. Starting even before we are born, female eggs have a surprisingly long journey to mature and reach ovulation. Let’s break it down.

Women are born with all the eggs for their entire lifetime. In contrast to men, who produce new sperm throughout their lives, women don’t produce new eggs after birth. The eggs lay dormant in the ovaries for years, until they are initiated into the maturation process that takes just one of them to ovulation-ready stage each month.

In the last 15 years, a few studies have identified ovarian stem cells, hinting at the possibility that women may keep making eggs throughout their lives from ovarian stem cells. However, this new theory has been challenged, partially because other research teams have been unable to reproduce the results. While some scientists consider this a possibility, a 2020 study that examined 24,000 cells in the ovarian cortex could not identify a single ovarian stem cell that may become new eggs. As of now, the consensus is that women probably don’t produce new eggs.

Egg production begins before women are born. In fact, we reach the peak of egg count when we are still in the uterus, at around 20 weeks after conception. At that stage, female fetus may have as many as 6-7 million immature eggs in the ovaries. By the time babies are born, the pool of immature eggs has shrunk to 1-2 million on average.

After birth, more eggs are lost to apoptosis. Scientists have estimated around 200,000 eggs remain in our ovaries by puberty.

Once women reach puberty and menstrual cycles set in, a handful of immature eggs are “recruited” each month into the process of maturation. Most of the eggs that starts the process do not reach ovulation, with (usually) just one egg reaching full maturation each cycle. In the entire female reproductive lifespan, roughly 400-450 will ever reach ovulation. At menopause, scientists estimate about 1,000 eggs remaining in the ovaries.

As many of us know, female egg count declines gradually in our teens and 20s, then starts a more rapid decline at around age 35. This is one reason female fertility declines with age – there simply aren’t as many eggs with potential to become a baby. The other side of the coin is the quality of these eggs, which takes us to how immature eggs develop once they are “recruited” into the maturation process.

Just like a sperm cell, each egg cell needs one set of 23 chromosomes, so that an embryo from an egg and a sperm would have the correct two sets of 23 chromosomes. A special type of cell division called meiosis produces egg cells with one set of chromosomes, ensuring the accuracy of this genetic math.

Eggs go through two cycles of meiotic cell division. The first meiosis starts in utero, but stops before birth. From birth until the time each egg is activated to begin the final maturation process, the immature egg is held in a state of little to no biological activity. A group of cells called granulosa cells surround the immature egg, providing structural support. This dormant period – called meiotic arrest – can last up to 50 years in humans.

After puberty, a handful of immature eggs start their final maturation journey toward ovulation. This is called ovarian follicle maturation or recruitment. Once recruited, eggs (called primary oocytes at this stage) and follicles (the fluid-filled sacks that hold the eggs and support cells) grow in size, shape and function.

This maturation process is tightly controlled by the endocrine system. Numerous reproductive hormones (including androgens, surprisingly) activate and inhibit processes at precise moments to ensure the healthy maturation of eggs.

For example, follicles develop receptors to follicle-stimulating hormone (FSH) at this stage, which will later drive the eggs’ maturation process. Eggs resume the first cycle of myosis, and once complete, primary oocytes become secondary oocytes with just one set of chromosomes.

Right after the first cycle of meiosis is complete, the eggs start the next phase of meiosis. At this point, the eggs are called secondary oocyte. However, they are again held in meiotic arrest until fertilization. It takes 45-60 days from the initial recruitment to the secondary follicle stage – and another 45-60 days until a single egg from the same cohort to reach ovulation.

Because our eggs age with us, some age-related egg damage may be inevitable. However, some reproductive endocrinologists think that the immature eggs held in dormant state are much less vulnerable to damages than other cells with active metabolism.

In addition, studies have suggested that the 90- to 120-day maturation process may be a window of opportunity to support egg health, whether by supporting the production of cellular energy necessary to drive the biological processes involved in egg maturation, by providing antioxidant protection to metabolically active eggs, or by potentially supporting the hormonal environment of the ovaries.

The key is to realize that the process is longer than a single menstrual cycle. If you get pregnant in November, for example, the egg that become the embryo in November probably started its journey toward ovulation in August – or even earlier. It makes sense to start early once pregnancy enters your mind.

Rutanya Glusman · Answer

Oocyte maturation is defined as a re-entry into meiosis that occurs just prior to ovulation and subsequent fertilization.

Parzan Maaney · Answer

An oocyte (UK: /ˈoʊəsaɪt/, US: /ˈoʊoʊ-/), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation.

The formation of an oocyte is called oocytogenesis, which is a part of oogenesis.[1] Oogenesis results in the formation of both primary oocytes during fetal period, and of secondary oocytes after it as part of ovulation.

Oocytes are rich in cytoplasm, which contains yolk granules to nourish the cell early in development.

During the primary oocyte stage of oogenesis, the nucleus is called a germinal vesicle.[2]

The only normal human type of secondary oocyte has the 23rd (sex) chromosome as 23,X (female-determining), whereas sperm can have 23,X (female-determining) or 23,Y (male-determining).

The space within an ovum or immature ovum is located is the cell-nest.[3]

The cumulus-oocyte complex contains layers of tightly packed cumulus cells surrounding the oocyte in the Graafian follicle. The oocyte is arrested in Meiosis II at the stage of metaphase II and is considered a secondary oocyte. Before ovulation, the cumulus complex goes through a structural change known as cumulus expansion. The granulosa cells transform from tightly compacted to an expanded mucoid matrix. Many studies show that cumulus expansion is critical for the maturation of the oocyte because the cumulus complex is the oocyte's direct communication with the developing follicle environment. It also plays a significant role in fertilization, though the mechanisms are not entirely known and are species specific.[4][5][6]

Because the fate of an oocyte is to become fertilized and ultimately grow into a fully functioning organism, it must be ready to regulate multiple cellular and developmental processes. The oocyte, a large and complex cell, must be supplied with numerous molecules that will direct the growth of the embryo and control cellular activities. As the oocyte is a product of female gametogenesis, the maternal contribution to the oocyte and consequently the newly fertilized egg, is enormous. There are many types of molecules that are maternally supplied to the oocyte, which will direct various activities within the growing zygote.

The DNA of a cell is vulnerable to the damaging effect of oxidative free radicals produced as byproducts of cellular metabolism. DNA damage occurring in oocytes, if not repaired, can be lethal and result in reduced fecundity and loss of potential progeny. Oocytes are substantially larger than the average somatic cell, and thus considerable metabolic activity is necessary for their provisioning. If this metabolic activity were carried out by the oocyte's metabolic machinery, the oocyte genome would be exposed to the reactive oxidative by-products generated. Thus it appears that a process evolved to avoid this vulnerability of germline DNA. It was proposed that, in order to avoid damage to the DNA genome of the oocytes, the metabolism contributing to the synthesis of much of the oocyte's constituents was shifted to other maternal cells that then transferred these constituents to oocytes.[7][8] Thus, oocytes of many organisms are protected from oxidative DNA damage while storing up a large mass of substances to nurture the zygote in its initial embryonic growth.

During the growth of the oocyte, a variety of maternally transcribed messenger RNAs, or mRNAs, are supplied by maternal cells. These mRNAs can be stored in mRNP (message ribonucleoprotein) complexes and be translated at specific time points, they can be localized within a specific region of the cytoplasm, or they can be homogeneously dispersed within the cytoplasm of the entire oocyte.[9] Maternally loaded proteins can also be localized or ubiquitous throughout the cytoplasm. The translated products of the mRNAs and the loaded proteins have multiple functions; from regulation of cellular "house-keeping" such as cell cycle progression and cellular metabolism, to regulation of developmental processes such as fertilization, activation of zygotic transcription, and formation of body axes.[9] Below are some examples of maternally inherited mRNAs and proteins found in the oocytes of the African clawed frog.

The oocyte receives mitochondria from maternal cells, which will go on to control embryonic metabolism and apoptotic events.[9] The partitioning of mitochondria is carried out by a system of microtubules that will localize mitochondria throughout the oocyte. In certain organisms, such as mammals, paternal mitochondria brought to the oocyte by the spermatozoon are degraded through the attachment of ubiquitinated proteins. The destruction of paternal mitochondria ensures the strictly maternal inheritance of mitochondria and mitochondrial DNA (mtDNA).[9]

In mammals, the nucleolus of the oocyte is derived solely from maternal cells.[22] The nucleolus, a structure found within the nucleus, is the location where rRNA is transcribed and assembled into ribosomes. While the nucleolus is dense and inactive in a mature oocyte, it is required for proper development of the embryo.[22]

Maternal cells also synthesize and contribute a store of ribosomes that are required for the translation of proteins before the zygotic genome is activated. In mammalian oocytes, maternally derived ribosomes and some mRNAs are stored in a structure called cytoplasmic lattices. These cytoplasmic lattices, a network of fibrils, protein, and RNAs, have been observed to increase in density as the number of ribosomes decrease within a growing oocyte.[23]

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis.[24] In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for many years, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage appears to provide the informational redundancy needed to repair damage in the DNA of the germline.[24] The repair process used likely involves homologous recombinational repair.[24][25][26] Prophase arrested oocytes have a high capability for efficient repair of DNA damages.[25] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility.[25]

The spermatozoon that fertilizes an oocyte will contribute its pronucleus, the other half of the zygotic genome. In some species, the spermatozoon will also contribute a centriole, which will help make up the zygotic centrosome required for the first division. However, in some species, such as in the mouse, the entire centrosome is acquired maternally.[27] Currently under investigation is the possibility of other cytoplasmic contributions made to the embryo by the spermatozoon.

During fertilization, the sperm provides three essential parts to the oocyte: (1) a signalling or activating factor, which causes the metabolically dormant oocyte to activate; (2) the haploid paternal genome; (3) the centrosome, which is responsible for maintaining the microtubule system. See anatomy of sperm

lgjzj Yunus · Answer

Typically, only one oocyte each cycle will become a mature egg and be ovulated from its follicle. This process is known as ovulation.

A woman is born with all the oocytes she will ever have. This number decreases naturally with age. Age also reduces the quality and genetic stability of the oocytes. This is why it's harder to get pregnant after 35.

The fully mature ovum is visible to the human eye, measuring 0.1 mm. It is about the size of the period at the end of this sentence.

Medications known as fertility drugs can stimulate the ovaries to release multiple oocytes during a menstrual cycle. be used to stimulate the ovaries to produce multiple oocytes rather than and ovulating as mature eggs. This is the cause for the higher risk of multiple pregnancies when taking fertility drugs. For every ovum ovulated, there is a possibility it can become fertilized by a sperm cell. These fertilized ova can become embryos (and, eventually, if all goes well, babies.)

During fertility treatments, the doctor will conduct ultrasounds to monitor follicle growth. The oocyte maturation is also taking place, but oocyte maturation is not visible on ultrasound. This is why follicle growth is observed and not oocyte growth.

If too many follicles grow, your treatment cycle may be canceled to prevent the risk of multiple pregnancies or ovarian hyperstimulation syndrome (OHSS).

During in vitro fertilization (IVF), if ultrasound monitoring does not show enough follicle growth—which means not sufficient oocytes are maturing—the cycle may be canceled to avoid treatment failure.

Alternative spellings: oöcyte, ovocyte, ocyte.

Oogenesis is what an oocyte goes through as it develops into a mature ovum.

You may assume that oogenesis occurs over the course of a month since that is how often you ovulate. But you would be wrong!

While it is true that whatever egg is ovulated completes the oogenesis process the month it is released from the ovary, oocyte development began way before you were even born.

In fact, it started when you were a very young embryo.

These are the stages of oocyte growth.

The “seed” cell of every oocyte is the primordial germ cell.

These are embryonic cells that will eventually become either sperm or oocyte cells.

In the developing embryo, these cells move into the area that will eventually become either the testis or ovaries (also known as the gonads).

(Interesting side note: Research published in 2012 found that some of these early oocyte stem cells are present in adult women’s ovaries. There may be a way in the future to take these stem cells and create new oocytes. This would mean that women would no longer be limited to the eggs they were born with.)

Once the primordial germ cell arrives in the gonads, it is influenced by the surrounding cells to become oogonium (plural, oogonia).

Oogonia are diploid cells. This means they have two (di) complete sets of chromosomes. In the human cell, this is 23 pairs or a total of 46.

This is an important thing to know because the oocyte will eventually have only half or 23 chromosomes. (During fertilization, it will get the other 23 from the sperm cell to have a complete set once again.)

During the first five months of prenatal development, the oogonium increase in number through a process known as mitotic cell division.

Meiosis is unique to germ cells. It only occurs in young egg and sperm cells.

In more typical cell division—which known as meiosis—cells duplicate by creating clones of themselves, each with a full set of chromosomes.

For example, one skin cell going through mitosis would eventually lead to two skin cells, with similar genetic codes.

During mitotic cell division, the oogonium splits into two separate cells that contain:

This mitotic division is why every new life has a unique genetic make-up that is unlike anyone else.

However, it is not completely random. It is all based on the original genetic material the embryo received from its father and mother.

These cells continue to multiply until reaching their peak. The peak occurs when the developing fetus is about five-months-along.

At this point, the girl fetus has 7 million oocytes.

This number will begin to decrease after this point. At birth, a baby girl has only 2 million oocytes left.

Every oocyte will go through two separate meiotic cell divisions before becoming a mature ovum. Meiotic cell division leads to growth and maturity of the oocyte, and not additional oocytes.

Towards the end of prenatal development, the oocytes stop multiplying in number and begin to mature individually.

At this stage, they go through the first meiotic cell division. This cell division leads to oocyte growth—not more oocytes—like what happens with the oogonium.

But they don’t just speed through development to maturity right now.

The primary oocytes freeze in their development and remain frozen until reproductive hormones trigger the next stage.

Oogenesis will continue at the age of puberty.

Puberty jump-starts the next stage of oocyte maturity.

Not all the oocytes will go through these later stages of oocyte development together, of course. They more or less take turns over a woman’s reproductive years. Each month, a new set of primary oocytes begin to mature.

Once a primary oocyte is affected by reproductive hormones, it completes Stage I of the meiotic cell division. This is known as oocyte maturation.

At the end of this first stage of meiotic cell division, the cell splits into two separate cells: a small polar body and a large secondary oocyte.

The small polar body eventually deteriorates.

The secondary oocyte begins the next stage of maturation.

The oocyte now begins the second phase of meiotic cell division.

Eventually, the secondary oocyte will split again into two separate cells: another small polar body cell and a larger mature cell.

This larger mature cell is known as an ootid.

As before, the smaller polar body cell will eventually deteriorate.

Ovulation occurs when the oocyte has reached the ootid stage of development.

At the time of ovulation, an ootid is released from the follicle.

Human egg cells cannot move on their own. Instead, finger-like projections draw the oocyte towards and into the fallopian tube.

Once inside the fallopian tube, small hair-like projections known as cilia continue to draw the ootid along.

In the fallopian tube, if pregnancy occurs, the ootid is fertilized by a sperm cell.

Once this fertilization takes place, the ootid goes through its final stage of maturation and becomes an ovum, a fully mature human egg cell.

That's right; the oocyte actually can’t complete its full development without fertilization.

During fertilization, the ovum and sperm cell combine, each containing 23 chromosomes each.

Rather quickly (but not at the exact moment of fertilization), these chromosomes fuse together, creating a new cell with a full set of chromosomes.

This new cell is called a zygote.

Shafaqat yggo · Answer