At menopause, when production of female gametes stops, about 1, primary oocytes will remain. The majority of primary oocytes present at puberty never develop, for unknown reasons. Like sperm formation in males, the monthly maturation of a primary oocyte in females is under the control of chemical messages produced in the brain. GnRH gonadotropin releasing hormone is produced in the hypothalamus of the brain and triggers release of Luteinizing Hormone LH and Follicle Stimulating Hormone FSH from the anterior pituitary gland of the brain into the blood.
These are the same brain hormones that cause testosterone secretion and sperm production in males. FSH and LH in the blood stimulate a follicle in one of the ovaries to begin cell division. Follicles consist of primary oocytes and additional cells around them. FSH and LH cause the primary oocyte to renew and complete Meiosis I while the other cells of the follicle divide by mitosis.
As cell division proceeds and the follicle grows, it begins to produce estrogen. Estrogen has several effects. It causes the inner lining of the uterus to thicken and develop many blood vessels, making it suitable for implantation of an embryo, should one happen along. Finally, estrogen contributes to female characteristics such as breast development, distribution of body fat, and a hip structure appropriate for childbirth. The burst of LH and FSH decribed in the preceeding paragraph drives a burst of cell division in the developing follicle.
The follicle ruptures, releasing what is now a secondary oocyte that has completed Meiosis I. This is ovulation. Some of the follicle cells remain in the ovary to become the "corpus luteum", which continues to make estrogen and now also progesterone. If the secondary oocyte is not fertilized after ovulation, it eventually dies, the corpus luteum in the ovary degenerates, and the thickened wall of the uterus sloughs off with some accompanying bleeding.
The cycle will repeat in the next month. If the secondary oocyte is fertilized, a different train of events follows. The nucleus of the secondary oocyte proceeds through Meiosis II and the resulting egg nucleus fuses with the nucleus of the fertilizing sperm. The now diploid zygote will initiate cell division by mitosis in 30 hours.
It will also produce a chemical signal called Human Chorionic Gonadotropin HCG that is received by the corpus luteum and prevents its degeneration. Early pregnancy test kits are very sensitive tests for HCG. The corpus luteum, and later the placenta of the developing fetus, will produce low levels of estrogen and progesterone throughout pregnancy. The low levels of estrogen and progesterone produced by the corpus luteum and later by the placenta during pregnancy have the opposite effect of the high estrogen levels produced by the developing follicle close to ovulation.
This prevents additional ovulations from occurring during pregnancy. Fertilization of subsequent eggs during the months after a pregnancy has begun would be catastrophic for embryos and mother alike.
Figure A variety of methods for female contraception have been developed. One of the most widely used and effective methods is the oral contraceptive pill, which manipulates the hormonal controls of ovulation. Hormonal contraceptives of this type are safe and effective. They contain synthetic estrogen and progesterone that suppress LH and FSH release from the anterior pituitary, just as estrogen and progesterone produced by the corpus luteum and placenta do during pregnancy.
Hormonal birth control thus prevents ovulation by simulating pregnancy. Hormonal birth control of this type was developed in the s. The early formulations contained relatively high doses of hormones and were linked to strokes, blood clots, and other cardiovascular problems in a very few women. Modern formulations contain lower doses and have not been linked to any side effects. In females, most of gametogenesis occurs during embryonic development.
Primordial germ cells migrate into the ovaries at week 4 of development and differentiate into oogonia 46,2N. All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase of meiosis I until puberty. After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization. At fertilization , the secondary oocyte completes meiosis II to form a mature oocyte 23,1N and a second polar body.
In males, gametogenesis begins at puberty and continues into advanced age. Primordial germ cells 46,2N migrate into the testes at week 4 of development and remain dormant. At puberty, primordial germ cells differentiate into type A spermatogonia 46,2N. Type A spermatogonia divide by mitosis to form either more type A spermatogonia to maintain the supply or type B spermatogonia. Under the influence of estrogen released during the first half of the menstrual cycle , three changes take place in the uterine tubes to facilitate its capture of the egg:.
The uterine tubes move closer to the ovaries physical approximation. The fimbriae on the ends of the tubes beat more rapidly increased fluid current. The number of ciliated cells in the epithelium of the fimbriae increase increase in ciliation. Sperm are deposited in the upper vagina and must overcome several obstacles to reach an egg in the ampulla of one of the uterine tubes.
The egg itself is viable for only about 24 hours. Table 1 - Obstacles to Sperm Transport. The alkaline seminal fluid temporarily neutralizes the normal acidity pH 4.
The composition of cervical mucus changes during menstrual cycle. Sperm can most easily penetrate the thinner E-mucus that predominates during the last few days before ovulation, as opposed to the thicker G-mucus. Two modes of transport:. An unexpected error occurred. Previous Video In humans, prophase two initiates meiosis two, where dense, X-shaped chromosomes begin to appear in a haploid cell's nucleus. Every X consists of a centromere and joined sister chromatids, each containing a unique combination of maternal and paternal material caused by crossing over during meiosis one.
Meanwhile, in the cell's cytoplasm, the centrosomes travel to opposite poles, and the microtubule-based meiotic spindle apparatus again develops between them. Prometaphase two is characterized by nuclear envelope disintegration and free-floating chromosomes. Kinetochores also materialize on the centromeres and serve as anchors for extending microtubules. This results in the meiotic spindles securing sister chromatids in a pair to opposing centrosomes. Continuing to metaphase two, the microtubules orient the chromosomes along the equator and during anaphase two, these structures truncate, tugging sister chromatids apart while the cell also stretches.
With telophase two, the chromatids are delivered to opposite edges and each such aggregation constitutes one haploid unreplicated set of chromosomes, which untangle.
New nuclear envelopes appear and the cell divides via cytokinesis. Thus, meiosis two results in haploid cells, each with a distinct mixture of parental information that yield gametes capable of fertilization, and giving rise to genetically unique siblings. Meiosis II is the second and final stage of meiosis. It relies on the haploid cells produced during meiosis I, each of which contain only 23 chromosomes—one from each homologous initial pair.
Importantly, each chromosome in these cells is composed of two joined copies, and when these cells enter meiosis II, the goal is to separate such sister chromatids using the same microtubule-based network employed in other division processes. The result of meiosis II is two haploid cells, each containing only one copy of all 23 chromosomes. Depending on whether the process occurs in males or females, these cells may form eggs or sperm, which—when joined through the process of fertilization—may yield a new diploid individual.
Although the goal of meiosis II is the same in both males and females—to produce haploid egg or sperm cells—there are some critical differences in this process between the sexes. This asymmetry allows for two cells of unequal sizes to be produced following meiosis II: a large egg, and a smaller polar body that dissolves. This division of cytoplasm ensures that the egg contains enough nutrients to support an embryo. The position of the meiotic spindle apparatus is of concern for scientists involved in assisted reproductive technologies, like intracytoplasmic sperm injection ICSI.
Embryologists must take care to avoid injection into the area of the meiotic spindle apparatus, as this could damage the microtubule framework and lead to an abnormal number of chromosomes in the resulting embryo. Therefore, embryologists performing ICSI typically predict the location of the spindle based on the position of the polar body or directly visualize the structure using techniques like polarized light microscopy. Another unique feature of female meiosis is that the egg precursor cells undergo cell cycle arrest, first in prophase I, and then in metaphase II.
At puberty, female sex hormones release the egg cells from prophase I arrest, and meiosis II begins. Subsequently, egg cells arrested in metaphase II are released from the ovary into the fallopian tube, where meiosis only resumes if fertilization occurs. This means that the meiotic spindle apparatus is formed and associated with chromosomes, but does not complete the process of separating sister chromatids until after a sperm and egg precursor cell join.
The arrest of meiosis II poses a unique challenge to women who choose to have their eggs frozen, as many in vitro fertilization protocols require that these cells be isolated during metaphase II and then frozen. Given that problems with the meiotic spindle can cause chromosomal abnormalities like trisomies, considerable research has been dedicated to determining which egg-freezing procedures have only minimal effects on this structure.
To diminish damage to eggs, techniques have been developed where sugar or other cryopreservation agents are added to the freezing medium, which limits the formation of ice crystals that can harm cells upon thawing. To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian.
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