VISIT WEBSITE >>>>> http://gg.gg/y83ws?7402559 <<<<<<
When yolk platelets are formed in the future animal hemisphere, they move inward toward the center of the cell. Vegetal yolk platelets, however, do not actively move, but remain at the periphery of the cell for long periods of time, enlarging as they stay there. They are slowly displaced from the cortex as new yolk platelets come in from the surface. The mechanism of this translocation remains unknown. As the yolk is being deposited, the organelles also become arranged asymmetrically.
The cortical granules begin to form from the Golgi apparatus; they are originally scattered randomly through the oocyte cytoplasm, but later migrate to the periphery of the cell. The mitochondria replicate at this time, dividing to form millions of mitochondria that will be apportioned to the different blastomeres during cleavage. In Xenopus, new mitochondria will not be formed until after gastrulation is initiated. As vitellogenesis nears an end, the oocyte cytoplasm becomes stratified.
The cortical granules, mitochondria, and pigment granules are found at the periphery of the cell, within the actin-rich oocyte cortex. Within the inner cytoplasm, distinct gradients emerge.
While the yolk platelets become more heavily concentrated at the vegetal pole of the oocyte, the glycogen granules, ribosomes, lipid vesicles, and endoplasmic reticulum are found toward the animal pole. Even specific mRNAs stored in the cytoplasm become localized to certain regions of the oocyte. While the precise mechanisms for establishing these gradients remain unknown, studies using inhibitors have shown that the cytoskeleton is critically important in localizing specific RNAs and morphogenetic factors.
There seem to be two pathways for gettting mRNAs into the vegetal cortex Forristall et al. The first pathway moves messages such as those encoding the Vg1 protein, which are initially present throughout the oocyte, into the vegetal cortex in a two-step process Yisraeli et al. In the first phase, microtubules are needed to bring Vg1 mRNA into the vegetal hemisphere. In the second phase, microfilaments are responsible for anchoring the Vg1 message to the cortex.
These messages are compartmentalized into clusters associated with the germ plasm and transported to the vegetal cortex in a manner that appears to be independent of the cytoskeleton Figure This mechanism is known as the Metro message transport organizer pathway. Schematic representations of the two pathways for localizing mRNAs to the vegetal region of the Xenopus oocyte.
The respective mRNAs are shown in yellow. In the cytoskeletal Vg1 pathway, messages are first seen throughout the egg, but they are translocated more Vitellogenesis in amphibians is mediated primarily by estrogen. Estrogen instructs the liver to express and secrete vitellogenin, and this protein is absorbed from the blood by the young oocyte.
Vera may link Vg1 mRNA to a set of endoplasmic reticulum vesicles that are translocated to the vegetal cortex. In several species, the developing oocyte is a flagellated cell whose flagellum marks the future animal pole of the egg. This flagellum is lost during oogenesis.
Amphibian oocytes can remain for years in the diplotene stage of meiotic prophase. This state resembles the G 2 phase of the cell division cycle see Chapter 8. Resumption of meiosis in the amphibian primary oocyte requires progesterone. This hormone is secreted by the follicle cells in response to gonadotropic hormones secreted by the pituitary gland.
Within 6 hours of progesterone stimulation, germinal vesicle breakdown GVBD occurs, the microvilli retract, the nucleoli disintegrate, and the chromosomes contract and migrate to the animal pole to begin division. Soon afterward, the first meiotic division occurs, and the mature ovum is released from the ovary by a process called ovulation. The ovulated egg is in second meiotic metaphase when it is released Figure Schematic representation of Xenopus oocyte maturation, showing the regulation of meiotic cell division by progesterone and fertilization.
Oocyte maturation is blocked in the diplotene stage of first meiotic prophase by the lack of active MPF. Progesterone more How does progesterone enable the egg to break its dormancy and resume meiosis? To understand the mechanisms by which this activation is accomplished, it is necessary to briefly review the model for early blastomere division see Chapter 8. MPF contains two subunits, cyclin B and the p34 cdc2 protein. The p34 protein is a cyclin-dependent-kinase—its activity is dependent upon the presence of cyclin.
The mediator of the progesterone signal is the c-mos protein. Progesterone reinitiates meiosis by causing the egg to polyadenylate the maternal c-mos mRNA that has been stored in its cytoplasm Sagata et al. This message is translated into a kDa phosphoprotein, known as c-mos. This protein is detectable only during oocyte maturation and is destroyed quickly upon fertilization. Yet during its brief lifetime, it plays a major role in releasing the egg from its dormancy.
If the translation of c-mos is inhibited by injecting c-mos antisense mRNA into the oocyte , germinal vesicle breakdown and the resumption of oocyte maturation do not occur. The c-mos protein activates a phosphorylation cascade that phosphorylates and activates the p34 subunit of MPF Ferrell and Machleder ; Ferrell The active MPF allows the germinal vesicle to break down and the chromosomes to divide. However, the chromosomes then encounter a second block.
MPF can take the chromosomes through only the first meiotic division and the prophase of the second meiotic division. The oocyte is arrested again in the metaphase of the second meiotic division. This metaphase block is caused by the combined actions of c-mos and another protein, cyclin-dependent kinase 2 cdk2; Gabrielli et al.
These two proteins are subunits of cytostatic factor CSF , which is found in mature frog eggs, and which can block cell cycles in metaphase Matsui It is thought that CSF prevents the degradation of cyclin Figure The metaphase block is broken by fertilization. Evidence suggests that the calcium ion flux attending fertilization enables the calcium-binding protein calmodulin to become active. Calmodulin, in turn, can activate two enzymes that inactivate CSF: calmodulin-dependent protein kinase II, which inactivates p34, and calpain II, a calcium-dependent protease that degrades c-mos Watanabe et al.
Without CSF, cyclin can be degraded, and the meiotic division can be completed. In most animals insects being a major exception , the growing oocyte is active in transcribing genes whose products are 1 necessary for cell metabolism, 2 necessary for oocyte-specific processes, or 3 needed for early development before the zygote-derived nuclei begins to function.
In mice, for instance, the growing diplotene oocyte is actively transcribing the genes for zona pellucida proteins ZP1, ZP2, and ZP3. Moreover, these genes are transcribed only in the oocyte and not in any other cell Figure Expression of the ZP3 gene in the developing mouse oocyte. A radioactive probe to the ZP3 message found it expressed only in the ovary, and specifically in the oocytes. B-C When more The amphibian oocyte has certain periods of very active RNA synthesis.
During the diplotene stage, certain chromosomes stretch out large loops of DNA, causing the chromosome to resemble a lampbrush a handy instrument for cleaning test tubes in the days before microfuges.
These lamp brush chromosomes Figure Oocyte chromosomes can be incubated with a radioactive RNA probe, and autoradiography used to visualize the precise location where the gene is being transcribed.
Figure It is obvious that a histone gene or set of histone genes is located on one of these loops of the lampbrush chromosome Old et al. Electron micrographs of gene transcripts from lampbrush chromosomes also enable one to see chains of mRNA coming off each gene as it is transcribed Hill and MacGregor Amphibian lampbrush chromosomes are active in the diplotene germinal vesicle during first meiotic prophase.
A A lampbrush chromosome of the salamander Notophthalmus viridescens. B Localization more At this time, all the rRNAs and tRNAs needed for protein synthesis until the mid-blastula stage are made, and all the maternal mRNAs needed for early development are transcribed.
This stage lasts for months in Xenopus. Does the first polar body divide? Answer Expert Verified. The first polar body born out of the first meiotic division does not divide but degenerates further. The reason for this degeneration is the unequal division of cytoplasm. All the cytoplasm is contained in the secondary oocyte.
What is inside of a polar body? What is oogenesis and its stages? The effect of gametogenesis in females is associated with the mature female gamete. This is created through a process called oogenesis. This happens in the ovaries or female gonads. What is growth phase in oogenesis?
Growth stage: This is a very long stage of the primary oocyte. It will last for several years. Oogonium develops into a large primary oocyte. Primary oocyte is then surrounded by a layer of granulosa cells to form the primary follicle. A significant number of these follicles degenerate from birth to puberty. Do polar bodies divide? What do polar bodies contain? The first polar body contains a subset of bivalent chromosomes, whereas the second polar body contains a haploid set of chromatids.
One unique feature of the female gamete is that the polar bodies can provide beneficial information about the genetic background of the oocyte without potentially destroying it. What is the first polar body? The first polar body PB1 is extruded after the onset of the luteinizing hormone surge [1], and extrusion of the PB1 is an important hallmark of oocyte meiotic maturation.
According to some studies, about one in 10 women who are diagnosed with premature ovarian insufficiency POI get pregnant, for reasons that are not yet clear. During endometriosis, the abnormally growing endometrial tissue can cause inflammation, scarring, cysts, and organ damage, including damage to the ovary. And if the ovary is damaged, it can mean impaired egg production or ovulation—or none at all. The main symptom of infertility is the inability to get pregnant.
There may be no other outward signs or symptoms. Infertility can be treated with medicine, surgery, artificial insemination, or assisted reproductive technology. Many times these treatments are combined. In most cases infertility is treated with drugs or surgery.
It originates from the endoderm of the yolk sac and then proliferates and migrates to the gonads of the developing embryo. Primordial germ cells are the most primitive germ cells. These first appear in the wall of the yolk sac around the fourth week of development. These germ cells proliferate by the process of mitosis and migrate to the gonads of the developing female embryo. In the first month of the gestation period, there are about primordial germ cells in the ovary.
In the ovary, primordial germ cells further multiply to form oogonia. After a significant number 7 million of oogonia has been achieved, some oogonia enlarge, and others degenerate. At this point, mitotic division stops. These enlarged oogonia are surrounded by flat epithelial cells and are called primary oocytes.
Primary oocytes with a sheath of flat epithelial cells are called primordial follicles. As of now, mitosis goes, its chromosome number remains the same as that of primordial germ cells. Now, after the oogonium has changed to the primary oocyte, the mitosis process stops, and meiosis begins. When primary oocytes reach the diplotene stage of the prophase of Meiosis-I division, they arrest their further progression.
Till this stage, the progression of oogenesis takes place before birth. At birth, there are 2 million primary oocytes. Now, we can say that all the primary oogonia have been converted into primary oocytes. At birth, there are no oogonia. Most of the primary oocytes degenerate from birth till puberty, and their number from 2 million becomes 40, These all 40, primordial follicles having primary oocytes have been arrested at the diplotene stage of Meiotic-I.
During the monthly cycle, primordial follicles start growing, and many of them change to primary follicles. Primary follicles have a primary oocyte surrounded by multilayers of follicular cells, which in turn are surrounded by some connective tissue. Surrounding the primary oocyte, there is a glycoprotein layer called zona pellucida, which is being secreted by primary oocyte and follicular cells.
The primary follicle then forms a secondary follicle. The secondary follicle has a fluid-filled space called the antrum. Some follicular cells that remain around the ovum and zona pellucida are called cumulus oophorus.
Surrounding the secondary follicle is a layer of connective tissue called theca folliculi. Its cells which are more internally placed, are called theca interna, while those that are externally placed are called theca externa. In the end, usually, only one secondary follicle develops, and others get degenerated.
Comments