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Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. Fourth edition. Brand-new York: Garland Science; 2002.


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In one respect at least, eggs are the many remarkable of pet cells: once activated, castle can offer rise to a complete new individual in ~ a matter of job or weeks. No other cell in a greater animal has actually this capacity. Activation is typically the an effect of fertilization—fusion the a sperm v the egg. In part organisms, however, the sperm chin is no strictly required, and an egg have the right to be caused artificially by a range of nonspecific chemical or physics treatments. Indeed, part organisms, consisting of a few vertebrates together as part lizards, usually reproduce from eggs that come to be activated in the lack of sperm—that is, parthenogenetically.

Although one egg can provide rise come every cell type in the adult organism, that is chin a highly committed cell, uniquely equipped for the single role of generating a brand-new individual. The cytoplasm of one egg can even reprogram a somatic cabinet nucleus so the the cell core can straight the development of a new individual. That is just how the famed sheep Dolly to be produced. The nucleus of an unfertilized lamb egg to be destroyed and also replaced v the cell core of an adult somatic cell. An electric shock was supplied to activate the egg, and also the resulting embryo was implanted in the uterus the a surrogate mother. The resulting common adult sheep had the genome of the donor somatic cell and also was therefore a clone the the donor sheep.

In this section, we briefly think about some that the devoted features of one egg prior to discussing how it establishes to the allude of being ready for fertilization.


An Egg Is Highly devoted for elevation Development, with huge Nutrient make reservation and an intricate Coat

The egg of most pets are giant single cells, include stockpiles of all the products needed because that initial advance of the embryo with to the stage at i m sorry the new individual can start feeding. Before the feeding stage, the large cell cleaves into countless smaller cells, however no net expansion occurs. The mammalian embryo is an exception. It have the right to start to grow early by taking up nutrient from the mommy via the placenta. Thus, a mammalian egg, although still a big cell, go not need to be as big as a frog or bird egg, because that example. In general, eggs are generally spherical or ovoid, v a diameter of about 0.1 mm in humans and also sea urchins (whose feeding larvae are tiny), 1 mm come 2 mm in frogs and also fishes, and also many centimeters in birds and also reptiles (Figure 20-19). A common somatic cell, by contrast, has a diameter the only around 10 or 20 μm (Figure 20-20).


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Figure 20-20

The relative sizes of miscellaneous eggs. Size are contrasted with the of a usual somatic cell.


The egg cytoplasm includes nutritional reserves in the form of yolk, which is well-off in lipids, proteins, and polysaccharides and also is usually had within discrete structures referred to as yolk granules. In part species, every yolk granule is membrane-enclosed, conversely, in rather it is not. In egg that construct into huge animals outside the mother"s body, yolk have the right to account for an ext than 95% the the volume the the cell. In mammals, whose embryos are mainly nourished by their mothers, there is little, if any, yolk.

The egg coat is an additional peculiarity the eggs. It is a committed form that extracellular matrix consisting largely of glycoprotein molecules, some secreted by the egg and others deposit on the by neighboring cells. In numerous species, the major coat is a layer immediately surrounding the egg plasma membrane; in nonmammalian eggs, such together those that sea urchins or chickens, that is referred to as the vitelline layer, whereas in mammalian egg it is called the zona pellucida (Figure 20-21). This layer protects the egg from mechanically damage, and also in plenty of eggs it additionally acts as a species-specific barrier to sperm, admitting just those that the same or very closely related species.


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Figure 20-21

The zona pellucida. (A) Scanning electron micrograph that a hamster egg, reflecting the zona pellucida. (B) A scanning electron micrograph the a similar egg in i beg your pardon the zona (to which countless sperm space attached) has actually been peeled ago to expose the underlying plasma (more...)


Many egg (including those that mammals) contain committed secretory vesicles just under the plasma membrane in the external region, or cortex, that the egg cytoplasm. Once the egg is activated by a sperm, this cortical granules relax their materials by exocytosis; the contents of the granules plot to alter the egg cloak so as to prevent an ext than one sperm indigenous fusing through the egg (discussed below).

Cortical granules room usually spread evenly throughout the egg cortex, however in some organisms various other cytoplasmic materials have a strikingly asymmetrical distribution. Few of these localized materials later serve to help establish the polarity that the embryo, as questioned in chapter 21.


Eggs build in Stages

A emerging egg is dubbed an oocyte. Its differentiation right into a maturation egg (or ovum) entails a collection of changes whose timing is geared to the procedures of meiosis in i m sorry the germ cell go v their two final, highly specialized divisions. Oocytes have advanced special mechanisms because that arresting progression through meiosis: they continue to be suspended in prophase i for a an extensive period if the oocyte grows in size, and in many instances they later arrest in metaphase II when awaiting fertilization (although they can arrest at assorted other points, depending on the species).

While the details that oocyte development (oogenesis) differ from species to species, the general stages space similar, together outlined in figure 20-22. Primordial germ cells migrate to the forming gonad to end up being oogonia, i beg your pardon proliferate through mitosis because that a duration before differentiating into primary oocytes. In ~ this phase (usually before birth in mammals), the very first meiotic division begins: the DNA replicates so the each chromosome consists of two sister chromatids, the duplicated homologous chromosomes pair along their long axes, and also crossing-over occurs in between nonsister chromatids of these paired chromosomes. After this events, the cell stays arrested in prophase of division I of meiosis (in a state equivalent, together we formerly pointed out, to a G2 step of a mitotic department cycle) for a period lasting from a couple of days to plenty of years, depending upon the species. During this long duration (or, in some cases, in ~ the start of sex-related maturity), the main oocytes synthesize a coat and also cortical granules. In the situation of big nonmammalian oocytes, they also accumulate ribosomes, yolk, glycogen, lipid, and also the mRNA that will certainly later straight the synthetic of proteins compelled for at an early stage embryonic growth and the unfolding the the developmental program. In countless oocytes, the extensive biosynthetic activities are reflect in the framework of the chromosomes, i m sorry decondense and type lateral loops, taking on a characteristic “lampbrush” appearance, signifying that they are very busily involved in RNA synthetic (see numbers 4-36 and also 4-37).


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Figure 20-22

The stages of oogenesis. Oogonia build from primordial germ cells the migrate right into the emerging gonad at an early stage in embryogenesis. ~ a number of mitotic divisions, oogonia start meiotic division I, after ~ which castle are called primary oocytes. In mammals, (more...)


The following phase the oocyte breakthrough is dubbed oocyte maturation. It typically does not take place until sexual maturity, when the oocyte is created by hormones. Under these hormonal influences, the cabinet resumes its progress through division I of meiosis. The chromosomes recondense, the nuclear envelope breaks under (this is normally taken to note the start of maturation), and the replicated homologous chromosomes segregate in ~ anaphase I into two daughter nuclei, every containing fifty percent the original variety of chromosomes. To end division I, the cytoplasm divides asymmetrically to develop two cells that differ substantially in size: one is a little polar body, and also the various other is a large secondary oocyte, the precursor that the egg. In ~ this stage, each of the chromosomes is still composed of 2 sister chromatids. These chromatids do not separate until division II of meiosis, when they are partitioned into separate cells, as previously described. After this final chromosome separation at anaphase II, the cytoplasm of the big secondary oocyte again divides asymmetrically to develop the mature egg (or ovum) and also a 2nd small polar body, each through a haploid set of solitary chromosomes (see number 20-22). Since of these two asymmetrical departments of their cytoplasm, oocytes keep their big size in spite of undergoing the 2 meiotic divisions. Both of the polar bodies space small, and also they eventually degenerate.

In many vertebrates, oocyte maturation proceeds to metaphase that meiosis II and also then arrests till fertilization. In ~ ovulation, the arrested second oocyte is exit from the ovary and also undergoes a fast maturation step that transforms it into an egg that is all set for fertilization. If fertilization occurs, the egg is stimulated to finish meiosis.


Oocytes Use unique Mechanisms to flourish to Their big Size

A somatic cell v a diameter of 10–20 μm frequently takes about 24 hrs to dual its mass in ready for cell division. At this rate of biosynthesis, together a cell would certainly take a really long time to with the thousand-fold greater mass that a mammalian egg through a diameter of 100 μm. It would take also longer to reach the million-fold better mass of an insect egg through a diameter of 1000 μm. Yet some insects live only a couple of days and also manage to develop eggs v diameters also greater 보다 1000 μm. The is clear the eggs must have actually special mechanisms because that achieving their large size.

One basic strategy for rapid development is to have actually extra gene copies in the cell. Thus, the oocyte delays perfect of the an initial meiotic department so as to grow when it has the diploid chromosome set in duplicate. In this way, it has twice as lot DNA obtainable for RNA synthesis together does an mean somatic cell in the G1 step of the cabinet cycle. The oocytes that some species go to also greater lengths come accumulate extra DNA: they produce many extra copies of specific genes. We comment on in thing 6 just how the somatic cells of many organisms require 100 to 500 copies of the ribosome RNA genes in stimulate to create enough ribosomes for protein synthesis. Egg require also greater number of ribosome to support protein synthesis throughout early embryogenesis, and in the oocytes of many pets the ribosome RNA genes are specifically amplified; part amphibian eggs, for example, save on computer 1 or 2 million copies of these genes.

Oocytes may also depend partly on the synthetic activities of various other cells for their growth. Yolk, because that example, is commonly synthesized external the ovary and imported right into the oocyte. In birds, amphibians, and insects, yolk proteins space made by liver cell (or your equivalents), i beg your pardon secrete these proteins right into the blood. Within the ovaries, oocytes take up the yolk protein from the extracellular fluid by receptor-mediated endocytosis (see figure 13-41). Nutritive assist can likewise come from bordering accessory cells in the ovary. These can be of two types. In part invertebrates, some of the progeny of the oogonia come to be nurse cells rather of becoming oocytes. These cells typically are linked to the oocyte by cytoplasmic bridges with which macromolecules can pass directly into the oocyte cytoplasm (Figure 20-23). For the insect oocyte, the nurse cells manufacture countless of the products—ribosomes, mRNA, protein, and also so on—that vertebrate oocytes need to manufacture for themselves.


Figure 20-23

Nurse cells and also follicle cells linked with a Drosophila oocyte. The nurse cells and also the oocyte arise native a usual oogonium, which gives rise to one oocyte and 15 nurse cells (only 7 that which are seen in this plane of section). These cells continue to be (more...)


The other accessory cell in the ovary that help to nourish occurring oocytes are simple somatic cells referred to as follicle cells, which are uncovered in both invertebrates and vertebrates. They are arranged as an epithelial layer approximately the oocyte (Figure 20-24, and see figure 20-23), come which lock are linked only by void junctions, which permit the exchange of small molecules however not macromolecules. While these cells room unable to carry out the oocyte v preformed macromolecules with these connecting junctions, lock may aid to supply the smaller sized precursor molecules from i beg your pardon macromolecules space made. In addition, follicle cells commonly secrete macromolecules that contribute to the egg coat, or are taken increase by receptor-mediated endocytosis into the farming oocyte, or plot on egg cell-surface receptors to manage the spatial patterning and axial asymmetries that the egg (discussed in thing 21).


Figure 20-24

Electron micrographs of emerging primary oocytes in the hare ovary. (A) an early stage of primary oocyte development. Neither a zona pellucida no one cortical granules have actually developed, and the oocyte is surrounded by a single layer of planarization follicle (more...)


Summary

Eggs construct in stages from primordial germ cells that migrate right into the emerging gonad beforehand in advancement to become oogonia. ~ mitotic proliferation, oogonia come to be primary oocytes, which start meiotic department I and then arrest at prophase ns for days come years, depending upon the species. Throughout this prophase-I arrest period, main oocytes grow, synthesize a coat, and accumulate ribosomes, mRNAs, and proteins, regularly enlisting the help of various other cells, including surrounding accessory cells. In the process of maturation, major oocytes finish meiotic division I to form a little polar body and a large secondary oocyte, i m sorry proceeds right into metaphase of meiotic division II. There, in countless species, the oocyte is arrested until created by fertilization to finish meiosis and begin embryonic development.

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