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

By commitment with the publisher, this book is obtainable by the find feature, yet cannot be browsed.

In one respect at least, eggs room the many remarkable of animal cells: when activated, lock can provide rise to a complete brand-new individual within a matter of work or weeks. No various other cell in a higher animal has actually this capacity. Activation is normally the repercussion of fertilization—fusion the a sperm through the egg. In some organisms, however, the sperm chin is no strictly required, and an egg have the right to be set off artificially by a selection of nonspecific chemical or physics treatments. Indeed, part organisms, including a few vertebrates such as some lizards, typically reproduce from eggs that become activated in the absence of sperm—that is, parthenogenetically.

Although one egg can offer rise come every cell kind in the adult organism, it is itself a highly devoted cell, uniquely fitted for the single role of generating a brand-new individual. The cytoplasm of an egg can even reprogram a somatic cell nucleus so the the nucleus can direct the breakthrough of a brand-new individual. That is exactly how the well known sheep Dolly was produced. The cell nucleus of an unfertilized lamb egg was destroyed and also replaced through the cell core of an adult somatic cell. An electric shock was used to activate the egg, and the resulting embryo to be implanted in the uterus the a surrogate mother. The resulting typical adult sheep had the genome of the donor somatic cell and was because of this a clone of the donor sheep.

In this section, us briefly consider some the the committed features of an egg before discussing exactly how it develops to the allude of being ready for fertilization.

An Egg Is Highly dedicated for independent Development, with large Nutrient reserves and an intricate Coat

The egg of most pets are giant solitary cells, include stockpiles of all the materials needed for initial development of the embryo through to the phase at i beg your pardon the brand-new individual can begin feeding. Prior to the feeding stage, the huge cell cleaves into plenty of smaller cells, yet no net development occurs. The mammalian embryo is one exception. It have the right to start to grow early by acquisition up nutrient from the mom via the placenta. Thus, a mammalian egg, although still a large cell, does not need to be as large as a frog or bird egg, for example. In general, eggs are typically 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 many centimeters in birds and also reptiles (Figure 20-19). A common somatic cell, by contrast, has a diameter the only about 10 or 20 μm (Figure 20-20).



Figure 20-20

The family member sizes of assorted eggs. Sizes are compared with the of a common somatic cell.

The egg cytoplasm includes nutritional to make reservation in the kind of yolk, i m sorry is affluent in lipids, proteins, and polysaccharides and is usually contained within discrete structures called yolk granules. In some species, every yolk granule is membrane-enclosed, whereas in others it is not. In egg that construct into large animals exterior the mother"s body, yolk have the right to account for much more than 95% the the volume that the cell. In mammals, whose embryos are mostly nourished by your mothers, over there is little, if any, yolk.

The egg coat is an additional peculiarity the eggs. That is a specialized form that extracellular procession consisting mostly of glycoprotein molecules, some secreted by the egg and also others deposited on it by neighboring cells. In numerous species, the major coat is a layer automatically surrounding the egg plasma membrane; in nonmammalian eggs, such as those of sea urchins or chickens, the is called 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 in countless eggs it likewise acts as a species-specific obstacle to sperm, admitting only those that the exact same or closely related species.


Figure 20-21

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

Many egg (including those that mammals) contain dedicated secretory vesicles simply under the plasma membrane in the outer region, or cortex, that the egg cytoplasm. When the egg is caused by a sperm, this cortical granules release their contents by exocytosis; the contents of the granules action to change the egg coat so as to prevent more than one sperm indigenous fusing v the egg (discussed below).

Cortical granules room usually spread evenly transparent the egg cortex, however in part organisms other cytoplasmic contents have a strikingly asymmetrical distribution. Some of these localized contents later serve to aid establish the polarity the the embryo, as debated in thing 21.

Eggs develop in Stages

A emerging egg is called an oocyte. That is differentiation into a tires egg (or ovum) involves a collection of changes whose time is aligned to the measures of meiosis in which the germ cell go through their two final, highly specialized divisions. Oocytes have developed special mechanisms for arresting development through meiosis: they stay suspended in prophase ns for a an extensive period while 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, relying on the species).

While the details that oocyte advancement (oogenesis) vary from types to species, the basic stages room similar, together outlined in figure 20-22. Primordial germ cells move to the forming gonad to become oogonia, i m sorry proliferate by mitosis because that a duration before separating into main oocytes. In ~ this stage (usually prior to birth in mammals), the first meiotic department begins: the DNA replicates so the each chromosome consists of 2 sister chromatids, the duplicated homologous chromosomes pair follow me their lengthy axes, and also crossing-over occurs in between nonsister chromatids of this paired chromosomes. After this events, the cell remains arrested in prophase of department I the meiosis (in a state equivalent, together we formerly pointed out, to a G2 phase of a mitotic department cycle) for a period lasting indigenous a couple of days to plenty of years, depending on the species. During this long period (or, in part cases, in ~ the onset of sexual maturity), the major oocytes synthesize a coat and cortical granules. In the instance of huge nonmammalian oocytes, they additionally accumulate ribosomes, yolk, glycogen, lipid, and also the mRNA that will later direct the synthetic of proteins compelled for early embryonic growth and the unfolding that the developmental program. In countless oocytes, the intensive biosynthetic activities are reflect in the structure of the chromosomes, i beg your pardon decondense and type lateral loops, acquisition on a properties “lampbrush” appearance, signifying the they are an extremely busily involved in RNA synthesis (see figures 4-36 and 4-37).


Figure 20-22

The stages of oogenesis. Oogonia construct from primordial germ cells the migrate right into the arising gonad beforehand in embryogenesis. ~ a variety of mitotic divisions, oogonia begin meiotic department I, after which they are dubbed primary oocytes. In mammals, (more...)

The next phase that oocyte development is called oocyte maturation. It normally does not happen until sex-related maturity, as soon as the oocyte is engendered by hormones. Under these hormone influences, the cabinet resumes its progression through department I the meiosis. The chromosomes recondense, the nuclear envelope breaks down (this is usually taken to note the start of maturation), and the replicated homologous chromosomes segregate in ~ anaphase I into two daughter nuclei, each containing fifty percent the original variety of chromosomes. Come end division I, the cytoplasm divides asymmetrically to develop two cells that differ substantially in size: one is a small polar body, and also the other is a large secondary oocyte, the precursor the the egg. At this stage, every of the chromosomes is still composed of two sister chromatids. These chromatids execute not separate until division II of meiosis, when they room partitioned right into separate cells, as formerly described. ~ this last chromosome separation at anaphase II, the cytoplasm the the large secondary oocyte again divides asymmetrically to develop the mature egg (or ovum) and a second small polar body, each v a haploid collection of single chromosomes (see figure 20-22). Due to the fact that of these two asymmetrical divisions of your cytoplasm, oocytes maintain their large size regardless of undergoing the two meiotic divisions. Both of the polar bodies space small, and they eventually degenerate.

In many vertebrates, oocyte tires proceeds come metaphase of meiosis II and also then arrests till fertilization. In ~ ovulation, the arrested secondary oocyte is released from the ovary and also undergoes a rapid maturation action that transforms it into an egg the is prepared for fertilization. If fertilization occurs, the egg is engendered to complete meiosis.

Oocytes Use distinct Mechanisms to grow to Their big Size

A somatic cell with a diameter of 10–20 μm generally takes around 24 hours to double its mass in ready for cabinet division. In ~ this rate of biosynthesis, together a cell would certainly take a an extremely long time to with the thousand-fold higher mass the a mammalian egg through a diameter of 100 μm. It would take also longer to reach the million-fold better mass of bug egg with a diameter the 1000 μm. Yet some insects live just a few days and manage to produce eggs v diameters also greater 보다 1000 μm. It is clear that eggs must have actually special mechanisms for achieving their huge size.

One straightforward strategy for rapid expansion is to have extra gene duplicates in the cell. Thus, the oocyte delays perfect of the first meiotic division so as to grow when it has the diploid chromosome collection in duplicate. In this way, it has twice as lot DNA accessible for RNA synthesis together does an average somatic cabinet in the G1 step of the cabinet cycle. The oocytes the some species go to also greater lengths come accumulate extra DNA: castle produce numerous extra duplicates of certain genes. We talk about in chapter 6 just how the somatic cells of most organisms require 100 come 500 duplicates of the ribosome RNA gene in order to create enough ribosomes because that protein synthesis. Egg require even greater numbers of ribosomes to support protein synthesis during early embryogenesis, and in the oocytes that many animals the ribosome RNA genes are specifically amplified; part amphibian eggs, because that example, save 1 or 2 million copies of this genes.

Oocytes may also depend partially on the synthetic tasks of other cells for your growth. Yolk, because that example, is commonly synthesized exterior the ovary and imported right into the oocyte. In birds, amphibians, and also insects, yolk proteins room made by liver cells (or their equivalents), which secrete these proteins right into the blood. In ~ the ovaries, oocytes take up the yolk proteins from the extracellular fluid by receptor-mediated endocytosis (see number 13-41). Nutritive aid can additionally come from surrounding accessory cell in the ovary. These have the right to be of two types. In part invertebrates, few of the progeny of the oogonia become nurse cells instead of ending up being oocytes. These cells typically are connected to the oocyte by cytoplasmic bridges through which macromolecules can pass straight into the oocyte cytoplasm (Figure 20-23). Because that the insect oocyte, the nurse cell manufacture many of the products—ribosomes, mRNA, protein, and so on—that vertebrate oocytes need to manufacture because that 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 offers rise to one oocyte and 15 nurse cells (only 7 of which space seen in this airplane of section). These cells continue to be (more...)

The other accessory cell in the ovary that assist to nourish developing oocytes are plain somatic cells referred to as follicle cells, i m sorry are uncovered in both invertebrates and also vertebrates. They space arranged as an epithelial layer roughly the oocyte (Figure 20-24, and also see number 20-23), come which castle are connected only by void junctions, which allow the exchange of little molecules but not macromolecules. While this cells space unable to provide the oocyte v preformed macromolecules with these communicating junctions, lock may aid to supply the smaller precursor molecule from i m sorry macromolecules space made. In addition, follicle cells frequently secrete macromolecules that add to the egg coat, or are taken up by receptor-mediated endocytosis right into the growing oocyte, or action on egg cell-surface receptors to manage the spatial patterning and also axial asymmetries the the egg (discussed in thing 21).

Figure 20-24

Electron micrographs of arising primary oocytes in the hare ovary. (A) very early stage of primary oocyte development. Neither a zona pellucida nor cortical granules have actually developed, and also the oocyte is surrounding by a single layer of flattened follicle (more...)


Eggs develop in step from primordial germ cells that migrate into the occurring gonad at an early stage in breakthrough to end up being oogonia. ~ mitotic proliferation, oogonia end up being primary oocytes, which begin meiotic division I and also then arrest at prophase i for days come years, depending upon the species. Throughout this prophase-I arrest period, major oocytes grow, synthesize a coat, and accumulate ribosomes, mRNAs, and also proteins, regularly enlisting the aid of various other cells, consisting of surrounding accessory cells. In the procedure of maturation, primary oocytes complete meiotic division I to kind a little polar body and also a large secondary oocyte, i m sorry proceeds into metaphase that meiotic division II. There, in plenty of species, the oocyte is arrested until engendered by fertilization to finish meiosis and also begin embryonic development.

See more: How Much Does 1/2 Cup Of Water Weigh In Lbs? 1 Us Cup Of Water (Cup) = 236

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