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

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In one respect at least, eggs are the most remarkable of pet cells: when activated, lock can offer rise come a complete brand-new individual within a matter of days or weeks. No various other cell in a higher animal has actually this capacity. Activation is usually the consequence of fertilization—fusion the a sperm with 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 chemistry or physics treatments. Indeed, part organisms, consisting of a couple of vertebrates together as some lizards, usually reproduce from egg that become activated in the absence of sperm—that is, parthenogenetically.

Although one egg can offer rise come every cell type in the adult organism, the is chin a highly dedicated cell, uniquely fitted for the single role of generating a new individual. The cytoplasm of one egg can even reprogram a somatic cabinet nucleus so the the cell core can straight the advancement of a brand-new individual. The is just how the well known sheep Dolly was produced. The cell core of an unfertilized sheep egg to be destroyed and replaced through the nucleus of an adult somatic cell. An electric shock was offered to activate the egg, and also the result embryo to be implanted in the uterus the a surrogate mother. The resulting common adult sheep had the genome the the donor somatic cell and was as such a clone that the donor sheep.

In this section, we briefly consider some that the specialized features of an egg prior to discussing just how it develops to the suggest of being all set for fertilization.

An Egg Is Highly dedicated for elevation Development, with huge Nutrient to make reservation and an elaborate Coat

The egg of most pets are giant single cells, containing stockpiles of all the materials needed because that initial advance 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 gigantic cell cleaves into many smaller cells, but no net expansion occurs. The mammalian embryo is one exception. It have the right to start come grow early by acquisition up nutrients from the mom via the placenta. Thus, a mammalian egg, although still a large cell, walk not need to be as large as a frog or bird egg, because that example. In general, eggs are commonly spherical or ovoid, through a diameter of about 0.1 mm in humans and also sea urchins (whose feeding larvae room tiny), 1 mm to 2 mm in frogs and fishes, and also many centimeters in birds and also reptiles (Figure 20-19). A typical somatic cell, by contrast, has a diameter of only around 10 or 20 μm (Figure 20-20).



Figure 20-20

The loved one sizes of various eggs. Sizes are compared with that of a common somatic cell.

The egg cytoplasm consists of nutritional make reservation in the form of yolk, which is affluent in lipids, proteins, and polysaccharides and is usually contained within discrete structures called yolk granules. In part species, every yolk granule is membrane-enclosed, vice versa, in others it is not. In egg that construct into huge animals external the mother"s body, yolk can account for an ext than 95% the the volume that the cell. In mammals, who embryos are largely nourished by your mothers, over there is little, if any, yolk.

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


Figure 20-21

The zona pellucida. (A) Scanning electron micrograph of a hamster egg, mirroring the zona pellucida. (B) A scanning electron micrograph that a comparable egg in i beg your pardon the zona (to which numerous sperm space attached) has been peeled ago to expose the basic plasma (more...)

Many egg (including those the mammals) contain dedicated secretory vesicles simply under the plasma membrane in the external region, or cortex, of the egg cytoplasm. As soon as the egg is caused by a sperm, these cortical granules release their contents by exocytosis; the contents of the granules plot to change the egg cloak so as to prevent an ext than one sperm native fusing through the egg (discussed below).

Cortical granules space usually dispersed evenly transparent the egg cortex, but in some organisms various other cytoplasmic contents have a strikingly asymmetrical distribution. Few of these localized contents later offer to aid establish the polarity the the embryo, as discussed in chapter 21.

Eggs build in Stages

A developing egg is called an oocyte. That differentiation into a tires egg (or ovum) entails a series of changes whose timing is geared to the steps of meiosis in which the germ cells go through their 2 final, highly committed divisions. Oocytes have advanced special mechanisms for arresting progression through meiosis: they stay suspended in prophase i for a an extensive period while the oocyte grows in size, and also in many instances they later arrest in metaphase II when awaiting fertilization (although they have the right to arrest at various other points, depending on the species).

While the details that oocyte breakthrough (oogenesis) differ from species to species, the general stages space similar, as outlined in figure 20-22. Primordial germ cells migrate to the developing gonad to become oogonia, which proliferate through mitosis because that a duration before separating into primary oocytes. In ~ this phase (usually prior to birth in mammals), the an initial meiotic division begins: the DNA replicates so that each chromosome is composed of 2 sister chromatids, the duplicated homologous chromosomes pair follow me their lengthy axes, and crossing-over occurs between nonsister chromatids of these paired chromosomes. After this events, the cell stays arrested in prophase of department I the meiosis (in a state equivalent, as we previously pointed out, come a G2 step of a mitotic department cycle) because that a period lasting from a couple of days to numerous years, depending upon the species. During this long period (or, in some cases, in ~ the beginning of sexual maturity), the primary oocytes synthesize a coat and cortical granules. In the case of huge nonmammalian oocytes, they likewise accumulate ribosomes, yolk, glycogen, lipid, and also the mRNA that will later straight the synthesis of proteins compelled for early embryonic growth and the unfolding of the developmental program. In numerous oocytes, the extensive biosynthetic activities are reflect in the structure of the chromosomes, i m sorry decondense and form lateral loops, acquisition on a properties “lampbrush” appearance, signifying the they are very busily engaged in RNA synthesis (see numbers 4-36 and 4-37).


Figure 20-22

The stages of oogenesis. Oogonia construct from primordial germ cells that migrate into the arising gonad at an early stage in embryogenesis. ~ a variety of mitotic divisions, oogonia start meiotic division I, ~ which castle are referred to as primary oocytes. In mammals, (more...)

The following phase of oocyte advance is called oocyte maturation. It typically does not happen until sexual maturity, as soon as the oocyte is created by hormones. Under these hormonal influences, the cell resumes its development through division I of meiosis. The chromosomes recondense, the atom envelope breaks under (this is typically taken to note the beginning of maturation), and also the replicated homologous chromosomes segregate at anaphase I into two daughter nuclei, every containing half the original variety of chromosomes. Come end department I, the cytoplasm divides asymmetrically to produce two cells the differ greatly in size: one is a tiny polar body, and the other is a large secondary oocyte, the precursor the the egg. At this stage, each of the chromosomes is still created of 2 sister chromatids. These chromatids execute not separate until division II the meiosis, once they are partitioned right into separate cells, as formerly described. After ~ this final chromosome separation in ~ anaphase II, the cytoplasm the the huge secondary oocyte again divides asymmetrically to develop the mature egg (or ovum) and also a 2nd small polar body, each v a haploid set of solitary chromosomes (see figure 20-22). Since of these two asymmetrical departments of their cytoplasm, oocytes maintain their huge size regardless of undergoing the two meiotic divisions. Both of the polar bodies are small, and also they ultimately degenerate.

In many vertebrates, oocyte maturation proceeds come metaphase that meiosis II and also then arrests until fertilization. In ~ ovulation, the arrested second oocyte is exit from the ovary and undergoes a rapid maturation action that transforms it right into an egg the is prepared for fertilization. If fertilization occurs, the egg is engendered to finish meiosis.

Oocytes Use distinct Mechanisms to prosper to Their huge Size

A somatic cell through a diameter that 10–20 μm generally takes about 24 hours to double its fixed in preparation for cell division. At this price of biosynthesis, such a cell would certainly take a really long time to reach the thousand-fold greater mass that a mammalian egg with a diameter of 100 μm. It would take even longer to with the million-fold higher mass of an insect egg v a diameter the 1000 μm. Yet part insects live just a few days and manage to develop eggs v diameters also greater than 1000 μm. The is clear the eggs must have special mechanisms because that achieving their huge size.

One straightforward strategy for rapid expansion is to have actually extra gene duplicates in the cell. Thus, the oocyte delays perfect of the very first meiotic division so regarding grow while it contains the diploid chromosome set in duplicate. In this way, it has actually twice as lot DNA accessible for RNA synthesis as does an typical somatic cabinet in the G1 phase of the cabinet cycle. The oocytes the some species go to even greater lengths to accumulate extra DNA: lock produce countless extra duplicates of particular genes. We talk about in chapter 6 how the somatic cell of most organisms call for 100 come 500 copies of the ribosomal RNA genes in bespeak to produce enough ribosomes because that protein synthesis. Egg require even greater numbers of ribosome to assistance protein synthesis during early embryogenesis, and also in the oocytes the many animals the ribosom RNA genes room specifically amplified; some amphibian eggs, because that example, save 1 or 2 million duplicates of these genes.

Oocytes may additionally depend partly on the synthetic tasks of other cells for their growth. Yolk, because that example, is normally synthesized external the ovary and also imported right into the oocyte. In birds, amphibians, and insects, yolk proteins are made by liver cell (or your equivalents), i beg your pardon secrete these proteins into the blood. Within the ovaries, oocytes take it up the yolk protein from the extracellular fluid by receptor-mediated endocytosis (see number 13-41). Nutritive help can additionally come from surrounding accessory cells in the ovary. These have the right to be of two types. In some invertebrates, several of the progeny of the oogonia become nurse cells instead of coming to be oocytes. This cells typically are associated to the oocyte by cytoplasmic bridges v which macromolecules deserve to pass straight into the oocyte cytoplasm (Figure 20-23). For the insect oocyte, the nurse cell manufacture many of the products—ribosomes, mRNA, protein, and also so on—that vertebrate oocytes have to manufacture because that themselves.

Figure 20-23

Nurse cells and also follicle cells associated with a Drosophila oocyte. The nurse cells and also the oocyte arise from a typical oogonium, which offers rise come one oocyte and 15 nurse cell (only 7 of which are seen in this aircraft of section). These cells continue to be (more...)

The various other accessory cells in the ovary that assist to nourish developing oocytes are simple somatic cells referred to as follicle cells, i beg your pardon are discovered in both invertebrates and also vertebrates. They are arranged together an epithelial layer approximately the oocyte (Figure 20-24, and also see number 20-23), come which lock are linked only by space junctions, which permit the exchange of small molecules yet not macromolecules. While this cells room unable to administer the oocyte v preformed macromolecules v these communicating junctions, lock may assist to supply the smaller precursor molecule from i m sorry macromolecules are made. In addition, follicle cells commonly secrete macromolecules that contribute to the egg coat, or space taken increase by receptor-mediated endocytosis right into the growing oocyte, or act on egg cell-surface receptor to regulate the spatial patterning and also axial asymmetries that the egg (discussed in chapter 21).

Figure 20-24

Electron micrographs of developing primary oocytes in the hare ovary. (A) very early stage of major oocyte development. Neither a zona pellucida no one cortical granules have actually developed, and also the oocyte is surrounding by a solitary layer of planarization follicle (more...)


Eggs build in step from primordial germ cells the migrate into the arising gonad early on in breakthrough to end up being oogonia. After ~ mitotic proliferation, oogonia come to be primary oocytes, which begin meiotic division I and also then arrest at prophase i for days to years, depending upon the species. During this prophase-I arrest period, main oocytes grow, synthesize a coat, and also accumulate ribosomes, mRNAs, and also proteins, often enlisting the assist of other cells, consisting of surrounding accessory cells. In the process of maturation, major oocytes complete meiotic division I to kind a tiny polar body and also a big secondary oocyte, i beg your pardon proceeds right into metaphase of meiotic department II. There, in countless species, the oocyte is arrested until created by fertilization to finish meiosis and begin embryonic development.

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