In Plants, Which Of The Following Is Most Similar To The Yolk In Animal Eggs?

In one respect at least, eggs are the nearly remarkable of brute cells: once activated, they can give rise to a complete new private within a thing of days or weeks. No other jail cell in a higher animal has this capacity. Activation is ordinarily the consequence of fertilization—fusion of a sperm with the egg. In some organisms, yet, the sperm itself is not strictly required, and an egg can be activated artificially by a diversity of nonspecific chemical or concrete treatments. Indeed, some organisms, including a few vertebrates such every bit some lizards, normally reproduce from eggs that become activated in the absence of sperm—that is, parthenogenetically.

Although an egg tin can give rise to every cell type in the adult organism, it is itself a highly specialized prison cell, uniquely equipped for the single office of generating a new private. The cytoplasm of an egg tin can fifty-fifty reprogram a somatic cell nucleus and then that the nucleus can direct the development of a new individual. That is how the famous sheep Dolly was produced. The nucleus of an unfertilized sheep egg was destroyed and replaced with the nucleus of an adult somatic jail cell. An electric shock was used to activate the egg, and the resulting embryo was implanted in the uterus of a surrogate mother. The resulting normal developed sheep had the genome of the donor somatic jail cell and was therefore a clone of the donor sheep.

In this department, we briefly consider some of the specialized features of an egg before discussing how it develops to the point of being ready for fertilization.

An Egg Is Highly Specialized for Independent Development, with Large Nutrient Reserves and an Elaborate Coat

The eggs of most animals are giant unmarried cells, containing stockpiles of all the materials needed for initial development of the embryo through to the stage at which the new individual tin can begin feeding. Before the feeding stage, the giant cell cleaves into many smaller cells, but no internet growth occurs. The mammalian embryo is an exception. Information technology can starting time to grow early past taking upwards nutrients from the female parent via the placenta. Thus, a mammalian egg, although still a big cell, does non have to be as big equally a frog or bird egg, for example. In general, eggs are typically spherical or ovoid, with a diameter of about 0.ane mm in humans and sea urchins (whose feeding larvae are tiny), 1 mm to ii mm in frogs and fishes, and many centimeters in birds and reptiles (Figure 20-19). A typical somatic jail cell, by dissimilarity, has a diameter of but almost x or 20 μm (Effigy 20-20).

Effigy twenty-19

The actual sizes of three eggs. The man egg is 0.one mm in diameter.

Figure 20-20

The relative sizes of various eggs. Sizes are compared with that of a typical somatic prison cell.

The egg cytoplasm contains nutritional reserves in the grade of yolk, which is rich in lipids, proteins, and polysaccharides and is usually independent within detached structures chosen yolk granules. In some species, each yolk granule is membrane-enclosed, whereas in others information technology is not. In eggs that develop into large animals outside the mother's body, yolk tin can account for more 95% of the volume of the prison cell. In mammals, whose embryos are largely nourished past their mothers, there is piffling, if any, yolk.

The egg coat is another peculiarity of eggs. It is a specialized form of extracellular matrix consisting largely of glycoprotein molecules, some secreted by the egg and others deposited on it past surrounding cells. In many species, the major coat is a layer immediately surrounding the egg plasma membrane; in nonmammalian eggs, such as those of body of water urchins or chickens, it is chosen the vitelline layer, whereas in mammalian eggs it is called the zona pellucida (Figure twenty-21). This layer protects the egg from mechanical impairment, and in many eggs information technology also acts every bit a species-specific bulwark to sperm, admitting only those of the same or closely related species.

Figure 20-21

The zona pellucida. (A) Scanning electron micrograph of a hamster egg, showing the zona pellucida. (B) A scanning electron micrograph of a similar egg in which the zona (to which many sperm are attached) has been peeled dorsum to reveal the underlying plasma (more...)

Many eggs (including those of mammals) contain specialized secretory vesicles just under the plasma membrane in the outer region, or cortex, of the egg cytoplasm. When the egg is activated by a sperm, these cortical granules release their contents by exocytosis; the contents of the granules act to modify the egg coat and so as to foreclose more than i sperm from fusing with the egg (discussed below).

Cortical granules are usually distributed evenly throughout the egg cortex, but in some organisms other cytoplasmic components accept a strikingly asymmetrical distribution. Some of these localized components afterward serve to assist establish the polarity of the embryo, as discussed in Affiliate 21.

Eggs Develop in Stages

A developing egg is chosen an oocyte. Its differentiation into a mature egg (or ovum) involves a series of changes whose timing is geared to the steps of meiosis in which the germ cells become through their two final, highly specialized divisions. Oocytes accept evolved special mechanisms for absorbing progress through meiosis: they remain suspended in prophase I for a prolonged period while the oocyte grows in size, and in many cases they later arrest in metaphase II while awaiting fertilization (although they can arrest at various other points, depending on the species).

While the details of oocyte development (oogenesis) vary from species to species, the general stages are similar, every bit outlined in Figure 20-22. Primordial germ cells migrate to the forming gonad to become oogonia, which proliferate by mitosis for a catamenia before differentiating into primary oocytes. At this stage (usually before nascence in mammals), the showtime meiotic segmentation begins: the DNA replicates so that each chromosome consists of ii sister chromatids, the duplicated homologous chromosomes pair forth their long axes, and crossing-over occurs betwixt nonsister chromatids of these paired chromosomes. Afterwards these events, the cell remains arrested in prophase of sectionalisation I of meiosis (in a state equivalent, equally we previously pointed out, to a M_ii phase of a mitotic segmentation cycle) for a menstruum lasting from a few days to many years, depending on the species. During this long period (or, in some cases, at the onset of sexual maturity), the primary oocytes synthesize a glaze and cortical granules. In the example of large nonmammalian oocytes, they also accrue ribosomes, yolk, glycogen, lipid, and the mRNA that will afterward straight the synthesis of proteins required for early on embryonic growth and the unfolding of the developmental program. In many oocytes, the intensive biosynthetic activities are reflected in the structure of the chromosomes, which decondense and form lateral loops, taking on a characteristic "lampbrush" appearance, signifying that they are very busily engaged in RNA synthesis (see Figures iv-36 and 4-37).

Figure twenty-22

The stages of oogenesis. Oogonia develop from primordial germ cells that migrate into the developing gonad early in embryogenesis. Subsequently a number of mitotic divisions, oogonia brainstorm meiotic division I, afterward which they are called main oocytes. In mammals, (more...)

The next phase of oocyte development is called oocyte maturation. It commonly does not occur until sexual maturity, when the oocyte is stimulated by hormones. Under these hormonal influences, the jail cell resumes its progress through division I of meiosis. The chromosomes recondense, the nuclear envelope breaks down (this is more often than not taken to mark the outset of maturation), and the replicated homologous chromosomes segregate at anaphase I into 2 daughter nuclei, each containing half the original number of chromosomes. To stop division I, the cytoplasm divides asymmetrically to produce two cells that differ greatly in size: 1 is a small polar body, and the other is a large secondary oocyte, the precursor of the egg. At this stage, each of the chromosomes is still composed of two sister chromatids. These chromatids do not separate until partitioning II of meiosis, when they are partitioned into dissever cells, as previously described. After this final chromosome separation at anaphase II, the cytoplasm of the big secondary oocyte again divides asymmetrically to produce the mature egg (or ovum) and a 2nd small polar trunk, each with a haploid gear up of single chromosomes (run into Figure 20-22). Because of these two asymmetrical divisions of their cytoplasm, oocytes maintain their big size despite undergoing the two meiotic divisions. Both of the polar bodies are modest, and they eventually degenerate.

In most vertebrates, oocyte maturation proceeds to metaphase of meiosis Ii and so arrests until fertilization. At ovulation, the arrested secondary oocyte is released from the ovary and undergoes a rapid maturation step that transforms it into an egg that is prepared for fertilization. If fertilization occurs, the egg is stimulated to complete meiosis.

Oocytes Use Special Mechanisms to Grow to Their Large Size

A somatic cell with a diameter of 10–20 μm typically takes about 24 hours to double its mass in preparation for jail cell partitioning. At this rate of biosynthesis, such a cell would have a very long time to attain the thousand-fold greater mass of a mammalian egg with a bore of 100 μm. It would accept even longer to accomplish the million-fold greater mass of an insect egg with a diameter of m μm. Yet some insects live only a few days and manage to produce eggs with diameters even greater than chiliad μm. It is clear that eggs must have special mechanisms for achieving their big size.

One unproblematic strategy for rapid growth is to take extra gene copies in the jail cell. Thus, the oocyte delays completion of the first meiotic segmentation so as to grow while it contains the diploid chromosome set in indistinguishable. In this style, it has twice as much DNA bachelor for RNA synthesis every bit does an average somatic cell in the G₁ phase of the cell cycle. The oocytes of some species go to fifty-fifty greater lengths to accumulate extra Dna: they produce many actress copies of certain genes. We talk over in Affiliate 6 how the somatic cells of most organisms crave 100 to 500 copies of the ribosomal RNA genes in guild to produce plenty ribosomes for poly peptide synthesis. Eggs crave even greater numbers of ribosomes to support protein synthesis during early embryogenesis, and in the oocytes of many animals the ribosomal RNA genes are specifically amplified; some amphibian eggs, for example, contain one or 2 1000000 copies of these genes.

Oocytes may also depend partly on the synthetic activities of other cells for their growth. Yolk, for instance, is usually synthesized outside the ovary and imported into the oocyte. In birds, amphibians, and insects, yolk proteins are fabricated by liver cells (or their equivalents), which secrete these proteins into the blood. Within the ovaries, oocytes take upward the yolk proteins from the extracellular fluid by receptor-mediated endocytosis (see Figure xiii-41). Nutritive aid can also come from neighboring accessory cells in the ovary. These can be of two types. In some invertebrates, some of the progeny of the oogonia go nurse cells instead of becoming oocytes. These cells commonly are connected to the oocyte past cytoplasmic bridges through which macromolecules can pass directly into the oocyte cytoplasm (Figure 20-23). For the insect oocyte, the nurse cells industry many of the products—ribosomes, mRNA, protein, and then on—that vertebrate oocytes accept to manufacture for themselves.

Figure 20-23

Nurse cells and follicle cells associated with a Drosophila oocyte. The nurse cells and the oocyte arise from a common oogonium, which gives rise to one oocyte and 15 nurse cells (but seven of which are seen in this plane of section). These cells remain (more...)

The other accessory cells in the ovary that help to nourish developing oocytes are ordinary somatic cells chosen follicle cells, which are found in both invertebrates and vertebrates. They are arranged as an epithelial layer around the oocyte (Figure 20-24, and see Effigy 20-23), to which they are connected only by gap junctions, which allow the commutation of pocket-size molecules simply not macromolecules. While these cells are unable to provide the oocyte with preformed macromolecules through these communicating junctions, they may assistance to supply the smaller forerunner molecules from which macromolecules are fabricated. In addition, follicle cells often secrete macromolecules that contribute to the egg coat, or are taken upwardly by receptor-mediated endocytosis into the growing oocyte, or human action on egg prison cell-surface receptors to command the spatial patterning and axial asymmetries of the egg (discussed in Chapter 21).

Effigy 20-24

Electron micrographs of developing primary oocytes in the rabbit ovary. (A) An early phase of primary oocyte development. Neither a zona pellucida nor cortical granules accept developed, and the oocyte is surrounded by a single layer of flattened follicle (more...)

Summary

Eggs develop in stages from primordial germ cells that migrate into the developing gonad early in development to become oogonia. Afterwards mitotic proliferation, oogonia become primary oocytes, which begin meiotic sectionalisation I and so arrest at prophase I for days to years, depending on the species. During this prophase-I arrest period, primary oocytes grow, synthesize a coat, and accrue ribosomes, mRNAs, and proteins, often enlisting the help of other cells, including surrounding accessory cells. In the process of maturation, primary oocytes complete meiotic division I to course a minor polar torso and a large secondary oocyte, which gain into metaphase of meiotic division Ii. There, in many species, the oocyte is arrested until stimulated by fertilization to complete meiosis and begin embryonic evolution.

Source: https://www.ncbi.nlm.nih.gov/books/NBK26842/

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