Fertilization- is more a chain of events than a single, isolatedFertilizationphenomenon. Indeed, interruption of any step in the chain willalmost certainly cause fertilization failure. The chain begins with agroup of changes affecting the sperm, which prepares them for thetask ahead. Successful fertilization requires not only that a sperm andegg fuse, but that not more than one sperm fuses with the egg.Fertilization by more than one sperm - polyspermy - almostinevitably leads to early embryonic death. At the end of the chainare links that have evolved to efficiently prevent polyspermy.
How Does Fertilization Occur? The number of eggs produced by females of differentspecies depends on how fertilization occurs. In some species,such as the frog you see here, fertilization is external. In themating season, the male mounts the female and squeezes herabdomen, forcing the eggs out. The male deposits sperm on themass of eggs and fertilization occurs. This is a bit haphazard andthe developing tadpoles are subject to many predators, sothousands of eggs are deposited to ensure that some will survive. In other species, fertilization is internal...the male depositssperm within the reproductive tract of the female and fertilizationoccurs there. This type of fertilization should sound at leastvaguely familiar to you, since it applies to the human species.Chances of an egg being fertilized are greater with internalfertilization, so fewer eggs are produced by the female.
During the spawning season, a sea urchin male releasesbillions of sperm into seawater. The presence of the sperm triggersnearby females to release millions of eggs and fertilization isexternal. We can induce urchins to release their gametes by injectingdilute potassium chloride into the animals body through themembrane surrounding its mouth, which is on the lower side of thebody. Since you cant tell the sex of a sea urchin by looking at it,you have to look at the gametes the animal produces.
The male releases a whiter viscous mass of sperm from tinypores on the upper surface of the body. A female urchin produces ayellowish-orange mass of eggs when injected, and we can collect thethe eggs in a beaker of sea water. An electron micrograph shows urchin sperm swarmingaround an egg. As soon as one sperm penetrates the egg, aprotective membrane lifts off the surface of the egg. It is called thefertilization membrane.
The Acrosome Reaction* Binding of sperm to the zona pellucida is the easypart of fertilization.* The acrosome reaction provides the sperm withan enzymatic drill to get through the zona pellucida.* The same zona pellucida protein that serves as asperm receptor also stimulates a series of eventsthat lead to many areas of fusion between theplasma membrane and outer acrosomal membrane.* Membrane fusion (actually an exocytosis) andvesiculation expose the acrosomal contents, leadingto leakage of acrosomal enzymes from the spermshead.* Sperm that lose their acrosomes beforeencountering the oocyte are unable to bind to thezona pellucida and thereby unable to fertilize.
Stages of Fertilization• * Contact and recognition between sperm and egg• * Regulation of sperm entry into the egg• * Fusion of the genetic material of sperm and egg * Activation of egg metabolism to start development
First stage of fertilization*Fertilization occurs*Zygote implants itself in the lining of the uterus*Rapid cell division occurs*Embryonic stage lasts from 2 weeks to 8 weeks*Cells differentiate into three distinct layers: the ectoderm, the mesoderm, and the endoderm*Nervous system begins to develop*Embryo is 1/2 inch long
Sperm-Oocyte Binding Once a sperm penetrates the zona pellucida, it binds toand fuses with the plasma membrane of the oocyte. Bindingoccurs at the posterior (post-acrosomal) region of the spermhead. The molecular nature of sperm-oocyte binding is notcompletely resolved. A leading candidate in some species is adimeric sperm glycoprotein called fertilin, which binds to aprotein in the oocyte plasma membrane and may also inducefusion. Interestingly, humans and apes have inactivatingmutations in the gene encoding one of the subunits of fertilin,suggesting that they use a different molecule to bind oocytes.
Early Development After fertilization occurs, what happens next? Thediploid zygote is just the beginning of a new organism. It willhave to develop into a multicelled embryo by undergoing celldivisions called cleavage. Turn to Roman Numeral III of yourlab guide and read the definition of cleavage. Soon after fertilization, the zygote will undergo cleavageto form two cells. Since this is a mitotic division, each cell willhave a nucleus with genetic information identical to that of thezygote, but will be half the size of the zygote. These cells donthave time to grow.
They cleave again by mitosis to form four cells and again thecells are reduced in size. The divisions continue. The four-celled stage is followed by an eight-celled stage, then16, 32, and so on. As the number of cells increases, the cells migrateoutwards to form a hollow ball of cells. This hollow ball is called a blastula. In the sea urchin embryo,the blastula cells develop cilia and the blastula revolves in theseawater. The cells on one side of the blastula begin to push in orinvaginate. You can see this beginning to happen here. This indentationmarks the gastrula stage of development. In a late gastrula, theindentation will deepen. Since cells on the inside of the gastrulaencounter different conditions than those on the outside, they begin todifferentiate into different types. The formation of tissues has begun.
As development and differentiation continue, a free swimminglarval stage appears...it is called a Pluteus larva. The larva feeds,grows, and undergoes structural changes to form a juvenile sea urchinwhich resembles the adult. Sperm cells and egg cells both are specialized to perform theirspecific roles in the fertilization process. Regardless of whetherfertilization is external or internal, the fusion of a sperm and eggresults in a zygote that contains a mixture of parental geneticinformation. All zygotes, whether they are destined to become babyrhinos, sea urchins, or humans, are programmed to go through verysimilar early stages of development.
New cells are formed by cleavages, they differentiate andeventually the embryo takes on a recognizable form. Result of meiosis, fertilization and development is anoffspring that has the characteristics of its species, but has acombination of characteristics that make it unique.
Progression of development1. 1 cell --> many cells2. uniform cells --> specialized cells, differentiation3. simple multicelled shape --> complex multicelledshape4. Not just morphology, also physiology and behaviorDevelopment is fundamentally a process of control ofgene expression.DNA --> mRNA--> proteins -----> phenotype
Recognition of Egg and Sperm• * The chemoatraction of the sperm to the egg• * Exocytosis of the acrosomal vesicle• * Binding of the sperm to the extracellular envelope• * Passing of the sperm through the extracellular envelope• * Fusion of egg and sperm plasma membranes
Post-fertilization Events Following fusion of the fertilizing sperm with theoocyte, the sperm head is incorporated into the egg cytoplasm.The nuclear envelope of the sperm disperses, and thechromatin rapidly loosens from its tightly packed state in aprocess called decondensation. In vertebrates, other spermcomponents, including mitochondria, are degraded rather thanincorporated into the embryo. Chromatin from both the sperm and egg are soon encapsulated in a nuclear membrane, forming pronuclei.
Cell Cleavage 1. Sea Urchin- Cleavage begins. Mitosis occurs. Cytokinesis is uneven, partitioning different amounts and different portions of cytoplasm to progeny cells. Smaller cells mark the animal pole and larger cells (containing more yolk, among other things) mark the vegetal pole. 2. Morula forms early as a solid ball of cells. (< or = 32s). 3. As cell division progresses (500-2,000 cells), a blastula is formed that resembles a hollow ball. The space in the center of this ball is called a blastocoel. The blastocoel simplyvides an internal space for the movement of cells duringtrulation.
4. FrogSteps 1, 2 and 3 as in sea urchin except that the increasedamount of yolk causes the cells at the vegetal pole to dividemuch slower leading to more and smaller cells at animal pole.Blastocoel forms, not in the center, but toward the animal pole.5. ChickCleavage occurs in cells lying atop the large yolk mass so that adisk (blastocyst or blastodisc) forms. Top and bottom layersof cells separate forming the blastocoel cavity.
The presence of the blastocyst indicates thattwo cell types are forming: the embryoblast (innercell mass on the inside of the blastocele), and thetrophoblast (the cells on the outside of theblastocele).
Cell Cleavage and Blastomere The zygote now begins to cleave,with each division occurring into twocells called blastomeres. The zygotes firstcell division begins a series of divisions,with each division occurringapproximately every twenty hours. Eachblastomere within the zona pellucidabecomes smaller and smaller with eachsubsequent division. When cell division ungeneratedabout sixteen cells, the zygote becomes amorula (mulberry shaped). It leaves thefallopian tube and enters the uterinecavity three to four days after
Early Blastocytes About four days after fertilization, the morulaenters the uterine cavity. Cell division continues, and acavity known as a blastocele forms in the center of themorula. Cells flatten and compact on the inside of thecavity while the zona pellucida remains the same size.With the appearance of the cavity in the center, the entirestructure is now called a blastocyst.
Properties of Embryo During Cleavage and Blastulation•During cleavage and blastulation the embryo is simple in structureand alteration in its cell arrangement are straightforward.•Many properties of the embryo and its constituents cells changeduring this period.•In sea urchin, isolated blastomeres at 2- or 4- cell stage give ordevelope to complete individual.
•Chick blastoderms (30,000 cells) was cut into two halves, eachhalf was able to form complete chick embryo.•Once mammalian blastocyst has become subdivided into thetrophoblast and inner cell mass, these two regions are notequivalent to constitute complete individual.•Partial or complete separation of the inner cell mass will givetwinning phenomenon.•Tertraparental mice.•In others such as the mollusk, 2- or 4-cell embryos give rise toincomplete embryo.•Blastomeres isolated from mammalian embryo show steadilydecreasing ability to form complete individuals from 2- to 8-cellstage.