Fertilization Notes.doc

Fertilization Introduction to Components of Fertilization Fertilization can be described as including differing specific events. For this course fertilization will include those events prior to sperm penetration, the events of penetration, post-penetration events and finally events in preparation for the first cleavage division. In general fertilization is viewed as those processes that prepare the sperm for meeting and penetrating the egg, the components of penetration through extracellular and cellular components surrounding the egg and changes that occur to the sperm following its entry into the egg cytoplasm. The subsequent events consist of changes the egg undergoes in response to penetration by the spermatozoon. Preliminary to Sperm Penetration Before spermatozoa can penetrate eggs, they first must be transported from the male reproductive to the location of the eggs. HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "sperm transport" Sperm transport is accomplished by varied adaptations depending fundamentally on whether the species in question employs internal or external fertilization. In either case, behavioral adaptations exist to assure that sperm and eggs come into contact. Spawning fish, for example, exhibit behaviors that bring males to the location where females are expelling eggs into the aqueous environment and the males then deposit sperm in the same location. Many anuran amphibians exhibit a mating behavior called HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "amplexus" amplexus in which males clasp themselves around the female from above, squeezing to help release her eggs and releasing spermatozoa to mix with the eggs being expelled. Species that employ internal fertilization exhibit mating behaviors that result either in HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "cloacal apposition" cloacal apposition by which the male introduces spermatozoa into the female cloaca or in HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "coital mating" coital mating in which the male inserts an HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "intromittent organ" intromittent organ into the female reproductive tract for sperm deposition. The deposited spermatozoa must be capable of swimming in order to approach the eggs in both cases above. Studies in mammals have shown that sperm transport through the female reproductive tract is in part dependent upon the hormonal status of the female; HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "estrogen" estrogen dominance favoring rapid sperm transport and progesterone decreasing it. The rate of sperm transport has been studied in several mammals and it has been demonstrated that within minutes of mating, sperm are rapidly transported throughout the female reproductive tract mainly through contractions of the muscles lining the tract. A second, slower sperm transport then occurs and is probably responsible for transporting those sperm that will be penetrating eggs to the site of fertilization. HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "capacitation" Capacitation of Spermatozoa A process called sperm capacitation occurs at some point during sperm transport. Capacitation constitutes change to the spermatozoa occurring after the sperm leave the male reproductive tract and before they are capable of penetrating the eggs. It has been described in most detail for mammalian species and occurs over a period of about an hour to several hours following mating, depending upon the species. Classically capacitation results in a change in sperm HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "motility" motility or swimming quality; capacitated spermatozoa exhibit what is called HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "hyperactivated motility" hyperactivated motility . Some evidence suggests that during capacitation, the outer membranes of spermatozoa also undergo some changes which make possible a later process called the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "acrosome reaction" acrosome reaction . A type of capacitation has been demonstrated in some amphibian species. Evidence suggests that contact with the jelly material surrounding amphibian eggs induces the sperm to undergo capacitation. Penetration of Outer Layers When spermatozoa arrive at the egg, several barriers are encountered before contact with the egg cell can be accomplished. From the outside inward these include, for mammals as an example: 1. the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "cumulus oophorus" cumulus oophorus , a cloud of loosely associated follicular HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "granulosa cells" granulosa cells held together by the extracellular matrix, HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "hyaluronic acid" hyaluronic acid , 2. the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "corona radiata" corona radiata , one to several layers of more tightly associated follicular granulosa cells, 3. the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "zona pellucida" zona pellucida , an extracellular matrix of glycoprotein that directly surrounds the egg cell. Non-mammalian species often have less differentiated follicle cell layers around their eggs, but all have the zona-like matrix, called the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "vitelline layer" vitelline layer or envelope in several species. During the acrosome reaction, the outer plasma membrane of the spermatozoon disintegrates and then the underlying outer acrosomal membrane disrupts to release its contents. One component released is HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "hyaluronidase" hyaluronidase that acts to digest the hyaluronic acid cementing the cumulus cells together. With the release of this enzyme, spermatozoa can swim through the cumulus layer and probably pass between corona cells to reach the zona pellucida by the same mechanism. The acrosome reaction is completed by sperm reaching the zona pellucida. The remaining membrane on the front of the sperm head is the inner acrosomal membrane that contains membrane-bound enzymes which are proteases called HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "acrosins" acrosins or HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "lysins" lysins . By hyperactivated motility the spermatozoon pushes its front surface against the zona pellucida and the acrosins there digest a passage through that extracellular matrix. Once the zona pellucida has been penetrated, the spermatozoon is located in a narrow area called the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "perivitelline space" perivitelline space between the inside of the zona and the egg plasma membrane. Penetration Phenomena For several invertebrate species, spermatozoa contacting the vitelline envelope extend an HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "acrosomal filament" acrosomal filament from their front surface toward the egg. This acrosomal filament penetrates the vitelline envelope by enzymatic activity and comes into contact with the outer surface of the egg plasma membrane. In other species such as amphibians and mammals the entire sperm head passes through the zona or vitelline envelope as described above and comes to lie in the perivitelline space, finally settling down against the outer surface of the egg plasma membrane. In all cases, once the spermatozoon contacts the egg plasma membrane, the membranes of the two cells undergo a fusion process that results in an opening up of the egg plasma membrane. As membrane fusion continues around the surface of the spermatozoon, the internal cytoplasms of egg and sperm become continuous and the sperm nucleus passes into the egg interior. Sperm Changes Once the nucleus of the penetrating spermatozoon has entered the egg cytoplasm, its nuclear envelope disintegrates, exposing its HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "chromatin" chromatin . The sperm nucleus expands in volume as the nuclear envelope disappears and the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "sperm-specific chromatin protein" sperm-specific chromatin protein is replaced by HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "histones" histones . As these events are concluded, a new nuclear envelope forms around the sperm nucleus and it is then called a male or sperm pronucleus. Soon afterward, DNA synthesis takes place to replicate the pronuclear chromatin (still n for ploidy, but 2c for number of DNA molecules per homologous chromosome). HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "egg activation" Egg Activation In the specific type of oocyte maturation the oocyte was left after the first meiotic division as a secondary oocyte with the first polar body. It has become customary to call the oocyte an egg when it is ready to be penetrated by a sperm cell; that is, when it is mature. During penetration by the spermatozoon, the oocyte or egg is activated to undergo resumption of meiosis and completes the second meiotic division to produce the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "ootid stage" ootid stage and the second polar body (ploidy is n, DNA copy number is 1c). Soon after completion of the second meiotic division, the egg chromosomes become diffuse and a pronuclear envelope forms around them. As was described for the sperm pronucleus, the DNA of the egg pronucleus also undergoes replication to double the copies of DNA molecules to 2c for the haploid complement. In addition to meiotic resumption, a second event of egg activation consists of the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "block to polyspermy" block to polyspermy . Two blocks appear to exit, a fast block that takes place when a spermatozoon begins its membrane fusion with the egg plasma membrane and a slower block that involves biochemical changes to the outer matrix, the zona pellucida or vitelline membrane. The mechanism involved in establishing the fast block to polyspermy is not completely clear but probably involves changes in ion transport and plasma membrane electrical potential. The slow block has been more thoroughly studied at least in a few species, notably in sea urchins and amphibians. While a spermatozoon is penetrating the egg, the cortical granules fuse with the inside of the egg plasma membrane and finally rupture through it to release their contents into the perivitelline space by HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "exocytosis" exocytosis . Specific components of the cortical granule contents react with constituents of the surrounding vitelline envelope to "harden" it, thereby preventing attachment and penetration by other spermatozoa. The changes in the vitelline envelope of some species such as sea urchins and amphibians are dramatic and can be observed directly as evidence of penetration by a spermatozoon. In addition to the biochemical changes noted above, the sea urchin vitelline membrane of a penetrated egg swells dramatically out away from the egg membrane and is called the HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "fertilization membrane" fertilization membrane . Egg activation also is characterized by changes, both qualitative and quantitative, in patterns of protein synthesis, changes in organellar organization within the egg, cytoskeletal changes such as formation and organization of HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "microtubules" microtubules for movement of pronuclei and in some species the synthesis of RNA. The many events of egg activation appear to be controlled by intracellular calcium levels, signal transduction pathways and by cell cycle regulatory agents. Although egg activation is triggered naturally in most known species as a result of penetration by a spermatozoon, eggs can become activated without penetration. Egg activation in the absence of spermatozoa is called HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "parthenogenesis" parthenogenesis and in some species is a normal strategy for embryogenesis. In species in which parthenogenesis does not naturally occur, egg activation can be induced experimentally by needle pricking, temperature changes, electrical stimulation or by exposure of eggs to a variety of chemical agents. Preparations for First Cleavage The egg at the pronucleus stage is an embryo specifically called a HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "zygote" zygote . Because both pronuclei contain diffuse chromatin and are synthetically active, the zygote is clearly in HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "interphase" interphase of the cell cycle. At this developmental stage, the embryo contains a diploid set of replicated chromosomes although the homologous chromosomes are separated from each other in separate pronuclei. Toward the end of this stage (about 20 hours in mouse zygotes), the pronuclear chromatin begins to condense into discrete chromosomes and the pronuclei both migrate toward the center of the egg. Depending on the species, either the envelopes of male and female pronuclei fuse together and the condensing chromatin intermixes, or the nuclear envelopes disintegrate while the chromosomes condense separately in the two pronuclei before mixing together. In both cases the combining of the two sets of haploid pronuclear chromosomes into a single diploid group is called HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "syngamy" syngamy or HYPERLINK "file:///C:\\Users\\Bob\\Documents\\My%20Documents\\Excel%20Documents\\ASU\\Bio351\\HTML's\\TEXT\\gloss.htm" \l "amphimixis" amphimixis . The zygote at this stage has left interphase to enter the M-phase of the cell cycle. During syngamy a spindle of microtubules forms around the combining chromosomes and individual spindle fibers attach to the chromosomes to organize them onto a metaphase plate. Finally the spindle apparatus begins to function and one-half of each homologous chromosome migrates to each pole of the mitotic spindle. When this occurs, cleavage has begun and fertilization has been completed. PAGE