GOP presidential candidate Newt Gingrich has come under fire for his comment that human life begins at implantation, not fertilization. By contrast, hard-core abortion supporters tend to view pregnancy as an egregious invasion of women’s bodies.
When an abortion advocate engages you on the topic of when pregnancy begins, recognize it as a side issue and redirect the conversation to the core question: when human life begins.
Since spermatids (male gametes) in plants do not have motor ability, they are carried by pollen tubes (male gametophytes) (Fig. C) A pollen tube that has reached the embryo sac enters one synergid via the filiform apparatus in response to inducers secreted from the synergid. When activated animal sperms approach the ovum surface, they are blocked by special matrices called the zona pellucida or the vitelline membrane.
In addition to introducing a paternal gene set to an ovum, the sperm also activates the ovum. The membrane receptors of a mammalian sperm bind to the glycoprotein Z3, which is located in the zona pellucida, after which the acrosome reaction of the sperm occurs in the order of (1) to (5). When only the sperm involved in fertilization enters the ovum, the sperm nucleus undergoes nuclear protein conversion and swells, thus forming the male pronucleus (Fig.
Pregnancy Image Gallery A microscopic view of sperm implantation during in vitro fertilization.A See more pregnancy pictures. People just starting the fertility process -- if the woman doesn't have severely damaged fallopian tubes -- usually attempt other treatments before IVF. When 3 pronuclei are present instead of 2, then it is a triploid zygote - which has an extra set of 23 chromosomes - and can not make a normal baby. During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called capacitation, or priming.
As it is swept along the distal uterine tube, the oocyte encounters the surviving capacitated sperm, which stream toward it in response to chemical attractants released by the cells of the corona radiata.
When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to prevent polyspermy, which is penetration by more than one sperm.
The first mechanism is the fast block, which involves a near instantaneous change in sodium ion permeability upon binding of the first sperm, depolarizing the oocyte plasma membrane and preventing the fusion of additional sperm cells. Recall that at the point of fertilization, the oocyte has not yet completed meiosis; all secondary oocytes remain arrested in metaphase of meiosis II until fertilization.
Much less commonly, a zygote can divide into two separate offspring during early development. In Vitro Fertilization IVF, which stands for in vitro fertilization, is an assisted reproductive technology.
The embryos are then incubated until they either reach the eight-cell stage or the blastocyst stage.
IVF is a relatively new and still evolving technology, and until recently it was necessary to transfer multiple embryos to achieve a good chance of a pregnancy. Go to this site to view resources covering various aspects of fertilization, including movies and animations showing sperm structure and motility, ovulation, and fertilization. Hundreds of millions of sperm deposited in the vagina travel toward the oocyte, but only a few hundred actually reach it. The oocyte that is released by ovulation is protected by a thick outer layer of granulosa cells known as the corona radiata and by the zona pellucida, a thick glycoprotein membrane that lies just outside the oocyte’s plasma membrane. For this process to occur species-specifically without multiple fertilizations, close interaction between male and female cells is necessary. These tubes reach the embryo sac (a female gametophyte) after passing through the style (2), the placenta (3), the funiculus (4) and the micropyle (5). The pollen tube breaks down in the synergid, and two spermatids fuse with the central cell and the egg cell, respectively (double fertilization). In mammals, when a sperm fuses with the surface of an ovum, there is a local increase in the level of intracellular Ca2+, which is then transmitted to the entire ovum in a wave-like motion (known as a calcium wave; see Fig. Depending on the source of the problem, women can take antibiotics, fertility drugs or prescription hormones to increase the chances of pregnancy. In this procedure, the sperm (from the male half of the couple or a sperm bank) is collected and manually placed in the woman's uterus or fallopian tubes. People are most likely to use IVF when low sperm count or blocked or damaged fallopian tubes are the culprit.
Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the vagina. Fertilization must occur in the distal uterine tube because an unfertilized oocyte cannot survive the 72-hour journey to the uterus.
Some sperm undergo a spontaneous acrosomal reaction, which is an acrosomal reaction not triggered by contact with the zona pellucida. This is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes. The fast block sets in almost immediately and lasts for about a minute, during which time an influx of calcium ions following sperm penetration triggers the second mechanism, the slow block.
However, in approximately 1 percent of ovulation cycles, two eggs are released and both are fertilized. In vitro, which in Latin translates to “in glass,” refers to a procedure that takes place outside of the body. A normal ovulation cycle produces only one oocyte, but the number can be boosted significantly (to 10–20 oocytes) by administering a short course of gonadotropins. The sperm are prepared by washing to remove seminal fluid because seminal fluid contains a peptide, FPP (or, fertilization promoting peptide), that—in high concentrations—prevents capacitation of the sperm. In the United States, fertilized eggs are typically cultured to the blastocyst stage because this results in a higher pregnancy rate. Today, however, transferred embryos are much more likely to implant successfully, so countries that regulate the IVF industry cap the number of embryos that can be transferred per cycle at two.
More than 40 percent of women under 35 succeed in giving birth following IVF, but the rate drops to a little over 10 percent in women over 40. The number of sperm that reach the oocyte is greatly reduced because of conditions within the female reproductive tract. When capacitated sperm make contact with the oocyte, they release the digestive enzymes in the acrosome (the acrosomal reaction) and are thus able to attach to the oocyte and burrow through to the oocyte’s zona pellucida. Sperm may be viable for up to 4 days; therefore, it is entirely possible that capacitated sperm are still residing in her uterine tubes and could fertilize the oocyte she has just ovulated.
Moreover, due to limitations in the current state of medical technology, most women do not become aware of their pregnancies until weeks after both fertilization and implantation have taken place, making it a moot point in the context of abortion.
From this perspective, it seems bizarre to claim that a woman can be carrying a genetically distinct human being in her womb, yet not be pregnant.
When pregnancy is viewed as an invasion rather than a natural part of life, it’s no wonder that abortion advocates define pregnancy as beginning at implantation. The activation of pollen tubes and their extension to the embryo sac (a female gametophyte) have been studied in torenia, whose embryo sac is denuded, making experimental procedures easy. Since pollen tubes sometimes stray in the regions circled by the broken lines, it is believed that signals that guide the tubes exist in these areas. The Z3 protein species-specifically binds to Z3 receptors on the plasma membrane of sperms (Fig.
The centrosome located at the base of the flagellum also plays a pivotal role in creating the asteroid body*5 in the ovum. Assisted reproductive technology (ART) is a group of fertility treatments that involve both the sperm and the egg. Surgery could be the answer for women who suffer from structural problems in the fallopian tubes or uterus. Another pre-ART option is intrauterine insemination, or IUI, where the sperm is placed directly into the woman's uterus using a catheter, which gives the sperm a head start in the race to the egg. This new single cell, called a zygote, contains all of the genetic material needed to form a human—half from the mother and half from the father. Almost immediately, millions of these sperm are overcome by the acidity of the vagina (approximately pH 3.8), and millions more may be blocked from entering the uterus by thick cervical mucus. They also deplete cholesterol molecules embedded in the membrane of the head of the sperm, thinning the membrane in such a way that will help facilitate the release of the lysosomal (digestive) enzymes needed for the sperm to penetrate the oocyte’s exterior once contact is made. As you will recall from your study of the oogenesis, this oocyte (specifically a secondary oocyte) is surrounded by two protective layers.
The digestive enzymes released by this reaction digest the extracellular matrix of the corona radiata. In this process, referred to as the cortical reaction, cortical granules sitting immediately below the oocyte plasma membrane fuse with the membrane and release zonal inhibiting proteins and mucopolysaccharides into the space between the plasma membrane and the zona pellucida. The unneeded complement of genetic material that results is stored in a second polar body that is eventually ejected.
Two zygotes form, implant, and develop, resulting in the birth of dizygotic (or fraternal) twins.
Although the zygote can split as early as the two-cell stage, splitting occurs most commonly during the early blastocyst stage, with roughly 70–100 cells present. The course begins with follicle-stimulating hormone (FSH) analogs, which support the development of multiple follicles, and ends with a luteinizing hormone (LH) analog that triggers ovulation.
If there are severe problems with the sperm—for example, the count is exceedingly low, or the sperm are completely nonmotile, or incapable of binding to or penetrating the zona pellucida—a sperm can be injected into an egg. Many sperm are overcome by the acidity of the vagina, others are blocked by mucus in the cervix, whereas others are attacked by phagocytic leukocytes in the uterus.
One of the sperm will then break through to the oocyte’s plasma membrane and release its haploid nucleus into the oocyte. If we could observe Raul’s sperm about an hour after ejaculation, however, we’d see that they appear to be moving only sluggishly. On Tuesday morning, she experiences the twinge of mid-cycle pain that she typically feels when she is ovulating.
Nevertheless, it is an interesting debate, because it serves as a proxy for pro- and anti-abortion attitudes about pregnancy.
In human sperms, this activation (known as sperm capacitation) is caused by bicarbonate ions in the genital duct of females.
Pollen tubes acquire the ability to extend toward an egg cell by passing through the style - a female tissue (Fig.
In the activated ovum, hydrolase is secreted from the cortical granules in the cell to the zona pellucida, thus changing its nature to block the entry of other sperms.
Using the microtubules of the asteroid body as rails, the female pronucleus is then carried to the center of the asteroid body. On the male side of the equation, one option is medication that increases sperm production.
Sperm must undergo the process of capacitation in order to have the “capacity” to fertilize an oocyte.
The corona radiata is an outer layer of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation.
Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida.
Zonal inhibiting proteins cause the release of any other attached sperm and destroy the oocyte’s sperm receptors, thus preventing any more sperm from binding.
Because dizygotic twins develop from two eggs fertilized by two sperm, they are no more identical than siblings born at different times. These two scenarios are distinct from each other, in that the twin embryos that separated at the two-cell stage will have individual placentas, whereas twin embryos that form from separation at the blastocyst stage will share a placenta and a chorionic cavity.
For example, a woman may produce normal eggs, but the eggs cannot reach the uterus because the uterine tubes are blocked or otherwise compromised.
Right before the ova would be released from the ovary, they are harvested using ultrasound-guided oocyte retrieval.
The oocyte’s membrane structure changes in response (cortical reaction), preventing any further penetration by another sperm and forming a fertilization membrane.
When Raul’s sperm eventually encounter Darcy’s oocyte, they appear to be incapable of generating an adequate acrosomal reaction. The bicarbonate ions enter the sperm and promote the production of cAMP, which changes the composition of lipids and glycoproteins in the plasma membrane, lowers membrane potential and increases intracellular metabolic activity and motor activity, thereby making the sperm fertile. On the other hand, the Ca2+ signal drives the developmental program of the ovum, leading the fertilized egg to the next stages of development. In sea urchins, the male and female pronuclei fuse together, forming a 2n fertilized nucleus. In IVF, the sperm fertilizes the egg outside the body, and doctors implant it into the woman's uterus in hopes of a successful pregnancy. According to the Centers for Disease Control, more than 48,000 babies were born in 2003 through ART -- 99 percent of these with IVF [source: CDC]. Men can also take antibiotics and prescription hormones or try a few lifestyle changes, like wearing looser underwear and avoiding hot showers and saunas. Thus, the race into the uterine tubes, which is the most typical site for sperm to encounter the oocyte, is reduced to a few thousand contenders. If they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte’s thick outer layer of cells.
The underlying zona pellucida (pellucid = “transparent”) is a transparent, but thick, glycoprotein membrane that surrounds the cell’s plasma membrane. This initiates a process called the acrosomal reaction in which the enzyme-filled “cap” of the sperm, called the acrosome, releases its stored digestive enzymes. Rather, hundreds of sperm cells must undergo the acrosomal reaction, each helping to degrade the corona radiata and zona pellucida until a path is created to allow one sperm to contact and fuse with the plasma membrane of the oocyte. The mucopolysaccharides then coat the nascent zygote in an impenetrable barrier that, together with hardened zona pellucida, is called a fertilization membrane. The two haploid nuclei derived from the sperm and oocyte and contained within the egg are referred to as pronuclei.
A man may have a low sperm count, low sperm motility, sperm with an unusually high percentage of morphological abnormalities, or sperm that are incapable of penetrating the zona pellucida of an egg. They go through the process of capacitation, which improves their motility and alters the membrane surrounding the acrosome, the cap-like structure in the head of a sperm that contains the digestive enzymes needed for it to attach to and penetrate the oocyte.
Fertilization is complete upon unification of the haploid nuclei of the two gametes, producing a diploid zygote. This alone, however, does not guide pollen tubes correctly to the ovum; such tubes can send spermatids to the embryo sac only in the presence of species-specific elements secreted from synergids in the embryo sac (Fig. 12-15), in which hydrolase stored in the acrosomal vesicle of a sperm is released to the zona pellucida and degrades it, allowing the sperm to penetrate. Other forms of ART include intracytoplasmic sperm injection (ICSI), gamete intrafallopian transfer (GIFT) and zygote intrafallopian transfer (ZIFT). Their journey—thought to be facilitated by uterine contractions—usually takes from 30 minutes to 2 hours. These enzymes clear a path through the zona pellucida that allows sperm to reach the oocyte. If you consider the loss of millions of sperm between entry into the vagina and degradation of the zona pellucida, you can understand why a low sperm count can cause male infertility. Fusion of the ovum plasma membrane and the sperm plasma membrane then occurs, and the sperm nucleus enters the ovum.
After fertilization, meiosis II resumes, and the male and female pronuclei approach each other using the same mechanism seen in sea urchins.
If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte’s plasma membrane ([link]).
The pronuclei then migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material intermingles. The pollen tube enters the embryo sac via a synergid and sends one spermatid to the egg cell and the other spermatid to the diploid central cell, thus allowing fertilization (double fertilization).
In some organisms, the ovum is activated without a sperm and develops into a complete organism. However, they do not fuse together, and chromosomes derived from the two nuclei start to exhibit the same behavior following the breakdown of the nuclear membrane during division.
Thus, fertilization can still occur if intercourse takes place a few days before ovulation. The plasma membrane of that sperm then fuses with the oocyte’s plasma membrane, and the head and mid-piece of the “winning” sperm enter the oocyte interior. This step completes the process of fertilization and results in a single-celled diploid zygote with all the genetic instructions it needs to develop into a human. Studies on species related to torenia have shown that the response of pollen tubes to the elements secreted from synergids is species-specific, indicating that these elements lead to the species-specificity of fertilization. The cell that thus becomes the 2n type again performs cleavage repeatedly and undergoes the developmental process. In comparison, an oocyte can survive independently for only approximately 24 hours following ovulation.
Intercourse more than a day after ovulation will therefore usually not result in fertilization.
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