RhymeZone: zygote related words
What's the difference between Gamete and Zygote? Gamete refers to the individual haploid sex cell, i.e, the egg or the sperm. Zygote is a diploid cell formed. 3. Thus, a zygote is clearly capable of giving rise to all the different kinds. The formation of a zygote by the union of genetically different gametes. Objective To investigate the relationship between the first polar body. During the process of fertilization, a series of reactions triggers the fusion of gametes to produce a diploid cell called a zygote. Once the sperm reaches the egg.
They are direct quotes and references from some of the most highly respected human embryology textbooks, and represent a consensus of human embryologists internationally. Basic human embryological facts To begin with, scientifically something very radical occurs between the processes of gametogenesis and fertilization—the change from a simple part of one human being i.
That is, upon fertilization, parts of human beings have actually been transformed into something very different from what they were before; they have been changed into a single, whole human being.
During the process of fertilization, the sperm and the oocyte cease to exist as such, and a new human being is produced. To understand this, it should be remembered that each kind of living organism has a specific number and quality of chromosomes that are characteristic for each member of a species. The number can vary only slightly if the organism is to survive.
For example, the characteristic number of chromosomes for a member of the human species is 46 plus or minus, e.
Every somatic or, body cell in a human being has this characteristic number of chromosomes. Even the early germ cells contain 46 chromosomes; it is only their mature forms - the sex gametes, or sperms and oocytes - which will later contain only 23 chromosomes each. This is necessary so that after their fusion at fertilization the characteristic number of chromosomes in a single individual member of the human species 46 can be maintained—otherwise we would end up with a monster of some sort.
To accurately see why a sperm or an oocyte are considered as only possessing human life, and not as living human beings themselves, one needs to look at the basic scientific facts involved in the processes of gametogenesis and of fertilization. It may help to keep in mind that the products of gametogenesis and fertilization are very different.
The products of gametogenesis are mature sex gametes with only 23 instead of 46 chromosomes. The product of fertilization is a living human being with 46 chromosomes. Gametogenesis refers to the maturation of germ cells, resulting in gametes. Fertilization refers to the initiation of a new human being. The timing of gametogenesis is different in males and in females.
The later stages of spermatogenesis in males occur at puberty, and continue throughout adult life. The process involves the production of spermatogonia from the primitive germ cells, which in turn become primary spermatocytes, and finally spermatids—or mature spermatozoa sperms.
These mature sperms will have only half of the number of their original chromosomes—i. By birth, only about- 2 million remain. By puberty, only aboutremain. The process includes several stages of maturation—the production of oogonia from primitive germ cells, which in turn become primary oocytes, which become definitive oocytes only at puberty. In fact, it does not reduce its number of chromosomes until and unless it is fertilized by the sperm, during which process the definitive oocyte becomes a secondary oocyte with only 23 chromosomes.
Many people confuse meiosis with a different process known as mitosis, but there is an important difference. Mitosis refers to the normal division of a somatic or of a germ cell in order to increase the number of those cells during growth and development.
Gamete Disease - The Igloo
The resulting cells contain the same number of chromosomes as the previous cells—in human beings, Meiosis refers to the halving of the number of chromosomes that are normally present in a germ cell - the precursor of a sperm or a definitive oocyte - in order for fertilization to take place. The resulting gamete cells have only half of the number of chromosomes as the previous cells—in human beings, He also sits on the international board of Nomina Embryologica which determines the correct terminology to be used in human embryology textbooks internationally: These cells are produced in the gonads, i.
During the differentiation of gametes, diploid cells those with a double set of chromosomes, as found in somatic cells [46 chromosomes] are termed primary, and haploid cells those with a single set of chromosomes [23 chromosomes] are called secondary. The reduction of chromosomal number Spermatogenesis, the production of spermatozoa, continues from immediately after puberty until old age. It takes place in the testis, which is also an endocrine gland, the interstitial cells of which secrete testosterone.
Previous to puberty, spermatogonia in the simiferous tubules of the testis remain relatively inactive. After puberty, under stimulation from the interstitial cells, spermatogonia proliferate When these undergo their first maturation division meiosis 1they become secondary spermatocytes.
The second maturation division meiosis 2 results in spermatids, which become converted into spermatozoa. Oogonia derived from primordial germ cells multiply by mitosis and become primary oocytes. The number of oogonia increases to nearly seven million by the middle of prenatal life, after which it diminishes to about two million at birth.
From these, several thousand oocytes are derived, several hundred of which mature and are liberated ovulated during a reproductive period of some thirty years.
Prophase of meiosis 1 begins during fetal life but ceases at the diplotene state, which persists during childhood. After puberty, meiosis 1 is resumed and a secondary oocyte The secondary oocyte is a female gamete in which the first meiotic division is completed and the second has begun. From oogonium to secondary oocyte takes from about 12 to 50 years to be completed.
Meiosis 2 is terminated after rupture of the follicle ovulation but only if a spermatozoon penetrates. Hence a human ovum does not [really] exist. Thus, for fertilization to be accomplished, a mature sperm and a mature human oocyte are needed.
Before fertilization,8 each has only 23 chromosomes. They each possess "human life," since they are parts of a living human being; but they are not each whole living human beings themselves.
They each have only 23 chromosomes, not 46 chromosomes—the number of chromosomes necessary and characteristic for a single individual member of the human species. Furthermore, a sperm can produce only "sperm" proteins and enzymes; an oocyte can produce only "oocyte" proteins and enzymes; neither alone is or can produce a human being with 46 chromosomes. Thus these terms themselves would qualify as "scientific" myths. The commonly used term, "fertilized egg," is especially very misleading, since there is really no longer an egg or oocyte once fertilization has begun.
What is being called a "fertilized egg" is not an egg of any sort; it is a human being. The zygote is characteristic of the last phase of fertilization and is identified by the first cleavage spindle. It is a unicellular embryo.
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The fusion of the sperm with 23 chromosomes and the oocyte with 23 chromosomes at fertilization results in a live human being, a single-cell human zygote, with 46 chromosomes—the number of chromosomes characteristic of an individual member of the human species. This cell results from the union of an oocyte and a sperm. A zygote is the beginning of a new human being i. The expression fertilized ovum refers to a secondary oocyte that is impregnated by a sperm; when fertilization is complete, the oocyte becomes a zygote.
In fact, this genetic growth and development has been proven not to be directed by the mother. In sum, a mature human sperm and a mature human oocyte are products of gametogenesis—each has only 23 chromosomes.
They each have only half of the required number of chromosomes for a human being. They cannot singly develop further into human beings.
They produce only "gamete" proteins and enzymes. They do not direct their own growth and development. And they are not individuals, i. They are only parts—each one a part of a human being. On the other hand, a human being is the immediate product of fertilization. It simply divides and grows bigger and bigger, developing through several stages as an embryo over an 8-week period.
Several of these developmental stages of the growing embryo are given special names, e. Given these basic facts of human embryology, it is easier to recognize the many scientifically inaccurate claims that have been advanced in the discussions about abortion, human embryo research, cloning, stem cell research, the formation of chimeras, and the use of abortifacients—and why these discussions obfuscate the objective scientific facts. The following is just a sampling of these current "scientific" myths.
But human sperms and human ova are human life, too. So prolifers would also have to agree that the destruction of human sperms and human ova are no different from abortions—and that is ridiculous!
As pointed out above in the background section, there is a radical difference, scientifically, between parts of a human being that only possess "human life" and a human embryo or human fetus that is an actual "human being. Destroying a human sperm or a human oocyte would not constitute abortion, since neither are human beings.
The issue is not when does human life begin, but rather when does the life of every human being begin. A human kidney or liver, a human skin cell, a sperm or an oocyte all possess human life, but they are not human beings—they are only parts of a human being.
As demonstrated above, the human embryonic organism formed at fertilization is a whole human being, and therefore it is not just a "blob" or a "bunch of cells.
As demonstrated above, scientifically there is absolutely no question whatsoever that the immediate product of fertilization is a newly existing human being.
A human zygote is a human being. It is not a "potential" or a "possible" human being. The immediate product of fertilization is genetically already a girl or a boy—determined by the kind of sperm that fertilizes the oocyte. If the sperm contains 22 autosomes and 2 X chromosomes, the embryo will be a genetic female, and if it contains 22 autosomes and an X and a Y chromosome, the embryo will be a genetic male. These are a few of the most common myths perpetuated sometimes even within quasi-scientific articles—especially within the bioethics literature.
As demonstrated above, the human embryo, who is a human being, begins at fertilization—not at implantation about daysdays, or 3 weeks. Thus the embryonic period also begins at fertilization, and ends by the end of the eighth week, when the fetal period begins. The embryonic period proper during which the vast majority of the named structures of the body appear, occupies the first 8 postovulatory weeks.
This "scientific" myth is perhaps the most common error, which pervades the current literature. The term "pre-embryo" has quite a long and interesting history. Time To Tell The Truth! But it roughly goes back to at least in the bioethics writings of Jesuit theologian Richard McCormick in his work with the Ethics Advisory Board to the United States Department of Health, Education and Welfare,18 and those of frog developmental biologist Dr.
Clifford Grobstein in a article in Scientific American,19 and most notably in his classic book, Science and the Unborn: Choosing Human Futures The term "pre-embryo" was also used as the rationale for permitting human embryo research in the British Warnock Committee Report ,21 and then picked up by literally hundreds of writers internationally, including, e.
Note that none of these is even a scientist, with the exception of Karen Dawson, who is just a geneticist. Oddly, the influential book by Singer, Kuhse, Buckle, and Dawson, Embryo Experimentation,22 which uses the term "pre-embryo," and which contains no scientific references for its "human embryology" chart or its list of "scientific" termsalong with the work of theologian McCormick and frog developmental biologist Grobstein, was used in the United States as the scientific basis for the National Institutes of Heath NIH Human Embryo Research Report.
It is particularly in the writings of these and other bioethicists that so much incorrect science is claimed in order to "scientifically" ground the "pre-embryo" myth and therefore "scientifically" justify many of the issues noted at the beginning of this article.
This would include abortion, as well as the use of donated or "made-for-research" early human embryos in destructive experimental human embryo research such as infertility research, cloning, stem cell research, the formation of chimeras, etc. To begin with, it has been demonstrated above that the immediate product of fertilization is a human being with 46 chromosomes, a human embryo, an individual member of the human species, and that this is the beginning of the embryonic period.
However, McCormick and Grobstein24 claim that even though the product of fertilization is genetically human, it is not a "developmental individual" yet—and in turn, this "scientific fact" grounds their moral claim about this "pre-embryo. It should be noted that at the zygote stage the genetic individual is not yet developmentally single—a source of only one individual.
As we will see, that does not occur until a single body axis has begun to form near the end of the second week post fertilization when implantation is underway. Implantation takes place at days. The "single body axis" to which he refers is the formation of the primitive streak, which takes place at 14 days. McCormick often confuses these different periods in his writings.
Developmental studies show that the cells of the outer wall become the trophoblast feeding layer and are precursors to the later placenta. Ultimately, all these cells are discarded at birth. The clear implication is that there is absolutely no relationship or interaction between these two cell layers, and so the "entity" is not a "developmental individual" yet. Some exchange occurs between these groups. The cells of this germ disc the inner cell layer develop into the embryo proper and also contribute to some of the extraembryonic membranes.
Similarly, it is not factually correct to state that all of the cells from the outer trophoblast layer are discarded after birth. They develop from the zygote but do not participate in the formation of the embryo or fetus—except for parts of the yolk sac and allantois. Part of the yolk sac is incorporated into the embryo as the primordium of the gut.
The allantois forms a fibrous cord that is known as the urachus in the fetus and the median umbilical ligament in the adult. It extends from the apex of the urinary bladder to the umbilicus. They are genetically a part of the individual and are composed of the same germ layers. Consequently, it is also scientifically incorrect to claim that only the inner cell layer constitutes the "embryo proper. Finally, McCormick claims that this "pre-embryo" has not yet decided how many individuals it will become, since the cells are totipotent and twinning can still take place.
Therefore, they argue, there is no "individual" present until days and the formation of the primitive streak, after which twinning cannot take place.
These are described in more detail. The embryo is usually defined as coming into existence at fertilization and becoming a fetus when organogenesis is completed eight weeks after fertilization. These borders are not sharply defined. The definition of an embryo thus cannot avoid being operational and context—dependent.
The term conceptus is useful to denote any entity resulting from fertilization, when no reference to a more specific stage is intended. An additional complication results from the significant overlap between the final stages of female gametogenesis, fertilization, and initial cleavage. Gametogenesis involves a special type of cell division called meiosis.
When primordial germ cells which are diploid—i. In the first meiotic division, there are genetic exchanges within each group of homologous chromosomeswhich then separate into diploid daughter cells. In the second meiotic division, there is no further round of DNA duplication. Each chromosome in a pair is allotted to a separate daughter cell, now haploid.
Each primordial germ cell thus gives rise to four daughter haploid cells. In the male, all four cells resulting from meiosis ultimately become functional spermatozoa.
In contrast, in the female, only one of the daughter cells becomes an oocyte, the other three cells are discarded as polar bodies. In addition, female meiosis is not completed until after fertilization has occurred. During each ovarian cycle of the sexually mature female, one oocyte progresses partially through meiosis but is arrested in the middle of the second meiotic division at the time it is discharged from the mature ovarian follicle into the oviduct.
If the oocyte is fertilized, meiosis is completed. Within the newly fertilized egg, the male and female pronuclei undergo a protracted migration towards each other, while DNA is duplicated within both. Thereafter, both nuclear envelopes disappear and the chromosomes derived from the male and female gamete are involved in the first cleavage division. Thus the first genuine diploid nucleus is observed at the two—cell stage only 30 hours after initial contact of sperm and oocyte.
While fertilization usually occurs close to the ovary, the conceptus is gently nudged towards the uterus, a voyage lasting about five days. Both through recombination of gene segments during the first meiotic division, and through random assortment of homologous chromosomes in gametes, genetic novelty is generated.
In other words, gametes are genetically distinctive in relation to their diploid progenitors and do not simply reflect the genetic structure of their parent organism.Fertilization terminology - gametes, zygotes, haploid and diploid.
In a sense, gametes are distinctive "individuals" in relation to the organism that produces them. Fertilization creates genetic novelty of a different sort, by combining two independent paternal genomes. The zygote is genetically distinctive because it represents the meeting of two independent parental lineages.
Thus genetic novelty appears twice per turn of the human life cycle. During cleavage, the zygote divides into smaller embryonic cells. At the 16—cell stage, the embryo is called a morula and a first differentiation into two cell types is initiated.
The trophoblast is the cell layer that will soon connect with the uterine wall, whereas the inner cell mass includes the cells of the later stage embryo.
At the blastocyst stage, a central cavity blastocoel is formed. If a blastomere is removed from the inner cell FIGURE 1 mass of a blastocyst as, for instance, in preimplantation diagnosisthe blastocyst is still able to produce a complete late embryo and fetus. This illustrates a fundamental principle called regulation, or regulative development. Within the early embryo, cell fates are not definitely fixed but largely depend on interactions with neighboring cells, so that development adjusts to the presence or absence of specific environmental cues.
The molecular basis and the genes responsible for these cues are increasingly well known. At the blastocyst stage, the inner mass cells are pluripotent i. Recent research does suggest that individual blastomeres acquire some degree of molecular specificity quite early. However, this inherent "bias" that tends to drive every blastomere towards a specific cellular fate can easily be overridden at this stage.
Around day 6, the blastocyst has hatched from the surrounding zona pellucida the outer envelope of the ovum and is ready for implantation. As it attaches to the endometrium, two distinctive layers appear in the inner cell mass. The ventral layer hypoblast contributes to the primitive yolk sac. The dorsal layer soon differentiates between the embryonic epiblast that will contribute to the embryo—to—be, and the amniotic ectoderm lining the newly appearing amniotic cavity day 7—8. This two—layered structure is called the embryonic disk.
All this happens as the blastocyst burrows deeper into the uterus wall and the trophoblast comes into close contact with maternal blood vessels. The trophoblast also produces human chorionic gonadotropin hCGwhich is the substance detected in pregnancy tests and is essential to the maintenance of pregnancy. Abnormal conceptuses are very common until that stage and are eliminated, usually without detectable signs of pregnancy. Inversely, fertilization occasionally results in a hydatidiform mole.
This structure consists of trophoblastic tissue and therefore mimics the early events of pregnancy hCG is producedwithout their being any actual embryonic tissue present.
The term pre—embryo was often used to mark the embryonic stages described so far. This term is sometimes shunned in contemporary discourse, as it has been suspected to be a semantic trick to downgrade the standing of the very early embryo. Yet even writers like Richard A. McCormick belonging to the Catholic tradition, sets great store by the moral standing of the earliest forms of prenatal development, have expressed doubts about the validity of this suspicion More importantly, doing away with the term "pre—embryo" does not solve the two underlying conceptual problems that this term addresses.
The first ensues from the cellular genealogy linking the zygote to the later stage embryo and fetus. Only a small part of the very early embryo is an actual precursor to the late embryo, fetus, and born child. Whatever terminology one wishes to use, no account of early development can avoid sentences such as this, written by Thomas W. Sadler in"[t]he inner cell mass gives rise to tissues of the embryo proper," or terms such as the embryo—to—be. This is an inescapable consequence of the fact that the late embryo includes only a small subset of all the cells that originate with the zygote and blastocyst Figure 1 shows the complex genealogy of embryonic and extraembryonic tissues in human development.
The second problem arises from the fact that the early embryo has a degree of freedom as regards its final numerical identity. Until about 12 days after fertilization, twinning can occur. In other words, until that stage, a single embryo still has the potential to divide in two embryos, ultimately developing into two separate persons.
Therefore there is no intrinsic one—to—one relationship between the zygote and the late embryo, as there is between the late embryo, the fetus, and the born human. Gastrulation begins with a wave of cellular movements that start at the tail end of the embryo and extend progressively forward.
Future endoderm and mesoderm cells slip inside the embryonic disk through a groove called the primitive streak day