The day of ovulation determines the length of a menstrual cycle. The common belief that stress can affect the timing of a period is half true. Stress can affect ovulation which then affects hormone release which begins the next menstrual period ; but, stress around the time of an expected period will not make it late - that date was decided 12-16 days earlier.OvulationEach month, a mature egg is released from one of a woman's two ovaries -- this is called ovulation. Ovulation takes place about two weeks after the first day of the last menstrual period.
When stem cells divide, one becomes a specialised cell,while the other remains a stem cell. Specialised cells areall the different cells in our body that perform a function.They are all the cells we see on us and in us,such as lung cells, skin cells, hair cells, and kidney cells."
original work, are licensed under a SIZE: 0.1 - 0. 15 mmTIME PERIOD: 1 day post-ovulation Fertilization begins when a sperm penetrates an oocyte (an egg) and it ends with the creation of the zygote. The fertilization process takes about 24 hours.A sperm can survive for up to 48 hours. It takes about ten hours to navigate the female reproductive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the upper part of the vagina, only 1%, 3 million, enter the uterus. The next step is the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced. Penetration of the zona pellucida takes about twenty minutes.Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization.
Fertilization FertilizationFertilization, the process by which male and female gametes fuse, occurs in theampullary region of the uterine tube. This is the widest part of the tube and38 Part One: General Embryologyis close to the ovary (Fig. 2.4). Spermatozoa may remain viable in the femalereproductive tract for several days.Only 1% of sperm deposited in the vagina enter the cervix, where theymay survive for many hours. Movement of sperm from the cervix to the oviductis accomplished primarily by their own propulsion, although they may be assistedby movements of fluids created by uterine cilia. The trip from cervixto oviduct requires a minimum of 2 to 7 hours, and after reaching the isthmus,sperm become less motile and cease their migration. At ovulation, spermagain become motile, perhaps because of chemoattractants produced by cumuluscells surrounding the egg, and swim to the ampulla where fertilizationusually occurs. Spermatozoa are not able to fertilize the oocyte immediatelyupon arrival in the female genital tract but must undergo (a) capacitation and(b) the acrosome reaction to acquire this capability.Capacitation is a period of conditioning in the female reproductive tractthat in the human lasts approximately 7 hours. Much of this conditioning,which occurs in the uterine tube, entails epithelial interactions between thesperm and mucosal surface of the tube. During this time a glycoprotein coatand seminal plasma proteins are removed from the plasma membrane thatoverlies the acrosomal region of the spermatozoa. Only capacitated sperm canpass through the corona cells and undergo the acrosome reaction.The acrosome reaction, which occurs after binding to the zona pellucida,is induced by zona proteins. This reaction culminates in the release of enzymesneeded to penetrate the zona pellucida, including acrosin and trypsin-like substances
passage of sperm through corona radiata *penetration of zona pellucida *fusion of plasma membranes of oocyte and sperm *completion of 2nd meiotic division of oocyte and formation ofthe female pronucleus *formation ofthe male pronucleus *asthe pronuclei fuse into a single diploid aggregation,theootid becomes a zygote
Human fertilizationis the union of a humanoid egg and sperm, usually occurring in the ampulla of the uterine tube. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development. The process of fertilization involves a sperm fusing with an ovum—usually following ejaculation during copulation. It is possible, but less probable, for fertilization to occur without copulation, artificial insemination, or In vitro fertilization.Upon encountering the ovum, the acrosome of the sperm produces enzymes which allow it to burrow through the outer jelly coat of the egg. The sperm plasma then fuses with the egg's plasma membrane, the sperm head disconnects from its flagellum and the egg travels down the Fallopian tube to reach the uterus.In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitroOocyte Fertilized••••• original work, are licensed under a SIZE: 0.1 - 0. 15 mmTIME PERIOD: 1 day post-ovulation Fertilization begins when a sperm penetrates an oocyte (an egg) and it ends with the creation of the zygote. The fertilization process takes about 24 hours.A sperm can survive for up to 48 hours. It takes about ten hours to navigate the female reproductive track, moving up the vaginal canal, through the cervix, and into the fallopian tube where fertilization begins. Though 300 million sperm may enter the upper part of the vagina, only 1%, 3 million, enter the uterus. The next step is the penetration of the zona pellucida, a tough membrane surrounding the oocyte. Only one sperm needs to bind with the protein receptors in the zona pellucida to trigger an enzyme reaction allowing the zona to be pierced. Penetration of the zona pellucida takes about twenty minutes.Within 11 hours following fertilization, the oocyte has extruded a polar body with its excess chromosomes. The fusion of the oocyte and sperm nuclei marks the creation of the zygote and the end of fertilization.
The phases of fertilization include phase 1, penetration of the corona radiata;phase 2, penetration of the zona pellucida; and phase 3, fusion of theoocyte and sperm cell membranes.PHASE 1: PENETRATION OF THE CORONA RADIATAOf the 200 to 300 million spermatozoa deposited in the female genital tract,only 300 to 500 reach the site of fertilization. Only one of these fertilizes theegg. It is thought that the others aid the fertilizing sperm in penetrating thebarriers protecting the female gamete. Capacitated sperm pass freely throughcorona cells (Fig. 2.5).PHASE 2: PENETRATION OF THE ZONA PELLUCIDAThe zona is a glycoprotein shell surrounding the egg that facilitates and maintainssperm binding and induces the acrosome reaction. Both binding and theacrosome reaction are mediated by the ligand ZP3, a zona protein. Releaseof acrosomal enzymes (acrosin) allows sperm to penetrate the zona, therebycoming in contact with the plasma membrane of the oocyte (Fig. 2.5). Permeabilityof the zona pellucida changes when the head of the sperm comesin contact with the oocyte surface. This contact results in release of lysosomalChapter 2: First Week of Development: Ovulation to Implantation 39enzymes from cortical granules lining the plasma membrane of the oocyte.In turn, these enzymes alter properties of the zona pellucida (zona reaction)to prevent sperm penetration and inactivate species-specific receptor sites forspermatozoa on the zona surface. Other spermatozoa have been found embeddedin the zona pellucida, but only one seems to be able to penetrate the oocytePHASE 3: FUSION OF THE OOCYTE ANDSPERM CELL MEMBRANESThe initial adhesion of sperm to the oocyte is mediated in part by the interactionof integrins on the oocyte and their ligands, disintegrins, on sperm. Afteradhesion, the plasma membranes of the sperm and egg fuse (Fig. 2.5). Becausethe plasma membrane covering the acrosomal head cap disappears during theacrosome reaction, actual fusion is accomplished between the oocyte membraneand the membrane that covers the posterior region of the sperm head(Fig. 2.5). In the human, both the head and tail of the spermatozoon enter thecytoplasm of the oocyte, but the plasma membrane is left behind on the oocytesurface. As soon as the spermatozoon has entered the oocyte, the egg respondsin three ways:1. Cortical and zona reactions. As a result of the release of cortical oocytegranules, which contain lysosomal enzymes, (a) the oocyte membranebecomes impenetrable to other spermatozoa, and (b) the zona pellucidaalters its structure and composition to prevent sperm binding andpenetration. These reactions prevent polyspermy (penetration of morethan one spermatozoon into the oocyte).2. Resumption of the second meiotic division. The oocyte finishes its secondmeiotic division immediately after entry of the spermatozoon. Oneof the daughter cells, which receives hardly any cytoplasm, is known asthe second polar body; the other daughter cell is the definitive oocyte.Its chromosomes (22+X) arrange themselves in a vesicular nucleusknown as the female pronucleus (Figs. 2.6 and 2.7).3. Metabolic activation of the egg. The activating factor is probably carriedby the spermatozoon. Postfusion activation may be considered toencompass the initial cellular and molecular events associated with earlyembryogenesis.The spermatozoon, meanwhile, moves forward until it lies close to thefemale pronucleus. Its nucleus becomes swollen and forms the male pronucleus(Fig. 2.6); the tail detaches and degenerates. Morphologically, the maleand female pronuclei are indistinguishable, and eventually, they come intoclose contact and lose their nuclear envelopes (Fig. 2.7A). During growth ofmale and female pronuclei (both haploid), each pronucleus must replicate itsDNA. If it does not, each cell of the two-cell zygote has only half of the normalamount of DNA. Immediately after DNA synthesis, chromosomes organize onthe spindle in preparation for a normal mitotic division. The 23 maternal and23 paternal (double) chromosomes split longitudinally at the centromere, andsister chromatids move to opposite poles, providing each cell of the zygotewith the normal diploid number of chromosomes and DNA (Fig. 2.6, D andE ). As sister chromatids move to opposite poles, a deep furrow appears on thesurface of the cell, gradually dividing the cytoplasm into two parts (Figs. 2.6Fand 2.7B).The main results of fertilization are as follows: Restoration of the diploid number of chromosomes, half from the fatherand half from the mother. Hence, the zygote contains a new combinationof chromosomes different from both parents. Determination of the sex of the new individual. An X-carrying spermproduces a female (XX) embryo, and a Y-carrying sperm produces a male(XY) embryo. Hence, the chromosomal sex of the embryo is determinedat fertilization. Initiation of cleavage. Without fertilization, the oocyte usually degenerates24 hours after ovulation.
Conception: From Egg to EmbryoAt the moment when a lone sperm penetrates a mature egg, conception or fertilization takes place. To better understand the incredible process of conception, take a journey with us from tiny egg to growing embryo.
Thus, if all functions properly, only one sperm can enter the ovum since only one receptor can be activated.
The corona radiata surround an ovum or unfertilized egg cell, and consist of two or three strata (layers) of follicular cells. They are attached to the outer protective layer of the ovum , the zonapellucida, and their main purpose in many animals is to supply vital proteins to the cell.They are formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle, and originate from the squamous granulosa cells present at the primordial stage of follicular development. The corona radiata is formed when the granulosa cells enlarge and become cuboidal, which occurs during the transition from the primordial to primary stage. These cuboidal granulosa cells, also known as the granulosa radiata, form more layers throughout the maturation process, and remain attached to the zona pellucida after the ovulation of the Graafian follicle. For fertilization to occur, sperm cells rely on hyaluronidase (an enzyme found in the acrosome of spermatozoa) to disperse the corona radiata from the zona pellucida of the secondary (ovulated) follicle, thus permitting entry into the perivitelline space and allowing contact between the sperm cell and the nucleus of the oocyte.The corona radiata are layers of follicle cells, that protect the secondary oocyte as it passes through the ruptured follicular wall, on its way to the infundibulum of the uterine (AKA fallopian) tubes. In order for fertilization to occur, the sperm must break through this layer of follicular cells by secreting the enzyme hyaluronidase. It takes the secretions of dozens of sperm to weaken the layer enough for one sperm to penetrate.
OoplasmOoplasm (also: oöplasm) is the yolk of the ovum, a cell substance at its center, which contains its nucleus, named the germinal vesicle, and the nucleolus, called the germinal spot.The ooplasm consists of the cytoplasm of the ordinary animal cell with its spongioplasm and hyaloplasm, often called the formative yolk; and the nutritive yolk or deutoplasm, made of rounded granules of fatty and albuminoid substances imbedded in the cytoplasm.Mammalian ova contain only a tiny amount of the nutritive yolk, for nourishing the embryo in the early stages of its development only. In contrast, bird eggs contain enough to supply the chick with nutriment throughout the whole period of incubation
The zona pellucida (plural zonaepellucidae, also egg coat) is a glycoproteinmembrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the oocyte, external but of essential importance to it. The zona pellucida first appears in multilaminar primary oocytes. It is secreted by both the oocyte and the follicular cellsThis structure binds spermatozoa, and is required to initiate the acrosome reaction.In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes to be replaced by the underlying layer of trophoblastic cells.The zona pellucida is essential for oocyte death and fertilization.In some older texts, it has also been called zona striata and stratum lucidum
The cortical reaction occurs in fertilisation when a spermcell unites with the egg's plasma membrane, (zona reaction).This reaction leads to a modification of the zona pellucidathat blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.The cortical reaction is exocytosisof the egg's cortical granules. Cortical granules are secretory vesicles that reside just below the egg's plasma membrane. When the fertilizing sperm contacts the egg plasma membrane, it causes calcium to be released from storage sites in the egg, raising the intracellular free calcium concentration. This triggers fusion of the cortical granule membranes with the egg plasma membrane, liberating the contents of the granules to the extracellular space. Fusion begins near the site of sperm contact, and then as the wave of calcium release sweeps around the egg, a wave of cortical granule fusion results. The contents of the granules vary with the species, and are not fully understood
Deuterostomes (taxonomic term: Deuterostomia; from the Greek: "second mouth") are a superphylum of animals. They are a subtaxonof the Bilateriabranch of the subregnumEumetazoa, and are opposed to the protostomes. Deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth. Deuterostomes are also known as enterocoelomatesbecause their coelom develops through enterocoely.
FunctionTrophoblasts are specialised cells of the placenta that play an important role in embryo implantation and interaction with the decidualised maternal uterus. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetal circulation via the umbilical cord. This core is surrounded by two layers of trophoblast; a single layer of mononuclear cytotrophoblast that fuse together to form the overlying multinucleated syncytiotrophoblast layer that covers the entire surface of the placenta. It is this syncytiotrophoblast that is in direct contact with the maternal blood that reaches the placental surface, and thus facilitates the exchange of nutrients, wastes and gases between the maternal and fetal systems.
A morula (Latin, morus: mulberry) is an embryo at an early stage of embryonic development, consisting of cells (called blastomeres) in a solid ball contained within the zona pellucida.The morula is produced by embryonic cleavage, the rapid division of the zygote. Once the zygote has divided into 32 cells, it begins to resemble a mulberry, hence the name morula (Latin, morus: mulberry). Within a few days after fertilization, cells on the outer part of the morula become bound tightly together with the formation of desmosomes and gap junctions, becoming nearly indistinguishable. This process is known as compaction. The cells of the morula then secrete a viscous liquid[specify], causing a central cavity to be formed, forming a hollow ball of cells known as the blastocyst.The blastocyst's outer cells will become the first embryonic epithelium (the trophectoderm). Some cells, however, will remain trapped in the interior and will become the inner cell mass(ICM), and are pluripotent. In mammals (except monotremes), the ICM will ultimately form the "embryo proper", while the trophectoderm will form the placenta and other extra-embryonic tissues.
Ovulation Occurs in Two Phases THE FIRST PHASE of the ovulation cycle is the follicular phase. It begins the first day of the last menstrual period (LMP) and stops at the next ovulation. This phase varies greatly for each woman. THE SECOND PHASE is the luteal phase. It begins the day of ovulation and lasts until the next menstrual cycle begins. Governed by hormone release, it follows a more regular timeline of between 12 to 16 days following ovulation.
Where are the Stem Cells? The blastocyst embryo with its two cell types: the inner cells that can initiate all cells of the body, and the outer cells destined to become the placenta, is the source of embryonic stem cells (esc). Embryonic Stem Cells (esc) have the potential to become any type of human body cell. Stem cells are the only cells of the body which divide into two unique entities. Typically when cells divide they produce an identical cell or "sister" cell. But stem cells make a new stem cell AND a new body type cell. This property makes them extremely unique. Blastocysts only become available for use as stem cells when donated by couples using in-vitro fertilization (IVF) techniques. A woman in an IVF program who has multiple eggs fertilized in a petri dish, can choose to donate extra blastocysts to research. These stem cells are useful to children and adults needing interventions beyond conventional medicine.
WEEK 1 Fertilization Cleavage Formation of the blastocyst Differentiation into the Cytotrophoblast / Syncytiotrophoblast
Embryology Review Week 1 – Fertilized Egg – Day 1.25: Two Cells Cleavage Compaction – Day 3: Morula Inner Cell Mass Trophoblasts – Day 5: Blastocyst Cavitation Hatching – Day 6: Implantation
PHASES OF FERTILIZATION DEFINITIONSSets1.penetrate corona radiata,2. penetrate zona pellucida,3.penetrate oocyte membrane .1 passage of sperm through corona radiata 2. penetration of zona pellucida 3. fusion of plasma membranes of oocyte and sperm 4. completion of second meiotic division of oocyte, formation of female pronucleus 5. formation of male pronucleus 6. pronuclei fuse to form zygote
Fertilization Contact of sperm & secondary oocyte occurs in uterine tube – Sperm penetration – Oocyte completes meiosis II – Nuclear fusion
Fertilization: Sperm produces hyaluronidase to penetrate the follicular cell layer of the corona radiata (see diagram below). It will then interact with only one of many receptors. At a ZP3 receptor, the head of the sperm releases its contents (acrosin) and burrows through the zona pellucida and perivitelline space - the acrosomal reaction.
Fertilization Sperm penetration Why so many?
Passage Into the Fallopian Tube Once the egg is released from the ovary, it travels into the fallopian tube where it remains until a single sperm penetrates it during fertilization
The Laborious Journey of the Sperm An average ejaculate discharges 40-150 million sperm which eagerly swim upstream toward the fallopian tubes on their mission to fertilize an egg. Fast-swimming sperm can reach the egg in a half an hour, while other may take days. The sperm can live up to 48-72 hours. Only a few hundred will even come close to the egg, due to the many natural barriers and hurdles that exist in the female reproductive tract.
Fertilization: Sperm Penetrates Egg If a sperm cell meets and penetrates an egg, it will fertilize the egg. The fertilization process takes about 24 hours. it. At the moment of fertilization, the genetic makeup isWhen fertilization happens, changes occur on the surface of the egg to prevent othersperm from penetratingcomplete, including the sex of the infant.
Once a receptor has been activated, a series of reactions to prevent polyspermy (the entrance of more than one sperm).will be initiated. – First, the cell surface will be depolarized. – Then, cortical granules (lysosomes) released into the perivitelline space will hydrolyze the other receptors.
Fusion of the plasma membranes of the oocyte and sperm occurs; the sperm nucleus is released into the cytoplasm of the oocyte; the rest of the sperm degenerates Entrance of sperm into the oocyte causes the secondary oocyte to complete its second meiotic division (2 polar bodies at this point) Male pronucleus forms and swells; pronuclei membranes become porous
Both the male and female pronuclei are essential fornormal development. Evidence for this exists when one orthe other is absent, as in the case of a hydatiform mole.
CORONA RADIATAThe corona radiata surround an ovumorunfertilized egg cell, and consist of two orthree strata (layers) of follicular cells. Theyare attached to the outer protective layer ofthe ovum, the zona pellucida, and theirmain purpose in many animals is to supplyvital proteins to the cell.They are formed by follicle cells adhering to the oocyte before it leavesthe ovarian follicle, and originate from the squamous granulosa cellspresent at the primordial stage of follicular development.The corona radiata is formed when the granulosa cells enlarge and becomecuboidal, which occurs during the transition from the primordial to primarystage.These cuboidal granulosa cells, also known as the granulosa radiata, formmore layers throughout the maturation process, and remain attached to thezona pellucida after the ovulation of the Graafian follicle.
Cone of attraction andVitelline membrane Where the spermatozoon is about to pierce, the yolk (ooplasm) is drawn out into a conical elevation, termed the cone of attraction. Once the spermatozoon has entered, the peripheral portion of the yolk changes into a membrane, the vitelline membrane, which prevents the passage of additional
Acrosome reaction The acrosome reaction must occur to mobilise enzymes within the head of the spermatozoon to degrade the zona pellucida. example: hyaluronidase.
Cortical reaction Once the sperm cells find their way past the zona pellucida, the cortical reactionoccurs: cortical granules inside the secondary oocyte fuse with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glyco -proteins in the zona pellucida to cross-link with each other—that is, the enzymes cause the ZP2 to hydrolyseinto ZP2f—making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm.
Fusion After the sperm enters the cytoplasm of the oocyte, the cortical reaction takes place, preventing other sperm from fertilizing the same egg. The oocyte now undergoes its second meiotic division producing the haploid ovum and releasing a polar body. The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material.
capacitation To become competent to accomplish these tasks, ejaculated mammalian sperm must normally be modified by conditions in the female reproductive tract, a process called, which requires about 5–6 hours in humans. Capacitation is triggered by bicarbonate ions (HCO3–) in the vagina, which enter the sperm and directly activate a soluble adenylyl cyclase enzyme in the cytosol. The cyclase produces cyclic AMP, which helps to initiate the changes associated with capacitation. Capacitation alters the lipid and glycoprotein composition of the sperm plasma membrane, increases sperm metabolism and motility, and markedly decreases the membrane potential (that is, the membrane potential moves to a more negative value so that the membrane becomes hyperpolarized). Once a capacitated sperm has penetrated the layer of follicle cells, it binds to the zona pellucida The zona usually acts as a barrier to fertilization across species, and removing it often eliminates this barrier.
Cell membranes The cell membranesof the secondary oocyte and sperm fuse.
Transformations In preparation for the fusion of their genetic material both the oocyte and the sperm undergo transformations as a reaction to the fusion of cell membranes. The oocyte completes its second meiotic division. This results in a mature ovum. The nucleus of the oocyte is called a pronucleusin this process, to distinguish it from the nuclei that are the result of fertilization. The sperms tail and mitochondria degenerate with the formation of the male pronucleus. This is why all mitochondria in humans are of maternal origin.
Replication The pronuclei migrate toward the centre of the oocyte, rapidly replicating their DNA as they do so to prepare the embryo for its first mitotic division.
Mitosis The male and female pronuclei dont fuse, although their genetic material do. Instead, their membranes dissolve, leaving no barriers between the male and female chromosomes. During this dissolution, a mitotic spindle forms between them. The spindle captures the chromosomes before they disperse in the egg cytoplasm. Upon subsequently undergoing mitosis (which includes pulling of chromatids towards centrioles in anaphase) the cell gathers genetic material from the male and female together. Thus, the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes. Each of the two daughter cells resulting from that mitosis has one replica of each chromatid that was replicated in the previous stage. Thus, they are genetically identical.
Spindle microtubules are stained in green with anti- tubulin antibodies, and DNA is labeled in blue with a DNA stain. (A) A meiotic spindle in a mature, unfertilized oocyte. (B) This fertilized egg is extruding its second polar body and is shown about 5 hours after fusion with a sperm. The sperm head (left) has nucleated an array of microtubules. (C) The two pronuclei have come together. D) By 16 hours after fusion with a sperm, the centrosome that entered theegg with the sperm has duplicated, and the daughter centrosomes haveorganized a bipolar mitotic spindle.The chromosomes of both pronuclei are aligned at the metaphase plate of thespindle. As indicated by the arrows in (C) and (D), the sperm tail is associatedwith one of the centrosomes.
Summary Mammalian fertilization begins when the head of a sperm binds in a species-specific mannerto the zona pellucida surrounding the egg. This induces the acrosome reactionin the sperm, which releases the contents of its acrosomal vesicle, exposing enzymesthat help the spermto digest its way through the zonato the egg plasma membranein order to fuse with it. The fusion of the sperm with the egg induces a Ca2+ signal in the egg. The Ca2+ signal activates the egg to undergo the cortical reaction, in which cortical granules release their contents, including enzymes that alter the zona pellucida and thereby prevent the fusion of additional sperm. The Ca2+ signal also triggers the development of the zygote, which begins after sperm and egg haploidpronuclei have come together, and their chromosomes have aligned on a single mitotic spindle, which mediates the first division of the zygote.
Diseases Various disorders can arise from defects in the fertilization process. Polyspermy results from multiple sperm fertilizing an egg. However, some researchers have found that in rare pairs of fraternal twins, their origin might have been from the fertilization of one egg cell from the mother and two sperm cells from the father. This possibility has been investigated by computer simulations of the fertilization process.
cleavage In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total or entire cleavage) or meroblastic (partial cleavage). The pole of the egg with the highest concentration of yolk is referred to as the vegetal pole while the opposite is referred to as the animal pole. Cleavage differs from other forms of cell division in that it increases the number of cells without increasing the mass. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases.
The Cells Begin to Divide The fertilized egg begins dividing rapidly, growing into many cells. It leaves the fallopian tube and enters the uterus three to four days after fertilization. Rarely, the fertilized egg does not leave the fallopian tube; this is called a tubal pregnancy or ectopic pregnancy and is a danger to the mother.
Cleavage•Following mitosis, two blastomeresform, each one totipotent (capable offorming its own organism)•Continuing along the uterine tube,the zygote divides to four, then eightcells•Size of individual blastomeresdecreases with progressive divisionsas zygote maintains size•A ball of 12 or more cells(32), themorula, enters the uterus around day3
Mechanism The rapid cell cycles are facilitated by maintaining high levels of proteins that control cell cycle progression such as the cyclins and their associated cyclin-dependent kinases (cdk). The complex CyclinB /cdc2 a.k.a. MPF (maturation promoting factor) promotes entry into mitosis. The processes of karyokinesis (mitosis) and cytokinesis work together to result in cleavage. The mitotic apparatus is made up of a central spindle and polar asters made up of polymers of tubulinprotein called microtubules.
The asters are nucleated by centrosomes and the centrosomes are organized by centrioles brought into the egg by the sperm as basal bodies. Cytokinesis is mediated by the contractile ring made up of polymers of actinprotein called microfilaments. Karyokinesis and cytokinesis are independent but spatially and temporally coordinated processes. While mitosis can occur in the absence of cytokinesis, cytokinesis requires the mitotic apparatus. The end of cleavage coincides with the beginning of zygotic transcription. This point is referred to as the midblastula transition and appears to be controlled by the nuclear: cytoplasmic ratio (about 1/6).
Types of cleavageDETERMINATE Determinate is the form of cleavage in most protostomes. It results in the developmental fate of the cells being set early in the embryo development. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete embryo.
INDETERMINATE A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes - when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.
HOLOBLASTIC In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells (cells with a small even distribution of yolk) or in mesolecithal cells (moderate amount of yolk in a gradient) – – bilateral holoblastic, – radial holoblastic, – rotational holoblastic, and – spiralholoblastic, cleavage.T These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.
MEROBLASTIC In the presence of a large amount of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage. Two major types of meroblastic cleavage are discoidal and superficial. – DISCOIDAL In discoidal cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blastodisc, on top of the yolk. Discoidal cleavage is commonly found in monotremes, birds, reptiles, and fish that have telolecithal egg cells (egg cells with the yolk concentrated at one end). Superficial – I SUPERFICIAL CLEAVAGE, mitosis occurs but not cytokinesis, resulting in a polynuclear cell. With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. Superficial cleavage occurs in arthropods that have centrolecithalegg cells (egg cells with the yolk located in the center of the cell).
BLASTOCYST The is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid- filled blastocyst cavity known as the blastocoele or the blastocystic cavity. The human blastocyst comprises 70-100 cells. Blastocyst formation begins at day 5 after fertilization in humans, when the blastocoele opens up in the morula, a process known as hatching.
Formation of the Blastocyst Compaction: Blastomeres clump together; the amount of cytoplasm is reduced Adhesion: E- cadherin gene is expressed; these proteins facilitate intercellular adhesion and communication Zona pellucida is shed, allowing for cell growth and for uterine fluid to infiltrate A fluid-filled cavity ( blastocoele) forms in the center, pushing cells to the periphery The outer cell layer, the trophoblast ("tropho" - to nourish), may have a nutritive role as the future embryonic part of the placenta The inner cell mass, the future embryo, forms as a ball of cells on one side of the spherical blastocyst
Parts of the blastocyst Formation of the blastocyst
Parts of the blastocyst The blastocyst consists of two primary cell types: the inner cell mass, also known as the "embryoblast" (this part of the embryo is used in stem cell research) the trophoblast, a layer of cells surrounding the inner cell mass and the blastocyst cavity (blastocoele) The former is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The latter combines with the maternal endometrium to form the placenta in eutherian mammals
Inner cell mass In early embryogenesisof most eutherian mammals, the inner cell mass (abbreviated ICMand also known as the embryoblast or pluriblast, the latter term being applicable to all mammals) is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus. This structure forms in the earliest steps of development, before implantation into the endometrium of the uterus has occurred. The ICM lies within the blastocoele(more correctly termed "blastocyst cavity," as it is not strictly homologous to the blastocoele of anamniote vertebrates) and is entirely surrounded by the single layer of cells called trophoblast
Trophoblasts (from Greek trephein:to feed, and blastos: germinator)are cells forming the outer layer of ablastocyst, which provide nutrients to theembryo and develop into a large part ofthe placenta.They are formed during the first stage ofpregnancyand are the first cells todifferentiate from the fertilized egg. Thislayer of trophoblasts is also collectivelyreferred to as "the trophoblast",or, aftergastrulation,the trophectoderm, as it isthen contiguous with the ectoderm of theembryo.am. Amniotic cavity.b.c. Blood- clot.b.s. Body-stalk.ect. Embryonic ectoderm.ent.Entoderm.mes. Mesoderm.m.v. Maternal vessels.tr. Trophoblast.u.e. Uterineepithelium.u.g. Uterine glands.y.s. Yolk-sac.
A blastocoel(e) or blastocele (alsocalled blastocyst cavity,cleavagecavity or segmentation cavity) is the fluid-filled central region of a blastocyst. A blastocoele forms during embryogenesis when a zygote (a fertilized ovum) divides into many cells through mitosis. A blastocoel can be described as the first cell cavity formed as the embryo enlarges. It is essential for later gastrulation.
Formation of the blastocyst The morula is a solid ball of about 16 undifferentiated, spheri cal cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other. This is called compaction and is likely mediated Blastulation. 1 - morula, by cell surface adhesion 2 - blastula. glycoproteins.
First stages of segmentation of a fertilized mammalian ovum. Semidiagrammatic. z.p. Zona pellucida. p.gl. Polar bodies.a. Two-cell stage.b. b. Four-cell stage.c. c. Eight-cell stage.d. d, e. Morula stage
Implantation After entering the uterus, the fertilized egg attaches to the uterine lining, or endometrium. This process is called implantation. The cells continue to divide.
Differentiation into theCytotrophoblast/Syncytiotrophoblast(day 7) Near the end of the first week, the blastocyst implants in the posterior wall of the uterus in the presence of high hormone levels; the blastocyst attaches at the embryonic pole (inner cell mass) so the embryo will eventually be attached dorsally. Occasionally, the blastocyst will implant on another spot in the uterus, potentially causing problems (placenta previa).
Early Development Implantation – 6-7 days after fertilization – Trophoblast forms placenta – ―Inner cell mass‖ forms embryo
Pregnancy Hormones Human chorionic gonadotropin (hCG) is a hormone present in the blood within about a week of conception. It is the hormone detected in a blood or urine pregnancy test, but it usually takes three to four weeks for levels of hCG to be high enough to be detected by pregnancy tests. It is secreted by cells that develop into the placenta.