Implantation And Uterine Receptivity Flashcards
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- Implantation and Uterine Receptivity
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A. Zona pellucida Hatching: Before the blastocyst can implant
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it must escape from the ZP
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be achieved by a combination of physical and chemical forces. Physical forces result from blastocyst
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expansion due to both cellular hyperplasia and fluid accumulation in the blastocoel. Less clear is the
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potential role of embryo and/or uterine-derived proteases in ZP thinning and blastocyst escape. It may
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be that enzymatic degradation of the ZP is species specific. The ability of the human blastocyst to
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escape from its ZP in vitro would suggest that a uterine-derived enzyme in this species is not
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mandatory. A cascade of enzymes is suggested to be important for mouse and bovine ZP escape and
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involves the conversion of plasminogen to plasmin by embryo-derived plasminogen activator
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(Cannon and Menino
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1998). Plasmin appears to have ZP proteolytic activity and may be a
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contributor to ZP escape. Failure of the blastocyst to hatch from the zona pellucida has been identified
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as a potential cause of implantation failure in ART cycles. Assisted zona hatching may be clinically
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useful for patients with previous failed cycles
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poor embryo quality and women aged 38 years and older
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(Practice Committees of SART and ASRM
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2006). However
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previous failed cycle may have a better prognosis following hatching in a second treatment cycle than
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those who had suboptimal embryos.
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B. Blastocyst- Endometrium Interaction: The hatched blastocyst must attach to the endometrium. The
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maternal component (uterus) must provide a suitable environment to sustain embryo development.
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The embryonic component must complete the following:
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signal to prevent corpus luteum regression
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produce the pregnancy hormone
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beta-human chorionic gonadotropin (BhCG)
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> maintain uterine secretory activity and regulate blood flow
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achieve the immunologic privilege of the fetal allograph (the embryo inherits paternally derived
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components that would otherwise be immunologically recognized as “non-self” and
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degraded/rejected).
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1. Endometrial receptivity: Embryo implantation involves direct interaction of the blastocyst with
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the luminal epithelium of the receptive uterus. Endometrial receptivity is generally believed to last
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for about 4 days in the human (menstrual cycle days 20 to 24 in a 28 day cycle) and has been
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termed the implantation window
although the duration may be shorter and advanced in IVF
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cycles. Indeed
the timing of decidualization may be altered in patients with increased
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progesterone levels during ovarian stimulation. There is also an ongoing debate as to the impact
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of supraphysiological estrogen levels on endometrial function during IVF. It is likely that the effects
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of ovarian stimulation will have a patient- and cycle-dependent impact on endometrial function that
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includes embryo viability
ovarian stimulation outcomes
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myomas
and endometriosis. Successful implantation requires the appropriately timed arrival
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of a viable blastocyst intoa receptive endometrium. Initial attachment is believed to involve a
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receptor-ligand like interaction.
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2. Embryo survival: is dependent upon the precise coordination of its early development and
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transport with the changing receptivity of the uterus. Implantation involves the following steps:
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Transport of the embryo: the blastocyst arrives in the uterus.
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Orientation: the ICM is oriented towards endometrial epithelial lining.
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Hatching: the blastocyst hatches out of zona pellucida.
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> Apposition: the blastocyst in close contact with endometrial lining but not connected.
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> Adhesion: Connections of an unknown nature are established between the embryo and the
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endometrial epithelium. The main purpose of implantation is to ensure trophoblast cells
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anchor firmly into endometrial stroma. The highly invasive nature of human embryo
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implantation compared to other mammals has limited the transposition of data from animal
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model systems to the human implantation process.
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Invasion: Thin folds of trophectodermal cells intrude between the endometrial epithelial cells.
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> Digestion: Integrins
at the tips of the invadopodia (cells at the tips of the anchoring villi)
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anchor the trophoblast to the basement membrane
expanding the placental bed and
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simultaneously remodeling maternal spiral arteries.
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Syncytialization: Fusion of some trophectoderm cells (TE) to form a multinucleate syncytium
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that proliferates and invades the endometrial extracellular matrix. Syncytium formation is an
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exceptional process that takes place only in a few cell types; myoblast
osteoclast
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and trophoblasts. This invasive syncytium emerges during or soon after the trophoblast
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passes through the breached uterine epithelium into the decidualized stromal layer beneath.
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Its role in the implantation process appears to be hollowing out regions within the stroma
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creating blood pools that fill with fluid and cells from maternal blood and uterine glands. These
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"nutrient pools," in turn
stimulate the growth of new capillaries leading to the establishment of
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a placental circulation system. The transition from histiotropic nutrition (passive exchange of
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material between the mother and the embryo) to hemotropic nutrition is critical for survival of
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the fetus. The syncytiotrophoblast (STB) is responsible for nutrient and gas exchange in the
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human placenta.
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Villous Formation:After approximately 12 days of gestation the blastocyst has now invaded
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and sunk below the endometrial surface. Strands of cytotrophoblast (former TE) begin to form
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and penetrate through the primitive syncytium to form primary chorionic villi. These structures
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are subsequently invaded by extra-embryonic mesoderm to form secondary and tertiary villi.
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The cytotrophoblast cells associated with the villi continue to divide and provide a progenitor
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cell population for the villous STB. This process continues throughout pregnancy and is
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essential to maintain pregnancy viability. The villous STB is also the major site for production
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of placental hormones.
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The implantation process is completed 7 to 12 days after fertilization. The embryonic ICM divides
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rapidly forming a two-layered disc.
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3. Endometrial changes and biomarkers of implantation: The window of imp.
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by the inhibition of uterine epithelial proliferation
structural epithelial cell remodeling
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attenuated estrogen (E2) response. These changes occur via paracrine signaling between the
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uterine epithelium and stroma. While the steroid hormones
E2 and progesterone (P4)
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many of these changes
numerous cell adhesion molecules
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cadherins
are expressed by the endometrium and appear to be necessary for successful
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interaction of the embryo with the endometrium. Specifically
the endometrial integrin avB3 has
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been suggested to function as a cell-adhesion receptor-like molecule important for initial
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blastocystlendometrial interactions. avB3 and its extracellular matrix ligand
osteopontin (OPN)
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are two well-characterized endometrial receptivity biomarkers that are maximally expressed in
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human endometrial epithelial cells during the implantation window. This observation
together with
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detection of OPN secretion into the uterine cavity at this time
suggest these substrates play a role
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in the regulation of endometrial function and embryo implantation (Apparao et al.
2001; von Wolff
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et al.
2001). The bone mnorphogenetic proteins (BMPs) are also major regulators of the
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pathways that control pregnancy. Many studies have contributed to establishing a role for various
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signaling pathways in the implantation process (Zhang et al
2013); however
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are required to establish whether these pathways function independently or converge into larger
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networks. There is also evidence that women with various benign gynecologic disorders exhibit
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decreased uterine receptivity and abnormal expression of endometrial biomarkers (Donaghay and
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Lessey
2007). Implantation defects are a major cause of infertility in women; thus
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to identify the uterine signaling pathways involved and an understanding of their regulation will
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improve infertility interventions.
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ned
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4. Embryonic biomarkers:
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Human leukocyte antigen G (HLA-G) has been postulated to have a pivotal role during the
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implantation process and measurement of its expression may provide a future method of
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embryo selection (Kotze et al.
2013).
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Beta human chorionic gonadotropin (BhCG) can be detected in the maternal serum as early
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as 8 days after conception
and its serum level increases dynamically during early gestation.
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The doubling time of BhCG levels in viable pregnancies following IVF or spontaneous
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pregnancy s slightly less than 2 days and has been shown to be an important variable in
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pregnancy outcome (Shamonki et al.
2009). Abnormal conceptions are associated with
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significantly slower doubling times
although a range also exists for viable pregnancies that
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may have an initial BhcG doubling time that is slightly longer than 2 days. Increasing oocyte
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age has also been shown to impact pregnancy viability for a given initial BhCG level. A clinical
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pregnancy can be identified using ultrasound by the observation of an intrauterine pregnancy
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sac and fetal cardiac activity at approximately 6-8 weeks' gestation.
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C. Stem cells
Research Models
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Our understanding of the stages of gamete maturation
fertlization
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implantation is far from complete and in many cases has been restricted by the lack of suitable material
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with which to conduct experimental studies
as well as methods to correct the errors found. As
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mentioned above
the study of early embryonic development has been greatly restricted due the
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ethical restraints on the use of human embryos for research. Moreover
in vivo
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within the female reproductive tract.
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1. Embryonic stem cells: One of the first breakthroughs came with the derivation of pluripotent
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human embryonic stem cells (hESCs) from the ICM of blastocysts. These cells can be used to
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generate model systems in vitro with which to study key events of cellular remodeling and cell
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substrates involved in the regulation of these developmental processes
early human
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morphogenesis including segregation of the pluripotent embryonic and extra-embryonic lineages
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and morphogenetic re-arrangements (Shahbazi et al.
2016). hESCs have also been stimulated to
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differentiate into trophoblast to form an experimental structure that represents syncytial tissue
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encountered at the initiation of placental development (Yabe et al.
2016). hESCs also provide a
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reservoir of cells with significant therapeutic potential.
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2. Adult stem cells: These are undifferentiated cells (somatic stem cells) found throughout the body
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in children and adults that divide to replenish dying cells and regenerate damaged tissues. Aduit
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stem cells have the ability to divide or self-renew indefinitely and generate all the cell types of the
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organ from which they originate
potentialy regenerating the entire organ from a few cells. Unlike
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embryonic stem cells
their use in research and therapy is not controversial because they can be
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isolated from a tissue sample. While adult stem cells have the ability to differentiate into more than
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one cell type
they are often restricted to certain lineages
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cell treatments have been used for many years to treat leukemia and related bone/blood cancers
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successfully through bone marrow transplants
although harvesting and or culturing them up to the
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numbers required can be challenging.
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3 Ovarian stem cells: The accepted dogma is that in the human all oogonia enter meiosis during
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fetal development
that the complement of follicles is laid down prior to birth and no germ cells
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remain in the post-natal ovary. However
this dogma has been debated for many years
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last two decades cells have been isolated first in murine (White et al.
2012) and subsequently in
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human (Hummitzsch et al.
2015) post natal ovary that express both germline and stem cells
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markers. While the body of evidence for the presence of ovarian "stem cells" is mounting
opinion
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remains divided with regard to the existence
significance
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stem cells. Indeed
isolation of these cells is challenging
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existence. Data regarding their developmental potential is limited
but the demonstration of the
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production of functional murine oocytes from induced pluripotent stem cells (Morohaku et al.
2016)
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and the advances in ovarian follicle cuture techniques provide a basis for further study. Moreover
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adult stem cells have been identified in most organ systems
raising the likelihood that the ovary
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contains stem cells for component cell types (Telfer and Anderson
2019).
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4. Nuclear reprogramming: Nuclear reprogramming is a procedure that causes changes in gene
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expression that allow a cell of one type to develop into a cell of another type
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cells
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can be forced to become pluripotent by nuclear reprogramming. An exa 80/154
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nuclear transfer. This consists of injection of the nucleus of a somatic ce.
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oocyte. The resulting pluripotent cells are genetically matched with the cell donor (this technique is
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thereby often called "therapeutic cloning")
except for the mitochondrial DNA
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the egg. Cells with some stem cell characteristics can also be produced by means of cell fusion
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with a human embryonic stem cell.
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ated
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5. Induced pluripotent stem cells (iPSCs): were first reported in 2006 using mouse fibroblasts.
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iPSCs can be generated from differentiated cels by using retroviral-mediated expression of core
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transcription factors known to be required for maintenance of pluripotency and proliferation of
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embryonic stem cells. iPSCs exhibit similar features to embryonic stem cells
including cell
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morphology
cell-surface markers
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epigenetic marks of pluripotent cell-specific genes
but lack global gene expression signatures.
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iPSCs can give rise to cells derived from all 3 germ layers in vitro and in vivo
and murine iPSCs
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injected into murine blastocysts have been shown to contribute to embryonic development (Brignier
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and Gewirtz
2010).
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6. Mitochondrial transfer: has recently emerged to prevent the transmission of genetic disorders in
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mitochondrial DNA. Mitochondrial DNA has a circular structure and contains 37 genes. Several
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hundred rRNA/tRNA mutations and coding/control region point mutations have been identified
and
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mitochondrial disorders affect at least 1 in 5
000 births in the United States. Since mitochondria are
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inherited solely from the oocyte
damaged mitochondrial DNA will be transmitted to all offspring.
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causing disease varying in severity depending on the proporticon of healthy and damaged
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mitochondria the child randomly inherits. Disruption of essential metabolic pathways in persons
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suffering from mtDNA disease
especially in high energy-demanding organs
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disability and early death. With mitochondrial transfer
the damaged mitochondria in the prospective
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mother's 00cyte is replaced with healthy mitochondria froma donor oocyte This technique is highly
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controversial
as it combines DNA from three individuals (male
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generate a so-called "three-parent baby." Zhang and co-workers reported a live birth following the
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use of "mitochondrial replacement therapy' (MRT) where meiotic spindles from the carrier
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oocytes were transferred into the ooplasm of enucleated donor metaphase l oocytes (Zhang et
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al.
2017). Another approach proposed for removal of damaged mtDNA is to perform pronuclear
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transfer
although this technique is associated with even greater ethical concerns as it leads to
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zygotes being discarded. In 2015
the United Kingdom's Human Fertilisation and Embryology Act
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(the HFE Act) approved the use of mitochondrial donation techniques as part of in vitro
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fertilization ((VF) treatments. MRT is not currently approved in the United States.
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Considerations regarding its use fall under the oversight of the US Food and Dr
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(FDA)
its approval would require clinical trials under an investigational new dn 817 154 1
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all work has been halted currently.
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7. CRISPRICas9: More recently
genome-editing tools
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short palindromic repeat (CRISPR)-associated system (Cas) have been used to modify genes in
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model systems
including animal zygotes and human cells with potential benefit for basic research.
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This system has proven itself easy
expedient
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of ways that include genome editing
gene function investigation
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human cells. The hope is that this type of approach will be able to edit the genome in human
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embryos to correct genes that cause potentially fatal disease states
or modify DNA as a step
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toward preventing inherited diseases. Tests of the CRISPRICas9 in human tri-pronuclear zygotes
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showed efficient cleavage of endogenous genes and repair of the double stranded breaks
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generated. However
the repair template could be either the endogenous homologous gene or
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exogenous DNA sequence
which complicated the analysis of possible gerne editing outcomes and
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made it difficult to predict the consequence of gene editing. Furthermore
mosaicism and mutations
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at non-target sites were apparent in the edited embryos
underscoring the need to gain a more
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comprehensive understanding of the mechanisms of CRISPR/Cas9-mediated genome editing in
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human cells before its therapeutic application (Liang et al.
2015). Despite these reservations
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babies were born in China in late 2018 following CRIPSR genome editing
causing great concern
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to the potentially untested and premature use of the technique (Lovell-Badge
2019). This is a
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rapidly evolving field with much potential gain for removal of life threatening diseases and will
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remain the focus of current research strategies to
hopefully
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use (Cryanowski
2019).
