Human fertilization
Human fertilization | |
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Details | |
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Precursor | Gametes |
Gives rise to | Zygote |
Anatomical terminology |
Part of a series on |
Human growth and development |
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Development and psychology |
Human fertilization is the union of an
The process of
In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitro.
History
In Antiquity, Aristotle depicted the formation of new individuals occurring through fusion of male and female fluids, with form and function emerging gradually, in a mode called by him as epigenetic.[5].
Sperm and oocyte meet
Ampulla
Fertilization occurs in the ampulla of the
Sperm preparation
At the beginning of the process, the sperm undergoes a series of changes, as freshly ejaculated sperm is unable or poorly able to fertilize.[8] The sperm must undergo capacitation in the female's reproductive tract, which increases its motility and hyperpolarizes its membrane, preparing it for the acrosome reaction, the enzymatic penetration of the egg's tough membrane, the zona pellucida, which surrounds the oocyte.[9]
Corona radiata
The sperm binds through the corona radiata, a layer of follicle cells on the outside of the secondary oocyte. The corona radiata sends out chemicals that attract the sperm in the fallopian tube to the oocyte. It lies above the zona pellucida, a membrane of glycoproteins that surrounds the oocyte.[10]
Cone of attraction and perivitelline membrane
Where the spermatozoan is about to pierce, the yolk (
Zona pellucida and acrosome reaction
After binding to the corona radiata the sperm reaches the zona pellucida, which is an extracellular matrix of glycoproteins. A ZP3 glycoprotein on the zona pellucida binds to a receptor on the cell surface of the sperm head. This binding triggers the acrosome to burst, releasing acrosomal enzymes that help the sperm penetrate through the thick zona pellucida layer surrounding the oocyte, ultimately gaining access to the egg's cell membrane.[12]
Some sperm cells consume their acrosome prematurely on the surface of the egg cell, facilitating the penetration by other sperm cells. As a population, mature haploid sperm cells have on average 50% genome similarity, so the premature acrosomal reactions aid fertilization by a member of the same cohort.[13] It may be regarded as a mechanism of kin selection.
Recent studies have shown that the egg is not passive during this process. In other words, they too appear to undergo changes that help facilitate such interaction.[14][15]
Fusion
Cortical reaction
After the sperm enters the cytoplasm of the oocyte, the tail and the outer coating of the sperm disintegrate. The fusion of sperm and oocyte membranes causes
Fusion of genetic material
Preparation
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
The sperm's tail and
Fusion
The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material.
Blocks of polyspermy
When the sperm enters the perivitelline space, a sperm-specific protein Izumo on the head binds to Juno receptors on the oocyte membrane.[20] Once it is bound, two blocks to polyspermy then occur. After approximately 40 minutes, the other Juno receptors on the oocyte are lost from the membrane, causing it to no longer be fusogenic. Additionally, the cortical reaction will happen which is caused by ovastacin binding and cleaving ZP2 receptors on the zona pellucida.[21] These two blocks of polyspermy are what prevent the zygote from having too much DNA.
Replication
The pronuclei migrate toward the center of the oocyte, rapidly replicating their
Mitosis
Usually 23 chromosomes from
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.[citation needed]
Fertilization age
Fertilization is the event most commonly used to mark the beginning point of life, in descriptions of prenatal development of the embryo or fetus.[24] The resultant age is known as fertilization age, fertilizational age, conceptional age, embryonic age, fetal age or (intrauterine) developmental (IUD)[25] age.
The average time to birth has been estimated to be 268 days (38 weeks and two days) from ovulation, with a standard deviation of 10 days or coefficient of variation of 3.7%.[29]
Fertilization age is sometimes used postnatally (after birth) as well to estimate various risk factors. For example, it is a better predictor than postnatal age for risk of intraventricular hemorrhage in premature babies treated with extracorporeal membrane oxygenation.[30]
Diseases affecting human fertility
Various disorders can arise from defects in the fertilization process. Whether that results in the process of contact between the sperm and egg, or the state of health of the biological parent carrying the zygote cell. The following are a few of the diseases that can occur and be present during the process.
- Polyspermy results from multiple sperm fertilizing an egg, leading to an offset number of chromosomes within the embryo.[31] Polyspermy, while physiologically possible in some species of vertebrates and invertebrates, is a lethal condition for the human zygote.
- androgens. This results in the ovulation period between contact of the egg being postponed or excluded.[32]
- Autoimmune disorders can lead to complications in implantation of the egg in the uterus, which may be the immune system's attack response to an established embryo on the uterine wall.[32]
- Cancer ultimately affects fertility and may lead to birth defects or miscarriages. Cancer severely damages reproductive organs, which affects fertility.[32]
- Endocrine system disorders affect human fertility by decreasing the body's ability to produce the level of hormones needed to successfully carry a zygote. Examples of these disorders include diabetes, adrenal disorders, and thyroid disorders.[32]
- Endometriosis is a condition that affects women in which the tissue normally produced in the uterus proceeds to grow outside of the uterus. This leads to extreme amounts of pain and discomfort and may result in an irregular menstrual cycle.[32]
See also
- Development of the human body
- Spontaneous conception, the unassisted conception of a subsequent child after prior use of assisted reproductive technology
References
- ISBN 978-3-662-02817-9.[page needed]
- ^ Garrison FH (1921). An Introduction to the History of Medicine. Saunders. pp. 566–567.
- ^ "Go Ask Alice!: Pregnant without intercourse?". Archived from the original on 2011-12-22. Retrieved 2016-01-24.
- ISBN 978-1-85941-159-9.
Pregnancy is well known to occur from such external ejaculation ...
- ISBN 978-3-319-53258-5.
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- ^ "Fertilization". Archived from the original on 24 June 2010. Retrieved 28 July 2010.
- PMID 30105226.
- ^ Miles, Linda. "LibGuides: BIO 140 - Human Biology I - Textbook: Chapter 45 - Fertilization". guides.hostos.cuny.edu. Retrieved 2022-11-28.
- ^ "Fertilization of the Ovum". Gray's Anatomy. Archived from the original on 2010-12-02. Retrieved 2010-10-16.
- ^ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). "Fertilization". Molecular Biology of the Cell. 4th Edition.
- ^ Angier N (2007-06-12). "Sleek, Fast and Focused: The Cells That Make Dad Dad". The New York Times. Archived from the original on 2017-04-29.
- ISBN 978-0-309-04136-2.
- ISBN 978-0-08-050836-8.
- ISBN 9780128151457, retrieved 2022-11-28
- ^ "Fertilization: The Cortical Reaction". Boundless. Boundless. Archived from the original on 10 April 2013. Retrieved 14 March 2013.
- ^ "Prefertilization Events". www.med.umich.edu. Retrieved 2022-11-28.
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- ^ ISBN 978-0-8053-4989-4.
- ^ "Five Facts about XX or XY | Gender Selection Authority". Archived from the original on 2016-10-06. Retrieved 2016-07-31.
- ^ "BRIEF OF BIOLOGISTS AS AMICI CURIAE IN SUPPORT OF NEITHER PARTY" (PDF). United States Supreme Court. Retrieved 20 April 2023.
- PMID 15744391.
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- ^ S2CID 29063110.
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- .
- ISBN 978-0-323-14843-6.
- ^ a b c d e "Diseases That Cause Infertility". Center of Reproductive Medicine. Retrieved 6 March 2021.