Editing Fertilisation (section)

== Fertilisation in animals ==
{{see also|Sexual reproduction in animals|Animal sexual behaviour}}
The mechanics behind fertilisation has been studied extensively in sea urchins and mice.  This research addresses the question of how the [[spermatozoon|sperm]] and the appropriate egg find each other and the question of how only one sperm gets into the egg and delivers its contents. There are three steps to fertilisation that ensure species-specificity:

# Chemotaxis
# Sperm activation/acrosomal reaction
# Sperm/egg adhesion

=== Internal vs. external ===
Consideration as to whether an animal (more specifically a vertebrate) uses [[Internal fertilization|internal]] or [[external fertilization|external fertilisation]] is often dependent on the method of birth. [[Oviparity|Oviparous]] animals laying eggs with thick calcium shells, such as [[chickens]], or thick leathery shells generally reproduce via internal fertilisation so that the sperm fertilises the egg without having to pass through the thick, protective, tertiary layer of the egg. [[Ovoviviparity|Ovoviviparous]] and [[viviparity|viviparous]] animals also use internal fertilisation. Although some organisms reproduce via [[amplexus]], they may still use internal fertilisation, as with some salamanders. Advantages of internal fertilisation include minimal waste of gametes, greater chance of individual egg fertilisation, longer period of egg protection, and selective fertilisation. Many females have the ability to store sperm for extended periods of time and can fertilise their eggs at their own desire.{{fact|date=August 2021}}

Oviparous animals producing eggs with thin tertiary membranes or no membranes at all, on the other hand, use external fertilisation methods. Such animals may be more precisely termed ovuliparous.<ref>{{cite book |last1=Lodé |first1=Thierry |title=Les stratégies de reproduction des animaux |trans-title=Reproduction Strategies in Animal Kingdom |language=fr |date=2001 |publisher=Dunod |isbn=978-2-10-005739-9 }}{{pn|date=August 2021}}</ref> External fertilisation is advantageous in that it minimises contact (which decreases the risk of disease transmission), and greater genetic variation.

=== Sea urchins ===
[[File:Acrosome reaction diagram en.svg|thumb|400px|Acrosome reaction on a sea urchin cell.]]

Sperm find the eggs via [[chemotaxis]], a type of ligand/receptor interaction. Resact is a 14 amino acid peptide purified from the jelly coat of ''A. punctulata'' that attracts the migration of sperm.

After finding the egg, the sperm penetrates the [[egg jelly|jelly coat]] through a process called sperm activation. In another ligand/receptor interaction, an oligosaccharide component of the egg binds and activates a receptor on the sperm and causes the [[acrosomal reaction]]. The acrosomal vesicles of the sperm fuse with the plasma membrane and are released. In this process, molecules bound to the acrosomal vesicle membrane, such as bindin, are exposed on the surface of the sperm.  These contents digest the jelly coat and eventually the vitelline membrane. In addition to the release of acrosomal vesicles, there is explosive polymerisation of [[actin]] to form a thin spike at the head of the sperm called the [[acrosomal process]].

The sperm binds to the egg through another ligand reaction between receptors on the [[vitelline membrane]]. The sperm surface protein bindin, binds to a receptor on the vitelline membrane identified as EBR1.

Fusion of the plasma membranes of the sperm and egg are likely mediated by bindin. At the site of contact, fusion causes the formation of a [[fertilization cone|fertilisation cone]].

=== Mammals ===
Male [[mammal]]s internally fertilise females and [[ejaculation|ejaculate]] [[semen]] through the [[penis]] during [[copulation (zoology)|copulation]].<ref name="Hyman1992">{{cite book |author=Libbie Henrietta Hyman |url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583 |title=Hyman's Comparative Vertebrate Anatomy |date=15 September 1992 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=583–}}</ref><ref>{{Cite book |last=Naguib |first=Marc |url=https://books.google.com/books?id=KgTeDwAAQBAJ&pg=PA65 |title=Advances in the Study of Behavior |date=2020-04-19 |publisher=Academic Press |isbn=978-0-12-820726-0 |language=en}}</ref> After ejaculation, many sperm move to the upper vagina (via contractions from the vagina) through the [[cervix]] and across the length of the [[uterus]] to meet the ovum. In cases where fertilisation occurs, the female usually [[ovulation|ovulates]] during a period that extends from hours before copulation to a few days after; therefore, in most mammals, it is more common for ejaculation to precede ovulation than vice versa.{{Citation needed|date=July 2024}}

When sperm are deposited into the anterior vagina, they are not capable of fertilisation (i.e., non-capacitated){{clarification needed|reason=statements appear contradictory|date=January 2024}} and are characterised by slow linear motility patterns. This motility, combined with muscular contractions enables sperm transport towards the uterus and [[oviduct]]s.<ref>{{Cite journal|title=Sperm transport in the female reproductive tract|journal=Human Reproduction Update|volume=12|issue=1|pages=23–37|doi=10.1093/humupd/dmi047|pmid=16272225|year=2006|last1=Suarez|first1=S.S.|last2=Pacey|first2=A. A.|doi-access=free}}</ref> There is a pH gradient within the micro-environment of the female reproductive tract such that the pH near the vaginal opening is lower (approximately 5) than the oviducts (approximately 8).<ref>{{cite journal |last1=Ng |first1=Ka Ying Bonnie |last2=Mingels |first2=Roel |last3=Morgan |first3=Hywel |last4=Macklon |first4=Nick |last5=Cheong |first5=Ying |title=In vivo oxygen, temperature and pH dynamics in the female reproductive tract and their importance in human conception: a systematic review |journal=Human Reproduction Update |date=1 January 2018 |volume=24 |issue=1 |pages=15–34 |doi=10.1093/humupd/dmx028 |pmid=29077897 |doi-access=free }}</ref> The sperm-specific pH-sensitive calcium transport protein called CatSper increases the sperm cell permeability to calcium as it moves further into the reproductive tract. Intracellular calcium influx contributes to sperm capacitation and hyperactivation, causing a more violent and rapid non-linear motility pattern as sperm approach the oocyte. The [[capacitation|capacitated]] spermatozoon and the oocyte meet and interact in the ''ampulla'' of the [[fallopian tube]]. Rheotaxis, thermotaxis and chemotaxis are known mechanisms that guide sperm towards the egg during the final stage of sperm migration.<ref>{{cite journal |doi=10.1530/JOE-16-0302 |pmid=27875265 |title=Oviduct: Roles in fertilization and early embryo development |journal=Journal of Endocrinology |volume=232 |issue=1 |pages=R1–R26 |year=2016 |last1=Li |first1=Shuai |last2=Winuthayanon |first2=Wipawee |doi-access=free }}</ref> Spermatozoa respond (see [[Sperm thermotaxis]]) to the temperature gradient of ~2&nbsp;°C between the oviduct and the ampulla,<ref>{{cite journal |doi=10.1016/j.mce.2006.03.027 |pmid=16672171 |title=Sperm thermotaxis |journal=Molecular and Cellular Endocrinology |volume=252 |issue=1–2 |pages=115–9 |year=2006 |last1=Bahat |first1=Anat |last2=Eisenbach |first2=Michael |s2cid=22094855 |url=https://zenodo.org/record/995804 }}</ref> and [[Chemotaxis#Eukaryotic chemotaxis|chemotactic]] gradients of [[progesterone]] have been confirmed as the signal emanating from the [[cumulus oophorus]] cells surrounding rabbit and human oocytes.<ref>{{cite journal |doi=10.1371/journal.pone.0008211 |pmid=19997608 |pmc=2782141 |title=Molecular Mechanism for Human Sperm Chemotaxis Mediated by Progesterone |journal=PLOS ONE |volume=4 |issue=12 |pages=e8211 |year=2009 |last1=Teves |first1=Maria E |last2=Guidobaldi |first2=Hector A |last3=Uñates |first3=Diego R |last4=Sanchez |first4=Raul |last5=Miska |first5=Werner |last6=Publicover |first6=Stephen J |last7=Morales Garcia |first7=Aduén A |last8=Giojalas |first8=Laura C |bibcode=2009PLoSO...4.8211T |doi-access=free }}</ref> Capacitated and hyperactivated sperm respond to these gradients by changing their behaviour and moving towards the cumulus-oocyte complex. Other chemotactic signals such as formyl Met-Leu-Phe (fMLF) may also guide spermatozoa.<ref name=Gnessi1986>{{Cite journal |vauthors=Gnessi L, Fabbri A, Silvestroni L, Moretti C, Fraioli F, Pert CB, Isidori A | year = 1986 | title = Evidence for the presence of specific receptors for N-formyl chemotactic peptides on human spermatozoa| journal = J Clin Endocrinol Metab| volume = 63| pages = 841–846| pmid =3018025 |doi = 10.1210/jcem-63-4-841| issue = 4 }}</ref>

The [[zona pellucida]], a thick layer of extracellular matrix that surrounds the egg and is similar to the role of the vitelline membrane in sea urchins, binds the sperm. Unlike sea urchins, the sperm binds to the egg before the acrosomal reaction. [[ZP3]], a glycoprotein in the zona pellucida, is responsible for egg/sperm adhesion in humans. The receptor [[galactosyltransferase]] (GalT) binds to the N-acetylglucosamine residues on the ZP3 and is important for binding with the sperm and activating the acrosome reaction. ZP3 is sufficient though unnecessary for sperm/egg binding. Two additional sperm receptors exist: a 250kD protein that binds to an oviduct secreted protein, and SED1, which independently binds to the zona.  After the acrosome reaction, the sperm is believed to remain bound to the zona pellucida through exposed ZP2 receptors. These receptors are unknown in mice but have been identified in guinea pigs.{{fact|date=August 2021}}

In mammals, the binding of the spermatozoon to the GalT initiates the [[acrosome reaction]]. This process releases the [[hyaluronidase]] that digests the matrix of [[hyaluronic acid]] in the vestments around the oocyte.  Additionally, heparin-like glycosaminoglycans (GAGs) are released near the oocyte that promote the acrosome reaction.<ref>{{cite journal |last1=Lee |first1=C. N. |last2=Clayton |first2=M. K. |last3=Bushmeyer |first3=S. M. |last4=First |first4=N. L. |last5=Ax |first5=R. L. |title=Glycosaminoglycans in Ewe Reproductive Tracts and Their Influence on Acrosome Reactions in Bovine Spermatozoa in Vitro |journal=Journal of Animal Science |date=1 September 1986 |volume=63 |issue=3 |pages=861–867 |doi=10.2527/jas1986.633861x |pmid=3759713 |s2cid=853558 }}</ref> Fusion between the oocyte [[plasma membrane]]s and sperm follows and allows the sperm [[cell nucleus|nucleus]], the typical [[centriole]], and atypical [[centriole]] that is attached to the [[flagellum]], but not the [[mitochondrion|mitochondria]], to enter the oocyte.<ref>{{cite journal |doi=10.1038/s41467-018-04678-8 |pmid=29880810 |title=A novel atypical sperm centriole is functional during human fertilization |journal=Nature Communications |volume=9 |issue=1 |pages=2210 |year=2018 |last1=Fishman |first1=Emily L |last2=Jo |first2=Kyoung |last3=Nguyen |first3=Quynh P. H |last4=Kong |first4=Dong |last5=Royfman |first5=Rachel |last6=Cekic |first6=Anthony R |last7=Khanal |first7=Sushil |last8=Miller |first8=Ann L |last9=Simerly |first9=Calvin |last10=Schatten |first10=Gerald |last11=Loncarek |first11=Jadranka |last12=Mennella |first12=Vito |last13=Avidor-Reiss |first13=Tomer |bibcode=2018NatCo...9.2210F |pmc=5992222 }}</ref>  The protein [[CD9]] likely mediates this fusion in mice (the binding homolog). The egg "[[egg activation|activates]]" itself upon fusing with a single sperm cell and thereby changes its cell membrane to prevent fusion with other sperm. [[Zinc]] atoms are released during this activation.{{citation needed|date=July 2019}}
{{Commons|Fertilization#Mammalian fertilization|Mammalian fertilisation}}
This process ultimately leads to the formation of a [[diploid]] cell called a [[zygote]].  The zygote divides to form a [[blastocyst]] and, upon entering the uterus, [[Implantation (human embryo)|implant]]s in the endometrium, beginning [[pregnancy]]. Embryonic implantation not in the [[uterus|uterine]] wall results in an [[ectopic pregnancy]] that can kill the mother.

In such animals as rabbits, coitus induces ovulation by stimulating the release of the pituitary hormone gonadotropin; this release greatly increases the likelihood of pregnancy.

==== Humans ====
[[File:Human Fertilization.png|thumb|Fertilisation in humans. The sperm and ovum unite through fertilisation, creating a zygote that (over the course of 8-9 days) implants in the uterine wall, where it resides for nine months.]]
{{main|Human fertilisation}}

Fertilisation in humans is the union of a human [[ovum|egg]] and [[spermatozoon|sperm]], usually occurring in the [[ampulla of the fallopian tube]], producing a single celled [[zygote]], the first stage of life in the development of a genetically unique organism,<ref>{{Cite web |title=Zygote {{!}} Definition, Development, Example, & Facts {{!}} Britannica |url=https://www.britannica.com/science/zygote |access-date=2023-01-14 |website=www.britannica.com |language=en}}</ref> and initiating [[Human embryonic development|embryonic development]]. Scientists discovered the dynamics of [[human fertilisation]] in the nineteenth century.

The term ''conception'' commonly refers to "the process of becoming pregnant involving fertilisation or implantation or both".<ref>{{cite web |title=Conception |url=https://www.merriam-webster.com/dictionary/conception |website=Merriam-Webster |access-date=January 27, 2019}}</ref> Its use makes it a subject of [[beginning of pregnancy controversy|semantic arguments about the beginning of pregnancy]], typically in the context of the [[abortion]] debate.
<!-- In a statement by the [[American Association of Pro-Life Obstetricians & Gynecologists]] (AAPLOG), regarding the controversial [[morning-after pill]], AAPLOG claims:-->
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<!--:"[Again,] one must be careful of the terminology. Many now speak of "conception" as that moment when the human blastocyst, the early ball of approximately 100 cells, implants in the mother's uterus (womb).  The time from actual fertilisation (egg and sperm unite in the Fallopian Tube) until implantation, a period of about 7-10 days, is ignored, even though no genetic change occurs in the cells during this time period.  Many family planning specialists who have supported the terminology change can thus rationalize that the destruction of the human embryo between fertilisation and implantation should be labeled "contraception", rather than "early abortion".-->
Upon [[gastrulation]], which occurs around 16 days after fertilisation, the implanted blastocyst develops three germ layers, the endoderm, the ectoderm and the mesoderm, and the genetic code of the father becomes fully involved in the development of the embryo; later twinning is impossible.  Additionally, interspecies hybrids survive only until gastrulation and cannot further develop.
However, some human developmental biology literature refers to the ''[[conceptus]]'' and such medical literature refers to the "products of conception" as the post-implantation embryo and its surrounding membranes.<ref>{{cite book |author1=Moore, K. L. |author2=T. V. M. Persaud |year=2003 |title=The Developing Human: Clinically Oriented Embryology |publisher=W. B. Saunders Company |isbn=0-7216-6974-3 |url=https://archive.org/details/developinghumanc00moor_0 }}{{pn|date=August 2021}}</ref> The term "conception" is not usually used in scientific literature because of its variable definition and connotation.

=== Insects ===
[[File:Voo nupcial detail.jpg|thumb|upright|Red-veined darters (''[[Sympetrum fonscolombii]]'') flying "in cop" (male ahead), enabling the male to prevent other males from mating. The eggs are fertilised as they are laid, one at a time.]]
{{expand section|date=February 2015}}<!--at least cover dragonflies, bees-->

Insects in different groups, including the [[Odonata]] ([[dragonflies]] and [[damselflies]]) and the [[Hymenoptera]] ([[ant]]s, [[bee]]s, and [[wasp]]s) practise delayed fertilisation. Among the Odonata, females may mate with multiple males, and store sperm until the eggs are laid. The male may hover above the female during egg-laying (oviposition) to prevent her from mating with other males and replacing his sperm; in some groups such as the darters, the male continues to grasp the female with his claspers during egg-laying, the pair flying around in tandem.<ref>{{cite book |last=Dijkstra |first=Klaas-Douwe B. |title=Field Guide to the Dragonflies of Britain and Europe |year=2006 |publisher=British Wildlife Publishing |isbn=0-9531399-4-8|pages=8–9}}</ref> Among social Hymenoptera, [[honeybee]] queens mate only on mating flights, in a short period lasting some days; a queen may mate with eight or more [[drone (bee)|drones]]. She then stores the sperm for the rest of her life, perhaps for five years or more.<ref>{{cite book |last1=Waldbauer |first1=Gilbert |title=The Birder's Bug Book|date=1998 |publisher=Harvard University Press}}</ref><ref>{{cite web|last1=Agriculture and Consumer Protection|title=Beekeeping in Africa: Colony life and social organization|url=http://www.fao.org/docrep/t0104e/t0104e05.htm|publisher=FAO}}</ref><!--other bees ...-->