Fertilization

A sperm about to fertilize an egg from a mammal.

Fertilization is the process by which two gametes (male and female) fuse during sexual reproduction to create a zygote with a genome derived from both parents. The two main purposes of fertilization are the combination of genes derived from both parents and the generation of a zygote.

In the case of plants with pros seeds, it is necessary to differentiate the phenomenon of fertilization itself (intimate union of two sexual cells until their respective nuclei and, to a greater or lesser degree, their cytoplasms are confused), from the process that precedes it: pollination, in which the pollen grains, developed in the thecae contained in each anther of a stamen (male reproductive leaf), are transported by the wind or insects to the stigmas, where they germinate emitting a tube pollen growing towards the ovary. In this case it is not gametes, but spores, since each pollen grain contains two gametes or male reproductive cells.

Process

The details of fertilization are as diverse as the species; however, there are four events that are constant in all of them:

1. The first contact and recognition between male gamete and female gamete is of great importance to ensure that both are of the same species.
2. The regulation of the interaction between male and female gamete. The female gamete can only be matched with a male gamete. This can be achieved by allowing only one male gamete to merge with the female, which will prevent others from doing so.^{[chuckles]required]}
3. The fusion of genetic material from both gametes.
4. The formation of the zygote and the beginning of its development (the embryogenesis).

Modalities

Based on the similarities and differences between gametes

• Isogamic fertilization: Union of two identical gametes in size and structure. It occurs only in some groups like protozoa.
• Anesogenic fertilization: Union of two different gametes, both in size and in structure, one male and another female. It occurs in most groups.
• Oogamic fertilization: very different gametes: the feminine is large and immobile and provides all the nutritious reserves to the cigot, while the male is small and mobile.

According to the participating individuals

• Cross-fertilization: fertilization in which each gamete comes from a different individual. In some rare case, two individuals fertilize each other, as is the case in the terrestrial snails (or. Pulmonata).
• Self-fecundation: when the two gametes come from the same individual. In angiosperma plants, whose flowers are usually hermaphrodite, self-fecundation is frequent, almost always combined with cross-fertilization. Some species coexist with the normal certain special flowers that do not open, and fertilization occurs within the cocoon (cleistogamy).

Depending on where it is produced

• Internal fertilization: the sperm called introespermatozoid, is a type of advanced sperm (its morphology is of a head that contains the nucleus and the acrosoma, a neck where the centolos are, the middle part that is the flagelo surrounded by mytochondrial pods, and finally the terminal piece that is the continuation of the scourge but surrounded by fibrous pods coupled to the female body. It is when the union of the two gametes or sexual cells (spermatozoid and egg) is performed within the mother's body in the uterus or matrix, according to this the animals are classified into:

1. Oviparos: internal fertilization and external embryonic development within an egg with nutrients and calcareous shell, e.g., Monotremas (ornitorrinco, equidnas), many species of invertebrates, reptiles and birds.
2. Ovovivíparos: internal fertilization and incomplete embryonic development, leave the mother's body when they are still fetuses to complete their development outside the mother's body, e.g. Marsupials (comadrejas, zarigüeyas, kangaroos). The marsupio (membranous blouse) contains the breast glands for the feeding of the calves.
3. Vivipars: internal fertilization and internal embryonic development, e.g., the Eutherians or Mammals.

• External fertilization: typical of aquatic animals, there are two types of sperm for external fertilization: the acuaespermatozoid that is the one freely emitted by the organism to the aquatic environment where it lives and which fertilizes also free eggs, and the endoaquatic sperm, which is also emitted into the aquatic environment but is directed by the inhaling or feeding currents of the female, to fertilize the eggs emitted by this and These sperm are primitive (their morphology is based on an acrosome in the form of a hood, a subspheric nucleus, a small number of specific mitochondria with mitochondria ridges and a tail or flagelo whose organization is in microtubules in 9 external and 1 internal pairs that is originated by a distal centole). It is carried out by almost all marine invertebrates and the following species:

1. Fish: in reproduction, the eggs are abandoned by the female in the water at random and are immediately fertilized by the male. Fertilization is external because it occurs in the water. The eggs are floating in the water, some fall and are fixed at the bottom, but most serve as food to other fish. There are some species of fish, such as sharks and hammer fish, which have internal fertilization, that is to say, that is done inside the female's body.
2. Amphibians: they are oviparous and make an external fertilization. The males hug the females and these when they spend one or two days, release the eggs in the water. Then the male deposits his sperm on the eggs to achieve fertilization and subsequent development of the same. The eggs are soft and peelless, as they dry quickly, deposit them in the water or in wet places. The babies don't look like their parents, they look like fish and breathe like fish. They change shape, that is, they suffer metamorphosis. They pass from a renacuajo state, where they have no legs, to adult form acquiring the four legs.

Recognition between male and female gametes

Diagram that shows the steps of an acrosomal reaction as it occurs in the sea urchins.

The following steps occur:

1. Chemo-attraction from the female gamete to the male, through the secretion of soluble molecules that attract sperm. In a large number of species sperm are attracted to the female gamete, through the secretion of a chemical by the latter.
2. Exocytosis of the acrosomal vesicle of the male gamete so that the enzymes contained in this vesicle can be released.
3. Union of male gamete to extracellular layer covering the egg. The sperm is first joined to the female gamete, then the release of the contents of the acrosomal vesicle.
4. Step of the sperm through the extracellular matrix (membrane vitelin in the sea urchin; pelvic area in mammals) covering the egg.
5. Fusion of the female and male cell membranes.

Fertilization in plants

Moras during fertilization. Some remain active while others have already received the pollen and are withered.

Gymnosperms

Gymnosperms have both male and female flowers.

The female flower has one bract, one scale, and two ovules. They are grouped around a floral axis, forming the female cone. The ovule contains an embryonic sac with two archegonia, with two oospheres or female gametes each. The male flowers form male cones around a floral axis. They have a scale and two pollen sacs or microsporangia in which the stem cells that give rise to pollen grains are formed.

Within the pollen grains there are two antherozoids or male gametes and two air sacs that favor their dispersal until they reach the female flower.

It takes up to a year for the pollen grain to germinate after reaching the female flower. The pollen tube opens slowly through the nucellus of the ovule. Upon reaching the female gametophyte, it passes through the neck of the archegonia, enters the oosphere and discharges its contents into it. At that time fertilization occurs, one of the gametes unites with the nucleus of the oosphere to form the zygote (diploid). The other male gamete, the vegetative nucleus, and the other cells of the archegonia degenerate.

The embryo is surrounded by the endosperm (reserve cells) and protected by the integument of the ovule, which becomes lignified.

The embryo is not fully mature until two years after the appearance of the flowers, when the seeds are released.

In the pine seed, the integument is diploid, produced by the maternal sporophyte, the reserve tissue (primary endosperm) is haploid as it is part of the female gametophyte. The diploid embryo that forms after fertilization is the new sporophyte.

Angiosperms

After the carpel has been pollinated, the pollen grain germinates in response to a sugary fluid (mainly sucrose) secreted by the mature stigma. From each pollen grain, a pollen tube arises that creates a path through the style and goes towards the embryo sac or female gametophyte of angiosperms which is located inside the ovule.

Through the pollen tube the generative nuclei or male gametes travel to the micropyle. The pollen tube passes through it and discharges its contents near one of the synergids of the embryo sac. Once their contents have been discharged, the generative nuclei fuse with the oosphere and with the polar nuclei in a process known as double fertilization.

Many pollen grains can reach the stigma and germinate, but only one will fertilize.

After being fertilized, the ovary begins to grow and will become the fruit. In multi-seeded fruits, several pollen grains are needed to fuse with each ovule.

Fertilization in invertebrates

Among invertebrates there are different methods of reproduction

Asexual reproduction

This type of reproduction can be carried out through different processes: mitosis, binary fission, budding and fragmentation, among others, followed by the growth and development of the new individual.

There are adaptive advantages conferred on organisms that exhibit this type of reproduction. By being able to produce a greater number of offspring, they can take advantage of favorable conditions present in a certain environment; by exploiting the different food sources, the available space and other resources present in the environment. A clear example of asexual reproduction in colony-forming invertebrates are corals.

Sexual reproduction

This type of reproduction involves the formation of haploid cells, gametes (egg and sperm) by meiosis, and the subsequent fusion of two of these cells to form a diploid zygote. Most invertebrates release their gametes into the medium where they live, and this is generally water, and external fertilization takes place.

In these organisms, synchrony in the release of gametes is critical, since both eggs and sperm must be released at the same time for fertilization to occur. Some of the factors that are believed to be related to the synchrony of gamete release in marine animals are water temperature, light, phytoplankton abundance, the lunar cycle, and the presence of organisms from other species.

It is also important to mention that, although most invertebrates follow this pattern of external fertilization, there are also those that carry out this process internally. This type of organisms must have a greater development of their reproductive systems, which allow facilitating this process.

Fertilization in mammals

Fertilization in mammals is internal. The role played by the female reproductive tract is very important because it makes it easier for sperm to reach the end of the fallopian tube, thanks to the muscular movement exerted by the uterus.

When the sperm reaches the ampulla, they gain competence (they can lose it if they hang around it too long). Sperm can have different survival rates depending on their location within the female reproductive tract.

In mammals, fertilization triggers the process by which meiosis is completed in the egg and the mother's set of chromosomes becomes the pronucleus of the egg.

The egg is covered in several layers: the plasma membrane, cortical granules, and the zona pellucida. The sperm is motile, designed to activate the egg and at the same time insert its nucleus into the egg's cytoplasm. Both the egg and the sperm are structurally specialized for fertilization. The egg is specialized to prevent fertilization by more than one sperm, while the sperm is specialized to get as close as possible to the egg membrane. When the sperm and the egg fuse, a locking mechanism is activated in the egg in which another sperm is prevented from fusing (polyspermy lock). This is necessary because if more than one sperm were to fuse with the egg, there would be extra sets of chromosomes and centrosomes, resulting in abnormal development.

Training process

After the sperm enters the female reproductive tract, a capacitation process occurs, which seeks to facilitate fertilization by eliminating certain inhibitory factors and barriers. Training as such refers to the physiological changes by which the spermatozoon becomes competent to fertilize the female gamete. The molecular changes of capacitation are still largely unknown. Four major changes have been identified so far:

1. Alteration of the cell membrane of sperm, elimination of cholesterol by albumin proteins in the female reproductive tract. This apparently increases the pH that allows the sperm to experience acrosomal action.
2. Carbohydrates are also lost on the spermatic surface, which appears to facilitate recognition for proteins in the pelvic area.
3. The potential of the sperm cell membrane becomes more negative when potassium ions leave the sperm. Apparently it makes it easier for calcium channels to open up and thus enters calcium into sperm. Bicarbonate and calcium ions are related to the production of cAMP and facilitate membrane fusion events.
4. Protein phosphorylation

It has been documented (Timothy Smith 1998 and Susan Suárez 1998) that before entering the ampulla of the fallopian tube where fertilization occurs, uncapacitated spermatozoa bind to the membranes of the tube cells in the isthmus until who complete training and drop out. This temporary union appears to lengthen the life of spermatozoa and slows down capacitation. This could be aimed at avoiding polyspermy, and maximizing the chance that sperm will be in the ampulla to find the female gamete when ejaculation occurs days before ovulation.

Sperm motility

In different regions of the female reproductive tract, different molecules are secreted, which can influence sperm motility. For example, in some cases of rodents, when spermatozoa pass from the uterus to the fallopian troops, they become hyperactivated, swimming at higher speeds. The hyperactivation appears to be related to cAMP of a calcium channel in the sperm tail. This facilitates motility through viscous fluids such as those found in the fallopian tubes. Hyperactivity and hyaluronidase allows sperm to break through the cumulus layer.

Other factors secreted in the oviduct provide the directional component of sperm movement, it is speculated that these chemotactic factors are secreted by the ovarian follicle. It has also been seen that only capacitated spermatozoa manage to be attracted by the chemotactic follicular fluid.

Plasmatic membrane fusion of sperm and female gamete (mammals).

Barriers to be overcome by spermatozoa

Capacitation allows the sperm to overcome various barriers and achieve fertilization. The first barrier upon reaching the egg is a layer of cumulus cells in hyaluronic acid. Hyaluronidase activity on the surface of the sperm head helps it penetrate this barrier. The second barrier is the zona pellucida, which is a layer of glycoproteins. The spermatozoon manages to penetrate this barrier thanks to the acrosomal reaction (release of the contents of the acrosomal vesicle located in the head of the spermatozoon).

The zona pellucida

The zona pellucida plays a role analogous to the yolk membrane of invertebrates. This extracellular matrix, which is synthesized by the oocyte, has two main functions: to unite the spermatozoon and to start the acrosome reaction. This layer of the zona pellucida has three main glycoproteins ZP1, ZP2 and ZP3. The latter is a species-specific receptor for sperm union.

Union of the spermatozoon with the egg

The cell membrane that covers the sperm head has several proteins (an example is SED1), these proteins can bind hundreds of zona pellucida ZP3 glycoproteins (by carbohydrate chains linked by serine and threonine. By experimenting In the acrosomal reaction on the zona pellucida, sperm cells concentrate their proteolytic enzymes on the attachment site and digest a hole through this extracellular layer.When ZP3 binds to receptors on the sperm cell membrane, the acrosomal reaction is activated. One of the bound sperm proteins is galactosyltransferase-I, an intramembranous enzyme whose active sites face outward and binds to carbohydrate residues of ZP3.This in turn activates sperm membrane-specific G proteins, the which activate a cascade that opens the calcium channels and causes the exocytosis of the acrosomal content, this is mediated by e l Calcium from the acrosomal vesicle.

The acrosomal contents include β-N-acetylglucosaminidase and various proteases which break oligosaccharide chains of zona pellucida glycoproteins. This allows the sperm to pierce the zona pellucida and approach the plasma membrane of the egg. For continuous perforation without losing adhesion with the extracellular matrix, a secondary attachment to the zona pellucida must be achieved, through ZP2.

The acrosomal reaction also exposes proteins on the sperm surface that can bind to the egg membrane and allow the fusion of both membranes. Another important component is the protein fertilin which binds to an integrin-like receptor on the membrane. An egg receptor for sperm recognition is the CD9 protein, which initiates egg-sperm integration and is then a critical factor in the fusion of myocytes (muscle precursors) to form the multinucleated myotube of striated muscle. In mammals, the sperm does not contact the female gamete at its end, but rather on the side of the head, in the equatorial domain region of the sperm head.

To avoid polyspermy, as soon as the first spermatozoon reaches the plasmatic membrane of the egg and integration begins, the cortical granules are released, which contain enzymes that prevent the union of other spermatozoa with the zona pellucida. Unlike other organisms such as sea urchins, there is no change in membrane potential in mammals.

Egg activation from fertilization

Activation of the egg from fertilization activates a series of events that result in the beginning of development. The main events are: the egg completes meiosis, the egg nuclei and sperm unite to form a diploid zygote, and the fertilized egg enters mitosis. In the case of mice and humans, the membranes of the pronuclei disappear before their union.

As in sea urchins, egg activation is related to the release of free calcium ions in the egg (producing a calcium wave which is necessary and sufficient to start development). The wave of calcium begins at the point where the sperm entered and crosses the entire egg. There are oscillations in the calcium concentration for several hours after fertilization. The mechanism by which calcium release is initiated is not known, but it is believed that sperm induces protein-specific factors that initiate calcium release after cortical granule fusion. Increasing calcium concentrations initiates the development of the fertilized egg by activating proteins related to the cell cycle.

Summary

This process is developed through the following steps:

1. Spermtozoids are considered chemo-tactically attracted to the female gamete, by molecules emitted by cells of the follicles around them.
2. The sperm, with the intact acrosome, cross the cluster area and are specifically attached to the pelvic area or extracellular ovation cover. Three proteins have been identified that are related to the binding of the sperm to this matrix; ZP1, ZP2 and ZP3. The latter acts as the main and most important recipient of male gametes, in addition to the ability to induce acrosomal reaction.
3. After the union, the sperm will carry out the acrosomal reaction, through a cell exocytosis.
4. Because of this reaction, the sperm can now pierce the pelvic area, pass through it and reach the cell membrane of the egg. The passage through the different layers of extracellular membranes of the egg depends solely on the male gamete's own motion, helped by enzymes of the acrosome.
5. At the end of the process, the sperm joins the plasma membrane of the egg and merges with it.

Human fertilization

Fecundation

The fertilization process begins with the contact between the gametes. Said encounter usually occurs in the region of the ampulla that occupies the external third of the uterine tube.
First, the spermatozoon penetrates the corona radiata of the oocyte, until it comes into contact with the zona pellucida. The acrosomal reaction that allows it to enter the zona pellucida is generated in the sperm head.
Both the sperm tail and tubal mucosal enzymes contribute acrosomal hyaluronidase to open the way for sperm through the zona pellucida.^{[citation needed]} In addition to hyaluronidase, other acrosomal enzymes may contribute to zona pellucida penetration: certain esterases, acrosins such as arrocin, and neuraminidase.
More than one sperm is needed to fertilize the oocyte (collaborative effect). The zona granulosa network is not easy for a sperm to pass through. Sperm have haluiorinase to facilitate passage until reaching the zona pellucida. Some spermatozoa are releasing the capsule of the acrosome vesicle leaving a path. It is essential to have an intact acrosome to perfrorate the oocyte, since without an acrosome the spermatozoon will not be able to cross the egg membrane. Therefore, spermatozoa are needed to release their enzymes before reaching the oocyte so that they degrade the zona granulosa and thus some spermatozoon manages to reach the zona pellucida with its acrosome intact and can then join the oocyte.^{[ citation required]}

When the spermatozoon meets the zona pellucida, it binds to it. The induced acrosomal reaction or the ZP3 protein is then produced. The sperm cell membrane fuses with the outer membrane of the acrosome and the contents are released through pores. The released enzymes (mainly acrosin, similar to trypsin) gradually dissolve the zona pellucida and allow the passage of the sperm pushed by the flagellum at a speed of 1 micron (μm) per minute. The zona pellucida is 17±4.0 μm thick and it takes human sperm to pass through it between 10 and 30 minutes.^{[citation needed]}

Attachment to the zona pellucida is a decisive step in fertilization. When the reaction is complete, the spermatozoon is covered by the inner membrane of the acrosome. This change is essential for subsequent contact with the oocyte. The postacrosomal zone comes into contact with the microvilli of the oocyte. The membranes then fuse and the cytoplasms come into contact. The contents of the sperm enter the cytoplasm of the oocyte. ^{[citation required]}
Without a correct acrosomal reaction, the postacrosomal zone does not make adequate contact with the oocyte.

Both the middle piece and the flagellum of the sperm can enter the oocyte. As soon as a spermatozoon approaches the oocyte, the entry of another must be prevented to avoid polyspermy. This is caused by two mechanisms:^{[citation required]}

1. The union triggers a fast, depolarizing wave on the ovolema that alters the surface. A massive input of Na+ ions prevents new membrane fusions. In an early, immediate and transitory block.
2. A second wave of depolarizing caused by Ca++ ions causes the emptying of thousands of cortical vesicles to the periviteline space. They are lisosomes that contain numerous enzymes that harden the pelvic area. Hardening permanently prevents the entry of more sperm and protects the zygote.

Diagram of the stages of fertilization

The ability of zooids to lift the second meiotic block, which made it impossible for oocyte II to continue with the meiosis process, is notable. Once the zooid penetrates the zona pellucida and makes contact with the plasmatic membrane of the oocyte II, an intensification of the respiratory metabolism of this cell occurs, the second polar body is formed, which is a smaller cell without genetic material, the product the completion of the meiotic process.

From the moment of fertilization, the chromosome number is restored and the sex of the embryo is defined, depending on whether the spermatozoon carries an X chromosome or a Y chromosome (oocytes can only carry an X chromosome).

It is a common idea that a single sperm is needed to fertilize a single oocyte. It is known that the contribution of several spermatozoa is necessary to be able to fertilize an oocyte. Hyalurase is only secreted if the spermatozoon reaches the zona pellucida, but sometimes there are spermatozoa that carry out the acrosome reaction prematurely, in such a way that they degrade the hyarulonic acid that surrounds the oocyte, clearing the way for other spermatozoa. Thus, several spermatozoa are needed to fertilize a single oocyte. In addition, the joint hyperactive movement is believed to also aid in penetration into the oocyte.

Egg or zygote

After fertilization has occurred, the zygote (previously called an ovum when the spermatozoon enters and metaphase II has resumed) begins to undergo a series of events such as segmentation

The terms fertilization and conception

Although from a few years to date these terms have been distanced to refer to them as different stages of the gestation process, the terms fertilization and conception have been considered as synonyms, while the word fertilization refers to the entire process from the moment the spermatozoa enter the uterus, travel and find the egg. On the other hand, conception is the exact moment in which the spermatozoon enters the oocyte and triggers a series of changes that will lead to the development of the embryo.

In the Medical-biological, historical and etymological dictionary, edited by the University of Salamanca, conception is defined as the beginning of pregnancy, encompassing the union of the ovum and the spermatozoon, and in nesting or implantation of the egg in the uterus. Its origin comes from the Latin "con- union, contact, complete action; cep- catch, receive; and tion- action.