Process of Fertilization in Human Being
Fertilization is the process in which the union of two haploid gametes i.e. male gamete (sperm) and female gamete (ovum) takes place to form a diploid zygote. The concept of fertilization was given by Leeuwenhoek in 1683. In human beings, inside the ampullary isthmic junction of the oviduct (fallopian tube), fertilization takes place. During mating, the male releases semen into the female’s vagina near the cervix. This is called insemination. While releasing, single semen may have 300 million sperms. At the rate of 1.5 to 3 mm per minute, sperms swim in the fluid medium to reach the site. A million sperms are engulfed by the leucocytes of the vaginal epithelium. The mature Graafian follicle of an ovary eject oocyte. The fallopian funnel received the oocyte and transferred it into the Fallopian tube. After releasing from the ovary within 24 hours secondary oocyte can be fertilized. Already second meiotic division is in progress, so, sperm enters the secondary oocyte. Various changes occur in sperm before fertilization, which prepares it to fertilize the ovum. This is called copulation. It takes nearly 5 to 6 hours for capacitation.
Process of Fertilization
In fertilization, various steps take place. The steps involved are :
- Acrosomal reaction
- Cortical reaction
- Penetration of sperm
- Karyogamy (Amphimixis)
- Activation of zygote
The antifertilizin of sperm interchange with the fertilizin ( ZP3, ZP2 ) of the ovum and undergoes an acrosomal reaction to release the sperm lysins. Main sperm lysins are:-
- Hyaluronidase- It takes action on follicle cells.
- Corona penetrating enzyme- It liquefies corona radiata.
- Zona lysine or acrosin- It helps in the digestion of zona pellucida.
For acrosomal reaction, the best pH, Ca++, and Mg++ ions concentration, and temperature are necessary. In acrosomal reactions, Ca++ contributes at a major level. Fertilization does not take place in absence of Ca++. Through chemical acknowledgment, sticking sperm with the egg of the same species is known as agglutination. Because of acrosomal reaction, the plasma membrane of sperm combines with the plasma membrane of the secondary oocyte, in order that sperm enters the oocyte. This helps in preventing polyspermy (entry of more than one sperm into the oocyte). It makes sure that monospermy (entry of only one sperm enters into the oocyte) takes place. The acrosome contains some zona pellucida digesting enzymes that penetrate further into the ovum layers. When sperm goes deeper the acrosome shrinks.
The cortical reaction occurs just after the acrosomal reaction ends. Below the plasma membrane of the secondary oocyte, the cortical granules are present. The plasma membrane of the oocyte combines with these granules and ejects cortical enzymes. The enzymes help zona pellucida to become harder because it also prevents polyspermy. The cortical reaction is dependent on the calcium exocytotic process in which the content of secretory granules is free into the perivitelline space straight away after fertilization, which gives out to prevent polyspermic fertilization. Organization of SNARE ( soluble N – ethylmaleimide – sensitive factor attachment protein receptor ) proteins in the mooring and combination of the cortical granule membrane with the oolemma in porcine oocytes.
Penetration of Sperm
The secondary oocyte forms a projection at the point of contact of sperms, called the cone of reception or fertilization cone which collects sperm. The lateral centrosome of the sperm split and creates two centrosomes to give rise to the meiotic spindle for cell division. In humans, the secondary oocyte does not contain centrioles of its own. A representative, ovulated egg of mammals is surrounded by two layers an outer layer of nearly 5000 cumulus cells and an inner, thick extracellular matrix, the zona pellucida. To attain the egg plasma membrane, sperm should penetrate both layers in steps needing sperm motility, sperm surface enzymes, and likely sperm-secreted enzymes. Sperm too tie ephemeral to the egg zona pellucida and the egg plasma membrane and then fuse.
To complete the delayed second meiotic division, sperm entry restores the secondary oocyte. This form a haploid mature ovum and a second polar body. The nucleus of the sperm is called the male pronucleus. The nucleus of the ovum is called the female pronucleus. Intermixing of chromosomes of sperm and ovum is known as Karyogamy. After this, the zygote is formed which is a diploid unicellular cell that contains 46 chromosomes in humans. In this stage, the mother is considered pregnant. It is the end phase of the procedure of combining haploid eukaryotic cells in a junction while fertilization, and it mentions the combination of two nuclei in specific. Karyogamy happens only in eukaryotic cells and also helps in the formation of new daughter cells.
Activation of Zygote
Sperm entry restores the metabolism in the zygote. In order that cleavage begins instantly. The zygote activates to form proteins which happen only after breaking. The development of a zygote that diploid cell is emerged containing DNA knowledge.
Steps involved in Fertilization
In the maturation of mammalian spermatozoa, capacitation is the next-to-last intermediate step and it is needed to give them adequate to fertilize an oocyte. It is a biochemical event; the sperm proceed usually and becomes mature to capacitation. In vivo, capacitation happens after ejaculation, when the spermatozoa depart from the vagina and go into the superior female reproductive tract. The uterus helps in the stages of capacitation by secreting sterol-binding albumin, lipoproteins, and proteolytic and glycosidase enzymes like heparin. For determination of in vitro fertilization, capacitation happens by the maturation of spermatozoa that have either gone through ejaculation or have been taken out from the epididymis and maturates in a specified medium for some hours. There is dissimilar manners to carrying out the capacitation step: Simple washing, migration, density gradients, and filter. The impartiality is to separate as numerous mobile spermatozoa as possible and to remove dead spermatozoa. Later either in vivo or in vitro capacitation the sperm should go through the final maturation step activation, involving the acrosome reaction.
Non-mammalian spermatozoa decline and need this capacitation step and are prepared to fertilize an oocyte just after the release from the male.
Sperm – Zona Pellucida Binding
In this step, the sperm meets an ovum and binds towards the zona pellucida layer of the ovum to shape a receptor-ligand reaction. After this coupling reaction, no sperm can enter through this layer. The procedure in which the sperm ties to the zona pellucida glycoprotein coating of the egg. The procedure starts with the extension of the sperm plasma membrane to the zona pellucida and includes an extension of the acrosome inner membrane to the zona pellucida later the acrosomal reaction takes place.
Zona Pellucida Penetration
Sperms head are structured in a way to break the walls of the ovum. It restores ovum maturation, hanged up during the first stage of meiosis. Another stage occurs immediately after the egg is fertilized. For fertilization to happen, a spermatozoon requires to cross the zona pellucida, which is a glycoprotein coating adjoining the oocyte. Crossing the zona pellucida needs an acrosome reaction where enzymes free from the spermatozoon head narrowly digest and soften the zona pellucida. So that the spermatozoa can penetrate in depth.
For acrosomal reaction, the best pH, Ca++, Mg ++ ions concentration, and temperature are necessary. In acrosomal reaction, Ca++ contributes at a major level. Fertilization does not take place in absence of Ca++. Through chemical acknowledgment, sticking sperm with the egg of the same species is known as agglutination. Because of acrosomal reaction, the plasma membrane of sperm combines with the plasma membrane of the secondary oocyte, in order that sperm enters the oocyte. This helps in preventing polyspermy ( entry of more than one sperm into the oocyte ). It makes sure the monospermy ( entry of only one sperm enters into the oocyte ) takes place. The acrosome contains some zona pellucida digesting enzymes that penetrate further into the ovum layers. When sperm goes deeper, the acrosome shrinks.
In this step, after being frozen the egg is activated through the metaphase stage of meiotic division ll. Cortical granules intermix with the zona pellucida and bring about exocytosis. The cortical reaction occurs just after the acrosomal reaction ends. Below the plasma membrane of the secondary oocyte, the cortical granules are present. The plasma membrane of the oocyte combines with these granules and ejects cortical enzymes. The enzymes help zona pellucida to become harder because it also prevents polyspermy. The cortical reaction is dependent on the calcium exocytotic process in which the content of secretory granules is free into the perivitelline space straight away after fertilization, which gives out to prevent polyspermic fertilization.
This is the last step of fertilization and zona pellucida becomes hard. Zona reaction mentions a moderation of the zona pellucida in reaction to fertilization, which changes the solubility and tiny properties of the zona pellucida and guides to a slow block to polyspermy.
Significance of Fertilization
- It brings back the diploid number of chromosomes.
- Fertilization begins splitting.
- It presents the centrioles which have a shortage in the mature egg.
- Fertilization helps in determining the sex of the embryo.
- It unites the characteristics of both parents. This is called variation.
The fertilization age is the duration of fertilization. It normally happens within a day of ovulation, one after the other, and happens on average 14 days next the commencement of the continuing menstruation (LMP). There is also significant flexibility in this period, with a 9 to 95 % forecast period of ovulation of 9 to 20 days next to menstruation even for a mean woman who has an average LMP – to – ovulation time of 14 .6 . In a recommendation category act for whole women, the 95% forecast period of the LMP – to – ovulation is 8.2 to 20.5 days. The real flexibility connecting gestational age even as approximated from the commencement of the last menstrual period is considerably bigger due to the unpredictability of which the menstrual cycle brings about pregnancy. For instance, the menstruation can be short sufficient to give the incorrect image that ab earlier menstruation gave rise to the pregnancy, likely giving an approximate gestational age i.e. roughly one month enormous. In addition, vaginal bleeding happens throughout 15 – 25 % of first-session pregnancies and can be wrong as menstruation, possibly giving an approximate gestational age that is deficient.
Diseases affecting Human Fertility
When a woman doesn’t conceive a pregnancy after trying to conceive for one year is called infertility. In women endometriosis, uterine fibroids, and thyroid diseases cause infertility. In men low sperm count or low testosterone, increases the risk of infertility.
Risk factors for Infertility
- Age (women after 35 or men after 40)
- Eating disorders such as bingeing, anorexia nervosa, and bulimia.
- Immoderate use of alcohol.
- Subjection to environmental toxins, including lead and pesticides.
- Exercising in excess amount
- Radiation therapy.
- Sexually transmitted diseases (STDs)
How is Polyspermy restricted?
The quick obstacles to polyspermy we attained by swapping the electric potential of the egg plasma membrane. This membrane supplies a selective fence between the egg cytoplasm and the exterior environment, and the ionic concentration of the egg varies to a great extent from that of its neighborhood. This concentration dissimilarity is specifically significant for sodium ions. The back is the case with potassium ions. This situation is maintained by the plasma membrane, which continuously holds back the entry of sodium ions into the oocyte and stops potassium ions from leaking out into the surrounding.
Fast block Polyspermy
The fast block to polyspermy is attained by swapping the electric potential of the egg plasma membrane. This membrane supplies a particular fence between the egg cytoplasm and the exterior surroundings and the ionic concentration of the egg varies greatly from that of its habitat. This concentration dissimilarity is mainly notable for sodium and potassium ions. Seawater has a specially high sodium ion concentration, although the egg cytoplasm holds proportionate sodium. The opposite is the case with potassium ions. This state is kept up by the plasma membrane, which continuously impedes the appearance of sodium ions in the oocyte and stops potassium ions from discharging out into the surroundings. If we put an electrode into an egg and put down a second electrode outside it, we can calculate the constant dissimilarity in charge over the egg plasma membrane. This resting membrane potential is normally around 70mV, normally expressed as – 70mV because the interior of the cell is negatively charged with respect to the exterior.
Slow block Polyspermy
The eggs of sea urchins have another procedure to make sure that numerous sperm do not approach the egg cytoplasm. The fast block polyspermy is transitory since afterward, the membrane potential of the sea urchin egg endures constructive for only around a minute. This short potential shift is not enough to block polyspermy, which can still happen if the sperm leap to the vitelline envelope is not by any means removed. This taking away is fulfilled by the cortical granule reaction, a steady mechanical block to polyspermy that becomes active around a minute later the first victorious sperm-egg attachment.
FAQs on Fertilization
Question 1: Write a short note on fertilization.
Fertilization is the procedure of crossing of a sperm with an ovum .
Question 2: Mention the function of zona pellucida.
Through fertilization , a sperm approach in touch with the zona pellucida coating of ovum and bring about changes in the membrane and forms fertilization envelope . It assist stop the arrival of additional sperms . So , zona pellucida make sure that only one sperm fertilizes an ovum .
Question 3: How does the sperm penetrate through the zona pellucida in the human ovum?
The sperm penetrates through the zona pellucida by let go of sperm lysins present in acrosome .
Question 4: How long does sperm lives?
The sperm lives for 72 hours . The sperm moves in the upward direction to Fallopian tube to fertilize the ovum .
Question 5: Where does fertilization occur in humans?
In humans, the Fertilization of ovum occurs in ampullary isthmic junction of the fallopian tube .
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