Learning Outcome
At the end of this block, students should be able to:
- Correlate the knowledge of fertilization with the relevant clinical conditions
Specific learning objectives:
By the end of this session, students should be able to:
- Identify the fertilization site and describe ovum transport
- Describe the Sperm Transport through female genital tract
- Describe the process of Maturation of sperms
- Enumerate & Explain the phases of Fertilization
- Comprehend the results of fertilization
Problem:
- A woman has had several bouts of pelvic inflammatory disease and now wants to have children; however, she has been having difficulty becoming pregnant.
- What is likely to be the problem, &what would u suggest?
Fertilization Site
- The usual site of fertilization is the ampulla of the uterine tube which is the widest & longest part.
Ovum Transport & Viability
- The released ovum (secondary oocyte) remains viable (i.e. capable of being fertilized by a sperm) for approximately 24 hours.
- If not fertilized within this period, the oocyte degenerates and is lost.
Period of Fertility
- The human female begins to ovulate at about 14th year and continues to do so until about 48 years of age when the reserves of ova in both the ovaries becomes exhausted and the reproductive life comes to an end.
Sperm Transport
- At intercourse usually 200-600 million sperms are deposited in the posterior vaginal fornix close to the uterine cervix.
- About 1% of sperms deposited in vagina enter the cervix of the uterus.
- The sperms pass slowly through the cervical canal by movement of their tails.
- The enzyme vesiculase, produced by seminal glands, coagulates some of the semen & forms a vaginal plug preventing the backflow of semen into the vagina.
- Further advance of sperms, i.e., through the uterus and uterine tubes is assisted by muscular contractions of the walls of these organs.
- Sperms move 2-3mm/min & speed varies with pH.
- Move slowly in acid environment of vagina, but move rapidly in alkaline environment of uterus.
- Prostaglandins in semen stimulate uterine motility, & fructose is an energy source for the sperms.
- The sperms reach the ampullae of the uterine tubes in 30 min or in 6 days after being deposited in the vagina.
- After reaching the isthmus, sperms become less motile & cease migration
- At ovulation, sperms again become motile, perhaps because of chemoattractant produced by cumulus cells & swim to ampulla
- Most of the spermatozoa are lost in this long journey and only 200 sperms reach the ampulla.
- Arrival of spermatozoa in ampullary part of uterine tube before ovulation favors birth of female babies (y-sperms are fast and more fragile as compared to x-sperms)
- Early arrival of oocyte in ampullary part of uterine tube (i.e., before the arrival of spermatozoa) favors birth of male babies
- Oocyte reach the fertilization site 25 min after ovulation.
- Oocyte is viable for fertilization for only 24 hours
- Safe period regarding contraception is to avoid 3-4 days before as well as 3-4 days after ovulation
MATURATION OF SPERMS
1. Capacitation
- Capacitation is the final step of maturation, which occurs in the uterine tube and takes about 7 hours to complete.
- Prolonged contact of the sperm with the secretions of the uterine tube results in the removal of glycoprotein coat & seminal proteins from the surface of the acrosome.
- The capacitated sperms show no morphologic change, but are more active & have the capability to easily penetrate the barriers surrounding the ovum (i.e., corona radiata and zona pellucida).
Completion of capacitation permits acrosome reaction to occur
2. Acrosome Reaction must be completed before sperm can fuse with oocyte.
- It occurs after binding to the zona pellucida & is induced by zonal proteins
- The sperm plasmalemma fuses with the underlying external acrosomal membrane at several points.
- The fused membranes break away resulting in the formation of multiple perforations in the acrosome.
- Through these perforations degradative enzymes (hyaluronidase, acrosin, estrase, neuraminidase) are released from the acrosome, which digest the zona pellucida and allow the sperm to reach the cell membrane of the oocyte.
The Acrosome Reaction
Binding of sperm to the zona pellucida is the easy part of fertilization. The sperm then faces the daunting task of penetrating the zona pellucida to get to the oocyte. Evolution’s response to this challenge is the acrosome – a huge modified lysosome that is packed with zona-digesting enzymes and located around the anterior part of the sperm’s head – just where it is needed.
The acrosome reaction provides the sperm with an enzymatic drill to get throught the zona pellucida. The same zona pellucida protein that serves as a sperm receptor also stimulates a series of events that lead to many areas of fusion between the plasma membrane and outer acrosomal membrane. Membrane fusion (actually an exocytosis) and vesiculation expose the acrosomal contents, leading to leakage of acrosomal enzymes from the sperm’s head.
As the acrosome reaction progresses and the sperm passes through the zona pellucida, more and more of the plasma membrane and acrosomal contents are lost. By the time the sperm traverses the zona pellucida, the entire anterior surface of its head, down to the inner acrosomal membrane, is denuded. The animation to the right depicts the acrosome reaction, with acrosomal enzymes colored red.
Sperm that lose their acrosomes before encountering the oocyte are unable to bind to the zona pellucida and thereby unable to fertilize. Assessment of acrosomal integrity of ejaculated sperm is commonly used in semen analysis.
FERTILIZATION
It is a complex sequence of coordinated molecular events that begins with contact between a sperm & an oocyte, & ends with the intermingling of maternal & paternal chromosomes at metaphase of 1st cleavage.
- This union marks the beginning of a new individual and normally occurs in the ampullary part of the uterine tube
- Best period for conception is around ovulation i.e.,14th day of typical 28 day menstrual cycle
Phases of fertilization:
- Penetration of Corona Radiate
- Penetration of Zona Pellucida
- Fusion of Oocyte and Sperm cell membranes
The fertilization process is completed in about 24 hours and comprises the following events:
1. Penetration of Corona Radiata.
- Of the 200 to 300 million spermatozoa normally deposited in the female genital tract, only 300 to 500 reach the site of fertilization; Out of these, many begin to penetrate through the corona radiata.
- Hyaluronidase released from the acrosome dissolves the intercellular substance present between the cells of corona radiata and this allows the spermatozoa to advance toward the zona pellucida.
- Tubal mucosal enzymes also assist dispersal.
- Movements of the tail are also important in the penetration.
- Many spermatozoa succeed in penetrating the corona radiata completely and make contact with the zona pellucida.
2. Penetration of the Zona Pellucida.
- When a sperm reaches the zona pellucida, it binds to a human-specific, glycoprotein sperm receptor molecule (ZP 3) in the zona.
- The enzymes (esterases, acrosin & neuraminidase) released from the acrosome digest the zona pellucida and allow the sperm to reach the cell membrane of the oocyte.
- ONE HITS REST MISS
3. Fusion of plasma membranes of oocyte & sperm
- Initial adhesion of sperm to the oocyte is mediated in part by the interaction of integrins on the oocyte and their ligands, disintegrins, on sperm. The plasma membranes of oocyte & sperm fuse & break down at the areas of fusion.
- Head & tail of sperm enter the cytoplasm of oocyte, sperm membrane remains behind.
As soon as the spermatozoon has entered the oocyte, the egg responds in three ways
1. Cortical reaction and zona reaction.
- Once the sperm penetrate the zona pellucida, cortical & zona reaction occur.
- The cortical reaction entails the fusion of the cortical granules of the oocyte with the oolemma.
- The cortical granules release an enzyme that diffuses into the zona pellucida and hydrolyses the ZP3 receptor.
- This reaction, called zona reaction,prevents other sperms from binding and, thus, makes the zona pellucida impenetrable for additional spermatozoa.
- The contents of cortical granules also cause changes in plasm membrane that makes it impermeable to other sperms.
- This mechanism prevents polyspermy (fertilization of the ovum by more than one spermatozoon).
2. Completion of the second meiotic division & formation of female pronucleus.
- Fusion of the sperm plasmalemma with the oolemma also causes the oocyte to resume meiosis.
- The secondary oocyte completes the second meiotic metaphase and quickly proceeds through the anaphase to form the definitive ovum and the nonfunctional second polar body.
- Following decondensation of maternal chromosomes, nucleus of mature oocyte is referred to as female pronucleus.
3. Metabolic activation of the egg.
- Activating factor is carried by spermatozoon
- Activation encompasses the initial cellular & molecular events associated with early embryogenesis
Formatiom of male pronucleus.
- Within the cytoplasm of the oocyte, the nucleus of sperm enlarges to form the male pronucleus & the tail degenerates.
- Morphologically male & female pronuclei are indistinguishable
- During growth of pronuclei, they replicate their DNA.
- The oocyte containing two haploid pronuclei is called an ootid.
- The male and female pronuclei move to lie in close proximity to each other.
Fusion of two pronucei into a single diploid aggregation of chromosomes; and the ootid becomes a zygote.
- In humans actual fusion of the male and female pronuclei does not take place.
- Instead, the two pronuclei replicate their DNA and lose their membranes.
- The two chromosome groups then move together to assume position on the first cleavage spindle.
Phases of fertilization
- 1. Passage of sperm through the corona radiate.
- 2. Penetration of the zona pellucida.
- 3. Fusion of plasma membranes of oocyte & sperm
- 4. Cortical reaction and zona reaction.
- 5. Completion of the second meiotic division & formation of female pronucleus.
- 6. Formatiom of male pronucleus.
- 7.Fusion of two pronucei into a single diploid aggregation of chromosomes; and ootid becomes a zygote.
Results of Fertilization
1. Restoration of diploid number of chromosomes. Fusion of two haploid gametes produces the zygote, a diploid cell with 46 chromosomes.
2. Sex determination.
- The sex of embryo is determined at fertilization by the kind of sex chromosome carried by the fertilizing sperm.
- Fertilization by an X-carrying sperm makes a female embryo, while fertilization by a Y-carrying sperm produces a male embryo.
3. Initiation of cleavage.
- Fertilization initiates development by stimulating the zygote to undergo a series of rapid mitotic divisions.
- This mitotic segmentation of the fertilized ovum is called cleavage.
1.What is the role of the corpus luteum, and what is its origin?
- The role of the corpus luteum is to produce hormones essential for preparing the uterus for pregnancy and then for maintaining that pregnancy until the placenta becomes fully functional (approximately the beginning of the fourth month).
- Initially, progesterone is the primary hormone produced, and it causes the uterus to enter the progestational (secretory) phase.
- Later, both estrogen and progesterone are produced to maintain pregnancy.
- The corpus luteum originates from the theca interna (derived from ovarian stromal cells) and from granulosa cells that remain in the ovary after ovulation.
2. What are the three phases of fertilization, and what reactions occur once fusion of the sperm and oocyte membranes takes place?
uThe three phases of fertilization are:
(1) penetration of the corona radiata,
(2) penetration of the zona pellucida, and
(3) fusion of the oocyte and sperm cell membranes.
Once fusion occurs, the egg undergoes the cortical and zona reactions to prevent polyspermy.
Cortical granules next to the oocyte plasma membrane release lysosomal enzymes that alter the cell membrane and the zona pellucida, such that additional sperms cannot enter the egg.
3. What are the primary causes of infertility in men & women???
- Infertility occurs in approximately 20% of married couples.
- A major cause of infertility in women is blockage of the uterine (fallopian) tubes caused by scarring from repeated pelvic inflammatory disease; in men, the primary cause is low sperm count.
- In vitro fertilization (IVF) techniques can circumvent these problems, although the success rate (approximately 20%) is low.
4. Problems:
- A woman has had several bouts of pelvic inflammatory disease and now wants to have children; however, she has been having difficulty becoming pregnant.
- What is likely to be the problem, &what would u suggest?
Key:
Key:
- Pelvic inflammatory diseases, such as gonorrhea, are a major cause of occluded oviducts (uterine tubes).
- Although the patient may be cured, scarring closes the lumen of the tubes and prevents the passage of sperm to the oocyte and of the oocyte to the uterine cavity.
- IVF can overcome the difficulty by fertilizing the woman’s oocytes in culture and transferring them to her uterus for implantation.