Fertilization and implantation Flashcards
What is fertilization?
Fertilization is the process by which male and female gametes fuse, which occurs in the ampullary region of the uterine tube (ampulla tubae uterinae).
Where does the fertilization occur?
Fertilization occurs in the ampullary region of the uterine tube (ampulla tubae uterinae).
For how long are spermatozoa viable in the female reproductive tract?
Spermatozoa may remain viable in the female reproductive tract for several days.
Which percantage of sperm deposited in the vagina enter the cervix uteri?
Only 1% of sperm deposited in the vagina enter the cervix uteri
How does the sperm get from cervix uteri to tuba uterina?
Movement of sperm from cervis uteri to tuba uterina occurs by muscular contractions of the uterus and tuba uterina and very litte by their own propulsion.
How long does the trip from cervix uteri to tuba uterina take?
The trip from cervix uteri to tuba uterina can occur as rapidly as 30 minutes or as slow as 6 days.
Can spermatozoa fertilize the oocyte immediately upon arrival?
No, it must undergo capacitation and the acrosome reaction to acquire this capability.
Describe the capacitation?
Capacitation is a period of conditioning in the female reproductive tract that in the human last approximately 7 hours.
This conditioning occurs in the tuba iteroma amd involves epithelial interactions between the sperm and the mucosal surface of the tube. During this time, a glycoprotein coat and seminal plasma proteins are remobed from the plasma membrane that overlies the acrosomal region of the spermatozoa.
Only capacitated sperm can pass through the corona radiata cells and undergo the acrosome reaction.
Describe the acrosome reaction
The acrosome reaction, which occurs after binding to the zona pellucida, is induced by zona proteins. This reaction culminates in the release of enzymes needed to penetrate the zona pellucida, including acrosin- and trypsin-like substances.
Name the phases of fertilization
Phase I - penetration of the corona radiata cells
Phase II - penetration of the zona pellucida
Phase III - fusion of the oocyte and spermatozoon cell membranes
Describe phase I of fertilization
Phase I: Penetration of the corona radiata
Of the 200-300 million spermatozoa normally deposited in the vagina, only 300-500 reach the site of fertilization. Only one of these fertilized the egg. It is though that the others aid the fertilizing sperm in penetrating the barriers protecting the female gamete. Capacitated sperm pass freely through corona radiata cells.
Describe phase II of fertilization
Phase II: Penetration of the zona pellucida
The zona is a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction. Both binding and the acrosome reaction are mediated by the ligand ZP3, a zona protein. Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona, thereby coming in contact with the plasma membran of the oocyte.
Permeability of the zona pellucida changes when the head of the sperm comes in contact with the oocyte surface. This contact results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte. In turn, these enzymes alter properties of the zona pellicida to prevent sperm penetration and inactivate species-specific receptor sites for spermatozoa on the zona surface.
Other spermatozoa have been found embedded in the zona pellucida, but only one seems to be able to penetrate the oocyte.
Describe phase III of fertilization
Phase III: Fusion of the oocyte and spermatozoon cell membranes
The initial adhesion of sperm to the ooyte is mediated in part by the interaction of integrins on the oocyte and their ligands, disintegrins, on sperm.
After adhesion, the plasma membranes of the sperm and egg fuse. Because plasma membrane covering the acromsomal head cap disappears during the acrosome reaction, actual fusion is accomplished between the oocyte membran and the membran that covers the posterior region of the sperm head.
In the human, both the head and the tail of the spermatozoon enter the cytoplasm of the oocyte, but the plasma membran is left behind on the oocyte surface.
What happens as soon as the spermatozoon has entered the oocyte (part of phase III)?
As soon as the spermatozoon has entered the oocyte, the egg respons in three ways:
1. Cortical and zona reactions: As a result of the release of cortical oocyte granules, which contain lysosomal enzymes, (1) the oocyte membrane becomes inpenetrable to other spermatozoa, and (2) the zona pellucida alters its structures and composition to prevent sperm binding and penetration. These reactions prevens polyspermy (penetration of more than one spermatozoon into the oocyte)
2. Resumption of the second meiotic division: The oocyte finishes its second meiotic division immediately after entry of the spermatozoon. One of the daughter cells, which receives hardly any cytoplasm, is known as the second polar body; the other daughter cell is the definitive oocyte. Its chromosomes (22 + X) arrange themselves in a vesicular nucleus known as the female pronucleus
3. Metabolic activation of the egg: The activating factor is probably carried by the spermatozoon. Activation encompasses the initial cellular and molecular events associated with early embryogenesis. The spermatozoon, meanwhile, moves forward until it lies close to the female pronulcues. Its nucleus becomes swollen and forms the male pronucleus; the tail detaches and degenerates.
What happens to the female pronucleus and male pronucleus when they come into close contact?
When they come into close contact they lose their nuclear envelopes
What happens immediately after DNA synthesis?
Immediately after DNA synthesis, chromosomes organize on the spindle in pereparation for a normal mitotic division. The 23 maternal and 23 paternal chromosomes split longitudinally at the centromere, and sister chromatids move to opposite poles, providing each cell of the zygote with the normal diploid number of chromosomes and DNA.
As sister chromatids move to opposite poles, a deep furrow appears on the surface of the cell, gradually dividing the cytoplasm into two parts.
What are the main results of fertilization?
The main results of fertilization are as follows:
- Restoration of the diploiud number of chromosomes, half from the father and half from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents.
- Determination of the sex of the new individual. An X-carrying sperm produces a female (XX) embryo, and a Y-carrying sperm produces a male (XY) embryo. Therefore, the chromosomal sex of the embryo is determined at fertilization.
- Initiation of cleavage. Without fertilization, the oocyte usually degenerates 24 hours after ovulation.
Without fertilization, how long is the oocyte viable after ovulation?
Without fertilization, the oocyte usually degenerates 24 hours after ovulation.
What happens when the zygote has reached two-cell stage?
Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, increasing the numbers of cells. These cells, which become smaller with each cleavage divisions, are known as blastomeres.
Describe the process of compaction
Until the eight-cell stage, the blastomeres form a loosely arranged clump. After the third cleavage the blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions. This process is called compaction and segregates inner cells, which communicate extensively by gap junctions, from outer cells.
What happens approximately 3 days after fertilization?
Approximately 3 days after fertilization, cells of the compacted embryo divide again to form a 16-cell morula.
Which parts can the morula be divided into?
Inner cells of the morula constitute the inner cell mass, and surrounding cells compose the outer cell mass.
- The inner cell mass gives rise to tissues of the embryo proper
- The outer cell mass forms the trophoblast, which later contributes to the placenta
What does the inner cell mass give rise to?
The inner cell mass gives rise to the tissues of the embryo proper
When is the two-cell stage aproximately reached?
The two-cell stage is reached approximately 30 hours after fertilization
When is the four-cell stage aproximately reached?
The four-cell stage is reached at approximately 40 hours after fertilization
When is the 12- to 16-cell stage aproximately reached?
The 12- to 16-cell stage is approximately reached 3 days after fertilization
What happens when fluid begins to penetrate through the zona pellucida into the intercellular spaces of the inner cell mass?
The intercellular spaces gradually become confluent, and finally, a single cavity, the blastocele, forms.
At this time, the embryo is a blastocyst. Cells of the inner cell mass, now called the embryoblast, are at one pole, ant those of the outer cell mass, or trophoblast, flatten and form the epithelial wall of the blastocyst.
How and when does the implantation begin?
Trophoblastic cells over the embryoblast pole begin to penetrate between the epithelial cells of the uterine mucosa (endometrium) on about the sixth day.
- New studies suggest that L selectin on trophoblast cells and its carbohydrate receptors on the uterine epithelium mediate initial attachment of the blastocyst to the uterus.
Describe the invasion by the trophoblast
Following capture by selectins, further attachment and invasion by the trophoblast and the extracellular matrix molecules laminin and fibronectin.
- Integrin receptors for laminin promote attachment
- Integrin receptors for fibronectin stimulate migration
What does the wall of the uterus consists of?
The wall of the uterus consists of three layers:
1. Endometrium or mucosa lining hte inside wall
2. Myometrium, a thick layer of smooth muscle
3. Perimetrium, the peritoneal covering lining the outside wall
Which stages does the endometrium pas through during the menstrual cycle?
- Follicular or proliferative phase
- Progestational or secretory phase
- Menstrual phase
Explan all the events during the first week of human development
- Oocyte immediately after oculation
- Ferilization, approximately 12 to 24 hours after ovulation
- Stage of the male and female pronuclei
- Spindle of the first mitotic division
- Two-cell stage (approximately 30 hours of age)
- Morula containing 12 to 16 blastomeres (approximately 3 days of age)
- Advanced morula stage reaching the uterin lumen (approximately 4 days of age)
- Early blastocyst stage (approximately 6 days of age)
When does the proliferative phase begin?
The proliferative phase begins at the end of the menstrual phase and is under influence of estrogen, and parallels growth of the ovarian follicles.
When does the secretory phase begin?
The secretory phase begins approximately 2 to 3 days after ovulation in response to progesterone produced by the corpus luteum.
What happens to the endometrium if fertilization does not occur?
If fertilization does not occur, shedding of the endometrium (stratum functionalis - stratum compactum and stratum spongiosum) marks the beginning of the menstrual phase.
What happens to the endometrium if fertilization does occur?
If fertilization does occur, the endometrium assists in implantation and contributes to formation of the placenta. Later in gestation, the placenta assumes the role of hormone production, and the corpus luteum degenerates.
During fertilization, the spermatozoon must penetrate…
- The corona radiata
- The zona pellucida
- The oocyte cell membrane
As soon as the spermatocyte has enteret the oocyte…
- The oocyte finished its second meiotic division and forms the female pronucleus
- The zona pellucida becomes impenetrable to other spermatozoa
- The head of the sperm separates from the tail, swells, and forms the male pronucleus
What happens when the female pronucleus and male pronucleus have been formed?
They replicate their DNA
What happens after both pronuclei have replicated their DNA?
After both pronuclei have replicated their DNA, paternal and maternal chromosomes intermingle, split longitudinally, and go through a mitotic division, giving rise to the two-cell stage.
What is cleavage?
Cleavage is a series of mitotic divisions that results in an increase in cells, blastomeres, which become smaller with each division.
What happens after three divisions of cleavage?
After three divisoins, blastomeres undergo compaction to become a tightly grouped ball of cells with inner and outer layers.
- Compacted blastomeres divide to form a 16-cell morula
When does the morula enter the uterus?
The morula enters the uterus on the third or fourth day after fertilization
What happens as the morula enters the uterus on the third or fourth day after fertilization?
As the morula enters the uterus on the third or fourth day after fertilization, a cavity begins to appear, and the blastocyst forms, which also contains an inner cell mass and an outer cell mass.
In what menstrual phase does the implantation occur?
The uterus at the time of implantation is in the secretory phase
