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