1. From the ovulation until the implantation

Learning objectives

When does ovulation occur, and what event triggers it?
How does LH cause ovulation?
Where does fertilization most commonly occur?
What must happen to the sperm cell before it can fertilize the egg?
Describe the process of capacitation
Describe the acrosome reaction
Describe the zona reaction
What occurs after the spermatozoon has entered the oocyte?
What is cleavage?
When are the two-cell and four-cell stages reached?
What is the morula, and when is it formed?
Which event marks the blastocyst stage?
When is the zona pellucida disintegrated?
Where does implantation most commonly occur?
What is the bilaminar disc?
What will the epiblast form, and where does it lie?
What will the hypoblast form, and where does it lie?
What is the anterior visceral endoderm, and what is its function?
When is the dorsal-ventral axis determined?
When is the cranio-caudal axis determined?

Ovulation

Ovulation usually occurs in the middle of the cycle, at day 14. In the days leading up to day 14 the ovarian follicle has reached the graafian stage. A surge in LH approximately one day before ovulation stimulates multiple processes which altogether cause the secondary oocyte to be extruded from the graafian follicle, which is the ovulation.

LH stimulates collagenase, an enzyme which digests collagen fibres surrounding the follicle. LH also increases the level of prostaglandins in the ovary, which causes local muscle contractions in the ovarian wall. At day 14 these processes cause the secondary oocyte to be extruded from the graafian follicle together with the cumulus oophorus.

The granulosa cells comprising the cumulus oophorus form the corona radiata around the zona pellucida. The corona radiata functions as a barrier which spermatozoa must penetrate, as we’ll see.

Fertilization

Fertilization is the process by which one spermatozoon fuses with the secondary oocyte. This process most commonly occurs in the ampullary region of the fallopian tube.

The journey of a sperm cell

Spermatozoa may remain viable in the female reproductive tract for several days, but fertilization itself most commonly occurs within 1 day of ovulation. Movement of sperm from the cervix to the uterine tube is mostly due to muscular contractions of the uterus and uterine tube, not due to the spermatozoa’s own propulsion.

After reaching the isthmus of the uterine tube the sperm stop and wait. At ovulation the sperm become motile again, causing them to swim towards the ampulla, where they will meet the ovulated oocyte.

Capacitation and the acrosome reaction

Before sperm cells can fertilize the oocyte, they must undergo two processes, capacitation and the acrosome reaction.

During capacitation the spermatozoa interact with the mucosal surface of the uterine tube. This interaction causes the spermatozoa to lose a glycoprotein coat and seminal plasma proteins on their surface. Capacitation allows sperm to pass through the corona radiata.

The acrosome reaction occurs when the spermatozoon has passed through the corona radiata and bound to the zona pellucida. A protein in the zona pellucida called ZP3 induces the acrosome reaction. The acrosome is a small structure on the head of the spermatozoon which contains enzymes.

During the acrosome reaction these enzymes are released from the acrosome, causing the zona pellucida at that point to be degraded. This allows the spermatozoon’s cell membrane to fuse with the oocyte’s cell membrane, forming a zygote.

The zona reaction

As soon as one spermatozoon has reached the oocyte’s cell membrane, the zona reaction occurs. Upon this contact enzymes will be released from the plasma membrane of the oocyte, which changes the properties of the zona pellucida to prevent other sperm from penetrating it. The zona reaction makes sure that only one sperm can fertilize the egg.

After fusion between the oocyte’s and the spermatozoon’s plasma membrane

As soon as the spermatozoon has entered the oocyte the oocyte finishes meiosis II, forming the mature ovum. The genetic material of the mature ovum forms the female pronucleus. The head of the spermatozoon forms the male pronucleus. At this point each of the pronuclei are 1n1c.

DNA is then replicated in both pronuclei, making them 1n2c as their chromosomes are replicated into sister chromatids. After this replication, the pronuclei fuse, making them 2n4c.

The sister chromatids are then split longitudinally at the centromere and moved to opposite sides of the cell. The cell is then divided in two at the middle, yielding two genetically identical 2n2c cells, still enclosed within the zona pellucida. The zygote is now in the two-cell stage, which is reached approximately 30 hours after fertilization. Each of these cells are known as blastomeres.

Results of fertilization

After fertilization, three results have been achieved:

A zygote is formed
Restoration of the diploid number of chromosomes
Determination of the chromosomal sex, depending on whether the spermatozoon carried an X or a Y chromosome
Cleavage is initiated

The road to implantation

Cleavage

Cleavage is the process where the zygote rapidly divides by mitosis. Because the zygote is still confined by the zona pellucida its volume and size remain unchanged as the cells multiply.

The two-cell stage is reached approximately 30 hours after fertilization, and 10 hours later the zygote is in the four-cell stage.

When the 16-cell stage is reached the zygote is now a morula. This occurs 3 days after fertilization. At this stage not all cells are in contact with the zona pellucida, separating the blastomeres into the inner and outer cell mass. The inner cell mass will give rise to the embryoblast and later the embryo itself, while the outer cell mass will form the trophoblast and later the placenta.

Blastocyst formation

Approximately during the fourth day the zygote will enter the uterine cavity, and at this time the zona pellucida disappears. Fluid from the uterine cavity enters between the cells and form a single large cavity in the zygote, the blastocele. At this point the zygote is known as a blastocyst. The trophoblast lines the outside of the blastocyst.

Implantation

The uterus is now in the secretory phase. Now that the zona pellucida is gone the blastocyst can begin implantation. During the sixth day after fertilization the finger-like projections of trophoblast cells starts to penetrate between the epithelial cells of the uterine mucosa, the endometrium. Implantation occurs most commonly into the anterior or posterior wall of the uterus.

The bilaminar disc

Now that the blastocyst has implanted, the blastocyst will begin to differentiate further. Cells in the embryoblast differentiate into epiblast and hypoblast cells.

The epiblast, like its name suggests, lies above the hypoblast, in contact with the trophoblast cells. These cells will form the embryo itself.

The hypoblast, like its name suggests, lies below the epiblast, in contact with the blastocele. The hypoblast will give rise to the yolk sac. Some of the hypoblast cells differentiate into anterior visceral endoderm.

The blastocyst now has two layers, and so the bilaminar disc has been formed.

The embryonic axes

When talking about embryonal axes (plural of axis) we mean how the embryo determines where the axes of the foetus will be. There are three embryonic axes, the dorsal-ventral axis, the cranio-caudal axis, and the left-right axis. The first two axes are determined during this phase of development.

The dorsal-ventral axis

The dorsal-ventral axis is formed as the embryoblast divides into the epiblast and the hypoblast. The cells of the epiblast in contact with the trophoblast “know” that they should form the dorsal surface of the foetus, while the cells of the epiblast in contact with the hypoblasts know that they should form the ventral surface.

The cranio-caudal axis

The anterior visceral endoderm of the hypoblast aids in forming the cranio-caudal axis. These cells secrete antagonists to the nodal signalling pathway, like Cerberus and lefty1. These proteins act on adjacent epiblast cells and specifies that this end of the embryo should be the cranial end.

Later, a structure called the primitive streak will also be involved in determining the cranio-caudal axis.

Summary

When does ovulation occur, and what event triggers it?
- Day 14
- Ovulation is triggered by the LH surge
How does LH cause ovulation?
- LH stimulates collagenase, which digests collagen around the follicle
- LH stimulates prostaglandins, causing muscles in the ovarian wall to extrude the oocyte
Where does fertilization most commonly occur?
- In the ampullary region of the fallopian tube
What must happen to the sperm cell before it can fertilize the egg?
- It must undergo capacitation and the acrosome reaction
Describe the process of capacitation
- Spermatozoa interact with the endometrium, causing the spermatozoa to lose a glycoprotein coat and seminal plasma proteins
- Capacitation allows the spermatozoa to pass through the corona radiata
Describe the acrosome reaction
- When the spermatozoa pass the corona radiata and binds to the zona pellucida enzymes is released from the acrosome
- These enzymes degrade the zona pellucida at that point, allowing the spermatozoon’s plasma membrane to fuse with the oocyte’s plasma membrane
Describe the zona reaction
- After one spermatozoon has fertilized the oocyte enzymes will be released from the plasma membrane of the oocyte
- These enzymes change the properties of the zona pellucida to prevent other sperm from penetrating it
What occurs after the spermatozoon has entered the oocyte?
- The pronuclei of the spermatozoon and the oocyte replicate their DNA
- The pronuclei then fuse, the sister chromatids are split longitudinally, and the cell is divided into two
What is cleavage?
- Cleavage is the process by which the zygote rapidly divides by mitosis
When are the two-cell and four-cell stages reached?
- Two-cell stage 30 hours after fertilization
- Four-cell stage 40 hours after fertilization
What is the morula, and when is it formed?
- The morula is the 16-cell stage
- It is separated into the inner cell mass (embryoblast) and the outer cell mass (trophoblast)
- It is formed 3 days after fertilization
Which event marks the blastocyst stage?
- The formation of a large cavity in the zygote, the blastocele
When is the zona pellucida disintegrated?
- 4 days after fertilization, when the zygote enters the uterine cavity
Where does implantation most commonly occur?
- The anterior or posterior wall of the uterus
What is the bilaminar disc?
- The bilaminar disc is the next stage of the embryoblast, which has two layers: the epiblast layer and the hypoblast layer
What will the epiblast form, and where does it lie?
- The epiblast will form the embryo itself
- It lies above the hypoblast, in contact with the trophoblast cells
What will the hypoblast form, and where does it lie?
- The hypoblast will form the yolk sac
- It lies below the epiblast, in contact with the blastocele
What is the anterior visceral endoderm, and what is its function?
- The anterior visceral endoderm is a part of the hypoblast which lies at the cranial end
- It will secrete antagonists to the nodal signalling pathway at the cranial end, which specifies to the cells at this end that this should develop into the cranial end
When is the dorsal-ventral axis determined?
- During formation of the bilaminar disc
When is the cranio-caudal axis determined?
- During formation of the anterior visceral endoderm