First Week of Development: Ovulation to Implantation Flashcards
3 events take place during the 1st week of development
Fertilization
II. Cleavage and Blastocyst Formation
III. Implantation
What is Fertilization?
The process that male and female gametes fuse and give rise to the formation of zygote
What is the usual site for fertilization?
the ampulla of the uterine tube
How long does fertilization usually take place?
24 hours
Spermatozoa are not able to fertilize the oocyte immediately upon arrival in the female genital tract.
→ What are the 2 processes that they have to undergo to acquire this capability?
(1) capacitation and (2) the acrosome reaction t
What is capacitation?
glycoprotein coat and seminal plasma pro- teins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa.
6 stages of fertilization
- Passage of the sperm through the corona radiata
- Penetration of the zone pellucida
- Fusion of plasma membranes of the oocyte and sperm
- Completion of the 2nd meiotic division of oocyte and formation of female pronucleus
- Formation of the male pronucleus
- The 2 pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote
Fertilization has 6 stages.
What is stage 1? Describe the process.
Passage of a sperm through the corona radiata
- For sperms to pass through the corona radiata, they must have been capacitated
Which type of sperms can pass freely through the corona radiata?
Only capacitated sperms
Fertilization has 6 stages.
What is stage 2? Describe the process.
Penetration of the zona pellucida
- The intact acrosome of the sperms binds with a zone glycoprotein (ZP3) on the zona pellucida (on oocyte surface)
What is zona?
a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction.
What is zona?
a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction.
What is acrosome reaction?
- occurs after binding to the zona pellucida
- induced by zona proteins.
- This reaction culminates in the release of enzymes needed to penetrate the zona pellucida, including acrosin- and trypsin-likesubstances
What happen to permeability of zona pellucida when a sperm comes in contact with the oocyte surface?
It changes
When a sperm comes in contact with the oocyte surface? Which enzymes are released? The role of
- Lysosomal enzymes are released from cortical granules lining the plasma membrane of the oocyte
- → Alter properties of the zona pellucida
When a sperm comes in contact with the oocyte surface
→ Lysosomal enzymes are released from cortical granules lining the plasma membrane of the oocyte
→ The role of these enzymes?
- Alter properties of the zona pellucida to
- prevent sperm penetration
- inactivate binding sites for spermatozoa on the zona pellicida surface
Fertilization has 6 stages.
What is stage 3? Describe the process.
Fusion of plasma membranes of the oocyte and sperm
- The plasma or cell membranes of the oocyte and sperm fuse and break down at the area of fusion
- The head and tail of sperm enter
- The plasma membrane of sperm remains behind
Fertilization has 6 stages.
What is stage 4? Describe the process.
Completion of the second meiotic division of oocyte and formation of female pronucleus
- Penetration of the oocyte by a sperm activates the oocyte into completing the second meiotic division and forming a mature oocyte and a second polar body
- The nucleus of the mature ovum/oocyte is now called the female pronucleus
Fertilization has 6 stages.
What is stage 5? Describe the process.
Formation of the male pronucleus
→ Within the cytoplasm of the oocyte, the nucleus of the sperm enlarges to form the male pronucleus and the tail of the sperm degenerates
Origin of all mitochondria in zygote (maternal or paternal?)
Since all sperm mitochondria degenerate, all mitochondria within the zygote are of maternal origin (i.e., all mitochondrial DNA is of maternal origin)
What is an ootid?
The oocyte containing two haploid pronuclei
Fertilization has 6 stages.
What is stage 6? Describe the process.
The 2 pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote
→ The chromosomes in the zygote become arranged on a cleavage spindle in preparation for cleavage of the zygote
What is cleavage?
a series of repeated mitotic divisions of the zygote resulting in a rapid increase in the number of cells (embryonic cells)
What are blastomeres?
a type of cell produced by cell division (cleavage) of the zygote after fertilization
When does cleavage normally occur?
When the zygote passes along the uterine tube toward the uterus
Division of the zygote into blastomeres begins when?
begins approximately 30 hours after fertilization (day 2)
Cleavage normally occurs as the ___ passes along the uterine tube toward the uterus
Zygote
Cleavage normally occurs as the zygote passes along the ___ to __
uterine tube
the uterus
Once the zygote has reached the two-cell stage, it undergoes a series of ___
mitotic divisions
Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, in- creasing the numbers of cells. These cells, which become smaller with each cleavage division, are known as ___
blastomeres
How can the zygote reach eight-cell stage?
Division of zygote into blastomeres
- 1st cleavage → division of zygote → 2-cell stage
- 2nd cleavage → 2 cell stage to 4 cell stage
- 3rd cleavage → 4 cell stage to eight-cell stage
Which process happening after 8-cell stage?
Compaction
What is compaction?
The process that segregates inner cells, which communicate extensively by gap junctions, from outer cells.
How can an 8-stage cell becomes a morula cell?
3 days after fertilization
→ cells of the compacted embryo divide again to form a 16-cell morula (mulberry).
How many blastomeres can a morula contain?
16 - 32 blastomeres
Describe the morula cell (inner, outer, what do they give rise to?)
- Inner cells of the morula constitute the inner cell mass → give rises to tissues of the embryo proper
- surrounding cells compose the outer cell mass → form trophoblast → contributes to placenta
The process that happen when the morula enters the uterus (approx. 4 days after fertilization)? (name)
Blastocyst formation
What happen immediately when the morula enters the uterine cavity?
a fluid-filled space called the blastocystic cavity appears inside the morula
→ The fluid passes from the uterine cavity through the zona pellucida to form this space
BLASTOCYST FORMATION
Gradually, the intercellular spaces become confluent, and finally, a ___ formsGradually, the intercellular spaces become confluent, and finally, a single cavity, the blastocele, forms
single cavity, the blastocele
BLASTOCYST FORMATION
During this process, what are cells of the inner cell mass and outer cell mass called?
- Inner cell mass → embryoblast
- outer cell mass → trophoblast
BLASTOCYST FORMATION
What happen by the late blastocyst stage (5th day; the stage where both embryoblast and trophoblast are present)?
(Hint: zona pellucida;
- The zona pellucida has disappeared → allowing implantation to begin
BLASTOCYST FORMATION
Is there any increases in the size of embryo during blastocyst formation? Why?
Although cleavage increases the number of blastomeres, note that each of the daughter cells is smaller than the parent cells
→ As a result, there is no increase in the size of the developing embryo until the zona pellucida degenerates
BLASTOCYST FORMATION
Why does the zona pellucida have to degenerate?
The zona pellucida must degenerate for implantation to occur
Under the influence of fibroblast growth factors (FGFs) at the early blastocyst stage, cells in the embryoblast differentiate into (1)___ and (2)___ cells
epiblast
hypoblast
What is anterior Visceral endoderm (AVE)?
some of the hypoblast cells are specified to form the anterior Visceral endoderm (AVE)
→ migrate to what will become the cranial end of the embryo
The role anterior Visceral endoderm (AVE) cells?
responsible for secreting nodal antagonists, including cerberus and leftyl
→ act on adjacent epiblast cells to specify the cranial end of the embryo.
3 layers of the wall of uterus
- Endometrium or mucosa lining the inside wall
- Myometrium, a thick layer of smooth muscle
- Perimetrium, the peritoneal covering lining the outside wall
3 layers of the wall of uterus
→ What is endometrium?
mucosa lining the inside wall
3 layers of the wall of uterus
→ What is Myometrium?
a thick layer of smooth muscle
3 layers of the wall of uterus
→ What is Perimetrium?
the peritoneal covering lining the outside wall
IMPLANTATION
Approximately 6 days after fertilization (day 20 of a 28-day menstrual cycle), the blastocyst attaches to the__
endometrial epithelium
IMPLANTATION
During menstrual cycle, what are the 3 phases that the uterine endometrium passes through?
- Follicular or proliferative phase
- Secretory or progestational phase
- Menstrual phase
IMPLANTATION
During menstrual cycle, what happen during Follicular or proliferative phase?
- Begins at the end of the menstrual phase, is under the influence of estrogen
- Parallels growth of the ovarian follicles.
IMPLANTATION
During menstrual cycle, what happen during Secretory or progestational phase?
begins approximately 2 to 3 days after ovulation in response to progesterone produced by the corpus luteum.
IMPLANTATION
During menstrual cycle, what happen during Secretory or progestational phase?
Only happen if fertilization does not occur
→ shedding of the endometrium (compact and spongy layers) marks the beginning of the menstrual phase
→ blood escapes from superficial arteries
→ small pieces of stroma and glands break away.
→ the basal layer is the only part of the endometrium that is retained
IMPLANTATION
During menstrual cycle, what happen if fertilization does occur?
the endometrium assists in implantation → contributes to formation of the placenta.
IMPLANTATION
Later in gestation, what does placenta do? What happen to corpus luteum?
- The placenta assumes the role of hormone production
- The corpus luteum degenerates.
IMPLANTATION
At the time of implantation, the mucosa of the uterus is in the __ phase
- The placenta assumes the role of hormone production
- The corpus luteum degenerates.
IMPLANTATION
At the time of implantation, the mucosa of the uterus is in the secretory phase.
→ What are the 3 layers that can be recognized the endometrium?
- a superficial compact layer,
- an intermediate spongy layer
- a thin basal layer
IMPLANTATION
If the oocyte is not fertilized, (1)____ and (2)___ spaces gradually become packed with (3)____ cells
- venules
- sinusoidal
- blood
IMPLANTATION
What are basal arteries? Their role?
- The arteries that supply the basal layer
- functions as the regenerative layer in the rebuilding of glands and arteries in the proliferative phase
IMPLANTATION
As soon as it attaches to the endometrial epithelium, the trophoblast starts to proliferate rapidly and gradually differentiates into 2 layers
→ What are these 2 layers?
- The arteries that supply the basal layer
- functions as the regenerative layer in the rebuilding of glands and arteries in the proliferative phase
What is the role of the corpus luteum, and what is its origin? (Langman)
What are the 3 phases of fertilization, and what reactions occur once fusion of the sperm and oocyte membranes takes place? (Langman)
