Embryology Lecture 2 Ovulation to Implantation: Week 1 Flashcards
Fertilization is a process of several events and typically takes place in the ______ of the uterine tube
ampulla
True or False: Fertilization occurs within 48 hours of ovulation.
False, 24
What are the 3 main phases of fertilization?
3 Phases:
- Penetration of corona radiata
- Binding and penetration of zona pellucida
- Fusion of membranes of secondary oocyte and spermatozoa
What happens during penetration of corona radiata?
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¨The sperm uses both chemical and physical means
The action of membrane-bound enzyme hyaluronidase on its coat
Swimming motion of its flagellum
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What happens during the binding and penetration of zona pelluicida? (3 steps)
Sperm binds with ZP3 receptor present on zona pellucida
This triggers acrosome reaction resulting in the release of acrosomal enzymes:
ACROSIN (acrosin digests zona pellucida at the site of discharge, enabling the sperm to reach the cell membrane)
Sperm contact with the cell membrane of a secondary oocyte triggers CORTICAL REACTION:
Causes release of cortical granules (lysosome) from oocyte cytoplasm:
- both zona pellucida and oocyte membrane become impermeable to other sperms (prevents polyspermy)
What happens during the fusion of membranes of secondary oocyte and spermatozoon?
Entire sperm except the cell membrane enters into the cytoplasm of oocyte
Sperm mitochondria and tail degenerate
All mitochondria in the embryo (and all mitochondrial DNA) come from the mother
As soon as the sperm enters the oocyte…..
¨Second meiotic division completes resulting in the formation of:
Definitive oocyte:
Oocyte has a vesicular nucleus:
Pronucleus
Oocyte is metabolically activated
Oocyte pronucleus fuses with sperm pronucleus
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Summary of Fertilization Phases
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Summary of Fertilization Phases Flow Chart
A new fertilized human oocyte
What are the 3 results of fertilization?
- Diploid chromosome number is restored
- Sex of the fetus is determined (XX-female, XY- male)
- Cleavage begins (a series of mitotic cell divisions within the zygote)
What happens during the two-cell stage of cleavage?
¨Individual cells:
blastomeres
Mitotic divisions maintain diploid(2n) chromosome configuration
Cells become smaller
Blastomeres are totipotent (can become any type of cell)
What happens in the 4 cell stage of cleavage?
Second cleavage results in 4 equivalent blastomeres
Still in zona pellucida
What happens in the 8 cell stage of cleavage?
Third cleavage results in 8 blastomeres
The blastomeres are still equivalent
Zona pellucida present
What happens after 8 cell stage?
Compaction is caused by:
Increased cell-cell adhesion
Outer layer of cells with tight junctions
Inner layer communicate by gap junctions
Cells that are forced to the outside are destined to become trophoblast cells that will form the placenta
The i_nner cells_ will form the embryo and are called the inner cell mass
BLASTOMERES ARE TOTIPOTENT CELLS
16 cell stage embryo looks like a mulberry and is called ______. Explain its characteristics
Still surrounded by zona pellucida
Consists of:
Inner cell mass or embryoblast:
Gives rise to embryo proper
Outer cell mass or trophoblast:
Forms the placenta and provides nutrition to the embryo
Forms 3-4 days after fertilization
Explain what forms after the morula?
Cavitation
A fluid-filled cavity forms inside the morula
This cavity is known as the blastocyst cavity or blastocele, and the morula is now called a blastula or blastocyst
The inner cell mass is now known as the embryoblast and the outer cell mass becomes the trophoblast
Note that the embryo is still contained in the zona pellucida
What happens during late blastocyst?
At day 5 the blastocyst shows polarity:
Embryonic or animal pole
Side of blastocyst to which inner cell mass is attached
Abembryonic pole
Opposite side
A mature blastocyst consists of what 3 structures?
A mature blastocyst consists of:
Inner cell mass ( embryoblast), trophoblast (outer cell mass) and blastocele
Human embryo development in culture
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How does twinning work?
A: Cleavage of an early embryo with each half developing as a completely separate embryo.
B: Splitting of the inner cell mass of a blastocyst and the formation of two embryos enclosed in a common trophoblast. This is the most common mode of twinning.
C: The inner cell mass does not completely separate, or if portions of the inner cell mass secondarily rejoin, conjoined twins may result.
______ of the embryo from zona pellucida occurs just prior to implantation
Inability to _______ results in Infertility
_______ results in abnormal implantation in the uterine tube
Hatching
hatch
Premature hatching
One major function of the ______ is to prevent premature implantation
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zona pellucida
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______ is the process by which the blastocyst embeds itself into the endometrium of the uterus
Implantation
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Descrobve what normal implantation is
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•Normal implantation site: Posterior superior wall of the endometrium in the
functional layer, during the secretory phase of the menstrual cycle
• Time:
7days after fertilization
• Before implantation trophoblast proliferate and start differentiating into cytotrophoblast and syncytiotrophoblast
Describe the process of normal implantation
The polar trophoblasts touch the endometrium and secrete proteolytic enzymes which dissolve the endometrium
The trophoblast becomes two-layered:
Syncytiotrophoblasts
Cytotrophoblasts
Syncytiotrophoblasts invade the endometrial stroma
The embryo is partially embedded in the endometrium
What is placental Previa? How can you diagnose it?
When implantation occurs in the lower parts of the uterus towards the cervix
This makes it easy for the placenta to tear, and the mother can die from hemorrhage, or the placenta may grow to obstruct the cervical canal
Can be diagnosed with ultrasound, and the baby is delivered via Cesarean section
What is the term for pregnancy outside of the uterus?
Ectopic pregnancy
Tubal – most common
Ovarian
Abdominal
Ectopic Implantation Summary
Endometriosis: Presence of endometrium like tissue in abnormal locations. Endometriosis most commonly involves ovaries, fallopian tubes and the tissue lining the pelvis.
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Week 1 General Summary
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Summary of 1st Week of Development (Day 1- 7)
