Embryology 3: Fertilisation & Implantation MCQ

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Questions and Answers

What is the primary function of the syncytiotrophoblast during early pregnancy?

To transport nutrients directly to the embryo

To protect the embryo from maternal immune response

To erode the endometrium and establish a pathway for the blastocyst (correct)

To produce progesterone for endometrial maintenance

What does the corpus luteum primarily produce to support early pregnancy?

Interleukins

Progesterone (correct)

Estrogen

Human Chorionic Gonadotropin (hCG)

Which type of trophoblast is primarily responsible for invasive behavior during implantation?

Syncytiotrophoblast (correct)

Decidual cells

Endometrial stroma

Cytotrophoblast

What is the role of the lacunae formed by the syncytiotrophoblast?

To fill with maternal blood that provides nutrients to the embryo Signup and view all the answers

Which factor is produced early in pregnancy that can be found in maternal blood by day 8?

Human Chorionic Gonadotropin (hCG) Signup and view all the answers

What is the purpose of the decidual reaction in the endometrium?

To support the implantation process and maintain pregnancy Signup and view all the answers

Which of the following describes the main role of interleukins produced by decidual cells?

To support implantation or inhibit invasive processes Signup and view all the answers

What might a woman mistake for her menstrual period during early pregnancy due to the embryonic process?

Light blood loss caused by erosion of endometrial vessels Signup and view all the answers

What is one of the outcomes of fertilization?

Restoration of the normal diploid number Signup and view all the answers

What role does the zona pellucida play during fertilization?

It acts as an immunological barrier to the embryo. Signup and view all the answers

What is the primary effect of cleavage on the zygote?

It increases the nuclear/cytoplasmic ratio. Signup and view all the answers

How long does it typically take for cleavage to begin after fertilization?

Around 30 hours post-fertilization Signup and view all the answers

What are inner blastomeres?

Blastomeres not in contact with the zona pellucida Signup and view all the answers

What is accomplished during the process of compaction?

Maximization of cell-cell contact among blastomeres Signup and view all the answers

What is produced after cleavage leads to 16 blastomeres?

Morula Signup and view all the answers

What is a consequence of the breakdown of protoamines in the male nucleus?

Decondensation of genetic material Signup and view all the answers

What is the main role of the decidual reaction in early pregnancy?

To nourish the embryo and provide an immunologically privileged site Signup and view all the answers

Which of the following structures is external to the embryo during early implantation?

Decidua basalis Signup and view all the answers

What is the primary factor that differentiates the epiblast from the hypoblast?

Transcription factors Signup and view all the answers

What does an ectopic pregnancy typically lead to?

Rupturing of the uterine tube and possible hemorrhage Signup and view all the answers

Which type of decidua lines the amniotic sac?

Decidua capsularis Signup and view all the answers

Which of the following is NOT a common risk factor for ectopic pregnancy?

Excessive exercise Signup and view all the answers

What characterizes the normal site of implantation in the uterus?

Posterior wall of the uterine cavity Signup and view all the answers

What is the embryonic shield primarily composed of?

Epiblast and hypoblast Signup and view all the answers

What is the primary function of hyaluronidase released by the acrosome during fertilisation?

To break down hyaluronic acid in the corona radiata Signup and view all the answers

Where does the optimal environment for the sperm and oocyte meeting occur?

In the ampullary region of the Fallopian tube Signup and view all the answers

What prevents polyspermy after fertilisation?

Granules produced by the oocyte Signup and view all the answers

What role do zonal proteins like ZP3 play in the fertilisation process?

They are species-specific binding sites for sperm Signup and view all the answers

Which phase of fertilisation involves the attachment of one spermatozoon to the oocyte?

Phase 3 Signup and view all the answers

What occurs during Phase 2 of fertilisation after spermatozoa penetrate the zona pellucida?

Enzymes digest the membrane of the oocyte Signup and view all the answers

Which structures of the spermatozoon enter the cytoplasm of the egg during fertilisation?

Head, mid piece, and tail Signup and view all the answers

What is the size relationship between the egg and the sperm cell?

The egg is 10 million times the volume of the sperm cell Signup and view all the answers

What is the main function of the parietal endoderm?

To support the amnion and other structures Signup and view all the answers

Where is the amniotic cavity formed?

In the epiblast Signup and view all the answers

What is the relationship between the epiblast and the yolk sac?

The yolk sac is ventral to the epiblast Signup and view all the answers

What forms the outer wall of the blastocyst?

Chorion Signup and view all the answers

What happens to the yolk sac by the 13th day of development?

It becomes much smaller Signup and view all the answers

Which structure is primarily responsible for enclosing the amniotic cavity?

Epiblast Signup and view all the answers

Why is the yolk sac not useful for nutrient provision in humans?

The embryo receives nutrients directly from the mother's capillaries Signup and view all the answers

What structure connects the yolk sac and the amniotic sac to the mesoderm?

Body stalk Signup and view all the answers

What is the primary function of the primitive blood vessels formed in the extraembryonic mesoderm?

To facilitate the exchange of materials between mother and embryo Signup and view all the answers

What occurs as the cytotrophoblast extends into the syncytiotrophoblast?

Development of the primary villus Signup and view all the answers

Which statement about spontaneous abortion (SA) is correct?

More than 50% of SA result from chromosomal abnormalities. Signup and view all the answers

What complication might arise if red blood cells from the embryo enter the mother's circulation?

Potential incompatibility issues between maternal and fetal blood Signup and view all the answers

What is a possible consequence of something from the mother's blood reaching the fetus?

Introduction of medications or toxins into the fetal blood Signup and view all the answers

What is the most common timing for spontaneous abortion?

In the first trimester, especially around the 3rd week Signup and view all the answers

What is the role of the tertiary villus in early pregnancy?

To enable nutrient and gas exchange between maternal blood and the embryo Signup and view all the answers

What might cause a higher incidence of spontaneous abortion in certain fetuses?

Presence of chromosomal abnormalities or specific birth defects Signup and view all the answers

Study Notes

Fertilization Process

Egg cell is significantly larger than a sperm cell (10 million times the volume)
Fertilization takes place in the ampullary region of the Fallopian tube
The folds in the Fallopian tube slow down the oocyte, creating an optimal environment
Many changes are needed for fertilization

Phase 1 of Fertilization

Spermatozoa swim through the granulosa cells of the corona radiata
Hyaluronidase is released from the acrosome, breaking down the hyaluronic acid
Spermatozoa swim rapidly toward the zona pellucida
Tubal enzymes may aid the process

Phase 2 of Fertilization

Spermatozoa reach the zona pellucida and attach to it
Spermatozoa bind to species-specific zonal proteins (mainly ZP3)
Enzymes (like acrosin) are released from the acrosome, digesting the zona pellucida membrane
Sperm head fuses with the zona pellucida
Sperm enters the perivitelline space between the zona pellucida and the plasma membrane
Only one sperm will attach to the oocyte's plasma membrane

Phase 3 of Fertilization

One sperm attaches to the oocyte's microvilli
Sperm's plasma membrane fuses with the oocyte's membrane
Sperm head, midpiece, and tail enter the oocyte's cytoplasm (mitochondria remain outside)
Juno receptors from the oocyte membrane are ejected to prevent polyspermy (multiple sperm fertilizing an egg)
Paternal mitochondria are eliminated

Phase 4 of Fertilization

Prevention of polyspermy:
- Fast block: membrane depolarization in a few seconds, preventing further sperm entry
- Permanent block: Changes in calcium concentration trigger cortical granules to release their contents into the perivitelline space, causing the zona pellucida to swell and harden

Phase 5 of Fertilization

Oocyte completes meiosis II and releases a second polar body
Two pronuclei (oocyte's and sperm's) form, fuse
Fertilized egg (zygote) ready for cleavage

Importance of Calcium

Calcium changes induce meiosis resumption, pronucleus formation and polyspermy prevention

Cleavage

Series of cell divisions increasing the cell number, without the zygote increasing in size
Occurs around 30 hours after fertilization
Resulting cells are called blastomeres
Blastomeres compact to form a morula, then a blastocyst (within 3 days)

Blastocyst Formation

Inner cell mass (embryoblast): forms the embryo
Trophoblast: forms the placenta
Blastocoele: fluid-filled cavity within the blastocyst
Zona pellucida sheds allowing blastocyst to hatch

Early Communication with Mother

Trophoblast produces the Early pregnancy factor to prevent maternal immune attack

Blastocyst Polarity

Embryonic (inner cell mass) pole and abembryonic (opposite) pole
Trophoblast on embryo side is polar trophoblast; other side is mural trophoblast

Implantation

Blastocyst attaches firmly to the endometrium
Three stages:
- Apposition: blastocyst approaches the endometrium
- Adhesion: close contact with adhesion molecules
- Invasion: blastocyst penetrates the endometrium
Implantation window: occurs 6 days after the LH surge
Cytokines aid implantation readiness: LIF
Immunotolerance regulation of the embryo as a semi-allograft

Trophoblast

Syncytiotrophoblast: invasive, erodes endometrium for implantation
Cytotrophoblast: provides nourishment for embryo: cells fuse to form the primary, secondary and tertiary villi

Chorionic Cavity

Cavity outside of the embryo formed from the extaembryonic mesoderm and cytotrophoblast
The yolk sac and body stalk form, connecting the embryo with the developing placenta

Uterine Mucosa

Provides the suitable environment for implantation before the fertilized egg's arrival.
Endometrium prepared for implantation at the end of menstrual cycle.

Ectopic Pregnancy

Implantation occurs outside the uterine cavity, most often in the fallopian tubes (and potentially other locations)
May cause hemorrhage.

Early Pregnancy

Corpus luteum produces progesterone to maintain endometrial function
Human Chorionic Gonadotropin (hCG) maintains the corpus luteum and detectable in maternal blood by day 8 and urine by day 10.
Immune response regulated by maternal immune system.
Blood vessels formation inside the chorion for improved nutrient absorption

Second Week (of Development)

Inner cell mass forms the epiblast and hypoblast
Amniotic cavity and yolk sac formation
Beginning of primary, secondary, and tertiary villi formation in the chorion
Early circulatory systems begin developing to supply oxygen and nutrients.

Important Notes:

The process described is for human fertilization.
The provided information summarizes the initial steps of prenatal development.

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Description

Test your knowledge on the biological processes and structures involved in early pregnancy. This quiz covers key concepts such as the syncytiotrophoblast, corpus luteum, and cleavage stages. Ideal for students studying human development or reproductive biology.