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Here is a structured exam-ready summary of Embryology Lecture 9: Cleavage & Blastocyst.
Embryology L2 - Cleavage & Blastocyst: Exam Summary
Course: Cytology & Embryology | Geomedi 2026 | Dr. Giorgi Berishvili
1. Human Development Stages (Overview)
| Period | Timeframe |
|---|---|
| Germinal | Fertilization - Day 14 |
| Embryonic | Week 3 - 8 |
| Fetal | Week 9 - Birth |
| Neonate | First month after birth |
| Infant | 1 month - 1 year |
| Childhood | Infancy to puberty |
| Adolescence | Starting from puberty |
| Adulthood | 18-21 years |
| Senescence | Non-programmed, time-dependent decline |
2. Cleavage
- The zygote begins cleavage divisions without growth, producing blastomeres
- Blastomeres are totipotent (can form any cell type)
- Compaction occurs at the 8-cell stage - tight junctions form between outer cells, locking inner cells inside
- The compacted embryo divides into a 16-cell morula with:
- Inner cell mass (ICM) - forms the embryo proper
- Outer cell mass - forms the trophoblast (contributes to placenta)
Cleavage Mechanism
- Mammals undergo rotational cleavage
- Cleavage is developmentally asymmetrical - cellular polarity and positioning dictate divergent lineage fates
- Cleavage is also asynchronous (cells don't divide at exactly the same time)
3. Maternal vs. Zygotic Transcripts
- mRNA and rRNA steadily accumulate during oocyte maturation
- Most maternal RNA is degraded by the 2-cell stage
- By the 4-cell stage, >1,500 embryonic genes are activated (embryonic genome activation)
- Embryonic genes are initially inhibited by methylation
- Demethylation begins after fertilization and peaks at the morula stage
4. Embryo's Journey to the Uterus
- Fertilization occurs in the oviduct (near the ovary)
- First division: ~24 hours after fertilization
- Compaction: Day 4
- Embryo loses the zona pellucida (ZP) after entering the uterus
- Forms the blastocyst and begins implantation
5. Blastocyst Formation
- Uterine fluids enter the inner cell mass by cavitation, forming the blastocyst cavity
- Embryoblast cells accumulate at one pole (inner cell mass)
- Outer trophoblasts flatten to form the epithelial wall
- By Day 6: Trophoblast cells over the embryoblast pole begin penetrating the uterine mucosa - start of implantation
How Water Enters the Blastocyst
- Ion channels pump Na⁺ into the extracellular space
- Increased Na⁺ concentration draws in H₂O by osmosis
- Na⁺ and H₂O are replenished from uterine cavity fluid
6. Establishment of Polarities
- Embryoblast cells differentiate into:
- Epiblast (dorsal) - will form the embryo proper
- Hypoblast / Primitive endoderm (ventral, adjacent to blastocyst cavity)
- This establishes dorso-ventral polarity
- Some hypoblast cells become Anterior Visceral Endoderm (AVE) - establishes the cranial-caudal axis
7. Implantation - Day by Day
Day 8
- Trophoblast differentiates into:
- Cytotrophoblast - inner mononuclear layer
- Syncytiotrophoblast - outer multinuclear zone (no distinct cell boundaries)
- Embryoblast differentiates into bilaminar germ disc:
- Hypoblast - small cuboidal cells adjacent to blastocyst cavity
- Epiblast - high columnar cells adjacent to amniotic cavity
- Amniotic cavity forms within epiblast by cavitation
- Endometrial stroma = highly vascularized innermost part of the uterus
Day 9
- Vacuoles appear in the syncytium → merge → form trophoblastic lacunae ("lacunar stage")
- Exocoelomic cavity (primitive yolk sac) begins to form, surrounded by exocoelomic membrane
- Exocoelomic cavity replaces the blastocyst cavity
Day 10-12
- Blastocyst is completely embedded in endometrial stroma
- Syncytiotrophoblast contacts maternal capillaries (sinusoids) → uteroplacental circulation established
- Extraembryonic mesoderm forms (likely from hypoblast) between trophoblast and exocoelomic cavity
- Large cavities form within it = extraembryonic coelom
- Decidual reaction (decidualization): endometrial cells proliferate, accumulate glycogen and lipids, and vascularize to support implantation
Day 13
- Cytotrophoblast cells proliferate into syncytium forming primary villi
- Hypoblast proliferates → forms secondary (definitive) yolk sac (lined with visceral/extraembryonic endoderm)
- Cavities in extraembryonic mesoderm merge → form chorionic cavity (extraembryonic coelom), surrounded by chorionic plate
- This squeezes old exocoelomic cavity into exocoelomic cyst
- Embryo is connected to the surrounding structure via connecting stalk (future umbilical cord)
8. In Vitro Fertilization (IVF)
- Fertilization of egg + sperm in a laboratory dish → embryo transferred to uterus
- Preimplantation Genetic Diagnosis (PGD):
- One blastomere is removed from an early cleavage IVF embryo
- DNA tested for pathogenic mutations, structural abnormalities, and disease predispositions
- Both nuclear and mitochondrial DNA can be tested
Key Terms to Know for Exam
| Term | Definition |
|---|---|
| Blastomere | Cell produced by cleavage of the zygote |
| Compaction | 8-cell stage; tight junctions form between outer cells |
| Morula | 16-cell solid ball |
| Cavitation | Process of fluid accumulation forming the blastocyst cavity |
| Embryoblast | Inner cell mass; forms the embryo proper |
| Trophoblast | Outer cells; forms placenta |
| Cytotrophoblast | Inner mononuclear layer of trophoblast |
| Syncytiotrophoblast | Outer multinuclear invasive layer |
| Epiblast | Upper layer of bilaminar disc (gives rise to embryo) |
| Hypoblast | Lower layer (gives rise to extraembryonic structures) |
| Lacunar stage | Day 9; vacuoles form in syncytiotrophoblast |
| Decidualization | Endometrial stromal transformation to support implantation |
| Primary villi | Day 13; cytotrophoblast columns into syncytium |
| AVE | Anterior Visceral Endoderm; establishes cranial-caudal axis |
| Connecting stalk | Future umbilical cord |
Sources cited in lecture: Sadler (Langman's Medical Embryology, 14th ed.), Carlson (Human Embryology & Developmental Biology, 5th ed.), Gilbert (Developmental Biology), Moore, Wolpert.
Give me MCQ type questions from this pdf
Here are 30 MCQs based on the Embryology L2 - Cleavage & Blastocyst lecture, organized by topic:
MCQ Practice - Cleavage & Blastocyst
Section 1: Cleavage & Morula
1. What is the term for the cells produced by cleavage divisions of the zygote?
- A) Oocytes
- B) Blastomeres ✅
- C) Trophoblasts
- D) Epiblasts
2. At what cell stage does compaction occur?
- A) 2-cell stage
- B) 4-cell stage
- C) 8-cell stage ✅
- D) 16-cell stage
3. What type of junction forms between outer cells during compaction?
- A) Gap junctions
- B) Desmosomes
- C) Tight junctions ✅
- D) Adherens junctions
4. The 16-cell solid ball of cells is called:
- A) Blastocyst
- B) Gastrula
- C) Morula ✅
- D) Zygote
5. Blastomeres are described as totipotent. This means they:
- A) Can only form trophoblast
- B) Can only form the embryo proper
- C) Can form any cell type ✅
- D) Are already committed to a specific lineage
6. The inner cell mass of the morula eventually gives rise to:
- A) The placenta
- B) The trophoblast
- C) The embryo proper ✅
- D) The exocoelomic membrane
7. The outer cell mass of the morula forms the:
- A) Epiblast
- B) Hypoblast
- C) Trophoblast ✅
- D) Amniotic cavity
Section 2: Cleavage Mechanism & Gene Activation
8. What type of cleavage is characteristic of mammals?
- A) Radial cleavage
- B) Spiral cleavage
- C) Rotational cleavage ✅
- D) Bilateral cleavage
9. Mammalian cleavage is described as:
- A) Synchronous and symmetrical
- B) Asynchronous and asymmetrical ✅
- C) Synchronous and asymmetrical
- D) Asynchronous and symmetrical
10. Most maternal RNA transcription products are degraded by which stage?
- A) Morula stage
- B) 4-cell stage
- C) 2-cell stage ✅
- D) Blastocyst stage
11. By the 4-cell stage, approximately how many embryonic genes are activated?
- A) >500
- B) >1500 ✅
- C) >5000
- D) >100
12. Initially, embryonic genes are inhibited by:
- A) Acetylation
- B) Phosphorylation
- C) Methylation ✅
- D) Ubiquitination
13. Demethylation of the embryonic genome is maximum at which stage?
- A) 2-cell stage
- B) 4-cell stage
- C) Blastocyst stage
- D) Morula stage ✅
Section 3: Blastocyst Formation & Journey
14. Where does fertilization normally occur?
- A) Uterine cavity
- B) Cervix
- C) Oviduct ✅
- D) Ovary
15. Approximately when does the first cleavage division occur after fertilization?
- A) 6 hours
- B) 12 hours
- C) 24 hours ✅
- D) 48 hours
16. The process by which fluid accumulates to form the blastocyst cavity is called:
- A) Gastrulation
- B) Cavitation ✅
- C) Compaction
- D) Implantation
17. The inner cell mass of the blastocyst is also known as the:
- A) Trophoblast
- B) Embryoblast ✅
- C) Syncytiotrophoblast
- D) Cytotrophoblast
18. What drives water entry into the blastocyst?
- A) Active transport of H₂O
- B) Na⁺ pumping creating an osmotic gradient ✅
- C) K⁺ channel activity
- D) Direct pinocytosis from uterine glands
19. On approximately which day do trophoblast cells begin penetrating the uterine mucosa?
- A) Day 3
- B) Day 4
- C) Day 6 ✅
- D) Day 8
Section 4: Bilaminar Disc & Implantation
20. The two layers of the bilaminar germ disc are:
- A) Ectoderm and endoderm
- B) Epiblast and hypoblast ✅
- C) Cytotrophoblast and syncytiotrophoblast
- D) Mesoderm and ectoderm
21. The hypoblast is also known as:
- A) Primitive ectoderm
- B) Primitive endoderm ✅
- C) Extraembryonic mesoderm
- D) Epiblast
22. The amniotic cavity forms within which layer?
- A) Hypoblast
- B) Cytotrophoblast
- C) Epiblast ✅
- D) Syncytiotrophoblast
23. Which structure establishes the cranial-caudal axis of the embryo?
- A) Trophoblast
- B) Epiblast
- C) Anterior Visceral Endoderm (AVE) ✅
- D) Extraembryonic mesoderm
24. On Day 9, vacuoles appear in the syncytiotrophoblast forming:
- A) Primary villi
- B) Chorionic cavity
- C) Trophoblastic lacunae ✅
- D) Exocoelomic cyst
25. The primitive yolk sac is also called the:
- A) Secondary yolk sac
- B) Exocoelomic cavity ✅
- C) Chorionic cavity
- D) Amniotic cavity
26. Uteroplacental circulation is established when syncytiotrophoblast comes into contact with maternal:
- A) Lymphatics
- B) Veins
- C) Arteries
- D) Sinusoids (capillaries) ✅
27. What is the decidual reaction?
- A) Formation of primary villi
- B) Endometrial stromal cells proliferating and accumulating glycogen/lipids to support implantation ✅
- C) Breakdown of the zona pellucida
- D) Formation of the chorionic cavity
28. On Day 13, cytotrophoblast cells penetrate into the syncytium forming:
- A) Secondary villi
- B) Tertiary villi
- C) Primary villi ✅
- D) Chorionic villi
29. The connecting stalk visible on Day 13 is the future:
- A) Placenta
- B) Amnion
- C) Umbilical cord ✅
- D) Chorion
Section 5: IVF & Clinical
30. In Preimplantation Genetic Diagnosis (PGD), what is removed for testing?
- A) The entire blastocyst
- B) Polar body only
- C) One blastomere from an early cleavage embryo ✅
- D) A trophoblast biopsy only
Quick Answer Key
| Q | Ans | Q | Ans | Q | Ans |
|---|---|---|---|---|---|
| 1 | B | 11 | B | 21 | B |
| 2 | C | 12 | C | 22 | C |
| 3 | C | 13 | D | 23 | C |
| 4 | C | 14 | C | 24 | C |
| 5 | C | 15 | C | 25 | B |
| 6 | C | 16 | B | 26 | D |
| 7 | C | 17 | B | 27 | B |
| 8 | C | 18 | B | 28 | C |
| 9 | B | 19 | C | 29 | C |
| 10 | C | 20 | B | 30 | C |
Tip for exam: Pay special attention to the day-by-day timeline of implantation (Days 6, 8, 9, 10-12, 13) - these are very commonly tested. Also know the difference between cytotrophoblast vs syncytiotrophoblast and epiblast vs hypoblast.
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