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Here is a complete exam-focused summary of the lecture:
Embryology Lecture 1 - Gametogenesis & Fertilization
Geomedi University | Dr. Giorgi Berishvili, PhD
1. Soma vs Germline
- The body (soma) is a temporary carrier of the germline - the cell lineage that produces gametes.
- Only germ cells (egg and sperm) pass genetic information to offspring, not somatic cells.
- Classic quote: "A hen is only an egg's way of making another egg." (Samuel Butler)
2. Germ Cell Attributes
| Function | Mechanism |
|---|---|
| Transmit genetic info | Meiosis halves chromosome number (diploid → haploid) |
| Preserve genetic integrity | Suppression of mutations; enhanced apoptosis |
| Generate genetic diversity | Crossing over + random reassortment of homologs |
3. Meiosis
Overview
- Two sequential divisions (Meiosis I and II) with no DNA replication between them.
- Reduces diploid (2n) to haploid (n).
Prophase I - Five Sub-Stages (KEY for exams)
| Stage | Key Events |
|---|---|
| Leptonema | Chromosomes become visible as thin threads; homology search begins |
| Zygonema | Chromosomes condense further; rough pairing of homologs; synaptonemal complex begins forming |
| Pachynema | Full synaptonemal complex formed; crossing over takes place (avg. 35 crossovers/cell in humans) |
| Diplonema | Synaptonemal complex dissolves; chromatids remain linked at chiasmata; in human oocytes, this stage can last 40+ years |
| Diakinesis | Terminalization (chiasmata move to ends); nucleolus and nuclear envelope break down; chromosomes attach to spindle fibers |
Crossing Over (Pachynema)
- Occurs between non-sister chromatids of homologous chromosomes (bivalents/tetrads).
- Enabled by the synaptonemal complex.
- Results in genetic recombination.
- Average crossovers: Males = 26/cell; Females = 44/cell.
Subsequent Meiosis I Stages
- Metaphase I: Tetrads align on metaphase plate.
- Anaphase I: Cohesin degrades; homologs pulled to opposite poles (chromosome disjunction).
- Telophase I: Nuclear membrane reforms; cell divides into two daughter cells; no S phase follows.
- Nondisjunction: Failure to separate homologs → extra chromosome → e.g., Down syndrome (trisomy 21).
Meiosis II (similar to mitosis)
- Prophase II: Dyads (sister chromatid pairs) present.
- Metaphase II: Centromeres on equatorial plate; kinetochores point in opposite directions.
- Anaphase II: Centromeric cohesins degrade; chromatid disjunction - sister chromatids pulled apart.
- Telophase II: Cytokinesis; each cell now has a unique mix of maternal/paternal genetic info.
4. Gametogenesis
Origin of Germ Cells
- All gametes arise from primordial germ cells (PGCs), formed during early embryogenesis (~Day 24).
- PGCs migrate to the gonadal ridge to form gonads.
- Misdirected PGCs usually die; if they survive, they can form teratomas.
5. Oogenesis
| Stage | Key Points |
|---|---|
| PGCs arrive at gonadal ridge | Differentiate into oogonia |
| Oogonia divide mitotically | Arrange in clusters with follicular cells |
| Some oogonia enter meiosis | Become primary oocytes |
| Most oogonia eliminated | By apoptosis; only a few remain as primary oocytes in primordial follicles |
| Primary oocytes | Arrested in meiotic prophase until puberty |
| Folliculogenesis | Primordial → primary → antral follicle; 90% die by apoptosis |
| LH surge | Induces preovulatory growth phase |
| FSH | Induces follicle maturation |
| Meiosis I completion | Produces secondary oocyte + first polar body |
| Secondary oocyte | Arrested in metaphase II ~3 hours before ovulation |
| Meiosis II completion | Only if fertilization occurs; otherwise degenerates within 24 hours |
Key Structures
- Zona pellucida (ZP): Glycoprotein coat secreted by granulosa cells and oocyte; mediates sperm binding, species specificity, and blocks polyspermy.
- Granulosa cells: Supply oocyte with growth factors via gap junctions.
- Theca layer: Surrounding connective tissue (stromal cells).
- Antrum: Fluid-filled space between granulosa cells in vesicular/antral follicle.
6. Spermatogenesis
| Stage | Key Points |
|---|---|
| Sex cords (gonadal ridge) | Contain PGCs; supported by Sertoli cells (nourishment) and Leydig cells (testosterone) |
| Regulation | LH from pituitary → spermatogenesis; FSH → Sertoli cells → testicular fluid |
| At puberty | Sex cords acquire lumen → seminiferous tubules; PGCs → spermatogonia |
| Meiosis I | Spermatogonia → primary spermatocytes → secondary spermatocytes |
| Meiosis II | Secondary spermatocytes → haploid spermatids |
| Spermiogenesis | Spermatids → mature spermatozoa |
Spermiogenesis Steps
- Acrosome formation (helps penetrate egg)
- Nucleus condensation
- Formation of neck and flagellum
- Mitochondrial sheath around flagellum (energy)
- Shedding of most cytoplasm (residual body)
- Fully formed spermatozoa enter seminiferous tubule lumen → epididymis → acquire full motility.
7. Fertilization
Barriers Sperm Must Penetrate
- Corona radiata - cloud of cumulus cells (broken down by enzymatic activity)
- Zona pellucida - broken down by enzymes released in the acrosomal reaction
- Sperm surface receptors bind egg surface ligands → membrane fusion → sperm nucleus enters cytoplasm.
Sequence of Events
- Sperm penetrates corona radiata and zona pellucida.
- Acrosomal reaction releases enzymes.
- Membrane fusion; sperm nucleus enters cytoplasm.
- Egg completes meiosis II.
8. Post-Fertilization Events
Prevention of Polyspermy
- Cortical reaction: Cortical granules release lysosomal enzymes → membrane becomes impenetrable.
- Zona reaction: ZP hardens, alters structure to prevent further sperm binding.
Pronucleus Formation & Fusion
- Egg chromosomes form female pronucleus.
- Sperm nucleus becomes male pronucleus.
- Spermatozoa loses its tail.
- Male mitochondria are destroyed by ubiquitination (→ maternal-only mitochondrial inheritance).
- Sperm centrosomes form microtubules connecting to egg microtubules.
- Two pronuclei migrate toward each other and fuse into a single diploid nucleus.
Results of Fertilization (3 Key Outcomes)
- Restoration of diploid chromosome number
- Determination of sex (X-sperm = female; Y-sperm = male)
- Initiation of cleavage
9. Special Topics
Age Effects on Mutation Rates
- Sperm DNA undergoes far more replication cycles than oocyte DNA.
- Therefore, many mutations are more often paternal in origin.
Mitochondrial Inheritance
- Mitochondria (and mtDNA disorders) are inherited from the mother only (paternal mitochondria are ubiquitinated).
- Homoplasmy: Daughter cells receive a pure population of mtDNA.
- Heteroplasmy: Daughter cells receive a mixture (some mutant, some normal) mtDNA.
- Distribution at cell division is random - called replicative segregation.
High-Yield Exam Points
- Diplonema in oocytes can last 40+ years (arrested until ovulation).
- Secondary oocyte is arrested in metaphase II until fertilization.
- If unfertilized, oocyte degenerates in 24 hours.
- Nondisjunction in Meiosis I → Down syndrome (trisomy 21).
- Only maternal mitochondria are inherited (paternal are ubiquitinated).
- LH surge → ovulation; FSH → follicle maturation.
- Misdirected PGCs can cause teratomas.
Give me MCQ type questions from this pdf
Based on the lecture content I already extracted, here are comprehensive MCQ questions covering all topics from the PDF:
MCQ Practice Questions - Gametogenesis & Fertilization
Section 1: Meiosis - Basics
1. Which of the following correctly describes the germline?
- A) It ages along with the soma
- B) It is a temporary carrier of genetic information
- C) A non-aging lineage of cells that give rise to gametes
- D) It inherits characteristics from somatic cells
✅ Answer: C - The germline is a non-aging lineage; the body (soma) is merely its temporary carrier.
2. What is the primary purpose of meiosis in germ cells?
- A) To repair DNA mutations
- B) To reduce chromosome number from diploid to haploid
- C) To increase mitochondrial number
- D) To produce identical daughter cells
✅ Answer: B - Meiosis reduces chromosome number by half, allowing fertilization to restore the diploid number.
3. Which event does NOT occur during meiosis to generate genetic diversity?
- A) DNA recombination during crossover
- B) Random distribution of homologous chromosomes
- C) DNA replication between Meiosis I and II
- D) Reassortment of chromosomes in daughter cells
✅ Answer: C - There is NO DNA replication (no S phase) between Meiosis I and Meiosis II.
Section 2: Prophase I Sub-stages
4. During which stage of meiotic prophase I does crossing over actually take place?
- A) Leptonema
- B) Zygonema
- C) Pachynema
- D) Diplonema
✅ Answer: C - Crossing over occurs at pachynema when the synaptonemal complex is fully formed.
5. Chiasmata are visible during which stage of prophase I?
- A) Leptonema
- B) Zygonema
- C) Pachynema
- D) Diplonema
✅ Answer: D - At diplonema, the synaptonemal complex dissolves and chromatids remain intertwined at chiasmata.
6. Which stage of prophase I is characterized by the beginning of homology search?
- A) Pachynema
- B) Zygonema
- C) Leptonema
- D) Diakinesis
✅ Answer: C - At leptonema, chromosomes become visible as thin threads and homology search begins.
7. In human oocytes, the diplonema stage can last for:
- A) A few hours
- B) Days to weeks
- C) More than 40 years
- D) Until birth only
✅ Answer: C - In human oocytes, diplonema can last more than 40 years (arrested until just before ovulation).
8. The synaptonemal complex begins to form during which stage?
- A) Leptonema
- B) Zygonema
- C) Pachynema
- D) Diplonema
✅ Answer: B - The synaptonemal complex begins forming at zygonema and is fully formed by pachynema.
9. During diakinesis, the movement of chiasmata toward the ends of the tetrad is called:
- A) Synapsis
- B) Disjunction
- C) Terminalization
- D) Replicative segregation
✅ Answer: C - Terminalization is the process where chiasmata move toward the ends of the tetrad during diakinesis.
10. A "bivalent" consists of:
- A) 2 chromosomes, 2 chromatids
- B) 2 homologous chromosomes, 4 chromatids
- C) 1 chromosome, 2 chromatids
- D) 4 chromosomes, 4 chromatids
✅ Answer: B - A bivalent (also called a tetrad) = 2 paired homologs, each made of 2 sister chromatids = 4 chromatids total.
Section 3: Meiosis I & II Stages
11. During Anaphase I, what holds sister chromatids together while homologs are pulled apart?
- A) Zona pellucida
- B) Synaptonemal complex
- C) Cohesin
- D) Chiasmata
✅ Answer: C - Cohesin keeps sister chromatids together during Anaphase I; it is degraded in Anaphase II.
12. The separation of homologous chromosomes during Anaphase I is called:
- A) Chromatid disjunction
- B) Chromosome disjunction
- C) Replicative segregation
- D) Terminalization
✅ Answer: B - Separation of homologs in Anaphase I = chromosome disjunction. Separation of sister chromatids in Anaphase II = chromatid disjunction.
13. Down syndrome (Trisomy 21) most often results from:
- A) Crossing over during pachynema
- B) Nondisjunction during Meiosis I
- C) Failure of acrosomal reaction
- D) Nondisjunction during Meiosis II
✅ Answer: B - Down syndrome most often results from nondisjunction of Chromosome 21 pair during Meiosis I.
14. What is unique about the interphase between Meiosis I and Meiosis II?
- A) It is very long with extensive DNA repair
- B) DNA replication (S phase) occurs
- C) No DNA replication takes place
- D) Crossing over occurs again
✅ Answer: C - There is NO S phase between Meiosis I and II, as chromosomes already consist of two chromatids.
15. During Metaphase II, the kinetochores of sister chromatids point:
- A) Toward the same pole
- B) In opposite directions
- C) Toward each other
- D) Randomly
✅ Answer: B - In Metaphase II, kinetochores of sister chromatids point in opposite directions (unlike Meiosis I).
Section 4: Origin of Germ Cells & Oogenesis
16. Misdirected primordial germ cells (PGCs) that survive may develop into:
- A) Fibromas
- B) Teratomas
- C) Seminomas
- D) Adenomas
✅ Answer: B - Misdirected PGCs that survive can develop into teratomas.
17. When PGCs arrive at the gonadal ridge in females, they differentiate into:
- A) Primary oocytes
- B) Secondary oocytes
- C) Oogonia
- D) Granulosa cells
✅ Answer: C - PGCs at the gonadal ridge differentiate into oogonia, which then divide mitotically.
18. Primary oocytes are arrested at which stage of meiosis until puberty?
- A) Metaphase II
- B) Metaphase I
- C) Meiotic prophase I
- D) Telophase I
✅ Answer: C - Primary oocytes remain arrested in meiotic prophase I until puberty.
19. After the LH surge, Meiosis I is completed, producing:
- A) A mature ovum and two polar bodies
- B) A secondary oocyte and a first polar body
- C) Two equal-sized secondary oocytes
- D) A primary oocyte and a polar body
✅ Answer: B - Completion of Meiosis I (unequal division) produces one secondary oocyte + one first polar body.
20. At what point does the secondary oocyte arrest during its maturation?
- A) Prophase II
- B) Anaphase II
- C) Metaphase II
- D) Telophase II
✅ Answer: C - The secondary oocyte arrests in Metaphase II approximately 3 hours before ovulation.
21. If the secondary oocyte is NOT fertilized, it degenerates approximately:
- A) 6 hours after ovulation
- B) 12 hours after ovulation
- C) 24 hours after ovulation
- D) 48 hours after ovulation
✅ Answer: C - An unfertilized secondary oocyte degenerates approximately 24 hours after ovulation.
22. The zona pellucida is secreted by:
- A) Theca cells only
- B) Granulosa cells and the oocyte
- C) PGCs only
- D) Leydig cells
✅ Answer: B - The zona pellucida is a glycoprotein layer secreted by both granulosa cells and the oocyte.
23. What is the function of the zona pellucida? (Select the INCORRECT statement)
- A) Mediates sperm binding
- B) Confers species specificity
- C) Stimulates testosterone production
- D) Serves as a block to polyspermic fertilization
✅ Answer: C - Testosterone production is by Leydig cells. The ZP mediates sperm binding, species specificity, and blocks polyspermy.
24. The size of follicles at ovulation compared to primordial follicles grows from:
- A) 30 µm to 2 mm
- B) 30 µm to 20 mm
- C) 300 µm to 20 mm
- D) 3 mm to 20 mm
✅ Answer: B - Follicles grow from 30 µm (primordial) to 20 mm at ovulation.
Section 5: Spermatogenesis & Spermiogenesis
25. Which cells support and nourish PGCs in the male gonad?
- A) Leydig cells
- B) Granulosa cells
- C) Sertoli cells
- D) Theca cells
✅ Answer: C - Sertoli cells (epithelial origin) support and nourish PGCs. Leydig cells secrete testosterone.
26. Spermatogenesis is primarily regulated by which hormone from the pituitary?
- A) FSH only
- B) LH only
- C) Both LH and FSH
- D) Estrogen
✅ Answer: C - LH regulates spermatogenesis overall; FSH binds to Sertoli cells to stimulate testicular fluid production.
27. At puberty, sex cords acquire a lumen and become:
- A) Epididymis
- B) Seminiferous tubules
- C) Vas deferens
- D) Ejaculatory ducts
✅ Answer: B - Sex cords acquire a lumen at puberty and become seminiferous tubules.
28. The correct sequence of spermatogenesis is:
- A) Spermatogonia → Spermatids → Spermatocytes → Spermatozoa
- B) Spermatogonia → Spermatocytes → Spermatids → Spermatozoa
- C) PGC → Spermatids → Spermatogonia → Spermatozoa
- D) Spermatocytes → Spermatogonia → Spermatids → Spermatozoa
✅ Answer: B - Correct order: Spermatogonia → Primary spermatocytes (Meiosis I) → Secondary spermatocytes → Spermatids (Meiosis II) → Spermatozoa (spermiogenesis).
29. Which of the following is NOT part of spermiogenesis?
- A) Formation of acrosome
- B) Condensation of nucleus
- C) Crossing over between chromatids
- D) Shedding of cytoplasm (residual body)
✅ Answer: C - Crossing over occurs during meiosis (pachynema), not during spermiogenesis.
30. Where do spermatozoa acquire full motility?
- A) Seminiferous tubules
- B) Vas deferens
- C) Epididymis
- D) Ejaculatory duct
✅ Answer: C - Spermatozoa acquire full motility in the epididymis.
Section 6: Fertilization & Post-Fertilization
31. The acrosomal reaction releases enzymes that break down the:
- A) Corona radiata
- B) Zona pellucida
- C) Cell membrane of the egg
- D) Cumulus cells
✅ Answer: B - Acrosomal enzymes break down zona pellucida glycoproteins to allow sperm penetration.
32. Prevention of polyspermy is achieved by: (Select the BEST answer)
- A) Cortical reaction only
- B) Zona reaction only
- C) Both cortical reaction and zona reaction
- D) Acrosomal reaction
✅ Answer: C - Both the cortical reaction (membrane impenetrable) and zona reaction (ZP hardens) together prevent polyspermy.
33. What happens to paternal mitochondria after fertilization?
- A) They replicate rapidly
- B) They fuse with maternal mitochondria
- C) They are destroyed by ubiquitination
- D) They migrate to the nucleus
✅ Answer: C - Male mitochondria are destroyed by ubiquitination, which is why mitochondrial inheritance is maternal only.
34. Which of the following are the THREE key results of fertilization?
- A) Sex determination, cleavage initiation, zona formation
- B) Diploid restoration, sex determination, cleavage initiation
- C) Haploid restoration, sex determination, implantation
- D) Acrosome formation, diploid restoration, sex determination
✅ Answer: B - The three key results: (1) Restoration of diploid number, (2) Sex determination, (3) Initiation of cleavage.
35. The Ca²⁺ wave released after fertilization is associated with:
- A) Formation of the male pronucleus
- B) Metabolic activation of the egg
- C) Hardening of the zona pellucida
- D) Destruction of sperm mitochondria
✅ Answer: B - Ca²⁺ stored in cytoplasm is released as a wave that causes metabolic activation of the egg.
Section 7: Mitochondrial Inheritance & Mutations
36. A cell in which all mtDNA copies are identical (either all normal or all mutant) is called:
- A) Heteroplasmic
- B) Homoplasmic
- C) Aneuploid
- D) Polyploid
✅ Answer: B - Homoplasmy = pure population of mtDNA. Heteroplasmy = mixture of normal and mutant mtDNA.
37. Why are many mutations more often paternal rather than maternal in origin?
- A) Sperm DNA is more exposed to radiation
- B) Sperm DNA undergoes far more replication cycles than oocyte DNA
- C) Oocytes have better DNA repair enzymes
- D) Paternal chromosomes are larger
✅ Answer: B - Because sperm DNA undergoes far more replication cycles, there are more opportunities for replication errors.
38. Random distribution of mtDNA copies between daughter cells during cell division is called:
- A) Heteroplasmy
- B) Homoplasmy
- C) Replicative segregation
- D) Nondisjunction
✅ Answer: C - Replicative segregation is the random distribution of mtDNA copies at cell division.
39. A child is born with a mitochondrial disease. The most likely source of the mutation is:
- A) Father
- B) Mother
- C) Either parent equally
- D) A spontaneous new mutation only
✅ Answer: B - Mitochondria (and mtDNA disorders) are inherited exclusively from the mother.
40. The male pronucleus forms from:
- A) The egg chromosomes after fertilization
- B) The sperm nucleus after entering the egg cytoplasm
- C) The first polar body
- D) Cortical granules
✅ Answer: B - After the sperm nucleus enters the egg cytoplasm, it becomes the male pronucleus. The egg chromosomes form the female pronucleus.
Bonus Rapid-Fire (True/False Format)
| # | Statement | Answer |
|---|---|---|
| 41 | Crossing over occurs between sister chromatids | ❌ False - between NON-sister chromatids |
| 42 | FSH induces maturation of ovarian follicles | ✅ True |
| 43 | The secondary oocyte completes meiosis II before fertilization | ❌ False - only completed IF fertilized |
| 44 | Sertoli cells secrete testosterone | ❌ False - Leydig cells secrete testosterone |
| 45 | Y-carrying sperm produces a male offspring | ✅ True |
Total: 45 questions covering all major topics from the lecture. Good luck on your exam! 🎯
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