Lecture 2: Gametogenesis

Question 1

What is Gametogenesis?

Answer 1

• Formation of gametes by forming genetic material in nucleus and cytoplasm

Question 2

What are primordial germ cells?

Answer 2

Cells needing to migrate from yolk sac into gonads during early embryonic development
- earliest recognizable precursors of gametes

Question 3

How is Gametogenesis divided into 4 phases?

Answer 3

1. Primordial Germ cells (PGCs) migration to the gonads (ovaries or testes) —> similar to stem cells (form everything)
2. An increase in the number of PGCs by Mitosis —> need more pools for PCGs to migrate to in order to make more
3. A reduction in chromosomal material by Meiosis —> need to reduce # of chromosomes in each gametes in sperm to up # of chromosomes when sperm and egg meets; creating the right # of chromosome specific to species
4. Structural and functional maturation of gametes (oogenesis, spermatogenesis) —> help in differentiation in male and female; 1st phase = same, 2-4th phase = different process in male and female

Question 4

Phase 1: Origin of the PGCs

Answer 4

• PGCs originate in epiblast —> moves into embryo —> into embryonic region (yolk sac) where they will be determined —> re-enter embryo —> migrate to gonads —> genital ridges for thickening of PGCs to connect to

Question 5

Phase 1: the arduous journey in brief

Answer 5

Between 4-6 weeks:
- Exit yolk sac —> enter hindgut epithelium (in embryo) —> migrate through dorsal mesentery —> reach primordial of the gonads

Question 6

How do PCGs migrate and know where to go?

Answer 6

1. Amoeboid movements during initial migration —> Pseudopods; help slide through extracellular matrix
2. Cytoplasmic processes link adjacent PGCs
3. Chemoattractants secreted by genital ridges —> attract PCGs indicating they are going the right way
4. Migration by extending pseudopod (integrity-fibronectin interactions) —> interaction between the two help guide the movement of PGCs
5. Follow extracellular matrix “roadways” lined with fibronectin —> tells integrity where the next step is

Question 7

What are the 2 types of Teratomas?

Answer 7

Teratomas = incorrect migration of PCGs —> continuing to proliferate
1. Sacrococcygeal teratoma (malignant or benign tumours) in a fetus 44.8% of cases —> compatible with life
2. Massive oropharyngeal teratoma having 1.7% of cases —> not compatible with life

Question 8

Describe a Sacrococcygeal Teratoma (SCT).

Answer 8

• tumour located on base of tailbone (coccyx)
• more common in females than in males
• usually not malignant —> cured by surgery after birth

Discovered by:
- blood-test showing high alpha fetoprotein (AFP) amount
- sonogram indicating extra amniotic fluid —> polyhyramnios
- mirror syndrome —> mother shows same symptoms that fetus has

Question 9

Describe Oropharyngeal Teratomas.

Answer 9

• extremely rare
• associated with high neonatal mortality rate due to severe airway obstruction
• progress in antenatal diagnosis

Question 10

Phase 2: Increase in the Number of Germ cells by Mitosis

Answer 10

• After arriving in gonads, PGCs undergo mitosis divisions
• # increases exponentially from 100s to 1,000,000s
• proliferation differs between male and female
• oogonia: max # reached during gestation
• spermatogonia: able to divide postnatally

Question 11

Gender Differences in Germ Cell Mitosis.

Answer 11

Females:
- in 2nd to 5th month of pregnancy, oogonia undergo mitosis activity in embryonic ovary
- population of germ cells increases from few 1000s to nearly 7,000,000
- Atresia (precursors die) of oogonia begins and continues throughout life —> loose available eggs right at 20 weeks of pregnancy

Males:
- mitosis begins early in embryonic testes
- continuous throughout life —> seminiferous tubules lined with germ cells
- beginning at puberty, subpopulations of spermatogonia undergo periodic waves of mitosis —> progeny enter meiosis in synchronous groups after puberty

Question 12

Phase 3: Reduction in chromosomal Number by Meiosis

Answer 12

• Reduction # of chromosomes from 2N to 1N to maintain specific #s of species for generations
• Independent re-assortment of paternal and maternal chromosomes allowing better mixing of genetic characteristics
• further distribution of maternal and paternal genetic info through crossing-over during first meiotic division

Question 13

How does Genetic Recombination occur?

Answer 13

Crossing-over: consists of exchange of segments between 2 chromosomes during pachytene stage
- occurs in sex chromosomes as well
- takes place in small region of homologous between X and Y chromosome
- not purely random —> occurs at sites along chromosomes called “hot spots”

Hot Spots:
- location based on configurations of proteins organizing chromosomes early in meiosis
- Cohesin: Hypermethylation of histone proteins identifies hot spots where DNA strands break and are repaired after crossing-over is completed
- Condensin: compacts chromosomes, necessary for both miotic and meiotic divisions

Question 14

Similarities vs. Differences between mitosis and meiosis (I & II)

Answer 14

Mitosis:
- centromere between sister chromatids splits —> one chromatid from each chromosome migrates to each pole of miotic spindle
Results:
- 1 division and 1 duplication
- genetically equal daughter cells (diploid)

Meiosis:
- preparation: DNA duplication
- end of Meiosis I: genetically different daughter cells (diploid)
- no duplication between
- Meiosis II: centromeres between sister chromatids divide
Results:
- 1 division and 2 duplications
- genetically different daughter cells (haploid)