W7L4 Fri Germ cell

Question 1

Role and member of germ cell

Answer 1

• Germ cells transmit genetic information from one generation to the next
  § Include gametes (eggs + sperm) + primordial germ cells PGCs (precursors to gametes)

Question 2

life cycle of germ cell

Answer 2

-Specification
-migration to genital ridges
-sexual differentiation
-gametogenesis
-fetilisation

Question 3

mechanism of Primordial germ cell specificaiton

Answer 3

-Determinative (preformistic) : specified by inheritance of “germ plasm” from the egg -
-Regulative (epigenetic) * specified by cell-cell interactions and signalling later in development

Question 4

Determinative (preformistic)
PGC specification

Answer 4

Ø Germ plasma cytoplasm is rich in specialised RNA-binding proteins, RNA + mitochondria
Ø Contain inhibitors of transcription + translation (prevent somatic differentiation)
Ø Insects, nematodes, fish, birds + frogs

Question 5

Zebrafish determinative specification

Answer 5

Ø Zebrafish: PGCs localise to cleavage (2-cell stage)
→ early PGCs localise outside embryo proper (protection from differentiation signals within embryo)
→ migrate into embryo once gonads form

Question 6

Regulative PGC specification

Answer 6

Ø Ancestral form of germ cell specification
Ø Humans: PGCs in extremity region of yolk sac (protection from differentiation signals in embryo)
Ø Mouse: germ cell specification in posterior embryo via interactions b/w extremity + embryonic region (involve BMPs)→ migrate into extremity
Ø Mammals, urodeles (salamanders), most reptiles

Question 7

Germ cells vs. somatic cells

Answer 7

germ cells and totipotent/pluripotent cells express many of the same genes
-somatic cell give rise to fetus, germ cell give rise to germ line

Question 8

Key gene processes of PGC specification in mammals

Answer 8

Ø Prdm14:induce pluripotency genes (Sox2, Nanog) + repress somatic mesodermal genes (Hox, Brachyury promote diff.)
Ø Prdm1: promote germ cell-specific genes (Stella, Nanos3), extensive epigenetic remodelling

Question 9

Germ cell proliferation

Answer 9

• E6.25 mouse: ~6 PGCs
• By E13.5: ~25,000 germ cells
• proliferation requires numerous growth factors & proteins
• autocrine and paracrine signals (SCF/c-kit, FGFs, LIF)

Question 10

PGC migration

Answer 10

§ Migratory route guided by extra cellular matrix + chemoattractive/repulsive signals
Ø Extremity region → developing gut → dorsal mesentery → genital ridges (next to mesonephros)
§ Germ cells that go the wrong route apoptose (prevent tumour formation)

Question 11

Germ cell sexual differentiation

Answer 11

first step is meiosis * in mice at E13.5-14.5:
– female germ cells enter meiosis
– male germ cells enter mitotic arrest
* germ cell differentiation initially depends only on the somatic environment, then later also on their chromosomal composition (XX or XY)

Question 12

Meiosis

Answer 12

unique to germ cells
* recombination of genetic material
* production of haploid gametes

Question 13

Control of entry into meiosis in the mouse

Answer 13

§ In mesonephros, vitamin A converted into retinoic acid (RA) which diffuses into gonads
Ø Females: RA activates Stra8 which promotes initiation of meiosis
Ø Males: expression of CYP26B1 degrades RA preventing meiosis

Question 14

Non-equivalence of information
from eggs and sperm

Answer 14

• gametes carry the same genetic information but some of it is differentially modified between the sexes (= epigenetic “imprinting” of genes)
• evolutionary explanation: parental conflict in fetal investment
• imprints must be erased each generation in the PGCs

Question 15

Type of Non-equivalence of information
from eggs and sperm

Answer 15

Ø Gynogenetic (2 maternal genomes): small placenta results in block in embryonic development
Ø Androgenetic (2 paternal genomes): embryonic growth retarded
Ø Epigenetic imprints must be erased each generation in PGCs (establish new imprints)

Question 16

Epigenetic modifications

Answer 16

-heritable changes to DNA or chromatin structure but not DNA sequence
Ø DNA modifications: cytosine methylation of CpG islands often inhibit gene activation
Ø Histone modifications: active/open chromatin (gene expression) or inactive/condensed chromatin
Ø DNA methylation heritable b/w generations, histone modifications b/w cell divisions

Question 17

Mechanisms behind initiating and maintaining epigenetic modifications

Answer 17

§ Developmental signals
→ activate DNA binding proteins
→ help establish + maintain DNA methylation
→ help establish + maintain chromatin structure
→ upregulation of DNA binding proteins

Question 18

Epigenetic modification of germ cells importance

Answer 18

Epigenetic modification of germ cells is required for:
* correct germ cell-specific gene expression
* X chromosome reactivation in PGCs and inactivation during spermatogenesis
* imprinting erasure in PGCs and re-establishment during gametogenesis
* progression of meiosis
* gametogenesis

Question 19

Modification of imprint status

Answer 19

• DNA methylation is the main epigenetic modification in PGCs
• epigenetic erasure of imprinted loci before and as germ cells arrive at genital ridge
• new imprint status established after sexual differentiation:
• females: during prophase I of meiosis
• males: during mitotic arrest
• Removal of DNA methylation by TET proteins
• Re-establishment of DNA methylation by de novo DNA methyltransferases (DNMTs)

Question 20

Epigenetic control of gametogenesis

Answer 20

• spermatids: histones replaced by protamines
• oocytes contribute factors for post-fertilisation reprogramming (transcription factors and epigenetic modifiers)