lecture 20: germ cells

Question 1

What were early observations on sperm and eggs?

Answer 1

• entire body in the head of sperm etc
• really weird ideas in 17th C
• whole person inside sperm - person inside the sperm of the person
• preformist theory
• ovust stream thought the being existed inside the egg
• neoformist - organs and cells developed slowly over time

Question 2

What are germ cells?

Answer 2

• the gametes (eggs and sperm)
• primordial germ cells (PGCs) are precursors to the gametes
• transmit genetic information from one generation to the next
• basically the reason why we are here

Question 3

What is the life cycle of germ cells?

Answer 3

• specification
  
  • up-regulation of pluripotency genes
  • down-regulation of somatic genes
  • proliferation (mitosis)
• migration to the genital ridges (future gonads)
  
  • proliferation (mitosis)
  • erasure of epigenetic imprints
• sexual differentiation (mitotic arrest or meiosis)
  
  • epigenetic reprogramming
• gametogenesis (ova or spermatozoa)
• fertilisation

Question 4

What are mechanisms of PGC specification?

Answer 4

• determinative (preformistic)
  
  • depends on inheritance of germ plasm
• regulative (epigenetic)
  
  • germ cell fate specified by cell-cell interactions and signalling

Question 5

What is determinative (preformistic) PGC specification?

Answer 5

• insects, nematodes, fish, birds & frogs
• inheritance of germ plasm – cytoplasm rich in specialised RNA binding proteins, RNA and mitochondria
• germ plasm contains inhibitors of transcription and translation
• germ cells specified very early in development
• e.g. vasa positive cells in zebra fish

Question 6

What is regulative PGC specification?

Answer 6

• mammals, urodeles (e.g. salamanders)
• depends on signals from adjacent cell populations
• ancestral form of germ cell specification?

Question 7

How do germ cells differ from somatic cells in their cell lineages?

Answer 7

• need to upregulate pluripotent genes in germ cells, downregulate somatic genes
• in somatic cells → upregulate somatic genes

Question 8

What are key processes of PGC specification?

Answer 8

• increased expression of pluripotency genes e.g. Sox2 and Nanoh
  
  • Prdm1
• decreased expression of somatic mesodermal genes e.g. Hox genes, Brachyury
  
  • Prdm1
• increased expression of germ cell-specific genes e.g. stella, nanos3
  
  • prdm14
• extensive epigenetic remodelling
  
  • prdm14

Question 9

What is germ cell proliferation?

Answer 9

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

Question 10

What is PGC migration?

Answer 10

• migratory route guided by ECM
• chemoattractive and repulsive signals are also involved

Question 11

What is PGC migration in a wallaby foetus?

Answer 11

• wallaby foetus about 23 days into a 27 day
• migrate from the gut, through dorsal mesentery into the gonads, near mesonephros
• migration is induced by factors produced in the tissues of the foetus and corresponding factors or receptors in the germ cells, and the extracellular matrix
• series of signals that is both chemoattractive and chemorepulsive
• PGCs tend to migrate in clumps
• ECM has a big role:
  
  • laminin, fibronectin etc
  • changes with time
• e.g. body cells might express Stem Cell Factor, germ cells might express the receptor ckit, SDF1 by germ cells, receptor in body tissues
• quite regulated
• overlap between signals that encourage proliferation and stop cell death

Question 12

What is germ cell sexual differentiation?

Answer 12

• first step is meiosis
• in mice: at E13.5-14.5 female germ cells enter meiosis, males enter mitotic arrest
• germ cell differentiation depends on the somatic environment initially, then on the chromosomal component of the germ cells

Question 13

What is meiosis?

Answer 13

• unique to germ cells
• exchange of genetic material
• production of haploid gametes

Question 14

What is control of entry into meiosis?

Answer 14

• VitA (mesonephros) → RA → RA (ovary) → Stra8 → meiosis
• in testis Cyp26b1 inhibits RA meaning no Stra8 and no meiosis

Question 15

What is inequivalence of information from eggs and sperm?

Answer 15

• e.g. horse x donkey
  
  • mare x jack → mule
  • jenny x stallion → hinny
• gametes carry the same genetic information but some of it is differentially modified between the sexes (= epigenetic modification)

Question 16

What are epigenetic modifications?

Answer 16

• heritable changes to DNA or chromatin structure but not to DNA sequence

Question 17

What are mechanisms of epigenetic modifications?

Answer 17

• DNA modifications e.g. methyl groups on CpG islands
• histone modifications
  
  • active/open chromatin
  • inactive/condensed chromatin
• developmental signals establish specific pattern on → dna binding proteins → chromatin structure and DNA methylation
• DNA methylation help establush and maintain → chromatin structure → DNA binding proteins
• DNA methylation influences binding of → DNA binding proteins
• main epigenetic modification in germ cells is DNA methylation
• genomic imprinting – expression of a gene in a parent-of origin specific manner
• e.g.
  
  • female germ cells: 17 maternally methylated genes with an average level of ~40% DNA methylation
  • male germ cells: 4 paternally methylated genes with an average level of ~89% DNA methylation

Question 18

What is the epigenetic control of germ cell development?

Answer 18

• epigenetic reprogramming of germ cells is required for:
  
  • correct gene expression
  • X chromosome inactivation/reactivation
  • progression of meiosis
  • gametogenesis

Question 19

What is modification of imprint status?

Answer 19

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

Question 20

What is epigenetic control of gametogenesis?

Answer 20

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