lecture 20: germ cells Flashcards
1
Q
What were early observations on sperm and eggs?
A
- 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
2
Q
What are germ cells?
A
- 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
3
Q
What is the life cycle of germ cells?
A
- 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
4
Q
What are mechanisms of PGC specification?
A
- determinative (preformistic)
- depends on inheritance of germ plasm
- regulative (epigenetic)
- germ cell fate specified by cell-cell interactions and signalling
5
Q
What is determinative (preformistic) PGC specification?
A
- 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
6
Q
What is regulative PGC specification?
A
- mammals, urodeles (e.g. salamanders)
- depends on signals from adjacent cell populations
- ancestral form of germ cell specification?
7
Q
How do germ cells differ from somatic cells in their cell lineages?
A
- need to upregulate pluripotent genes in germ cells, downregulate somatic genes
- in somatic cells → upregulate somatic genes
8
Q
What are key processes of PGC specification?
A
- 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
9
Q
What is germ cell proliferation?
A
- 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)
10
Q
What is PGC migration?
A
- migratory route guided by ECM
- chemoattractive and repulsive signals are also involved
11
Q
What is PGC migration in a wallaby foetus?
A
- 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
12
Q
What is germ cell sexual differentiation?
A
- 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
13
Q
What is meiosis?
A
- unique to germ cells
- exchange of genetic material
- production of haploid gametes
14
Q
What is control of entry into meiosis?
A
- VitA (mesonephros) → RA → RA (ovary) → Stra8 → meiosis
- in testis Cyp26b1 inhibits RA meaning no Stra8 and no meiosis
15
Q
What is inequivalence of information from eggs and sperm?
A
- 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)
16
Q
What are epigenetic modifications?
A
- heritable changes to DNA or chromatin structure but not to DNA sequence
17
Q
What are mechanisms of epigenetic modifications?
A
- 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
18
Q
What is the epigenetic control of germ cell development?
A
- epigenetic reprogramming of germ cells is required for:
- correct gene expression
- X chromosome inactivation/reactivation
- progression of meiosis
- gametogenesis
19
Q
What is modification of imprint status?
A
- 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)
20
Q
What is epigenetic control of gametogenesis?
A
- spermatids: histones replaced by protamines
- oocytes contribute factors for post-fertilisation reprogramming (transcription factors and epigenetic modifiers)
