Gametogenesis and Sex determination Flashcards
Tell me about the two types of cells in organisms ?
- Somas look after germ cells
- Germ cells undergo meiosis and passes on genetic information to next generation
Tell me about organisation of the testis and ovary
What are germ cells?
- Germ cells: cells in your body that form gametes (sperm and oocytes)
- These are the most important cells in your body…… all your other (somatic) cells are there to keep your germ cells alive long enough to make more germ cells (in your children).
- Biologically, you have no greater function than to keep your germ cells alive!!!!
Tell me about the separation of germ cells from somatic cells in (plants, Cnidaria, tunicates and flatworms) vs. (vertebrates, nematodes and insects)
- Occurs at different times in different organisms
- Plants, and animals like Cnidaria, Tunicates and Flatworms
Somatic cells-> Germ Cells throughout life (have lots of stem cells which can differentiate into soma or germ cells throughout their life cycle not just in development- advantage= planaria can divide by fission, or take small part and will regrow completely and can form gametes to continue life cycle of further generations)
- Most animals (Vertebrates, Nematodes, Insects)
Germ cells formed at a specific and early points in development but not in the gonads they migrate there later
What molecular componenets define germ cells?
The proteins
- Vasa
- Nanos
- Tudor and Piwi
These seem to be highly conserved in animals
Tell me about Vasa proteins
Vasa proteins – bind mRNA and increase translation efficiency of germ cells specific messages- keep germ cells in their state
Tell me about Nanos proteins
Nanos proteins- bind mRNA and decreases translation efficiency specific messages present in somatic cells (mesoderm/ ectoderm/ endoderm)- Stop germ cells differentiating
Tell me about Tudor and Piwi proteins
Tudor and Piwi proteins- silence gene transcription- keep the cells in pluripotent (differentiate into many other cell types later on)
How germ cells become defined differs, Tell me about the two ways to form an early germ cell?
- Pre-determined (Autonomous) formation
Seen in many animals Nematodes, Insects, Fish, FrogsHere the germ cells are defined by the segregation of Vasa/ Nanos/ Tudor and Piwi which become concentrated in a cytoplasmic region of the egg – the germ plasm-
- (pre-determined/ autonomous -there is no external signal)
- Induced formation
Seen in Mammals
Here the of the germ cell formation occurs later and Vasa/ Nanos/ Tudor and Piwi are not present in the egg but their expression is induced by signalling molecules (paracrine molecules) from neighbouring cells
In some organisms germ cells are pre-determined, what does this mean?
- Egg gets fertilized by the sperm.
- Newly created embryo starts dividing: 1-cell, 2-cell 4-cell etc.…
- At some point in that early development a small group of cells becomes ‘earmarked’ (i.e., pre-determined) to become germ cells (and make sperm or eggs in the adult animal).
- So, removal of these cells from that embryo would result in a sterile adult animal
Tell me about pre-determined (autonomous) formation
Here Vasa/ Nanos/ Tudor and Piwi are already present in the egg and become segregated to specific regions of the cell – the germ plasm- and hence as the egg divides into specific cells of the developing embryo
In C.elegans, what are the Vasa proteins etc. concentrated into?
P granules
Tell me about P granules in C.elegans?
- These are retained in one cell only – the P cells P1 to P4
- If you destroy any of these P cells the animal will form no germ cells and be infertile
- Cell fate is restricted by “germ plasm”
- …no evidence of germ plasm in mammals
- The P granules- markers for the germ plasm segregate to the P4 cell
- They contain inhibitors of gene transcription and prevent the germ cells differentiation into somatic cells
- P have restricted fate as are keeping the germ plasm inside it.
In mammals, germ cell formation is inductive what does this mean?
There is no germ plasm
- Human embryo (~100 cells)
- Has NO pre-determined germ cells.
- Germ cells are made after embryo implants into uterus
In mammals, what do germ cells form?
And when do they become identifiable
Mammalian germ cells form as major body plan develop and become identifiable shortly after gastrulation (human, days 13-19, mouse, day 5-6).
Whats gastrulation? What does it form and what are these?
Gastrulation - series of cell migrations to positions where they will form the three primary cell layers:
- Ectoderm forms the outer layer (skin, brain, nervous system).
- Endoderm forms the inner layer (digestive and respiratory systems).
- Mesoderm forms the middle layer (muscles, bone, reproductive system, kidneys)
What are epiblast cells?
The epiblast cells are the functional progenitors of soma and germ cells which later differentiate into three layers: definitive endoderm, mesoderm and ectoderm. Stem cells derived from epiblast are pluripotent.
How are germ cells often referred to as in mammals?
Primoridal germ cells (PGCs)
In embryos, the structure which eventually forms the testis or the ovary is called what?
Genital ridge
Are PGCs made in the genital ridge?
- PGCs are NOT made in genital ridges.
- In all vertebrates PGCs migrate to the genital ridge.
What do PGCs start life as?
Pluripotent Epiblast cells (as do all cells in our body)
What do Epiblast cells later go on to form?
Epiblast cells (almost all of them) later go on to form mesoderm, ectoderm and endoderm at gastrulation
They are PLURIPOTENT because they have the ability to form all the 3 primary cell layers and hence all the somatic cells- they also have to ability to form PGCs
What causes some Epiblast cells to become PGCs?
What does this also induce?
BONE MORPHOGENETIC PROTEIN (BMP) produced in neighboring cells causes some (~6 in mouse) posterior epiblast cells to become PGCs.
ie BMP induces Vasa/ Nanos /Tudor and Piwi proteins to become expressed – it also retains the expression of pluripotency markers (Nanog and Sox2) in PGCs. Express these markers to keep in an undifferentiated state
Where do PGCs then migrate to?
These then migrate in many species to areas outside the embryo proper- In humans/mice to the yolk sac (an extra embryonic membrane)- why –? To separate these cells from paracrine differentiative signals in the rapidly forming embryo
How can PGC formation be induced?
The PGCs form in the posterior (back) part of the epiblast – if you replace these cells in an embryo with anterior epiblast cells (front) from an embryo expressing GFP - these new cells will go on to form GFP expressing PGCs
Also, if you remove BMP (4) the inducer – by gene knockout- you fail to get PGCs
Embryology in the mouse
Where are PGCs located in embryos
What are the stages or germ cell development in the mouse ?
In the induction stage of germ cell development in the mouse tell me the following about Bone morphogenetic proteins (BMPs)- (BMP4/8b/2)
- Where?
- Act on?
- Do what?
- How?
- Fragilis?
Bone Morphogenetic Proteins (BMPs) - (BMP4, BMP8b, BMP2)
Where? Extraembryonic cells
Act on? Epiblast cells.
Do what? Prevent germ cell differentiation into somatic cells.
How? Smad signaling to nucleus, makes Fragilis and Blimp1 proteins
Fragilis- is plasma membrane receptor (adhesion with E-cadherin, other cells?)
In the specification stage of germ cell development in the mouse tell me the following about the BLIMP1
- Where?
- Act on?
- Do what?
- How?
- Blimp1
BLIMP1
Where? Primordial Germ Cells
Act on? Primordial Germ Cells
Do what? Prevent germ cell differentiation into somatic cells.
Only 6 cells are BLIMP positive at E6.25 in mouse (100-200 in humans).
How? Transcriptional regulator
Blimp1 – is a transcription factor activating Nanos and maintaining Nanog expression (nanos is a marker for PGC)
PGC migration
In the migration stage of germ cell development in mice, tell me about NANOS3, KIT and DEAD END1
NANOS3, KIT and DEAD END1
- Nanos3 & Dead end1 - prevent apoptosis during migration (cells more likely to stay alive)
- Kit- PGCs express receptor for Kit-ligand. Prevents apoptosis, stimulates proliferation, and may help line path to gonad.
- Loss of Kit leads to sterile animals (or kit ligand)
- During the migration the PGC increase in numbers to ~5000 cells
For many years a region of the Y chromosome was known to be important in testis development in mammals, what is now known as?
This region was called the TDF (Testis Determining Factor)
Where is the TDF located?
The TDF was known to be at the very end of the short arm (‘p’) of Y chromosome
What is the TDF due to?
Later, the TDF was found to be due to a single gene called ‘SRY’. = Sex-determining Region on the Y Chromosome” and is a transcription factor
Tell me about a translocation error of TDF?
Error is translocated TDF in paternal side, usually translocated to X chromosome. If this is used to fertilise egg then get an XX genotype with a male phenotype, producing sperm that is X not Y which makes them infertile
XX(SRY)- male phenotype but infertile- PGCS acts as male but… other genes only needed for fully mature sperm
XY(SRY)- Female phenotype but infertile- PGCS act as female but… need 2 X active chromosomes for fully oogenesis- (see XO)
SRY gene converts an XX egg into a male mouse?
Y-chromosome gene essential or male development
