Vertebrate development Flashcards
What axis is distinguised in egg itself?
- Animal pole - named this as believed most of the nervous system derived from it
- Vegetal pole - denotes tissue that later forms digestive and ventral part of embryo
How is dorsal-ventral axis formed?
sperm entry specifically at a certain position and triggers reformation of the cytoskeleton
Important animal in developmental biology and why
Drosophila Melanogaster
- The larvae of this fly looks like this (worm like structure). We can already define segments by lit up areas - head and tail are clearly different from rest of the body.
- Segments so easily visualised under microscope that it was utilised by geneticists to find mutations that would affect this segmentation (Nobel prize)
Bicoid mutant of fly?
lacks anterior half of larvae
Oskar mutant of fly?
entire thorical and abdominal segments are missing
Gene cascade of early drosophila embryo
1)
Egg polarity genes - anterior and posteror ends - Bicoid protein high conc at anterior end - morphogen gradient. mRNA translated to protein when egg fertilised
There is a protein with opposite gradient to bicoid
Gene cascade of early drosophila embryo
2-4
2) Gap gene activation - subdivides embryo
3) Gap gene activates pair rule gene - further subdivides
4) Segment polarity genes - define anterior and posterior end of each segment
Gene cascade of early drosophila embryo activates what class of genes? What is their dunction
Homeotic selector genes (HSG). Need to specify where a head if forming, thorax, abdominal etc and the different organs within those structures.
What is co-linearity
The order of genes on the chromosome is the same as the anterior border of their expression domains in the developing embryo. The reason for this colinearity is not yet completely understood
What are Hox genes?
Transcription factors (TFs), the harbour a specific gene regulatory domain. Those TFs have the ability to bind to DNA with a. specific protein structure -in the case of Hox genes is called a homeobox.
How are Hox genes orientated in drosophila
come in a cluster. There are 8 genes present here in this one chromosomal localisation.
- Co-linearilty in their positioning on chromosome and in their activation along anterior, posterior axis in the embryo
Hox gene arrangement in vertebrates
4 clusters rather than 1
Most 3’ localised are activated anterior
Best seen in neural spine tube
What does hox gene bind to?
specific DNA sequence and modulate gene expression of their target genes
- DNA binding domain called homeodomain
Homeodomain structure
- consists of 3 helical parts (blue I,II,III) and only helix number 3 makes contact to DNA. Reads sequence in DNA and makes hydrogen bonds to specific nucleotides.
It has a recognition sequence, present in the DNA (red) and the protein (blue).
Why is binding of Hox gene to homeodomain important?
Binding is important because TFs able to recruit RNA polymerase and entire apparatus required for transcription of a gene. These genes are able to activate genes required for development of specific segments. Activating them in a specific sequence along the anterior-posterior axis, we generate different structures of the developing embryo. Any misregulation results in serious alterations In the morphology
Main model organisms for developmental biology?
Frog, Chick, Mouse, Zebrafish
What is gastrulation?
- movement of cells into the interior
- Generation of novel neighbourhood relationship between cells
- Lay down of embryonic axis - defines where dorsal and ventral is positions
First step of development
Cell division
Germ layers
Ectoderm - traditionally blue. Makes nervous system and skin and neural crest cells
Endoderm yellow - making the entire digestive tract and organs derived from digestive tract
Mesoderm - Most variable germ layer. Bones, kidney, RBC, myocardium. Also layers of muscles around endoderm that are derived from mesoderm
Up until what cell stage are blastomeres totipotent in higher vertebrates and in frog egg
Up to 8 cell stage
- In frog egg - already fixed at fertilisation
What is the Spemann-Mangold Organizer
Shows that this tissue is able to induce a gastrulation movement but also instruct the cells in the neighbourhood to form a second embryo. Coined the term organizer
What does french flag model represent?
How morphogen gradients induces different cell identities in the embryo
- Set up as gradients - source where molecule (one e.g. of morphagen is bicoid) is at high conc and area where there is low conc and also area where there is a medium concentration.
- Cells eperience diff conc of this morphogen and respond by turning on different identities - blue, red or white identities.
So high concentrations of morphogens activate genes that are different from medium or low levels of the respective morphogen
Example of french flag model
TGF-beta signalling molecule
Zebra fish injected with mRNA for TGF-beta signalling molecules. Embryo was incubated. Response of modification was studied.
When injected with 4pg - different genes activated in different cells.
If only 10.4mg, only purple present.
If we inject v high amounts, then only red activated.
Cells are able to receive how much signal they receive and activate specific genes in response to those amounts.
SO cells have a dose dependant response (gene activation) Leads to v different structures.
