What are the first events in life?
Cleavage - haploid germ cells fuse to produce diploid zygote. Mitotic cell division produced blastomeres
Morula - Embryo then re arranges to form ICM (totipotent) and trophoblasts.
Blastocyst - Fluid fills the zona pollucida forming blastocyst cavity
Implantation of the blastocyst into the uterine wall at the trophoblast cells invade uterine wall.
Explain the process of gastrulation?
Cells from later edges move towards centre of embryonic disk. Cells converge in the middle of the epiblast and elongates anteriorly forming primitive streak. Cells undergo and ingress under epiblast forming endoderm. second lot of cells form mesoderm. Primitive streak stop elongating 75% axis. knot of rapidly dividing stem cells forms called a node. Epiblast cells that ingress at the node form structures of the head. Node regresses posteriorly leaving behind cells that form somite & notochord.
Explain secondary gastrulation
Node regresses posteriorly and interacts with the tailbud att the end of the streak. forms cells of the tails
What are the genetic models for experiments?
- Drosophila- short life span,small & easy to cause point mutations BUT not a vertebrate
- Xenopus laevis- Large eggs so transplantation or cells easy,point mutation easily induced
- Zebra fish - small easy to breed, tissue transplant possible,transparent embryo, point mutation easy
- Chick- Eggs easy to obtain, chickens not on site, easily experimentally mutated BUT genome no sequenced
- Mouse- Mammal, easy to cause knock out gene mutations, genetically more similar to humans BUT poor for experimental techniques (embryo hard to access).
what are the 5 steps leading to neural tube formation…
- Ectoderm epithelia change from cuboidal to columnar. Ectoderm layer thickens towards the edges.
- Cells along midline attach to the notochord forming a midline called Medial hinge point (MHP). Neural groove formed
- Epidermis either side of the neural groove push inwards forcing it to lift.
- Inwards pushing of the causes the formation of the dorsolateral hinge point (DLHP). dorsal aspect of the neural groove now points inwards.
- Edges fuse together by cells interdigitating and the ectoderm grows over the top.
Describe secondary neuralation and where it occurs…
- Only occurs in the sacral/caudal region
- Mesenchyme cells condense and form a medullary chord
- Numerous cavities form, eventually fusing to creat a lumen. (Cavitation)
Discuss neural crest induction..
- Ectoderm expresses Wnt6 and BMP4 which activate the neural plate to express Slug and FoxD3
- Slug and FoxD3 cause cell adhesions to be lost and therefore EMT
- Neural crest cells can now delaminate from the neural tube and migrate into the peripheries
What cells originate from neural crest …
- Pigment cells
- Bone & cartilage
- Peripheral nervous system (PNS)
Where do limbs originate from and what determines their position?
- Limb bud outgrowth is due to the action of FGF10
- Mesenchyme core and ectoderm surrounding
- HoxB5 gene is responsible for consistent limb positions (between cervical & thoracic and Lumber and sacral)
Describe proximal-distal outgrowth of the limb..
- Progress zone sits under the Apical ectodermal ridge
- The more time spent in the progress zone means that part of the limb will be more distal
- Hox genes also required to signal certain points in the skeleton e.g Hoxa9 needed for humerus and Hoxd9 needed for femur
Describe anterior-posterior patterning..
-FGF from the AER acts on the posterior edge of limb (zone of polarising activity) causing the expression SHH. A gradient of SHH is created, with high levels causing a the formation of little finger (posterior) and low levels indicating formation of the thumb (anterior)
Describe dorsal-ventral patterning ..
- Ectoderm covering limb mesenchyme expresses WNT7a
- induces the expression of lmx1 in dorsal mesenchyme and engrailed is expressed on the ventral side.
- Signals form AER control this process
Describe some ways in which limbs are sculpted ..
- Apoptosis between radius and ulna and also between digits
- Lack of Hox 12 and Hox 13 genes cause the formation of fins
- inhibiting BMP in between developing digits causes webbed feet.
- changes in Hox gene expression in developing snakes LPM causes no cervical thoracic vertebrae boundary, so no where for forelimb to form. No formation of AER So hindelimb doesn’t form
Describe somite maturation..
- Presomatic mesoderm buds off anteriorly to form somite
- MET to form epithelial outer crust
- In ventral somite EMT to form scleretome, dorsal part aspect remains mesenchyme and forms dermomyotome
- Dermomyotome then forms myotome, myotomal cells don’t divide, cells are taken from dermomyotome. Remaining cells form dermatome
Describe somite resegmentation …
- Only scleretome segments not myotome or dermatome
- Fissure develops separating somite into posterior and anterior. Posterior of one somite joins with anterior of adjacent somite BUT the original somite is still connected to its original opposite end by myotome as it didn’t segment.
