week 4: embryo folding

Question 1

summary

Answer 1

• due to different growth rates
• two lateral body folds in median plane across longitudinal axis
• cranio-caudal folding in ventral direction
• simultaneously
• endoderm moves towards midline and fuses including dorsal part of yolk sac to form primitive gut tube
• foregut closed by oropharageal membrane which ruptures at the end of 4th week to form mouth
• cloacal membrane closes hindgut

Question 2

median plane: folding at cranial end

Answer 2

• primodial heart and septum transversus folded into correct position
• folding the the cranial end: cardiogenic area (bar of mesoderm) translocate to future chest

Question 3

consequences of the foldings

Answer 3

• embryo changes shape from disc to cylinder
• rudiments move to the appropriate anatomical localisation (eg. heart tube)
• spaces form for the ensuing development of the body cavities

Question 4

median plane: folding at caudal end

Answer 4

• thin caudal rim of disc folds ventrally and becomes part of ventral surface
• connecting stalk is carried cranially until it merges which neck of yolk sac
• umbilical cord emerges from belly

Question 5

delimitation of the body of the embryo

Answer 5

• until gastrulation the disc connected to annexes through its entire outline
• delimitation: wrapping phenomenon: transforms embryo in a tube
• isolated from annexed which remain connected by connecting stalk
• defines outer surface of embryo
• at umbilical level the closure remains incomplete

Question 6

lateral mesoderm forms two membranes lining coelomatic cavities

Answer 6

• end of w4: intraembryonic coelomic cavity formed by folding
• encloses inner organs
• outer: somatic mesoderm forms paritetal/somatopleuric layer lining peritoneal cavity
• inner: splanchic mesoderm forms visceral!splanchnopleuric layer: covering abdomical organs
• splanchnocoele: embryonic body cavity

Question 7

peritoneum

Answer 7

• large serous membrane
• in male forms a closed sac
• in female is penetrated by the lateral ends of fallopian tubes
• serosa: free surface of peritoneum has a layer of flattened mesothelial cells kept moist and smooth by a thin film of serous fluid
• inner organs are hanging to mesentery

Question 8

GI

Answer 8

• primodial gut formed
• endoderm will form lining of GI tract
• GI is suspended in celomatic cavities

Question 9

neurulation: gives rise to CNS

Answer 9

• notochord induces part of ectoderm to become neural plate
• shaping and folding: neural fold, groove, crest
• elevation: neural folds approaching each other
• convergence, closure: neural tube forms neural canal, neural crest developing spinal ganglia

Question 10

closure of neural tube

Answer 10

• progressive along its axis (cranially and caudally)
• begins day 22 at cervical region
• closing cranial neuropores on day 24 and caudal on 26

Question 11

secondary neurulation

Answer 11

• following closure of caudal neuropore the neural tube extends caudally into sacral and coccygeal region
• merging of caudal neuropore forming tail bud
• tail bud initially condensate into a solid mass: medullary cord which then undergoes cavitation and merges with neural canal

Question 12

neural crest cells

Answer 12

• during closure some cells deriving from crest segregate
• forma migrating population
• undergo extensive migration along well defined pathways and have many derivatives
• enteric nervous system derives from NCC from vagal and lumbosacral regions

Question 13

somites

Answer 13

• in paraxial mesoderms
• somite segmentation requires a mesecnhymal to epithelial transition
• as somites are segmented new cells are added to the PSM caudally (cycles determined by cycling genes determining segmentation clock)
• different territories segregate: dermomyotome (dorsal, maintain epithelian features) and sclerotome
  histology:
• somite cavity is filled of paraxial presomitic mesoderm cells which maintain mesenchymal features
• cell division occurs in epithelial cells as well as in the mesenchymal cells in the cavity

Question 14

dermomyotome

Answer 14

• cells form dmt migrate from dorso-medial edge and a new layer: myotome: give rise to epaxial mucles
• those migrating from ventro-lateral give rise to hypaxial muscles
• cells remaining in the demamyotome become mesenchymal forming the dermatome contributing to the dermis

Question 15

sclerotome

Answer 15

• from ventral portion of somite
• cells detach from the somites
• cells from ventro-medial portion undergo EMT transition to form sclerotome
• sclerotome cells migrate to surround notochord and neural tube: vertebrae

Question 16

somite differentiation

Answer 16

• neighbour tissues induce somite differentiation
• occurs cranio-caudally at different time points
• each somite undergoes to similar differentiation program but the final morphology will depend on its position along A-P axis