Somites and Embryonic Folding

Question 1

Explain the formation of the intraembryonic coelom

Answer 1

1) Clefts will appear in the lateral plate mesoderm- above prechordal plate and going to the side in a horseshoe shape
2) These fuse to form a horseshoe shaped cavity- which is a useful space for thing to grow into
3) Intraembryonic coelom will eventually form 3 caivities- pericardial cavity (heart), pleural cavity (lungs), peritoneal cavity (peritoneum- liver/ kidney). There is continuity between the intra and extraembryonic coelom (coelom= basically gaps found in the intraembryonic mesoderm and extraembryonic mesoderm)
4) At the same time as spaces fusing intraembryonic mesoderm forms 3 lateral columns:
- Paraxial mesoderm- by the axis / midline
- Lateral plate- lateral in embryo
- Intermediate- between them
Lateral plate in continuity with extraembryonic mesoderm
5) 4th week: Paraxial mesoderm forms little cubes of tissue on either side of the neural tube- somites- each somite gets a spinal nerve, which innervates anything formed form the little block of tissue. Somites split into 3- medial part forms sclerotome (skeleton), intermediate part forms myotome (muscle), lateral part forms dermatome (skin- dermis)
6) Intermediate mesoderm forms nephrotome (genitourinary system) and lateral forms parietal (body wall) or visceral (wall of gut)
7) In forth week, somites form a craniocaudal sequence- 3 occipital, 8 cervical, 12 thorasic, 5 lumbar, 5 sacrial, 1-5 coccygeal- corresponds to no of spinal nerves present. Occipital somites forms the skull and tongue (musculature)
Each somite receives a segmental spinal nerve, which follows that tissue wherever it migrates
8) prosterior and anterior neuropore are the last part of the tube to close
9) By week 4 we also see heart bulge, limbs, face, umbilical chord forming
10) At the prochordal plate we see 2 tubes forming- 1 for each lung. Further down we see more of a slit at each side (peritoneal cavity?)
11) This slit clearly separates the lateral plate into the somatopleuric mesoderm (continuous with mesoderm of amniotic cavity–> striated/ voluntary muscle) and the splanchnopleuric mesoderm (continuous with mesoderm of yolk sack)- form smooth muscle of gut.
12) allantois forming- extension of the yolk sack goes into the connective stalk- evenutally forming umbilical chord.

Question 2

What is a good was to track how old the embryo is

Answer 2

• Count the number of somites formed by 28 days (week 4) all will be present.

Question 3

How can we tell if the baby had a neural tube defect?

Answer 3

This defect would lead to the neural tube not closing properly- release of material into mothers bloodstream- measured to indicate that the neural tube hasn’t closed and more investigation done.

Question 4

Explain the longitudinal fording of the embryo

Answer 4

1) Embryo forms flat oval disk with:
- Ectoderm
- Mesoderm
- Neural tube
- Mesoderm
- Notochord
- Mesoderm
- Endoderm
2 plates: cloacal plate (anus), prochordal plate (mouth)
Septum transversum (diaphragm), pericardial cavity (intraembryonic coelom)
2) Rapid growth of neural tube so now we have an extended brain-
- this forces the amnion to bend around
- Forces the septum transversum and intraembryonic coelom to bend around with it
- Prochordal plate now vertical
- So is cloacal plate
- Distinct folding- marked 1-5 along ectoderm: 4 above 3 and 5 above 2

So amniotic cavity pulled around embryo
3) - Yolk sack pinched off
- Prochordal plate and cloacal plate vertical with endoderm on top and ectoderm on bottom
- Now reversal of order: 54321
- Right order: Brain ahead of mouth (prechordal plate), ahead of heart (intraembryonic coelom), ahead of diaphragm (septum transversum- orange layer surrounding heart)
- Part of yolk sack made into gut tube
Yolk sac pinched off so continuous gut tube running through the embryo
- amnion only attached to embryo over small region (umbilicus)

Question 5

Lateral folding

Answer 5

1) Edges of amnium grow down towards the yolk sack- putting it under pressure
2) Folding greatly reduces communication between extra and intraembryonic coelom
3) Vitellointestinal duct joins gut tube (intestine) to the yolk sack (vitello) in the umbilicus region
4) Yolk sack becomes completely cut off
5) Gut tube suspended by the mesentery- on wither side intraembryonic coelom- where organs will form, behind surrounded by mesoderm is the notochord and neural tube
6) Amniotic cavity all the way around embryo- this will burst before pregnancy- waters broke