WEEK 13 (Gut tube and body cavities)

Question 1

Which layers does the Solid mesodermal layer divide into?

Answer 1

• Parietal (somatic) layer
• VisceraL (splanchnic) layer

Question 2

Where is the Parietal/Somatic layer found?

Answer 2

adjacent to the surface ectoderm and continuous with the extraembryonic parietal mesoderm layer over the amnion

Question 3

Together, the parietal (somatic) layer and overlying ectoderm are called the ______________________

Answer 3

Somatopleure

Question 4

Where is the Visceral/Splanchnic layer found?

Answer 4

adjacent to endoderm forming the gut tube and continuous with the visceral layer of extra embryonic mesoderm covering the yolk sac

Question 5

Together, the visceral/splanchnic layer and underlying endoderm are called the _________________

Answer 5

Splanchnopleure

Question 6

During the third and fourth weeks, what does the ectoderm and endoderm form?

Answer 6

• Ectoderm = Neural plate (brain + spinal cord)
• Endoderm = Gut tube

Question 7

What happens to the mesoderm during the 4th week?

Answer 7

• Holds the ectoderm and endoderm tubes together
• Splits into visceral/splanchnic and parietal/somatic layers
• Visceral layer = rolls ventrally + connected to gut tube
• Parietal layer = forms the lateral body wall folds

Question 8

At the end of the third week, Intraembryonic mesoderm differentiates into what?

Answer 8

• PARAXIAL MESODERM = forms somitomeres and somites -> form skull and vertebrae
• INTERMEDIATE MESODERM = urogenital system
• LATERAL PLATE MESODERM = forms body cavity

Question 9

What happens at the end of the fourth week?

Answer 9

The lateral body wall folds meet in the midline and FUSE to close the ventral body wall -> Closure is aided by growth of head and tail regions that cause embryo to curve into FOETAL POSITION -> Closure of gut tube except for connection to yolk sac called the VITELLINE (YOLK SAC) DUCT

Question 10

Describe the formation of serous membranes

Answer 10

1) Some cells of mesoderm form the PARIETAL LAYER the outside of the PERITONEAL, PLEURAL and PERICARDIAL CAVITIES
2) Some cells of mesoderm form the VISCERAL LAYER covering the ABDOMINAL ORGANS, LUNGS and HEART

Question 11

What is the Septum transversum?

Answer 11

A thick plate of mesodermal tissue occupying the space between the thoracic cavity and the stalk of the yolk sac

Question 12

What are the properties of the Septum transversum?

Answer 12

• Derived from visceral (splanchnic) mesoderm surrounding the heart
• Does not separate thoracic and abdominal cavities completely but leaves large openings (PERICARDIOPERITONEAL CANALS) on each side of the foregut

Question 13

What happens as a result of the rapid growth of the lungs?

Answer 13

PERICARDIOPERITONEAL CANALS become too small and the lungs begin to expand into the MESENCHYME of the body wall dorsally, laterally and ventrally

Question 14

Due to the expansion of the lungs, mesoderm of the body wall forms which components?

Answer 14

• Definitive wall of the THORAX
• PLEUROPERICARDIAL MEMBRANES (contain cardinal veins + phrenic nerves)

Question 15

In adults, what does the pleuropericardial membranes form?

Answer 15

Fibrous pericardium

Question 16

Describe the formation of the diaphragm

Answer 16

1) 5th week = PLEUROPERITONEAL FOLDS appear
2) 7th week = PLEAUROPERITONEAL FOLDS fuse with the SEPTUM TRANSVERSUM and MESENTERY of the oesophagus -> separates thoracic cavity from the abdominal cavity
3) 4th month = RIM derived from body wall forms the most peripheral part of the diaphragm

Question 17

What is the diaphragm derived from?

Answer 17

• Septum transversum (forms central tendon of diaphragm)
• Two pleuroperitoneal membranes
• Muscular components from somites (C3 - C5)
• Mesentery of the oesophagus