Development- Body and Coelom Formation

Question 1

What did the evolution of extra-embryonic or fetal membranes allow?

Answer 1

Allowed reptiles, mammals, and birds to move away from aquatic environments and to evolve to land

Question 2

How does the embryonic disc become covered by extra-embryonic membranes?

Answer 2

Body folding

Question 3

Function of extra-embryonic membranes

Answer 3

• Supply/storage of nutrients
• Gas exchange
• Excretion of wastes
• Mechanical protection
• Immune protection
• Hormone production (mammals)

Question 4

What are the extra-embryonic membranes?

Answer 4

• Chorion
• Amnion
• Yolk Sac
• Allantois

Question 5

Yolk sac

Answer 5

• Connected to midgut by yolk duct (yolk duct wall continuous with gut)
• Mediates nutrition in developing birds and reptiles
• No yolk within the membrane in higher mammals. But still important as it is the first hematopoietic organ (blood cell formation) in mammals and a source of primordial germ cells (or stem cells which will eventually form oocytes/sperm cells)

Question 6

What forms the yolk sac wall?

Answer 6

• Formed from endoderm and visceral mesoderm (splanchnic)
• Therefore termed Splanchnopleure

Question 7

Chorion

Answer 7

• Outermost extra-embryonic membrane
• Functions in gas exchange, respiration etc

Question 8

What is the chorion derived from?

Answer 8

• Derived from trophectoderm (near embryonic disc) and somatic mesoderm
• Therefore termed somatopleure

Question 9

Where is the chorion in reptiles and birds?

Answer 9

Up against the egg shell

Question 10

Steps of body folding cranial to caudal

Answer 10

1. Somatopleure pushes up forming chorioamniotic folds that wrap around the embryo forming the amnion and eventually the amniotic cavity
2. Splanchnopleure pinches inwards forming primitive gut, and the yolk sac and yolk duct

Question 11

Amnion and its importance

Answer 11

The most evolutionary significant membrane to form

Surrounds the embryo proper and provides an aquatic micro-environment which permits embryogenesis in water analogous to evolutionary ancestors.
- Prevents dessication
- Acts as a shock absorber

Question 12

Mesamnion (chorioamniotic raphe)

Answer 12

• Seam. Site of fusion of somatopleure/ chorioamniontic folds
• In pigs and ruminants, fusion.
• In horses and carnivores, no fusion occurs. The chorion and amnion connection is lost in horses and carnivores. This allows allantois to move in between the chorion and amnion and wrap around the fetus.

Question 13

Allantois

Answer 13

Acts to store urinary wastes form embryo’s developing urinary system and mediates gas exchange

Question 14

Size difference of allantois in reptiles vs. mammals

Answer 14

Reptiles and birds: can be quite large and apposed to chorion keeping toxic by-products of metabolism away from the embryo

Mammals: size of allantois depends on how well nitrogenous wastes are removed by chorionic placenta, but it can fill the entire extra-embryonic coelom

Question 15

Why does the head bend so much during cranial-caudal folding?

Answer 15

Head bends to create enough space for full development of large brain

Question 16

When does both the lateral and caudal-cranial folding take place?

Answer 16

Occurs early on, around 3 weeks will be complete

All folding occurs at the same time

Question 17

Steps of Body folding- Lateral

Answer 17

1. Chorioamniotic folds which eventually surround fetus fetus
   –>Results in amniotic cavity
2. Splanchnopleure pinching inwards
   –> Results in right and left intraembryonic coelom (provides embryonic borders)

Question 18

Formation of intraembryonic coelomic cavities

Answer 18

• Splanchnopleure pinches inwards
• Lateral plate mesoderm (located on outer edge of splanchnopleure) creates clefts or spaces along the sides of the embryo
• Clefts or spaces will eventually fuse and form intraembryonic coelomic cavities
• Will extend cranially in horseshoe shape
• The more cranial portion of the horseshoe-shaped coelom will be ventral to foregut and developing heart

Question 19

Formation of pericardial and pleural cavities

Answer 19

The right and left intraembryonic coelom will initially fuse and form attached left and right pleuro-pericardial cavity which will surround the developing heart and lungs

Eventually, the pleuro-pericardial folds grow medially into the left and right sides of pleuro-pericardial cavity and will fuse to separate and divide into the pleural and pericardial cavities

• Pericardial cavity will surround heart
• Pleural cavity will surround lungs (will be larger as lungs grow and push cavity)

Question 20

What will the pericardial cavity surround?

Answer 20

heart

Question 21

What will the pleural cavity surround?

Answer 21

lungs

Question 22

Peritoneal cavity

Answer 22

• Lateral folding results in the left and right intra-embryonic coeloms surrounding the developing gut
• Gut suspended by folds of the splanchnic mesoderm between the coelomic cavities lined by the mesothelium (future peritoneum)
• Later on, ventral mesentery caudal to duodenum and cranial to rectum atrophies – gut suspended only by dorsal mesentery which allows it to grown in length and rotate
• Results in a single peritoneal cavity

Question 23

Formation of the diaphragm

Answer 23

• As diaphragm forms, it separates the pleuro-pericardial cavity from the developing peritoneal cavity
• The space that forms from this separation is closed due to the growth of the pleuro-peritoneal folds from the lateral body wall that fuse to the septum transversum and the mesothelial fold suspending the esophagus (mesoesophagus)
• Results in primordial diaphragm

Question 24

Diaphragmatic hernias

Answer 24

When fusion fails, there is a persistent opening between the thoracic and abdominal cavities where abdominal viscera pass through this opening into the thoracic cavity

Two types:
1. Pleuro-peritoneal herniation
2. Peritoneal-pericardial herniation

Question 25

Pleuro-peritoneal herniation

Answer 25

• Failure of one or both pleuro-peritoneal folds to develop or fuse with mesoesophagus and septum transversum
• Usually occurs on left side
• Most common in humans resulting in viscera (stomach and intestines) being present in the pleural cavity

Question 26

Peritoneal-pericardial herniation

Answer 26

• Occurs in domestic animals, dogs and cats
• Result in defect in development of septum transversum leading to improper communication between peritoneal and pericardial cavities. Herniation of viscera such as the liver, pyloric region of stomach and the intestines into the pericardial cavity

Question 27

Schistosomus Reflexus

Answer 27

• A congenital fatal anomaly
• Primarily observed during embryonic development in ruminants
• Leads to significant dystocia that is less difficult in large ruminants compared to smaller ones

Question 28

Schistosomus Reflexus presentation

Answer 28

• Head, limbs, and tail are in close proximity resembling a distinctive ventral convex curvature
• Results in acute angulations of spinal column
• Limb fusion/stiffness (ankylosis)
• Failure of the closure of all or most of the ventral wall of the fetal body resulting in exteriorized viscera (lateral folding failure)

Question 29

Schistosomus reflexus and birth

Answer 29

• Malformed calves are unable to pass through the birth canal
• Fetus must be removed by c-section or fetotomy
• Leads to recovery of cow for future breeding

Question 30

Gastroschisis

Answer 30

• Failure of ventral body wall to close in the abdominal region can lead to movement of abdominal contents such as the intestines, outside of the body cavity through a fissure between umbilicus and sternum
• Often a smaller area so can be surgically repaired

Question 31

Congenital umbilical hernias

Answer 31

• Associated with abdominal ventral body wall defect
• Protrusion of viscera occurs around the umbilicus due to lack of muscular body wall in that region and can be found in small animals, pigs, cattle
• One of the most common defects in pigs
• Common in calves, especially Holstein Friesian cattle with frequency of 4-15% therefore may very likely be linked to genetics