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In which direction and where does the embryo fold in the 4th week?

Along the median and horizontal planes at the cranial, caudal and lateral aspects of the embryo to create the tube-within-a-tube.

What causes the folding of the embryo into a tube-within-a-tube body plan?

Rapid growth of embryo particularly of its Brain and Spinal cord and remodeling of the tissue layers

Does the yolk sak grow during the 4th wk?


Which side of the embryonic disc is the yolk sac attached to?

ventral rim

What causes the folding to be concentrated in the thin, flexible outer rim of the disc?

The developing notochord, neural tube, and somites stiffen the dorsal axis of the embryo

What gives rise to the ventral surface of the body?

The cranial, caudal, and lateral margins of the disc fold completely under the dorsal axial structures

Where do the folding rings become continuous?

These folds become continuous with one another as a ring of tissue at the position of the future umbilicus

What are the three areas of folding?

Cranial (head), caudal (tail), and lateral body fold

What folds the primordium of the brain?

Neural folds in cranial region forms the primordium of the Brain.

Describe the folding of the developing forebrain

The developing forebrain grows cranially beyond the oropharyngeal membrane and overhangs the developing heart.

What becomes the central tendon of the diaphragm?

septum transversum

Where does the pericardial coelom lie after folding?

After folding, the pericardial coelom lies ventral to the developing heart and cranial to the septum transversum.

Where does the tail fold come from?

Growth of distal part of neural tube. The tail region projects over the cloacal membrane (future anus)

connecting stalk

primordium of umbilical cord

Where is the connecting stalk attached to after folding?

ventral aspect of the embryo


endodermal diverticulum of the yolk sac is partially incorporated into the embryo

Origin of lateral folds

Result from the rapid growth of the spinal cord and the somites producing right and left lateral folds

Movement of lateral folds

Lateral folds moves toward the median plane forming a roughly cylindrical embryo.!

Evolution of connection between mid-gut and yolk sac?

- Initially there is a wide connection between mid gut and yolk sac
- After lateral folding, this communication is reduced to a yolk stalk or vitelline duct.

What does the mid-gut yolk sac connecting stalk become?

Umbilical cord!

What does the intraembryonic coelom give rise to?

i) Pericardial cavity
ii) Pleural cavity
iii) Peritoneal cavity

Pericardioperitoneal canal

connects the Pericardial cavities cranially and the Peritoneal cavities caudally.

Where do the Pericardioperitoneal canals lie?

lateral to primitive foregut and dorsal to Septum transversum

Cranial ridges

Pleuropericardial folds (forms the pericardial sac) separates pericardial cavities from pleural cavities

Caudal ridges

Pleuroperitoneal folds are located inferior to the lungs

Which Pericardioperitoneal is larger?

The left Pericardioperitoneal canal is larger than the right

By when are both Pericardioperitoneal canals closed?

7 weeks

Pleuropericardial membranes

The Pleuropericardial folds enlarge to form partitions that separate the Pericardial cavity from the Pleural cavities. Inner layer forms the Fibrous pericardium

What components does the diaphragm develop from?

i) Septum transversum—gives rise to the Central tendon of Diaphragm
ii) Pleuroperitoneal membranes
iii) Dorsal mesentery of esophagus-gives rise to crura of Diaphragm
iv) Lateral body walls (muscular ingrowths from the body wall)

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