Development of the heart

Question 1

Does the heart develop from the mesoderm, endoderm or ectoderm?

Answer 1

develops from cardiogenic mesoderm

- is originally at the cranial end of the neural tube

Question 2

Describe the development of the heart up to (+including) 22 days.

Answer 2

• At around 18-19 days, the heart begins to form
• Angiogenic cell clusters fuse to form right and left endocardial tubes
• Endocardial tubes are pushed into the thoracic cavity (as embryonic folding continues)
• endocardial tubes fuse together (primitive heart), and this is complete at ~22 days
• At the same time the endocardial tubes are forming, other major heart components are also being formed
• Once the primitive heart has been formed, it forms 5 distinct regions:
• -> truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus

Question 3

At what day does the heart start beating?

Answer 3

22 days

Question 4

Describe blood flow through the developing heart at 22 days.

Answer 4

Sinus venosus –> paired atria – atrioventricular sulcus –> primitive ventricle – interventricular sulcus –> bulbus cordis –> truncus arteriosus –> dorsal aorta

Question 5

State what these primitive structures give rise to in the developed heart:

1. Primitive ventricle
2. Truncus arteriosus
3. Bulbus cordis

Answer 5

1. expand to become left ventricle
2. aorta and pulmonary trunk
3. smooth parts of left and right ventricles (aortic vestibule and conus arteriosus)

Question 6

Describe the process of heart looping.

Answer 6

Days 22-24: heart tube doubles in length
Day 23-28: heart begins to form an ‘S’ shape
Day 23: primitive ventricle begins to twist and bend over - bulbus cordis follows
Day 24: primitive atria loops up behind ventricle towards truncus arteriosus

Question 7

How does dextrocardia occur? How common is it?

Answer 7

• Primitive heart tube folds to the left in a mirror image of normal looping
• Occurs when all the organs are reversed (situs inversus)
• Occurs in 1:7000 individuals

Question 8

When do the endocardial cushions form?

Answer 8

At the end of the 4th weeks

Question 9

What partitions the common atrium into two atria? When does this occur?

Answer 9

Septum primum (grows towards the posterior and anterior endocardial cushions)
Occurs ~ day 28

Question 10

How are the following structures formed:

1. ostium primum
2. ostium secundum
3. foramen ovale

Answer 10

1. free edge of the septum primum (it forms an opening)
2. from perforations in the upper septum primum
3. free edge of the septum secundum

*septum primum acts as a valve over the foramen ovale

Question 11

When do the ventricles fully form? What part of the ventricles are formed by endocardial cushions?

Answer 11

End of the 4th week
- ventricles expand, myocardium is added to the outside
Endocardial cushions form the membranous part of ventricles

Question 12

Describe how the aorta and pulmonary trunk become partitioned.

Answer 12

• Left and right bulbar ridges grow from the inner walls of the truncus arteriosus and bulbus cordis
• Twist around each other and fuse to form a spiral aortico-pulmonary septum
• Due to twisting, the aorta connected with the left ventricle and the pulmonary trunk with the right ventricle