1. DEVELOPMENT OF ATRIA & VENTRICLES
Dr. Mujahid Khan

2. Fate of Heart Tubes
The tubular heart elongates and develops alternate dilations and constrictions:
Truncus Arteriosus
Bulbus Cordis
Ventricle
Atrium
Sinus venosus

3. Partitioning of Heart
Partitioning of the atrioventricular canal, primordial atrium, and primordial ventricle begins around 4th week
Completed by the end of the 5th week

5. Partitioning of Atrioventricular Canal
Toward the end of the 4th week
Endocardial cushions form on the dorsal and ventral walls of the AV canal
The AV endocardial cushions approach each other and fuse
Dividing the AV canal into right & left canals
These canals partially separate the primordial atrium from the ventricle
Endocardial cushion works as AV valves

8. Endocardial Cushion
It develops from a specialized extracellular matrix (ECM) or cardiac jelly
The transformed endocardial cushions contribute to the formation of the valves and membranous septa of the heart

10. Partitioning of Primordial Atrium
Beginning at the end of the 4th week
Is divided into right & left atria by the formation and subsequent modification and fusion of two septa:
Septum primum
Septum secondum

11. Septum Primum It’s a thin crescent shaped membrane
Grows from the roof of the primordial atrium towards the fusing endocardial cushions
Dividing the common atrium into left & right halves

15. Foramen Primum
A large opening forms in the growing septum primum between its crescentic free edge and the endocardial cushion
It serves as the shunt, enabling oxygenated blood to pass from right to the left atrium
Becomes progressively smaller and disappears as the septum primum fuses with the fused endocardial cushions to form a primordial AV septum

17. Foramen Secondum
Perforation appears in the central part of the septum primum before the foramen primum disappears
Perforation coalesce to form another opening, the foramen secondum
Concurrently, the free edge of the septum primum fuses with the left side of the fused endocardial cushions
It ensures a continuous flow of oxygenated blood from right to the left atrium

19. Septum Secondum A crescentic muscular membrane
Grows from ventrocranial wall of atrium
Immediately to the right of the septum primum
It is thick and grows during 5th & 6th weeks
Gradually overlaps the foramen secondum in the septum primum

21. Septum Secondum It forms an incomplete partition between the atria
Consequently an oval foramen forms
Cranial part of the septum primum ( attached to the roof of left atrium) disappears gradually
Remaining part of septum primum (attached to endocardial cushion) forms a flap like valve of oval foramen

23. Oval Foramen
Before birth it allows most of the oxygenated blood entering the right atrium from IVC to pass into the left atrium
Prevents the blood flow in opposite direction
After birth it normally closes and the valve of the oval foramen fuses with septum primum
The interatrial septum becomes a complete partition between the atria

25. Oval Fossa
An oval depression in the lower part of the interatrial septum of the right atrium known as oval fossa
It’s a vestige of the oval foramen

26. Partitioning of Primordial Ventricle
Division of primordial ventricle is first indicated by a median muscular ridge, the primordial interventricular septum
Is a thick crescentic fold has a concave free edge
Initially most of its height results from dilation of the ventricles on each side of the IV septum

28. Partitioning of Primordial Ventricle
Medial walls of the enlarging ventricles approach each other and fuse to form the primordium of the muscular part of the IV septum
Active proliferation of myoblasts in the septum increase its size
Until the 7th week there is a crescent shaped interventricular foramen between the free edge of IV septum and the fused endocardial cushion

30. Interventricular Foramen
The IV foramen permits communication between the right and the left ventricles
It usually closes by the end of the 7th week as the bulbar ridges fuse with the endocardial cushion

31. Closure of IV Foramen
Formation of the membranous part of the IV septum result from the fusion of tissues from 3 sources:
The right bulbar ridge
The left bulbar ridge
The endocardial cushion

33. Interventricular Septum
The membranous part of the IV septum is derived from an extension of tissue from the right side of the endocardial cushion to the muscular part of the IV septum
This tissue merges with the aorticopulmonary septum and thick muscular part of the IV septum

34. Interventricular Septum
After closure of the IV foramen and formation of the membranous part of the IV septum, the pulmonary trunk is in communication with the right ventricle and the aorta with the left ventricle

35. Trabeculae Carnae
Cavitation of the ventricular walls forms a sponge-work of muscular bundles called trabeculae carnae
Some of these bundles become the papillary muscles and tendinous cords
The tendinous cords run from the papillary muscles to the atrioventricular valves

37. Tetralogy of Fallot Classic group of four cardiac defects:
Pulmonary stenosis
Ventricular septal defect
Dextroposition of aorta
Right ventricular hypertrophy