The document summarizes the development of the heart from the 3rd week of gestation. It describes how the heart forms from mesodermal cells that aggregate to form endothelial heart tubes. It then details the formation of the various septa within the heart including the interatrial septum (from the septum primum, intermedium, and secundum), interventricular septum (from the endocardial cushions fusing), and division of the atrioventricular canal. Endocardial cushions play an important role in septation and any defects can result in conditions like atrioventricular septal defects.

1. Introduction to development of
the heart
• It develops early in the middle of 3rd
week , from aggregation of splanchnic
mesodermal cells, in cardiogenic area,
ventral to pericardial coelom, and dorsal
to yolk sac.
• They form 2 angioblastic cords that
canalize to form 2 endocardial heart
tubes.

4. Development of sinus venosus
• In the middle of the fourth week, the sinus venosus receives
venous blood from the right and left sinus horns .
• Each horn receives blood from three important veins: (1)
the vitelline or the omphalomesenteric vein, (2) the
umbilical vein, and (3) the common cardinal vein.
• Communication between the sinus and the atrium is wide-
initially
• Later the entrance of the sinus shifts to the right
• This shift is caused primarily by left-to-right shunts of blood,
which occur in the venous system during the fourth and fifth
weeks of development.

5. Development of sinus venosus
• With obliteration of the right umbilical vein and the left
vitelline vein during the fifth week, the left sinus horn rapidly
loses its importance
• When the left common cardinal is obliterated at 10 weeks, all
that remains of the left sinus horn is the oblique vein of the
left atrium and the coronary sinus .

6. Development of sinus venosus
• As a result of left-to-right shunts of blood, the right sinus horn
and veins enlarge greatly.
• The right horn, which now forms the only communication
between the original sinus venosus and the atrium, is
incorporated into the right atrium to form the smooth-walled
part of the right atrium .
• Its entrance, the sinuatrial orifice, is flanked on each side by a
valvular fold, the right and left venous valves .
• Dorsocranially, the valves fuse, forming a ridge known as the
septum spurium .

7. Development of sinus venosus
• Initially the valves are large, but when the right sinus horn is
incorporated into the wall of the atrium, the left venous valve
and the septum spurium fuse with the developing atrial
septum .
• The superior portion of the right venous valve disappears
entirely.
• The inferior portion develops into two parts: (1) the valve of
the inferior vena cava and (2) the valve of the coronary
sinus.
• The crista terminalis forms the dividing line between the
original trabeculated part of the right atrium and the smooth-
walled part (sinus venarum), which originates from the right
sinus horn .

8. Cardiac septa
• The major septa of the heart are formed between the 27th
and 37th days of development, when the embryo grows in
length from 5 mm to approximately 16 to 17 mm.

9. CARDIAC SEPTA
• The other manner in which a septum is formed does not
involve endocardial cushions.
• a narrow strip of tissue in the wall of the atrium or ventricle
should fail to grow while areas on each side of it expand
rapidly, a narrow ridge forms between the two expanding
portions .
• When growth of the expanding portions continues on either
side of the narrow portion, the two walls approach each other
and eventually merge, forming a septum

10. CARDIAC SEPTA

11. Partitioning of the
primitive Heart
• Division of A-V canal , primitive
atrium & primitive ventricle begins
at the middle or end of 4th week and
completed by the end of 5th week.
• These processes occur concurrently.
• At the end of 4th week, 2 endocardial
cushions on dorsal & ventral walls of
atrioventricular canal , develop from
mesenchymal cells of cardiac jelly.

12. INTERATRIAL SEPTUM
• Begins in 5th week of gestation
• Septum primum begins to develop, growing toward the
endocardial cushions.
• The progressively diminishing space between the
endocardial cushions and the septum primum is known
as the ostium primum.
• Before the septum primum fuses with the endocardial
cushions, small perforations develop and coalesce in the
cephalic portion of the septum primum, which is known
as the ostium secundum.

13. 1-Septum primum :
•A thin crescent-shaped membrane grows
from the roof of common atrium.
•The lower margin of the septum is free and
concave.
•Anterior and posterior horns of the septum
fuse respectively with the ventral and dorsal
endocardial cushions of the primitive
atrioventricular canal.
2-Septum intermedium:
•The ventral and dorsal cushions of the
atrioventricular canal fuse to form a broad
anterioposterior partition, which divide the
canal into right and left atrioventricular
orifices.

14. • A foramen known as ostium
primum is formed between the
upper surface of septum
intermedium and lower border of
septum primum.
• Later, ostium primum is closed by
the fusion of two septa.
• Associated with the closure of
ostium primum, the upper and
dorsal part of septum disintegrates
forming a foramen known as ostium
secundum.

15. INTERATRIAL SEPTUM
• To the right of the septum primum, the septum secundum
starts to form as an invagination of the atrial wall.
• The septum secundum stops growing at the end of the
seventh week of gestation, leaving a posterior and inferior gap
known as the fossa ovalis.
• When the left venous valve and the septum spurium fuse with
the right side of the septum secundum, the free concave edge
of the septum secundum begins to overlap the ostium
secundum

16. 3-Septum secundum-
•It arises on the right side of septum
primum from the space of the right
atrium which is interval between
septum primum and septum spurium.
•The septum secundum incorporates
the whole of left venous valve and
extends vertically downwards.
•The lower margin grows sufficiently to
overlap the upper margin of the
septum primum.
•The valvular opening formed between
the lower margin of the septum
secundum and upper margin of the
septum primum is called foramen
ovale.

17. FORAMEN OVALE
• The opening left by the septum secundum is called the oval
foramen
• When the upper part of the septum primum gradually
disappears, the remaining part becomes the valve of the oval
foramen.

18. FORAMEN OVALE
• After birth, when lung circulation begins and pressure in
the left atrium increases, the valve of the oval foramen is
pressed against the septum secundum, obliterating the
oval foramen and separating the right and left atria.
• In about 20% of cases, fusion of the septum primum and
septum secundum is incomplete, and a narrow oblique
cleft remains between the two atria.

19. Features of the
interatrial septum
• On the right side:
1. Fossa ovalis: Oval depression in the lower part of the septum, and the floor is
formed by the septum primum.
2. Limbus fossa ovalis: a sickle shaped fold that surrounds the upper, anterior and
posterior margins of the fossa ovalis. It represents lower free margins of the
septum secundum.
3. Foramina venarium minimarium: Venae cordis minimi open through these
foramina.
4. Atrio-ventricular node: It is situated in the lower part of the septum above the
opening of coronary sinus.

20. • On the left side:
• 1. Presence of the semilunar fold with the concavity directed upwards; it
is a remnant of the upper margin of the septum primum.
• 2. Lunate impression above the fold is formed by septum secundum.
• 3. Foramina venarium minimarium.

21. Interatrial septum

22. Endocardial cushions
• dorsal & ventral swellings
• fuse, dividing the single AV
canal into paired canals
• involved in formation of
interatrial & interventricular
septa
• derived from neural crest
• involved in many CHDs

23. Septum primum grows from atrial
roof toward endocardial cushions
• Foramen primum: shunt that closes
• Foramen secundum: perforates
septum primum, allowing shunt
• Septum secundum grows down,
Over
lFrom Moore & Persaud 1998apping
foramen secundum

24. Septum secundum grows down,
overlapping foramen secundum
• Foramen ovale: between septum
primum & septum secundum
• Remaining portion of septum
primum
forms valve of foramen ovale

26. • Fetus
• • right side high pressure (high pulmonary resistance, etc.)
• • well oxygenated blood streams through foramen ovale
• • valve of foramen ovale closes with left atrial contraction
• • After birth
• • right side low pressure (low pulmonary resistance)
• • valve remains closed (physiological closure)
• • valve eventually fuses (anatomical closure): fossa ovalis

28. Formation of the Cardiac Septa
• The Atrioventricular (AV) septum
• Atrial septum
• Interventricular septum
• Aorticopulmonary septum

29. Molecular regulation of septal development
• NKX2.5.-the master gene for heart development
• BMPs 2 and 4
• WNT protein inhibitors-CRESCENT and CERBERUS
• FGF8
• Retinoic acid
• TBX5- DNA- binding motif known as the T-box. Expressed
later than NKX2.5, it plays an important role in septation.

31. During 5th week, the AV- endocardial
cushions meet and unite in the mid line
to form a septum and divide the
common A-V canal into right & left A-V
canals.
Endocardial cushions also form the
AV- valves + membranous septa of
interventricular septum.
Note in D, coronal section ,begining
of development of interatrial &
intervent. septa.

32. Formation of the Interatrial septum.
• The septum is developed from three sources:
• 1. Septum primum
• 2. Septum intermedium
• 3. Septum secundum

34. Development of muscular part of IVS:
Primordial muscular IVS arises in the floor of ventricle , as thick crescentic
fold with concave free edge.
This septum subdivides the original ventricular cavity incompletely into
right & left ventricles that communicate together through IV foramen.
It meets the fused AV cushions (Septum intermedium) and partially fuses
with them
This foramen closes by the end of 7th week as the 2 bulbar ridges fuse
with the endocadial cushion.

35. • 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

38. VSD
• Perimembranous(membranous/infracristal)70-
80%
• Muscular- 5-20%
Central- mid muscular
Apical
Marginal- along RV septal junction
Swiss cheese septum – multiple defects
• Inlet/ AV canal type-5-8%
• Supracristal/ subaortic- 5-7%
soto et al

39. ENDOCARDIAL CUSHION DEFECT
• COMMON AV CANALLEFT AND RIGHT SIDE
BY FUSION OF SUPERIOR AND INFERIOS
ENDOCARDIAL CUSHION
• FAILUREAV SEPTAL DEFECT WITH VARIOUS
COMBINATION

40. • PARTIAL AV CANAL DEFECTSEPERATE AV
ORIFICE
• COMPLETE AV CANAL DEFECTCOMMON AV
ORIFICE
• ISOLATED INLET VSD

41. 4th week – Formation of septa starts, IVS starts proliferating from
5th w the bulbar ridges & fused endocardial cushions.
 6th w. incorporation of the proximal part of bulbus cordis into the ventricles.
6th w. proliferation of endocardial cushions to diminish I V foramen.
7th w. fusion of bulbar ridges + extensions of endocardial cushions upward with
aortico-pulmonary septum and down with muscular I V septum to close I V foramen ,
so memb. IV septum is formed

42. DEVELOPMENT OF ARTERY

43. • First arteries to appear are Right and left primitive
aorta
• They are continous to the endothelial heart tubes
• Each primitive aorta have ventral portion lying in
the first pharygeal arch
• 1st,2nd and 5th arch dissapears

53. INTERRUPTED AORTIC ARCH

55. RIGHT AORTIC ARCH

56. DOUBLE AORTIC ARCH