Download 12.Atria & Ventricle..

DEVELOPMENT OF ATRIA
& VENTRICLES
Dr. Mujahid Khan
Fate of Heart Tubes
 The
tubular heart elongates and develops
alternate dilations and constrictions:
 Truncus Arteriosus
 Bulbus
Cordis
 Ventricle
 Atrium
 Sinus venosus
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
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
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
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
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
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
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
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
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
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
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
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
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
Interventricular Foramen
 The
IV foramen permits communication
between the right and the left ventricles
usually closes by the end of the 7th week
as the bulbar ridges fuse with the
endocardial cushion
 It
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
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
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
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
Tetralogy of Fallot
Classic group of four cardiac defects:
 Pulmonary
stenosis
 Ventricular septal defect
 Dextroposition of aorta
 Right ventricular hypertrophy