Talk:Intermediate - Atrial Ventricular Septation

From Embryology

Flash Version

<Flowplayer height="564" width="720" autoplay="false">Heart septation 003.flv</Flowplayer>

--Mark Hill 16:01, 2 October 2009 (EST) |

  • I don't like the coloured boxes appearing over the heart, difficult to read, they should appear beside the image in a white space. Same comment for all animations with this layout of text boxes appearing.
  • Try and keep labels off the top of the image unless it has a specific purpose for being there.
  • Same comment as before concerning opening title and initial description of the heart.
  • All image labels should all be lower case.

I have not made all the changes that I have suggested to the uploaded PPT draft files, but you can see what I want File:Cardiac septation intermediate draft.ppt

--Phoebe Norville 17:12, 24 September 2009 (EST) First content added:


All of the partitioning of the primitive heart occurs between the middle of the fourth week and the end of the fifth week. The following animation shows the processes involved in the division of the atrioventricular canal, atria and ventricles. These are described in more detail below.

Division of the Atrioventricular (AV) Canal

Two endocardial cushions form on the dorsal and ventral surfaces of the AV canal, referred to as the superior and inferior cushions respectively. The cardiac jelly in this region expands, while mesenchymal cells from the endocardium invade the cushions allowing them to grow and fuse. This fusion divides the common AV canal into the right and left canals, hence partially separating the primitive atrium and ventricle. Two smaller endocardial cushions also form on the lateral walls of the AV canal, which later help to form the mitral and tricuspid heart valves.

Septation of the Atria

Membranous tissue forming the septum primum grows from the roof of the atrium, dividing it into left and right halves. The space between the septum primum and the endocardial cushions is referred to as the foramen primum. Apoptosis-induced perforations appear in the centre of the septum primum to produce the foramen secundum. At this time, the strong, muscular septum secundum grows immediately to the right of the septum primum and gradually overlaps the foramen secundum during the fifth and sixth weeks of development. The incomplete partition of the atrium by the septum secundum forms the foramen ovale.

Blood flows through the foramen ovale and foramen secundum to the left atrium forming a right-to-left shunt. The remaining portion of the septum primum acts as the valve of the foramen ovale. Blood cannot flow in the opposite direction as the muscular strength of the septum secundum prevents prolapse of the septum primum.

Remodelling of the Inflow tract and Atria

The development of two left to right shunts in the venous system leads to an increase in the right horn of the sinus venosus and consequentially a decrease in left horn by the end of the fourth week. The sinuatrial orifice correspondingly shifts to the right and thus becomes located in the right atrium. Hence the right atrium receives the superior vena cava and inferior vena cava in the adult. The left sinus horn regresses to form the coronary sinus in humans. Thus the sinus venosus gradually becomes incorporated into the right atrium. It contributes to the smooth walled part of the adult right atrium, referred to as the sinus venarum. The trabeculated right atrium corresponds to the primordial atrium; the division between these structures is indicated by the inner crista terminalis and outer sulcus terminalis.

The primordial pulmonary vein develops in the dorsal wall of the LA. As the atrium increases in size it incorporates more of the branches of the pulmonary vein, culminating in its receiving the four pulmonary veins. The smooth wall of the adult LA originated from the primordial pulmonary vein, while the trabeculated wall represents the primordial atrium.

Septation of the Ventricles

Minor trabeculations appear during early development of the primordial ventricle. Following growth of the ventricles further trabeculations appear and grow as larger, muscular structures. Some researchers believe that as the trabeculations grow they coalesce resulting in the formation of the ventricular septum. However, the more commonly described theory of septation begins with the appearance of a primordial muscular interventricular (IV) ridge developing in the floor of the ventricle near the apex. As either side of the ventricle grows and dilates, their medial walls fuse forming the prominent IV septum. The foramen located between the cranial portion of the IV septum and the endocardial cushions, the IV foramen, closes by the end of the seventh week as the bulbar ridges fuse with the endocardial cushions.