6.(c) Diagrammatically describe the steps of development of heart in mammals.

Stages of Mammalian Heart Development

The development of the mammalian heart is a continuous process that can be broadly divided into several key stages, each involving significant morphological transformations.

1. Formation of the Cardiogenic Area and Endocardial Tubes (Days 18-20)

• The heart originates from the mesodermal germ layer, specifically from cells in the cardiogenic area located in the splanchnic mesoderm cranial to the neural folds.
• Under the influence of signaling molecules from the underlying endoderm, these mesodermal cells differentiate into cardiac myoblasts and blood islands.
• Two distinct angiogenic cell clusters form on either side of the neural plate, developing into cardiogenic cords.
• As lumens develop within these cords, they become hollow structures known as endocardial tubes.

Diagram 1: Formation of Endocardial Tubes

(Imagine a diagram showing two parallel tubes forming from the mesoderm, lateral to the neural plate, then moving medially.)

Early embryonic stage showing the formation and migration of two endocardial tubes.

2. Fusion of Endocardial Tubes to Form the Primitive Heart Tube (Days 20-22)

• Due to embryonic folding (lateral and cephalocaudal), the two endocardial tubes migrate towards the midline of the embryo within the thoracic cavity.
• By approximately day 22, these two tubes fuse to form a single, straight primitive heart tube.
• This primitive heart tube rapidly differentiates into five distinct regions from caudal (venous) to cranial (arterial):
  • Sinus Venosus: Receives venous blood.
  • Primitive Atrium: Develops into parts of both atria.
  • Primitive Ventricle: Forms the left ventricle.
  • Bulbus Cordis: Develops into the right ventricle and outflow tracts.
  • Truncus Arteriosus: Forms the ascending aorta and pulmonary trunk.
• At this stage, peristaltic-like contractions begin, propelling blood from the sinus venosus towards the truncus arteriosus.

Diagram 2: Primitive Heart Tube

(Imagine a diagram showing a single, straight tube with five labeled regions: Sinus Venosus, Primitive Atrium, Primitive Ventricle, Bulbus Cordis, Truncus Arteriosus.)

The five distinct regions of the primitive heart tube.

3. Cardiac Looping (Days 23-28)

• The primitive heart tube continues to elongate rapidly, but the pericardial cavity constrains its growth, forcing it to bend and fold upon itself. This process is called cardiac looping.
• The cranial portion (bulbus cordis and truncus arteriosus) bends ventrally, caudally, and to the right.
• The caudal portion (primitive atrium and sinus venosus) moves dorsally, cranially, and to the left.
• This bending transforms the straight tube first into a C-shaped loop, then into an S-shaped configuration, bringing the future atria and ventricles into their characteristic relative positions.
• Successful looping is crucial for the proper alignment of the cardiac chambers and outflow tracts, and defects can lead to congenital heart diseases.

Diagram 3: Cardiac Looping (C-shape and S-shape)

(Imagine a diagram showing the straight tube bending into a C-shape, then further into an S-shape, with arrows indicating the direction of movement of different segments.)

Sequential stages of cardiac looping from straight tube to S-shaped configuration.

4. Septation of Atria and Ventricles (Weeks 4-8)

After looping, the heart undergoes septation, forming the definitive four-chambered structure and separating the systemic and pulmonary circulations.

Atrial Septation:

• Begins around the end of week 4.
• Septum Primum: A crescent-shaped membrane grows downwards from the roof of the primitive atrium towards the endocardial cushions. An opening, the ostium primum, remains at its lower edge.
• As the septum primum fuses with the endocardial cushions, the ostium primum closes, but perforations appear in the upper part of the septum primum, forming the ostium secundum.
• Septum Secundum: A second, thicker, muscular fold forms to the right of the septum primum, also growing downwards. It does not completely close, leaving an opening called the foramen ovale.
• The foramen ovale, guarded by the septum primum acting as a valve, allows blood to shunt from the right atrium to the left atrium during fetal life, bypassing the non-functional lungs.

Ventricular Septation:

• Begins at the end of week 4.
• Muscular Interventricular Septum: Grows upwards from the floor of the primitive ventricle, partially dividing it. A gap, the primary interventricular foramen, remains at its superior aspect.
• Membranous Interventricular Septum: Formed by the fusion of the conotruncal septum (from the outflow tract) and the endocardial cushions, it closes the primary interventricular foramen.

Outflow Tract Septation:

• The truncus arteriosus and bulbus cordis are divided by the formation of the aortico-pulmonary septum, which spirals to separate the ascending aorta and pulmonary trunk. This spiral ensures the aorta arises from the left ventricle and the pulmonary artery from the right ventricle.

Diagram 4: Septation of Atria and Ventricles

(Imagine a diagram illustrating the formation of septum primum, ostium primum, ostium secundum, septum secundum, and foramen ovale in the atria, and the muscular and membranous interventricular septum in the ventricles.)

Development of atrial and ventricular septa, leading to a four-chambered heart.

5. Valve Formation (Weeks 5-9)

• Atrioventricular (AV) Valves: Tricuspid (right) and Bicuspid (mitral, left) valves develop from proliferations of the endocardial cushions within the atrioventricular canals.
• Semilunar Valves: Aortic and Pulmonary valves form from swellings of subendocardial tissue at the outflow tracts of the truncus arteriosus.

By the end of the eighth week, the mammalian heart has largely achieved its definitive four-chambered structure with functional valves, although maturation continues until birth and beyond.