Development Of The Heart And Blood Vessels Flashcards
What is a Morula?
DAY 4
It is a solid ball of cells formed as the zygote undergoes cleavage.
What is the Early Blastocyst?
DAY 6
It is a hollow ball of cells with a fluid-filled cavity. It comprises an inner cell mass, a blastocyte cavity and trophocytes forming the outer layer.
What is the Late Blastocyte?
DAY 10
It is the pre-embryo. It contains an embryonic disc (which are two layers of cells that will become the embryo proper) between the amniotic cavity and yolk sac.
What is the Gastrula?
DAY 16
It is the embryo with the three primary germ layers: the Ectoderm, the Mesoderm and the Endoderm.
List what is made from the three primary germ layers.
The Ectoderm becomes the skin (epidermis), brain, spinal cord (CNS), sensory organs, etc.
The Mesoderm becomes the heart, skeletal muscle, kidneys, urogenital organs, connective tissue, etc.
The Endoderm becomes the lining of the gastrointestinal tract and lungs, pancreas, liver, etc.
In a simplified way, it can be said that the Ectoderm gives rise to skin and the nervous system, the Endoderm to the intestinal organs, and the Mesoderm to the rest of the organs.
Describe the structure of the embryo at Day 18.
At the head end, we have a primary heart field with blood islands in it.
At the tail end, we have the primitive node, a group of cells on the anterior portion of the primitive streak. This node is where gastrulation first begins.
The heart develops from the primitive streak via the blood islands in the splanchnic mesoderm. - IMPORTANT
Note that organ placement is determined by the asymmetry along the primitive streak
Describe the formation of the heart tube across Days 20, 21 and 22
AT 20 DAYS:
The blood islands slowly fuse together into 2 tubes called cardiogenic cords.
AT 21 DAYS:
The tubes fuse down the middle to make one heart tube.
AT 22 DAYS:
Further development gives rise to a tube with asymmetric openings, forming an arterial and venous end.
There is the beginning of asymmetry in the bulge to the left.
At the superior end, we have the truncus arteriosus, and at the inferior end, we have the sinus venosus. The sinus venousus will attach to rudimentary circulation.
What is situs inversus?
It is a condition in which all the heart and other organs of the body are transposed through the sagittal plane to lie on the opposite side from normal.
Describe the cardiac looping across Days 22, 23, 24 and 35.
AT 22 DAYS:
The cells at each end of the heart proliferate, making the tube longer. The entire structure is in a confined space, so it begins to fold upon itself.
AT 23/24 DAYS:
The primitive atrium loops up above and behind the primitive ventricle. The looping process brings the primitive areas of the heart into the proper spatial relationship for development.
AT 24 DAYS:
The primitive atrium, when viewed from the side, is growing out of the back and becoming two atria.
The future right ventricle grows downwards at the front.
AT 35 DAYS:
The heart is beating by this stage and the foetal heart can be seen on the ultrasound. However, the four interior chambers of the heart are not yet divided.
Describe the division of the atrioventricular canal (at 30 days).
The septum primum grows along the midsagittal plane, separating the atria, except for a temporary space called the formen primum.
The left side of the atrium grows pulmonary veins, sending growing veins to the developing lungs.
The posterior and anterior endocardial cushions fuse, dividing the atria from the ventricles.
A ventricular septum will eventually grow upwards to form a division between the two ventricles. It will attach to the divison between the atria and the ventricles.
Describe a ventricular septum defect, and the effects it has.
One cause of a ventricular septum defect is the failure of the endocardial cushions to provide an anchor point for the developing ventricular septum.
The extent of the opening may vary from pin size to complete absence of the ventricular septum. We can see this abnormality using an echocardiogram.
This means that during ventricular contraction, some of the blood from the left ventricle will leak into the right ventricle, passing through the lungs and reentering the left ventricle via the pulmonary veins and left atrium. We can hear this turbulence when listening to the heart.
This has 2 main effects:
1) The systemic circulation doesn’t receive all the blood being pumped by the left ventricle.
2) The leakage of blood into the right ventricle elevates right ventricular pressure and volume, causing pulmonary hypertension. It can also cause right ventricular wall hyperplasia, increasing the hearts O2 demand.
Describe the formation of the atrial septa (across 40 and 43 days).
AT 40 DAYS:
The anterior and posterior endocardial cushions have fused, leaving the left and right atrioventricular canals.
Initially, the temporary hole in the septum primum called the foramen primum permits the right to left shunt of foetal blood, as there is no pulmonary circulation.
A second, more muscular and robust septum called the septum secondum grows down from the roof of the atria, just lateral to the septum primum. It obstructs the place where the foramen primum was. The foramen secondum is partially obscured by the septum secondum, but the foramen ovale still remains, providing an alternative left to right shunt.
AT 43 DAYS:
The septum secondum grows but leaves the foramen ovale, a space permitting the right to left shunt of blood.
The foramen secondum is partially obscured by the septum secondum, but the foramen ovale remains, providing an alternative left to right shunt as the foramen primum is closed.
The ventricular septum grows up to fuse with the now fused endocardial cushions.
What happens to the foramen ovale when we are born?
At birth, the lungs become functional. this means that the pulmonary vascular pressure decreases and the left atrial pressure exceeds that of the right, forcing the septum primum against the septum secondum, functionally closing the foramen ovale.
In time, the septa eventually fuse, leaving behind the fossa ovalis.
Describe the division of the outflow tract.
At this stage, there is no separation between the ventricles, so all the blood from the heart is leaving through one vessel. Eventually, they are going to be separated into the left and right sides (the pulmonary artery and the aorta).
The separation of the ventricles results from the union of the: conotruncal septum, endocardial cushions and the ventricular septum.
The conotruncal septum grows as a spiral down the conus arteriosus. There is a division at the top of the tube which spirally grows down to create two intertwining tubes. The conotruncal septum meets with the endocardial cushions and the ventricular septum. The three of them make the final full separation of the left and right sides.
Describe the Tetralogy of Fallot.
If the conotruncal septum is misaligned, you either get a differently sized aorta and pulmonary artery, or it won’t meet with the endocardial cushion correctly, giving rise to a septal defect. The ultimate complication is the Tetralogy of Fallot.
1) PULMONARY STENOSIS:
When the spiral valve has grown off centre, so we end up with a large aorta and small pulmonary artery, restricting blood to the lungs.
2) OVERRIDING AORTA:
When the aortic opening is positioned over a ventricular septal defect, it allows the blood from both sides of the heart to enter the aorta, which means that some deoxygenated blood is pumped around the body.
3) VENTRICULAR SEPTAL DEFECT:
During systole, blood from the left ventricle leaks into the right ventricle. This increases pressure in the pulmonary circulation and also causes a volume overload in the left ventricle.
4) RIGHT VENTRICULAR HYPERTROPHY:
The right ventricular wall increases in size to deal with the obstruction in the pulmonary artery. This increases the heart’s oxygen demand.
