Development of the heart and blood vessels Flashcards
Describe the human embryo development (first 3 stages up to a blastocyte)
- The fertilised egg contains two pronuclei. This is the first sign of fertilisation. The fusing of male and female chromosomes
- The developing embryo contains 6-8 cells after 3 days.
- After 5 days the developing embryo is called a blastocytes and contains an approximately 100 cells
What are morula, blastocytes and gastrula and describe their structural features?
Morula - solid ball of cells formed from when the zygote undergoes cleavage.
Early Blastocyte - hollow ball of cells that contains a fluid filled cavity. It contains inner cells mass, the blastocyte cavity and trophoblasts.
Late blastocyte - pre-embryo, with the embryonic disk, contains two layers of cells that become embryo proper. It contains the amniotic sac, embryonic disk and yoc sac.
Gastrula - embryo with the three primary germ layers of cells (ectoderm, mesoderm and endoderm.)
What do the endoderm, mesoderm and ectoderm form?
Ectoderm - skin (epidermis), brain spinal cord (CNS) and sensory organs
Mesoderm - heart, skeletal muscles, kidneys, urogenital, connective tissue
Endoderm - the lining of the gastrointestinal tract and lungs, pancreas and liver
Describe gastrulation and these two structures
The embryonic disk that forms in day 16 forms these two structures and the notochord forms the spinal cord.
Trilaminar embryo
Describe the formation of the heart tube
The heart develops from the primitive streak via the blood islands in the splanchnic mesoderm. Day 18.
The blood islands fuse together to form two heart tubes called cardiogenic tubes. Day 20.
The 2 tubes fuse together down the middle to form one heart tube. Day 21.
Further development gives rise to a heart tube with asymmetric openings forming an arterial end and a venous end. Beginning of asymmetry with a bulge to the left. Day 22.
What is Situs Inversus?
• If the 3rd image is reversed in the body, all other organs in the body are on the wrong side. This occurs due to asymmetric structure reversing.
Describe cardiac looping (day 22 to day 24)
Day 22 - the cells at each end of the heart proliferate making the tube longer. The entire structure is in a confined space so begins to fold upon itself.
Day 23 and 24 - The primitive atrium loops above and behind the primitive ventricle. The process of looping brings primitive areas of the heart into the proper spatial relationship for development
Day 24 - the primitive atrium seen from the front grows outwards and will form the 2 atrium
the future right ventricle will grow downwards at the front.
Describe the structure of the heart at day 35 of cardiac looping
On image
At 35 days old, you can see the beating of the heart under ultrasound. We have primitive ventricles and atria.
Describe the division of the atrioventricular canal
Day 30
Septum primum - grows along the midsagittal plane separating the atria, apart from a temporary space called the foremen primum.
The left side of the atrium will grow veins to the developing lungs.
The posterior and anterior endocardial cushions will come together separating the atria and the ventricles.
A ventricular septum will eventually grow upwards separating the two ventricles and attach to the endocardial cushions
What is Ventricular septal defect and what causes it?
During ventricular contraction some of the blood from the left ventricle leaks into the right ventricle, passes through the lungs and re-enters the left ventricle via the pulmonary veins and left atrium
One cause of ventricular septal defect is the failure of the endocardial cushions to provide an anchor point for the developing ventricular septum
What are the 3 effects from ventricular septal defect?
This has 3 main effects:
- Systemic circulation doesn’t receive all the blood being pumped by the left ventricle.
- Leakage of blood into the right ventricle elevates right ventricular pressure and volume, causing pulmonary hypertension.
- Increase the pulmonary pressure, this can damage the lungs and cause fluid to be squeezed out of the capillaries into the alveoli, called pulmonary oedema.
Describe the formation of the atrial septa (40 days)
Day 40
Initially, the temporary hole in the septum primum allows that shunt of blood from the right to the left atrium as there is no pulmonary circulation. However, this closes before a second hole called the foremen Secundum opens.
Septum Secundum then forms, a more muscular septum grows down from the roof of the atria lateral to the septum primum. This obscures the place where the foremen primum was.
The endocardial cushions then fuse to form a left and right atrioventricular canal
Describe the formation of the atrioventricular septa on day 43
The septum Secundum grows but leaves the foramen ovale, a space permitting the right to the left shunt of blood
The foremen secundum is partially obscured by the septum secundum but the foramen ovale remains to provide an alternative right to left shunt. The foramen primum is closed.
Ventricular septum - the ventricular septum grows up to fuse with the now fused endocardial cushions
What happens at birth to the heart?
At birth the lungs become functional, pulmonary vascular pressure decreases and the left atrial pressure exceeds that of the right forcing the septum primum against the septum secundum, functionally closing the foramen ovale. In time the septa eventually fuse. A patent foramen ovale is a common septal defect present to some extent in up to 25% of adults.
Give an overview of what is going to happen in the final stage - the division of outflow tract
At this stage there is no separation between the ventricles 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.
