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Case 3: Embryology 1, MAKE FUCKING COPIES OF THE DIAGRAMS…
Case 3: Embryology 1
Embryological Development of the Cardiovascular SystemDescribe the process of development of the Cardiovascular System and the Neural Tube
- Cardiovascular system begins to develop at +/- 20 days of gestation
- The heart beats at +/- 22 days
- The Splanchnic mesoderm forms many endothelial tubes
- These tubes will develop as part of the CVS
- The Cardiogenic Mesoderm, found in the U-Tube in the flat embryonic disk, will develop 2 Endocardial Tubes, that fuse into 1 Tube
- This 1 Tube has an Inflow channel (Caudal) and an Outflow Channel (Cranial) on its sides
- Along the 1 Tube, 4 Chambers will form from the Caudal end to the Cranial end
- These chambers are: Sinus Venosus, Primitive Atrium, Primitive Ventricle and Bulbus cordis
- Bulbus Cordis, placed at the Outflow Channel end, opens Cranially into the Truncus arteriosus
- Truncus arteriosus is the Primordium of the Ascending Aorta and Pulmonary Trunk
- The Heart will then loop to the Right and assume the general appearance of the Adult heart
- During the 4th to the 7th Weeks partitions of the 4 chambers in the heart will occur
- During the 4th and 5th Weeks, Pharyngeal arches will form in the neck region
- The Pharyngeal arches are then penetrated by 6 pairs of Aortic Arches arising from the aortic sac
- During the 6th to the 8th the Aortic arches will be transformed into the adult arrangement of the Carotid System, Subclavian Arteries and Pulmonary Arteries
- The critical period of development of the Heart is during day 20 to day 50 post fertilization
- Deviations from the normal pattern of development at any time WILL lead to one or more Congenital Defects
- Fetal cardiovascular system undergoes important modifications at birth
- Several developmental remnants visible are in the adult heart
Early Embryonic Circulatory SystemOutline the Early Embryonic Circulatory SystemIn the Early Embryonic Circulatory System there is a:
- Yolk Sac contains the Vitelline vessels (Vitelline Artery and Vitelline Veins)
- Blood is taken to the Yolk Sac where it is processed and taken back into the body from the Yolk Sac
- In the body the blood is taken to the heart where it is pumped to circulate in the body
- During this period of the Early Embryonic Circulatory System, a Placenta is developing
- Thus, causing the Yolk Sac to gradually disappear and become incorporated into the Mid-Gut and a small part will be pinched off and lost
- And the Vitelline vessels will stop featuring majorly in the CVS
- The Placenta, the 1 Umbilical Vein and the 2 Umbilical Arteries will take over as part of the Embryonic CVS
- 1 Umbilical Vein carries oxygenated blood with nutrients from the Placenta to the body
- 2 Umbilical Arteries carry deoxygenated blood with waste products to the Placenta for processing
Flat Embryonic DiscDescribe the distinct features of the Flat Embryonic Disc
- Cranial End at the Top has the:
- Cardiogenic Mesoderm, lying in the middle layer between the Ectoderm and the Endoderm
- Caudal to Cardiogenic Mesoderm is the Buccopharyngeal/ Oropharyngeal membrane which will develop into the mouth
- Down along the length of the Disc in the mesoderm there is a Notochord with Somites on either side and the Intermediate Mesoderm
- The Caudal at the Bottom has the:
- Cloacal Membrane which will form the anus
Cranio-Caudal FoldingIn Cranio-Caudal Folding:
- The Angiogenic Cell cluster which will form the Cardiogenic Mesoderm, begins to turn ventrally
- Going almost caudally, under the head so that the Heart becomes located in the developing neck region
- And then later migrate to the Thorax
Cranio-Caudal Folding Labels
- U-Tube also known as the Pleuropericardial Canal contains the:
- Buccopharyngeal Membrane, future mouth
- Future Pericardial cavity at it's tip, where the heart will migrate to.
- Future Pleural and Peritoneal cavities at the two tube ends
Somatopleure and Splanchnopleure
- Somatopleure is the Ectoderm and the Somatic mesoderm
- Forms musculoskeletal system and skin
- Has a Somatic Nerve Supply
- Splanchnopleure is the Endoderm and Splanchnic mesoderm
- Forms the Cardiac Muscle, Heart and Blood vessels
- Has an Autonomic Nerve Supply
Ectopia CordisOutline Ectopia Cordis
- Ectopia cordis is the developmental defect in the anterior wall of the trunk (thorax) resulting in the formation of the heart outside of the body
- This is treated by corrective surgery
The Importance of having a 4-chambered HeartOutline the Importance of having a 4-Chambered Heart4-Chambered Heart is required for:
- Gaseous Exchange and the Separation of the Left and Right Atria and Ventricles
- Separations of the Pulmonary Circulation and Systemic Circulation
- Create an Inflow end and Outflow end
- Create an Arterial flow and Venous flow
- Separation of Oxygen rich Blood and Oxygen Poor/Carbon dioxide rich blood
Outline the difference between Arterial and Venous Flow
- Arteries always carry blood away from the heart
- Veins always carry blood to the heart
- Pulmonary Veins
- Superior and Inferior Vena cava
The Development of the HeartOutline the development of the Heart with reference to diagrams
- Early Development
- Initially, the Mesoderm Cells travel through the Primitive Streak to the embryo's head forming a horseshoe shaped area with two limbs, this is known as the Primary Heart Field
- The Embryo now has an Ectoderm Layer, Mesoderm Layer and an Endoderm layer
- The Mesoderm Layer is split up into the following three parts from medial to lateral: Paraxial mesoderm, Intermediate mesoderm and Lateral plate mesoderm
- The Lateral Plate mesoderm splits into a Somatic Layer and Splanchnic Layer
- Heart tube will develop from the Splanchnic Layer of the Lateral plate mesoderm
- The Endoderm will secrete Vascular Endothelial Growth Factors (VEGFs)
- The VEGFs will then stimulate the Splanchnic mesoderm to differentiate thus resulting in the following:
- Mesoderm forming Hemocytoblasts encapsulated in Angioblasts cells
- A certain amount of Mesoderm growing into the Endoderm resulting in the formation of a Endocardial Tube on each Limb
- Formation of the Primitive Pericardial Cavity lateral to each Endocardial tube on the Mesoderm
- At its inferior end, each Endocardial Tube is connected to a Vitelline Vein from the yolk sac
- Mesodermal cells also form a pair of longitudinal vessels called Dorsal Aortae
- Lateral Folding
Describe the process of Lateral Folding
- During Week 3, In Lateral Folding the embryo then folds into a cylindrical shape as the Lateral borders meet at the midline
- The Left and Right Vitelline Veins also fuse forming the Sinus Venosus - Inflow Tract
- Dorsal Aortae will fuse forming an Aortic Sac - Outflow Tract
- The 2 Endocardial Tubes will then fuse resulting in the formation of 1 Primitive Heart Tube
- The 2 Primitive Pericardial Cavities will fuse to form 1 Primitive Pericardial Cavity around the Primitive Heart Tube
- Primitive Heart tube will then remain suspended to the Primitive Pericardial Cavity by a sheet of Mesoderm called the Dorsal Mesocardium
- The Primitive Heart Tube now has 2 layers:
- Endothelial Lining
- Cardiac myoblasts
- The Endothelial Lining will form the Endocardium
- Cardiac myoblasts will form the Myocardium
- Some myocardial cells in the Sinus Venosus begin to produce rhythmic electric discharge
- The Cardiac myoblasts wills secrete a Gelatinous Substance called the Cardiac Jelly between the Endocardium and Myocardium of the embryonic heart
- Cardiac Jelly will transform into the connective tissue of the Endocardium
- Cranio-Caudal Folding
Describe the process of Cranio-Caudal Folding
- In Cranio-Caudal Folding the Cylindrical embryo folds down its length to form a shrimp-like shape
- During this process, the heart is pushed towards the chest
- The heart tube reaches the Thorax and circulating blood can be seen travelling through the Heart Tube
Describe the structure of the Heart Tube before Looping
- From the Top to the Bottom. The heart tube has the following divisions
- Aortic Sac attached to 2 Outflow Tracts
- Aortic Sac forms 2 Dorsal Aortae
- Truncus Arteriosus is placed at the top of the heart tube, and pumps blood through the Aortic Sac into the Primitive Circulatory System
- Truncus Arteriosus forms the Pulmonary Artery and Ascending Aortic Arch
- Bulbus cordis
- Bulbus cordis forms the Right Ventricle and Outflow Tracts for BOTH Ventricles
- Primitive Ventricle
- Primitive Ventricle forms the Left Ventricle
- Primitive ventricle is separated from the Bulbus cordis by the Bulboventricular Sulcus
- Primitive Atrium
- Primitive atrium forms BOTH the Left and Right Atria
- Primitive Atrium and Primitive Ventricle are separated by the Atrioventricular Sulcus
- Sinus Venosus are attached to the Left and Right Sinus Horns that bring in the blood.
- Sinus Venosus Horns are made up of he following veins from Lateral to Medial
- Common cardinal Vein
- Umbilical Vein
- Vitelline Vein
- Cardiac Looping
Describe the process Cardiac Looping
- Cardiac Looping occurs during the 4th to 5th week of gestation
- During the process of Cardiac Looping the following changes will occur in the Heart Tube:
- The Heart Tube will fold into a "C" Shape
- The Truncus arteriosus and Bulbus Cordis will move Downwards and to the Right to form the Top portion of the "C"
- The Primitive Ventricle bends to the Right of the midline and slightly to the front to form the middle portion of the "C"
- Primitive Atrium and Sinus Venous then move Backwards and then Upwards to form the bottom of the "C"
- The Ventricle then enlarges and moves Left, crossing over the Midline again and covering the Primitive Atrium
- Cardiac Looping depends on the proteins called Dynein's
- The absence or damage of Dynein's results in Dextrocardia or Situs Inversus
- Dextrocardia is a congenital abnormality in which the Heart Tube in the developing embryo loops to the Right instead of the Left, resulting the in the mirror of the Normal Heart Position on the X-Ray.
- Sinus Venosus then allows some of its cells to move into the Pericardial Cavity.
- These cells will then form around the heart resulting in the formation of the Visceral Pericardium
- During Week 4 to Week 6 of gestation some of the cells of the Sinus Venosus will infiltrate the heart and form a Primitive Conduction System that allows the heart to beat
- This can be detected on the Transvaginal Ultra-sound during week
Circulation Systems of the EmbryoOutline and Label the different types of Circulatory Systems present in the embryo during Utero
- Embryonic Circulation
- 6 Aortic Arches in total
- Form Outflow Tracts
- Connect to the Aortic sac
- Anterior Cardinal vein
- Posterior Cardinal vein
- Common Cardinal vein
- Forms Inflow Tracts
- Connect to Atrium
- Vitelline Circulation
- Vitelline Vein
- Vitelline vein connects with the Common Cardinal Vein
- Vitelline Artery
- Vitelline Artery connects with the Dorsal Aorta
- Placental Circulation
- 1 Umbilical Vein that connects with the Common Cardinal Vein
- 2 Umbilical Arteries (Left and Right)
- Umbilical arteries connect with the Dorsal Aorta
The Shunts of the Embryo HeartOutline the shunts of the Embryo Heart
- Ductus arteriosus
- Placed between Pulmonary trunk and Aortic arch
- Shunts blood from the Pulmonary Trunk to arch of Aorta, thus bypassing the Pulmonary Circulation
- From the Pulmonary circulation to Systemic circulation
- Ductus Venosus
- A connection between the Umbilical vein bringing oxygen and nutrient rich blood from the Placenta and bypassing the Liver
- And therefore, entering the Inferior Vena Cava directly
- ALL systems benefit from this Ductus Venosus
- Foramen Ovale
- Placed between Right and Left Atria
- Shunts blood from the Right Atrium to the Left Atrium and thus bypasses Pulmonary Circulation and enters straight into Systemic Circulation
Why is the Pulmonary Circulation bypassed ?
- The embryo is developing in the liquid environment and there is no gaseous exchange in the heart
- Therefore, the bypass allows for a small amount of blood to be sent to the lungs of the embryo so that they can grow
- Gaseous exchange will occur in the Placenta
- Thus, protecting the lungs from High Intracardiac Pressure and the development of Pulmonary Hypertension
**NOTE: ALL shunts will need to seal off during the time of child birth
Foramen OvaleOutline the process of closure of the Foramen ovale, Ductus Arteriosus and Ductus Venosus
- During utero, the pressure in the Right atrium is higher than the pressure in the Left atrium
- This is because the lungs are not inflated by air
- The tissue are solid therefore there is tremendous resistance in the blood vessels to the passage of blood
- When the baby is born, and take the first breath. The Alveoli are inflated
- And the pressure in the Right atrium drops, and becomes lower than the pressure in the Left atrium
- The pressure drops forces the Foramen ovale to closes
- Changes in pH and Prostaglandins cause the smooth muscle of the Ductus arteriosus to spasm
- And then overtime the Ductus Arteriosus becomes a solid ligament
- Ductus Venosus also becomes constricted and overtime forms a Ligament
Partitioning of the HeartDescribe the development of the Atrioventricular valves (Tricuspid and Bicuspid Valves)
- If we make a cross section cut through the Atrioventricular Sulcus we will witness the following:
- Neural crest cells will enter the Heart tube and cause the Mesoderm to proliferate on the anterior and posterior walls of the Atrioventricular Canal
- Resulting in the formation of anterior and posterior Endocardial Cushions
- The endocardial cushions will then grow towards each other, fuse together and form a Septum Intermedium.
- The heart is now separated into the Left and Right Atrioventricular canals
- Endocardial cells from the Septum intermedium will then proliferate on the ventricular side of each canal forming leaflets of the Bicuspid/Mitral and Tricuspid Valves
- Remodeling, which involves the formation of the Annulus rings from endocardial cells, will occur and result in the formation of Valves Flaps
- The Tricuspid valve on the Right has 3 Leaflets and the Bicuspid/Mitral valve on the Left has 2 Leaflets
- As a consequence of remodeling, parts of the walls of the ventricles become so thin that the muscular cords connecting the valves with the wall are replaced by Connective Tissue
- This connective tissue is called Chordae Tendineae
- Canals are now divided into the Atria and Ventricles
What is the purpose of the Atrioventricular valves ?
- Atrioventricular valves aim for a unidirectional flow of blood. Thus, preventing the backflow of blood from the Ventricles back into the Atria
Describe the development of the Left and Right Atria
- Coming down from the between the future Left and Right Atria is a crescent-shaped shaped Septum called the Septum Primum
- The Septum Primum grows down towards the Septum intermedium. However, a gap remains between the Septum primum and Septum intermedium
- This gap is called the Ostium primum
- The Septum Primum will then continues to grow downwards and fuse together with the Septum Intermedium, thus closing the Ostium primum completely
- However, another gap will form towards the top of the Septum Primum called Ostium Secundum
- After the formation of the Ostium Secundum, another piece of tissue will grow just to the right of the Septum Primum. This is the Septum Secundum
- The Septum Secundum will grow downwards covering the Ostium Secundum and leaving a small opening.
- The small opening is called the Foramen Ovale
- Septum Secundum (Foramen ovale) will act as a one-way valve allowing blood flow from the Right to the Left atrium
- After birth, closure of the Foramen ovale is facilitated by a decrease in in the Right atrial pressure and an increase in the Left atrial pressure as a result of increased Pulmonary Venous Return
- Thus resulting in the formation of the Fossa ovalis
Describe the formation of the Left and Right Ventricles
- Ventricles increase in size and internal volume due to growth of myocardium
- Ventricular wall becomes remodeled, leading to trabeculation.
- At the apex of the heart, a tissue will start to develop and move upwards from the Apex, in an attempt to separate the Left and Right ventricle
- This is known as the Muscular Portion of the Interventricular Septum
- Muscular Portion of the Interventricular Septum does NOT reach the Septum intermedium, therefore creating a small gap.
- The gap is filled by a Septum made from 2 components: Further growth of the endocardial cells of the Septum intermedium and the Septum that divides the Truncus arteriosus
- This newly formed septum is called the Membranous Portion of the interventricular Septum
- This part of the septum accommodates the Atrioventricular bundle of the conducting system of the heart
- Therefore, only the myocardial connection between the Atria and Ventricle is the Conducting tissue
- This arrangement ensures the sequential contraction of the atria followed by the ventricles because the 2 pairs of chambers are otherwise separated from one another
Describe the development of the Aorta and Pulmonary Trunk
- Development of the Aorta and Pulmonary Trunk begins with a division in the Truncus arteriosus.
- Neural crest cells will invade the tube of the Truncus arteriosus and the Bulbus cordis.
- These neural crest cells will stimulate the formation formation of 2 endocardial cushions at the Truncus arteriosus, Bulbus cordis and in the anterior and posterior aspect of the Conus cordis
- The endocardial cushions then fuse and form a septum at the Truncus arteriosus, Bulbus cordis and in the middle of the tube called a Conus cordis
- Therefore, the link between the Truncus arteriosus and the Bulbus cordis will undergo a corkscrew motion resulting in the formation of the Septum called the Aorticopulmonary septum
- Aorticopulmonary septum then separates the Aorta and the pulmonary Trunk
Describe the development of Semilunar valves
- At the junction between the Truncus anterior and the Bulbus cordis known as the Conus Cordis, tubercles will form on the inner wall of the vessels in the position of the Semilunar Valves
- There are initially 4 tubercles, with one located on the Anterior, Posterior, Left and Right aspect of the vessel wall
- The Left and Right tubercles are located on the Septum
- Therefore, the tubercles located on the Septum will be divided into equal halves due to invagination of the entire vessel.
- With the Posterior Half going to the Aorta and the Anterior half going to the Pulmonary Trunk
- When the division is complete each vessel will have 3 Tubercles.
- Each vessels is now hollowed on the side facing away from the chambers to form cusps of the Semilunar valves
- Aortic semilunar valve forms part of the Left ventricular outflow tracts
- Pulmonary semilunar valve forms part of the Right ventricular outflow tracts
Describe the formation of the Superior and Inferior Vena Cava and Coronary Sinus
- The Superior and Inferior Vena cava and the Coronary sinus develop from the Sinus venosus
- Sinus venosus has a Left and Right Horn. Each horn has a Common Cardinal vein, Umbilical Vein and Vitelline vein
- The veins of the Left Sinus Horn break down leaving behind the Left Sinus Horn
- On the Right Horn, the Umbilical vein starts to degenerate leaving behind the Vitelline Vein and Common Cardinal Vein
- Afterwards, the Sinus venosus becomes asymmetric as the Left Horn shifts towards the Right Horn, and fuses with the Right Horn before entering the Sinus Venosus
- The fusion of the 2 Horns and the absorption of the Sinus venosus into the Primitive Atria results in the following
- Left Horn will shrink in size and persist as the Coronary Sinus
- Right Horn enlarges and becomes a smooth walled part of the Right atrium forming the openings for the Superior and inferior Vena cava
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Foetal CirculationDescribe Fetal Circulation with mention of the components involved
- Placenta is a low-resistance circuit and functions as an organ of gaseous exchange
- The entire Fetal systemic circulation is a low-resistance circuit
- The Lungs are filled with fluid and hypoxic vasoconstriction creates a high-resistance circuit
- The pressure on the right side of the heart is greater than the pressure on the left side of the heart
- Ductus arteriosus, Ductus venosus and Foramen ovale shunt blood away from Foetal Lungs and allows blood to bypass Pulmonary circulation into the Systemic Circulation
Foetal Blood Flow Outline the Foetal Blood Flow in Utero
- Blood moves from the Placenta through the Umbilical Vein which splits up into the Left and Right Umbilical veins
- Blood in the Left Umbilical Vein moves to the Portal vein and then flows to the Liver and then to the Inferior vena cava via the Hepatic Vein
- From the Inferior Vena cava the blood will enter the Right atrium
- Blood in the Right umbilical vein will flow to the Ductus venosus and bypasses the Liver to enter directly into the Inferior Vena Cava and then flow into the Right atrium
- From the Right atrium the blood flows into the Left atrium via the Foramen ovale
- However, a small amount of blood in the Right atrium will enter the Right ventricle and flow into the Pulmonary artery to the Lungs to promote Lung growth
- Blood in the Left side of the Heart is then shunted from the Pulmonary artery to the Aorta by the Ductus arteriosus, a small blood vessel
- From the Aorta, oxygenated blood is then delivered to the Systemic Circulation
- The Right and Left Common Iliac Arteries lead to Internal and External Iliac Arteries finally returning deoxygenated blood back to the placenta via the Umbilical arteries