Embryology Of The Heart Flashcards
What is gastrulation?
An early phase in embryonic development that occurs in the 3rd week
During this phase the embryoblast develops into a trilaminar structure called the gastrula
What are the 3 germ layers?
Ectoderm
Mesoderm
Endoderm
What is the ectoderm?
Outer layer
Gives rise to structures that are in contact with the outside of the body:
Central nervous system
Peripheral nervous system
Sensory epithelium of nose, ear and eyes
Epidermis of skin, hair and nails
Pituitary, mammary and sweat glands
Enamel of teeth
What is the mesoderm?
Inner layer
Has 3 parts
Paraxial plate mesoderm:
Gives rise to somites, which give rise to the supporting tissue of the body;
Myotome (muscle tissue)
Sclerotome (cartilage and bone)
Dermatome (dermis of the skin)
Intermediate plate mesoderm:
Generates the urogenital system;
Kidneys
Gonads and their respective duct
Lateral plate mesoderm:
Is found at the periphery of the embryo
Splits into two layers;
Somatic parietal layer mesoderm forms future body wall
Splanchnic (visceral) layer mesoderm forms circulatory system, connective tissue for glands, muscle, connective tissue and peritoneal components, of the wall of the gut
What is the endoderm?
Bottom layer
Gives rise to the:
Epithelial lining of the gastrointestinal tract, respiratory tract and urinary bladder
Parenchyma of the thyroid gland, parathyroid gland, liver and pancreas
Epithelial lining of the tympanic cavity and auditory tube
How does the primitive heart tube fold?
During the 3rd week of development the heart is formed from cells that form a horseshoe shaped region called the cardiogenic region
By Day 19 (3rd week), two endocardial tubes form
These tubes will fuse to form a single, primitive heart tube
Day 21, as the embryo undergoes lateral folding,the two endocardial tubes have fused to form a single heart tube
What does the heart tube grow and develop into?
Bulges
What is the bulbus cordis?
The proximal 1/3rd of the bulbus cordis gives rise to the muscular right ventricle
The conus cordis (lower part of bulbus cordis) gives rise to smooth outflow portion of the right and left ventricles
The truncus cordis (upper part of the bulbus cordis) gives rise to the proximal aorta and pulmonary trunk
What is the primitive/primordial ventricle?
Gives rise to the left ventricle
What is the primitive/primordial atrium?
Gives rise to the anterior part of the right atrium and the entire left atrium and the left and right auricles
What is the sinus venosus (right and left horns)?
Forms part of the right atrium, vena cava and coronary sinus
What is the aortic sac?
Forms the aorta and pulmonary artery
- By Day 22 the heart begins to beat
- By Day 23 the heart tube begins to fold:
The bulbus cordis moves inferiorly, anteriorly and the embryos right
The primitive ventricle moves to the embryos left side
The primitive atrium and the sinus venosus move superiorly and posteriorly, resulting in the sinus venosus being posterior to the primitive atrium
How is the interatrial septum formed?
In the developing foetus the lungs (and thus the pulmonary circulation) are not fully functional
- This creates increased vascular resistance in the pulmonary arterial circulation,
and increased pressure in the right side of the heart (which
supplies the pulmonary arterial circulation) - Thus, in the developing foetus, pressure is greater in the right side of the heart than the left side
- As a results, as blood enters the right atrium, much of this
blood is shunted to the left atrium - down its pressure
gradient - At the end of the fourth week, a crescent-shaped tissue
called the septum primum starts to grow towards the endocardial cushions - The diminishing opening between the septum primum and the endocardial cushion is called the foramen (ostium) primum. This opening allows blood to be shunted from the right atrium to the left atrium
- Before the foramen primum completely closes, enlarging perforations develop in the wall of the septum primum
- These enlarging perforations form a single opening called the foramen (ostium) secundum
- Thus, a new opening for right-to-left shunting of blood appears before the foramen
primum disappears - As the foramen (ostium) primum disappears, the foramen (ostium) secundum
enlarges - A second crescent-shaped ridge of tissues called
the septum secundum grows towards the endocardial cushions. The septum secundum is thick and muscular, compared to the thin,
membranous septum primum - By around the end of the 6th week, the septum
secundum finishes growing. The septum secundum contains a permanent opening on its posterior-inferior surface, called the foramen ovale - Blood will enter from the right atrium go through the foramen ovale and the septum secundum and enter the left atrium
- The foramen secundum enlarges and the upper part of the septum primum gradually degenerates
- The lower part of the septum primum remains and is now called the valve of the foramen ovale. It covers the foramen ovale and forms a flap that moves when blood flows from the right atrium to the left
atrium - At this point the path of blood is as follows; as blood enter the right atrium, it will be shunted from the right side to the left due to the pressure difference between the two sides in
the following manner:
1. Blood enters right atrium
2. Blood flows through the foramen ovale
3. Blood pushes the valve of the foramen ovale
to the left
4. Blood enters the left atrium - When the baby is born and takes its first breath, the lungs and pulmonary arterial
circulation become fully functional - consequently, the pressure in the right side of
the heart drops - Pressure is now greater in the left side of the heart than the right
- The blood in the left atrium pushes the valve of the foramen ovale
against the muscular septum secundum, thereby closing the
passageway between the two atria - The septum secundum and valve of the foramen ovale usually fuse and form a
solid interatrial septum about three months after birth - After the interatrial septum forms, there remains a
thinned, oval part of the septum where the foramen
ovale used to be. This thinned oval area in the
inertial septum is called the fossa ovalis - The fossa ovalis is a landmark in the adult heart that
represent where the valve of the foramen ovale
permanently covered the foramen ovale
How is the atrioventricular canal divided?
As the interatrial septum is forming the atrioventricular
canal is also forming
- The two endocardial tubes fuse to form a single heart
tube, which then folds and the heart begins to beat - At this stage in heart development, there is one
common atrium and one common ventricle - The atrium and ventricle are connected by an internal
opening called the atrioventricular canal - Blood first enters the atrium through the superior and inferior vena cava, then it passes through the atrioventricular canal, into the ventricle and then will exit the heart through the truncus arteriosus
- Masses of tissue called endocardial cushions grow from the sides of the atrioventricular canal to partition it into two separate openings
- As the endocardial
cushions grow together,
the atrioventricular canal
also is being repositioned
to the right side of the heart - The superior & inferior
endocardial cushions fuse, forming two separate opening that are now called the right & left atrioventricular canals - these canals will become the right & left atrioventricular openings of the heart - Now, as blood flows through the heart, it will pass from the
atrium, through both atrioventricular openings into the ventricle, and up though the truncus arteriosus
Explain the development of the aorta pulmonary trunk and interventricular septum
- Early in heart development blood will flow from the atria, through the left and right atrioventricular canals and
into the common ventricle - blood then leaves the heart via the truncus arteriosus - which will eventually be partitioned into an aorta and a pulmonary trunk - At the end of the fourth week, a muscular ventricular septum grows superiorly from the
floor of the ventricle. This septum divides this area in to left and right ventricles. - An opening still remains between the muscular
ventricular septum and the fused endocardial cushion. This opening is called the interventricular foramen - At the end of the fifth week, two ridges of tissue appear
on the sides of the truncus arteriosus. These masses of
tissue are called the conotruncal ridges (truncoconal
swellings) - These ridges grow towards each other and make a spiral
shaped septum, called the aorticopulmonary septum
which divided the truncus arteriosus into the aorta and
pulmonary trunk - As the conotruncal rises grow and fun to form the aorticopulmonary septum, they also grow inferiorly into the ventricles themselves
- The aorticopulmonary septum will fuse with the already fused endocardial cushions and the muscular ventricular septum
- Once the aorticopulmonary septum, endocardial
cushions and muscular ventricular septum fuse (by
week 8), they from the membranous ventricular
septum - this septum closes off the opening known as the interventricular foramen - Now, blood enters the right ventricle through the right atrioventricular opening and leaves via the newly developed pulmonary trunk
- Blood enters the left ventricle through the left atrioventricular opening and leaves via the newly developed aorta
