Embryology of the Heart and Structure and Embryology of the Circulation Flashcards
Gastrulation
• An early phase in embryonic development that occurs in the 3rd week
• During this phase the embyroblast, develops into a trilaminar (three-layered) structure called the gastrula
Another definition:
Mass movement and invagination of the blastula to form three layers – ectoderm, mesoderm (middle layer) and endoderm
3 germ layers
- Ectoderm (outer layer)
- Mesoderm (middle layer)
- Endoderm
What comes from each layer?
Ectoderm
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 eye
- Epidermis of skin, hair and nails
- Pituitary, mammary & sweat glands
- Enamel of teeth
there is SOME contribution from the cardiac neural crest cells from the ectoderm.
Mesoderm- described as 3 parts
- Paraxial plate mesoderm
- Intermediate plate mesoderm
- Lateral plate mesoderm
a. Somatic (parietal) layer mesoderm forms
b. Splanchnic (visceral) layer mesoderm forms
Endoderm
gives rise to the:
a. Epithelial lining of the gastrointestinal tract, respiratory tract and urinary bladder
b. Parenchyma of the thyroid gland, parathyroid glans, liver and pancreas
c. Epithelial lining of the tympanic cavity and auditory tube
Mesoderm- Paraxial plate mesoderm (1)
gives rise to somites
- Somites give rise to the supporting tissue of the body:
a. Myotome (muscle tissue)
b. Sclerotome (cartilage and bone)
c. Dermatome (dermis of the skin)
Mesoderm- Intermediate plate mesoderm (2)
generates the urogenital system - the kidneys, gonads,
and their respective duct systems
Mesoderm- Lateral plate mesoderm
a. Somatic (parietal) layer mesoderm forms (3a)
Lateral plate mesoderm: is found at the periphery of the embryo.
a. Somatic (parietal) layer mesoderm forms:
1. Future body wall
Mesoderm- Lateral plate mesoderm
b. Splanchnic (visceral) layer mesoderm forms:
b. Splanchnic (visceral) layer mesoderm forms:
- Circulatory system
- Connective tissue for glands
- Muscle, connective tissue and peritoneal components, of the wall of the gut
Folding of the primitive heart tube
- During the third week of development the heart is formed from cells that form a horseshoe shaped region called the cardiogenic region
- By day 19 (third 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
The heart tube grows and develops develops- 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 bulbus cordis) gives rise to the proximal aorta & pulmonary trunk
The heart tube grows and develops develops-Primitive/ primordial ventricle
gives rise to the left ventricle
The heart tube grows and develops develops- Primitive/ primordial atrium:
gives rise to the anterior part of the right atrium and the entire left atrium and the left and right auricles
The heart tube grows and develops develops- Sinus Venosus (right and left horns):
Forms part of the right atrium, vena cava and coronary sinus
The heart tube grows and develops develops- 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 (looping) -
- The bulbus cordis moves inferiorly, anteriorly and to the embryo’s right
- The primitive ventricle moves to the embryo’s left side
- The primitive atrium and the sinus venosus move superiorly and posteriorly - resulting in the sinus venosus being posterior to the primitive atrium
Week 3/4 (the primitive heart tube)
Visceral Mesoderm -> 2x Heart Tubes -> Fuse (Lateral folding) -> Craniocaudal folding (“Shrimp”) -> Heart tube has divisions now
Truncus arteriosus gives rise to
> Ascending aorta
Pulmonary trunk
Bulbus cordus gives rise to
> Smooth (outflow) parts of L & R ventricles
Primitive Ventricle gives rise to
> Forms majority of ventricles
Primitive atrium gives rise to
> both auricular appendages
Entire L atrium
Anterior part of R atrium
Sinus venosus gives rise to
> Smooth part of R atrium
Vena cavae
Coronary sinus
DEVELOPMENT OF THE INTERATRIAL SEPTUM
- 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
Foramen (ostium) primum
The diminishing opening between the septum primum and the endocardial cushion
Role of 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 (yellow arrow)
foramen (ostium) secundum
These enlarging perforations form a single opening
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
Septum Secundum
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
septum secundum finishes growing
Foramen Ovale
The septum secundum contains a permanent opening on its posterior inferior surface, called the foramen ovale
Next movement of blood
Blood will enter from the right atrium go through the
foramen ovale and the septum secundum and enter
the left atrium
Foraman ovale
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:
- Blood enters right atrium
- Blood flows through the foramen ovale
- Blood pushes the valve of the foramen ovale to the left
- 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
About 3 months after birth
- The septum secundum and valve of the foramen ovale usually fuse and form a solid interatrial septum
- 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
THE DIVISION OF THE ATRIOVENTRICULAR CANAL
- 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
Atrioventricular Canal
- The atrium and ventricle are connected by an internal
opening called the atrioventricular canal
Firstly,
- 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
Then,
Masses of tissue called endocardial cushions grow from the sides of the atrioventricular canal to partition it into two separate openings
Then,
As the endocardial cushions grow together, the atrioventricular canal also is being repositioned to the right side of the heart
Then,
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
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 4th 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 5th 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
By week 8
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
How does blood now enter the right ventricle?
through the right atrioventricular opening and leaves via the newly developed pulmonary trunk
How does the blood enter the left ventricle?
through the left atrioventricular opening and leaves via the newly developed aorta
AORTIC ARCH VESSELS DEVELOPMENT:
Occurs from 27 days to 7 weeks old
FETAL CIRCULATION
- Oxygenated blood from the placenta enters the foetus though the umbilical vein
- Most of the newly oxygenated blood bypasses the liver via the ductus venosus and combines with DEOXYGENATED BLOOD in the inferior vena cava
- Blood then join deoxygenated blood from the superior vena cava and empires into the right atrium
- Since pressure in the right atrium is larger than the presses in the left atrium, most blood will be shunted through the foramen ovale
- Some blood does travel from the right atrium to the left atrium via the pulmonary trunk but most blood moves directly to the aorta via the DUCTOS
ARTERIOSUS
Deoxygenated blood returns to the placenta via the UMBILICAL ARTERIES originating from the internal iliacs near the bladder
