Embryology Flashcards
What are the parts of the primitive heart tube?
Aortic roots Truncus arteriosus Bulbus cordis Ventricle Atrium Sinus venous
How does the cephalic portion of the primitive heart tube loop?
Ventrally, caudally and to the right
How does the caudal portion of the primitive heart tube loop?
Dorsally, carnally and to the left
What happens to the sinus venosus?
Venous turn shift to the right hand side, left sinus horn recedes. Right sinus horn is then absorbed by the enlarging right atrium
What does the right atrium develop from?
Most of the primitive atrium
Sinus venosus (engulfed)
Receives venous drainage from the body (venae cava) and the heart (coronary sinus)
How does the left atrium develop?
Small portion of the primitive atrium
Absorbs proximal parts of pulmonary veins
Sprouts pulmonary vein, then grows to absorb it and its first 4 branches
Receives oxygenated blood from the lungs
What is the ultimate fate of the fetal foramen ovale?
Fossa ovalis
What is the ultimate fate of the fetal ductus arteriosus?
Ligamentum arteriosum
What is the ultimate fate of the fetal ductus venosus?
Ligamentum venosum
What is the ultimate fate of the fetal umbilical vein?
Ligamentum teres (hepatis)
What is the ultimate fate of the fetal sinus venosus?
Right atrium, except for the left horn
What is the ultimate fate of the fetal primitive atrium?
Auricles of definitive atria
What is the ultimate fate of the fetal primitive ventricle?
Left ventricle
What is the ultimate fate of the fetal bulboventricular sulcus?
Primary interventricular foramen
What is the ultimate fate of the fetal bulbus cordis?
Proximal 1/3 - right ventricle (trabeculated)
Conus cords - outflow tract of left and right ventricles
Truncus arteriosus - roots and proximal aorta and pulmonary trunk
What are the derivatives of the 4th aortic arch?
Right - proximal part of right subclavian artery
Left - arch of aorta
What are the derivatives of the 6th aortic arch?
Right - right pulmonary arch
Left - left pulmonary arch and ductus arteriosus (connection to aorta)
What is the first step in the division of the heart into left and right channels?
The development of endocardial cushions in the atrioventricular region
Describe the division of the common atrium
Septum primum grows down towards the fused endocardial cushions. Contains the ostium primum (hole present before the septum primum fuses with the endocardial cushions)
Before the ostium primum closes, a second hole, the ostium secundum appears in the septum primum
Finally, a second, crescent shaped septum, the septum secundum grows, the hole in this septum is called the foramen ovale.
Describe the process of ventricular septation
2 components, muscular and membranous.
Muscular portion grows upwards towards the fused endocardial cushions.
Membranous portion is formed by connective tissue derived from endocardial cushions to fill the remaining gap.
How are the separate aorta/pulmonary arteries formed?
Endocardial cushions appear in the truncus arteriosus. As they grow towards each other, they twist around each other forming a spiral septum
How is the foramen oval closed after birth?
Before birth, RA pressure is greater than LA pressure
After birth, LA pressure is greater than RA pressure
This pushes the septum primum against the septum secundum, which then fuse together.
What orchestrates limb growth proximal to distal?
The apical ectodermal ridge (AER)
When do limb buds start to develop?
Towards the end of the 4th week (lower limb ~2 days behind upper limbs)
Where does the upper limb bud appear?
Opposite the caudal cervical spine segments
Where do the lower limb buds appear?
Opposite the lumbar and sacral spinal segments
What does the Apical Ectodermal Ridge do?
Critical for limb bud growth
Orchestrates limb growth proximal to distal
Appearance of paddles
Finally induces development of digits within hand/foot plates
Then AER regresses
How does the AER work?
AER releases inductive molecules to cells immediately beneath it, making them remain undifferentiated. Proximal mesenchyme begins to differentiate into constituent tissues
What does the AER mark the boundary of?
Boundary between dorsal and ventral limb ectoderm - ectoderm exerts dorsalising and ventralising influences over mesenchyme core
Where is the zone of polarising activity (ZPA) located?
Posterior base of the limb bud
What does the zone of polarising activity (ZPA) do?
Generates asymmetry in the limbs - anterior/posterior axis determination. Controls both patterning and maintains AER.
Describe how axial specification occurs in limbs
Anterior/posterior: ZPA
Proximal/distal: AER
Dorsal/ventral: Ectoderm
How are interdigital spaces formed?
AER breaks up and is maintained only over the tips of the digital rays. Interdigital spaces are progressively sculpted by apoptosis of the tissue between digits.
What are digital rays?
Mesenchyme condensations within plates forming cartilaginous models of digital bones
What is syndactyly?
The fusion of digits (may be just CT or bones can be fused)
What is polydactyly?
Extra digits (AR)
What is amelia?
Complete absence of a limb
What is meromelia?
Partial absence of one or more limb structures (phocomelia)
How do limb bones form?
Lateral plate mesoderm condenses and differentiates
Cartilage model forms
Endochondral ossification takes place
Eventually secondary ossification centres appear at epiphyses
How do limb muscles form?
Myogenic precursors migrate into limbs from somites, then coalesce into 2 common muscle masses around the newly formed skeletal elements
Ventral - flexor
Dorsal - extensor
Individual muscles then split from common masses
How does the musculature differ in the formation of muscles in the upper and lower limbs?
In upper limb, flexor is anterior
In lower limb, flexor is posterior
This is due to rotation of limbs in opposite directions (upper laterally, lower medially)
In which direction does the upper limb rotate?
Laterally
In which direction does the lower limb rotate?
Medially
When do spinal nerves enter the limb bud?
Early in development - without innervation development stalls
What does preaxial mean?
Towards thumb/big toe
What does postillai mean?
Towards little finger/little toe
What is an atrial septal defect?
A congenital heart defect in which blood flows between the atria of the heart
What is a ventricular septal defect?
There is a hole in the wall that separates the right and left ventricles of the heart
What is patent ductus arteriosus?
Congenital heart defect whereby the ductus arteriosus fails to close after birth, resulting in irregular transmission of blood between the pulmonary artery and the aorta.
When is patent ductus arteriosus occur?
Common in newborns with persistent respiratory difficulties such as hypoxia
In. Which direction does blood flow in patent ductus arteriosus?
From the aorta to the pulmonary artery, raising lung pressure and causing shortness of breath
What is patent foramen ovale?
A hole between the left and right atria of the heart, when this hole fails to close after birth
What is coarction of aorta?
Aortic narrowing, usually in the area where the ductus arteriosus inserts. Causes left ventricle to work much harder
What is tetralogy of fallot?
Four heart defect found together, as follows:
- Pulmonary stenosis (narrowing AR or below the pulmonary valve)
- Ventricular Septal Defect VSD (hole in wall between ventricles)
- Over-riding aorta (the entrance of the aorta lies over the VSD, so accepts blood from both sides of the heart
- Right ventricular hypertrophy (right ventricle becomes thickened as it has to force blood through the narrowed pulmonary artery)
What is tricuspid atresia?
Congenital heart defect in which the tricuspid heart valve is missing/does not open. This results in an atrial septal defect, and a small or missing right ventricle (hypoplastic) that can’t adequately pump blood into the lungs.
What is transposition of the great arteries?
Pulmonary artery is where the aorta should be, and the aorta is where the pulmonary artery should be. Often associated with a ventricular septal defect
