Embryology of the heart Flashcards
Explain the origin and development of the primitive heart tube
Looping AND remodelling
(Possibly prelude for notes)
- - Recall that early embryo relies on diffusion for delivery of nutrients
- At this stage, there is no need for vascularisation
- However, as embryo/blastocyst implants and grows within the endometrium, even at a few millimetres across, diffusion is not enough to meet the nutrient demand
- Following two to three weeks of development, several key processes have already occurred, including mplantation of the blastocyst into the uterine wall; the establishment of three distinct germ layers - the mesoderm, endoderm and ectoderm - through gastrulation; and encompass the beginnings of the neurulation process
- Recall that the three embryonic germ layers, developed through the process of gastrulation, form the basis for all the tissues and organs in the body:
- The endoderm will form the epithelium of the digestive tract, lungs, liver, pancreas, and thyroid
- The mesoderm will give rise to muscle, cartilage and bone, heart and circulatory system, gonads, and the urogenital system
- The ectoderm will give rise to outer components of the body, such as skin, hair, and mammary glands and part of the nervous system (see neurulation).
The mesoderm (like ectoderm) can be divided into several categories. It is from the lateral plate mesoderm that serous membranes, smooth muscles and connnective tissue of viscera, and the heart are derived.
The cardiogenic region, more specifically, is located in the splanchnic mesoderm, which helps create the heart.
However, at 18 days, the cardiogenic area is positioned cranial to the notochord. Thus a process of folding is required to get heart “to the right place.”
This process is called craniocaudal folding.
Note: at the end of this process, the neginnings of what will become the thorax have formed, as well as the septum transversum, a border between thorax and abdomen i.e. the precursor to the diaphragm.
From days 20-21, the process of endocardial tube fusion to primitive heart tube occurs. On day 22, several structures have formed inclyding the truncus arteriosus (which will become sthe pulmonary trunk and aortic arch), the bulbus cordis (right ventricle and outflow tract), the primitive ventricle (L ventricle) and primitive atrium (left and right atria).
the differentiation of the heart wall also occurs in this period?. By the end of this process, the endocardium, cardiac jelly (thick ECM layer), myocardium (from splanchnic mesoderm, secretes cardiac jelly), epicadium (sinus venosus mesothelial cells) have formed.
NOTE 2: the atria are located inferiorly to ventricles, thus more folding over days 23-39, in which the bulbus cordia moves to the right, ventricle moves left, and primitive atriu, and SV (base) extends back and up (i.e. on top of ventricle and bulbus cordis). Also aortic arch arteries form.
By the end of this process, primitive heart tube has familiar heart shape
At the same time?, lateral folding occurs to create to teh pericardial cavity. The heart tube is formed from the fusion of endocardial tubes, while the pericardial cavity is formed from the folding of splanchnic ie. visceral mesoderm, and parietal or somatic mesoderm.
heart starts function at 22 days
NOTE:
dextrocardia occurs when heart folding of 23-29 days occurs in opposite directions e.g. bublus cordis moves left instead of right, ventricle moves right instead of left etc.
Dextrocardia often occurs with situs inversus
Explain the partitioning of the atrioventricular canal
Septation as a process begins from day 28
- Cushion cells derived from neural crest cells that have migrated into the cardiac jelly, work to remodel and separate different compartments
- Conotruncal endocardial cushion cells are responsible for division of the atria and ventricles
- Bulges called arioventricular endocardial cushions form dosally and ventrally, and approach each other in order to fuse and create two openings i.e. for R and L av canals
- once fused in the midline it forms a structure called the septum intermedium, and allow blood flow through L and R canals
- However, atria and still connected, as well as ventricles
- Note: while formation of AV valves begins at 5 weeks, it takes eight weeks to complete, The vakves form as invaginations. The leaflets of the valves emerge from AV cushions of teh endocardial tissue. They differentiate and remodel to form the leaflets
- The chordae tendinae originate fromt he heart wall and consist of thin connective tissue (undergo degeneration and differentiation to become thin), and firmly root the valves in place
Explain the partitioning of the atria
- Partitioning of atria follows a similar process to the partitioning of the atrioventricular canal
- it requires the formation of the septum primum, and then the septum secundum
- The septum primum descends from the heart wall to approach the dorsal endocardial cushions
- in doing so, it creates the foramen primum
- following this the septum secundum develops
- this now creates two formaeminea: primum and secundum
- septum primum fuses to close off formamen secundum
- foramen secundum– forms just before full closure of foramina
- an opening is still required, this is to allow blood in atria to bypass ventricles
- a valve is formed from the septum primum, that enables unidirectional blood flwo. Valve shuts with changes in blood flow on birth, and fuses very quickly to form fossa ovale
It is during this process that septal defects can form e.g. patent foramen
Explain the separation (Septation) of the outflow tract and ventricles
Septation of outflow tract:
- formation of spiral aortico-pulmonary septum
- formed by fusion of conotruncal ridges in midline
- changes position along tract
- separate blood outflow for atria and ventricles
Septation of ventricles:
- formation of interventricular septum
- has two parts: muscular (arising from endocardial cushion cells) and membranous that fuse together
Describe the formation of semilunar valves
- semilumar valves arise from endocardial cushions i.e. from neural crest ceels in outflow tract
- in aorta, they come from the posterior, left and right cushions
- in the pulmonary trunk they arise from anterior, left and right cushions
- they form by twisitnf and separation to create two valves with three evenly divided leaflets
- twisitn and separation also brings aorta to right side
Describe the anatomy and function of fetal circulation
Fetal circulation:
- oxygenated blood flows via umbilical veins
- little blood flows through collapsed lungs
- foramina and ducts are used to bypass lungs and liver
There are two connections to the placenta:
- the umbilical veins
- two umbilical arteries (mixed blood content, medium oxygen)
There are also three shunts i.e. for bypassing:
- umbilical veins to IVC, via ductus venosus (flwo controlled by physiological sphincter, 50% of blood bypasses liver)
- right atrium to left atrium via foramen ovale (remains open as lots of blood flows)
- pulmonary trunk to aorta through ductus arteriosus
HI
Describe changes in neonatal circulation
Neonatal changes:
- opening of pulmonary vessles: in response to oxygen, ressistance drops suddenly
- umbilical veins and arteries are closed as a result of spontaneous ocnstricyinf and clamoing, some retained as ligaments e.g. ligamentum teres
- ductus arteriosus constricts in reponse to high oxygen (pa cahngs also cuases direction of blood flow to change), within 10-15 h
- ductus venosus closes after both
- both ductus venosus and ductus arteriosus are retained as ligamentum arteriosum, and ligamentum venosum
- foramen ovale closes, as left atrium pressure increases and right atrium pressure drops, the flexible septum primum is forced against the rigid septum secundum
- this initial closing occurs quickly, however, full fusion takes three months
Describe cardiac venous transformation
Several structures at day 24:
- left and right atria, with single orifice for pulmonary vein
- left and right ventricles
- situs venosus, which will give rise to sinus venarum of right atrium
- vitelline veins left and right (located medially). Right vitelline vein gives rise to inferior vena cava. Receives nutrients from placenta
- umbilical veins left and right. Left umbilical vein becomes ligametum teres hepatis. Receives nutrients from yolk sac.
- anterior and posterior cardinal veins (merge to form right and left common cardinal veins. Right collects all blood from embryo) . Anterior cardinal vein develops further to become superior vena cava, jugular veins and left brachiocephalic vein
- by day 50, several of these structures are ‘obliterated’ including right umbilical vein, and posterior cardinal vein, and left vitelline
- orifices of pulmonary veins develop further
- right and left auricles form (from proterusion of sinus venosum moving into wall)*
- aorta dn pulmonary artery slaready formed
By day 56
- right and left barchio, subclavian and jugular
- oblique vein of LAA
- coronary sinus
- superior and inferior vena cava
left veins, explains why symm
Discuss congenital heart defects
- Most common developmental defects
- septal defects make up 53%: e.g. atrial opening, or opening between ventricles
recall: patent ventricular foramen usu. occurs in membranous portion - AV canal defects make up 13% e.g. atrial and ventricular septal defects, commo n atrioventricular valves
- LVOTO in 25% of cases: issues of outflow tract obstruction
- conotruncal malformations (36%)
Label the following diagram
days 18-21
Label the following diagram
lateral folding, 19-24 days
Label the following diagram
craniocaudald folding
n.b. truncus arteruisus and sinus venarum
endo tube fusion to primitive heart tube
Differentiation of heart wall
23-29? days
Label the following
septum primum
septum secundum
