Development of Heart-I21-05-2013

Dr Laxman Khanal

“A loving heart is the beginning of all knowledge”- Thomas Carlyle

Before the topic

• the starting point is the formation of the trilaminar embryo.• At the extreme head end of the embryo the ectoderm and

endoderm are in contact without intervening mesoderm. This area is called the prochordal plate (“in front of the notochord”).

• the lateral plate mesoderm splits into somatopleuric and splanchnopleuric layer and form the coelomic cavity .

• In the cranial most part of lateral plate mesoderm the coelomic cavity does not extends , so two layer of mesoderm are continuous with each other. This part of unsplited mesoderm is called as septum transversum.

Some basic fact to know

• During first 20 days of development there is no cardiovascular system.

• It is the first organ of the body to start functioning.• Heart start to beat on day22.• You should understand the precise distinction

between arteries and veins. • The cardiovascular system is derived from:

Splanchnic mesoderm, which forms the primordium of the heart , Neural crest cells and proepicardium.

Start from the beginning

• The vascular system appears in the middle of the third week, when the embryo is no longer able to satisfy its nutritional requirements by diffusion alone.

• Progenitor heart cells lie in the epiblast immidiatly lateral to the primitive streak.

• From there, they migrate toward the cranium and positioned themselves just rostral to the oropharyngeal membrane and neural fold. Here they resides in the splanchnic layer of lateral plate mesoderm.

• In the splanchnic layer migrated cells are induced by underlying endoderm to form cardaic myoblasts.

• Blood islands also appear in the same mesoderm layer, which form the blood cells and vessels by vasculogenesis.

• With time these islands unites to form a horseshoe-shaped endothelial lined tube surrounded by cardiac myoblast. This region is called as cardiogenic field.

• Intra-embryonic cavity over it develops into pericardial cavity.

We are discussing after the folding of embryo

• Development of heart takes place in the cardiogenic area which is situated below the stomodeum, above the septum transversum, behind the pericardial sac and in front of the fore-gut.

See how heart tube is changing its position during folding of embryo.

Formation of primitive heart tube

• Cardiogenic area consists of angioblastic mesenchymal cell. These cells arrange in the form of two endothelial lined tubes, which are placed side by side.

• Endothelial tubes undergo cranio-caudal fusion and form single primitive heart tube.

• In addition to the cardiogenic region, other blood islands also appear bilaterally, parallel, and close to the midline of the embryo. These islands form a pair of longitudinal vessels, the dorsal aortae behind the fore gut.

BACK

The dorsal aorta form independently and then grow to meet the ventral output from the heart in the aortic arches.

FUSED DORSAL AORTA1 ST AORTIC ARCH (R)

ORAL PLATE

ATRIUMVENTRICLEVENOUS RETURN FROM

CARDINAL VEINS, VITELLINE VEIN AND ALLANTOIC (UMBILICAL)

VEIN

Layers of primitive heart tube• heart tube consists of three layers:

(a) the endocardium, forming the internal endothelial lining of the heart- formed from the blood island in cardiogenic filed.(b) the myocardium,- formed from the cardiac myoblasts. (c) the epicardium- formed from the proepicardium(formed by the mesothelial cells of the septum transversum).

• This outer layer is responsible for formation of the coronary arteries, including their endothelial lining and smooth muscle.

• The developing heart tube bulges more and more into the pericardial cavity.

• Initially, however, the tube remains attached to the dorsal side of the pericardial cavity by a fold of mesodermal tissue, the dorsal mesocardium.

• With further development, the dorsal mesocardium disappears, creating the transverse pericardial sinus, which connects both sides of the pericardial cavity. The heart is now suspended in the cavity by blood vessels at its cranial and caudal poles

Fusion of two primitive heart tubes and attachment of dorsal mesocardium.

cardiac looping / Dextral looping• Cardiac looping start from the day 23 and completed by

day 28.• What is it ?? – it is continuous process of growth of heart

tube due to change in cell shape result in bending of straight tube into adult form of heart.

• Why ?? – to accommodate the components of heart in the pericardial cavity.

- In the primitive heart tube, venous blood flows through the left ventricle prior to the right ventricle. This situation must be corrected because in the normal adult heart venous blood flows into the right ventricle.

How ??- cephalic portion bends ventrally, caudally and toward right side. Caudal end loops in exactly opposite direction( dorsally , cranially and toward left.)

Cardiac looping

• While the cardiac loop is forming, local expansions become visible throughout the length of the tube but -

• The atrioventricular junction remains narrow and forms the atrioventricular canal, which connects the common atrium and the early embryonic ventricle.

• The junction between the ventricle and the bulbus cordis, externally indicated by the bulboventricular sulcus , remains narrow. It is called the primary interventricular foramen.

Abnormalities of Cardiac Looping

• Dextrocardia, in which the heart lies on the right side of the thorax instead of the left, is caused because the heart loops to the left instead of the right.

• Dextrocardia may coincide with situs inversus, a complete reversal of asymmetry in all organs.

• it may be associated with laterality sequence or heterotaxy in which sidedness is random, such that some organs are reversed and others are not.

Molecular regulation of cardiac development

Venous end is specified by more concentration of Retinoic acid and arterial end by less concentration of retinoic acid.

Fate of primitive heart tube

Sinus venosus- • it is the caudal end of heart tube which communicate

with primitive atrium through sinoatrial orifice.• the sinus venosus receives venous blood from the

right and left sinus horns. Each horn receives blood from three important veins:

(1) the vitelline or the omphalomesenteric vein (2) the umbilical vein (3) the common cardinal vein.

Embryonic veins

•S-A orifice changes its shape from horizontal to vertical and position from center to right side.•Most of left horn of sinus venosus disappear except small part which form oblique vein of left atrium and coronary sinus.

Primitive atrium – give rise to trabeculated part of right and left atrium. Two atria get separated with eachother by the formation of interatrial septum.

Development of right atrium• As a result of left-to-right shunts of blood, the right

sinus horn and veins enlarge greatly. The right horn, which now forms the only communication between the original sinus venosus and the atrium, is incorporated into the right atrium to form the smooth-walled part of the right atrium-sinus venarum.

• Its entrance in right atrium is guarded by right and left venous valves. Dorso-cranially these valves fuse to form a ridge ( septum spurium)

Development of right atrium

• when the right sinus horn is incorporated into the wall of the atrium, the left venous valve and the septum spurium fuse with the developing atrial septum.

• The superior portion of the right venous valve disappears entirely. The inferior portion develops into two parts:

(1) valve of the inferior vena cava (2) valve of the coronary sinus

• Crista terminalis divide the trabeculated part ( primitive atrium) and smooth part( Rt. Horn of sinus venosus)

Development of left atrium

Right atrium – rough part by primitive atrium and smooth part by right sinus horn.

Left atrium- rough part by primitive atrium and smooth part by absorption of pulmonary veins.

Division of A-V canal

Common A-V canal divide into right and left halves by the thickenings of AV cushion that appear on its dorsal and ventral walls. Fusion of cushions form the septum intermedium.

Local proliferation of mesenchymal tissue under the endocardium around the Rt and Lt A-V canal forms the tricuspid and bicuspid valve respectively.

Defect in A-V canal division

• Ebstein's anomaly is caused by the failure of leaflets of the tricuspid valve to attach normally to the annulus fibrosus but are instead displaced inferiorly into the right ventricle.

• It results in a condition in which the right ventricle is divided into a large, upper, "atrialized" portion and a small, lower, functional portion.

• Risk factor is Lithium intake by mother • Definition- it is atrialization of right ventricle

Primitive ventricle- forms the trabeculated part of left ventricle.Bulbus Cordis

• Proximal 1/3rd – trabeculated part of right ventricle.• Middle 1/3rd ( conus)- outflow tract( smooth) of right and left

ventricles.• Distal 1/3rd ( truncus arteriosus)- ascending aorta and

pulmonary trunk by formation of spiral septum. It is continuous distally with the aortic sac, which is continuous with

the Rt and Lt pharyngeal arteries to join the pair of dorsal aortae.• The junction between the ventricle and the bulbus cordis,

externally indicated by the bulboventricular sulcus , remains narrow. It is called the primary interventricular foramen.

Conducting system of heart• Initially, the pacemaker lies in the caudal part of the left

cardiac tube. Later, to the sinus venosus , and as the sinus is incorporated into the right atrium, pacemaker tissue lies near the opening of the superior vena cava. Thus, the sinuatrial node is formed.

• The atrioventricular node and bundle (bundle of His) are derived from two sources:

(1) cells in the left wall of the sinus venosus and(2) cells from the atrioventricular canal. Once the sinus venosus is incorporated into the right atrium,

these cells lie in their final position at the base of the interatrial septum.

The trabeculated part of the right ventricle is derived from which of the following?

(A) Truncus arteriosus(B) Bulbus cordis(C) Primitive ventricle(D) Primitive atrium(E) Sinus venosus

“Because God has made us for Himself, our hearts are restless until they rest in Him.”

― Augustine of Hippo

Thank You