Early Development of the Heart

Introduction

• The cardiovascular system (CVS) is the first major system to develop in an embryo.
• Early development is essential to deliver oxygen and nutrients.
• A primordial uteroplacental circulation forms during the 3rd week.

Early Heart Development (Week 3)

• Angioblastic cords appear during the 3rd week.
• Canalization of the angioblastic cords results in the endocardial heart tubes.
• Two heart tubes fuse to form a single tubular heart by the end of the 3rd week.
• The heart begins to beat around day 22 to 23.

Formation of the Primitive Heart Tube

• Endothelial heart tubes fuse to form a single primitive heart tube.
• The primitive heart tube possesses a cranial (arterial) and a caudal end.
• As the heart tube fuses, surrounding mesenchyme thickens to form the myocardium and epicardium.
• The endothelial tube becomes the endocardium.
• Cardiac jelly separates the two layers.

Heart Tube Elongation and Chamber Formation

• The primitive heart tube elongates, develops dilatations and constrictions, forming an S-shaped, 5-chambered heart.
• The five chambers are the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and sinus venosus.

Connections of Heart Tube Chambers

• The truncus arteriosus is continuous cranially with the aortic sac.
• Pharyngeal arch arteries arise from the aortic sac.
• The sinus venosus is continuous caudally.
• The sinus venosus receives; the umbilical vein from the chorion, the vitelline vein from the umbilical vesicle, and the common cardinal vein from the embryo.

Bulboventricular Loop Formation

• The bulbus cordis and ventricle grow faster, which causes U-shaped bending and forms bulboventricular loop.
• As the heart bends the atrium and sinus venosus settle to lie dorsal to the truncus arteriosus, bulbus cordis, and ventricle

Partitioning of the Atrioventricular (AV) Canal

• The AV canal forms at the junction between the primitive atrium and ventricle.
• By the end of the 4th week endocardial cushions form on the dorsal and ventral walls of the AV canal.
• During the 5th week, these cushions merge.

Partitioning of the Primordial Atrium

• By the end of the 4th week, the primordial atrium is divided into right and left atria by two septa (primum and secundum).

Atrial Partitioning: Septum Primum

• Septum primum: A thin, crescent-shaped membrane.
• The membrane of the septum primum grows from the roof of the primordial atrium.
• It grows towards the fusing endocardial heart cushions
• The ostium primum is a gap that is left at its lower end.
• A gap in division acts as a shunt connecting right and left atria chambers.
• The Ostium primum becomes smaller, perforations appear in the central part of the septum and these are known as the ostium secundum

Atrial Partitioning: Septum Secundum

• Septum Secundum: A crescentic, muscular membrane growing downwards from the ventrocranial roof of the right atrium.
• Septum Secundum overlaps the foramen secundum in weeks 5-6.
• Incomplete partitioning results in formation of foramen ovale.
• The Cranial portion of septum primum disappears.
• Valve for Foramen Ovale: Caudal part becomes flap-like to from this valve.

Foramen Ovale Function

• Shunts blood from right to left before birth
• Due to increase in left atria pressures at birth there is complete partitioning of foramen ovale.

Changes in the Sinus Venosus

• The left horn of the sinus venosus decreases in size.
• The right horn of the sinus venosus increases in size.
• The left horn forms the coronary sinus.
• The right horn is incorporated into the wall of the right atrium which forms; the smooth part referred to as the sinus venarum
• The primitive atrium both region separated by the sulcus terminalis (externally) and crista terminalis (internally)

Left Atrium Formation

• The smooth part of the left atrium is derived from incorporation of the primordial pulmonary vein.
• Four pulmonary veins open into the left atrium.
• The primoridal atrium derives the left auricle.

Clinical Problems: Atrial Septal Defects (ASD)

• Atrial septal defects (ASD) are common congenital cardiac anomalies, present in 10-15% of patients.
• ASDs are more common in females than males (2-3:1).

Types of Atrial Septal Defects

• Ostium secundum defects
• Endocardial cushion defects with ostium primum defect
• Sinus venosus defects
• Common atrium

Partitioning of the Primordial Ventricle

• The interventricular septum (IV) forms as a muscular ridge growing from the floor of the ventricle near its apex.
• The IV septum grows through ventricular dilation and proliferation of myoblasts.
• The interventricular foramen (IV)is a crescent-shaped opening until the 7th week between the free edge of the septum and the fused endocardial.
• The IV foramen permits communication between the two ventricles.
• The IV foramen closes by the end of the 7th week.
• Bulbar ridges fuse with the endocardial cushion to close the IV foramen.
• Closure of the IV foramen occurs due to the; right and left bulbar ridges and, endocardiac cushion.

Membranous Interventricular Septum

• The membranous portion forms through tissue extension from the endocardial cushion to the muscular interventricular septum.
• After closure of the IV foramen, the pulmonary trunk communicates with the right ventricle.
• The aorta communicates with the left ventricle.
• Cavitation of ventricles gives rise to trabeculae carnae, papillary muscles, chordae tendinae.

Partitioning of the Bulbus Cordis and Truncus Arteriosus

• In the 5th week, active proliferation of mesenchyme occurs in the bulbar ridges in the bulbus cordis and the truncal ridges in the truncus arteriosus.
• Spiral Septum: The ridges Undergo 180° spiraling and form this septum.
• The aorticopulmonary septum (AP) divides the BC and TA into two arterial channels.
• These two arterial channels produce the ascending aorta and pulmonary trunk.
• Spiralling of the AP septum causes of the pulmonary trunk to twists around the ascending aorta.

Fate of the Bulbus Cordis

• Becomes incorporated into the wall of definitive ventricles.
• Right side becomes the conus arteriosus or infundibulum (origin of the pulmonary trunk).
• Left side it forms the vestibule of the aorta.

Ventricular Septal Defects

• Ventricular septal defects (VSDs) are the most common type of congenital heart disease (CHD).
• VSDs occur more frequently in males.
• Small VSDs may close spontaneously.
• Large VSDs allow left-to-right shunting of blood.
• Membranous VSD : a form of ventricular septal defect.
• Muscular VSDs may appear anywhere in the muscular part.
• Sometimes there are multiple small defects- Swiss cheese VSD.
• Absence of the IV septum results in a single ventricle.

Tetralogy of Fallot

• Τetralogy of Fallot: An abnormality resulting from unequal division within the conotruncal region.
• Displacement of the septum in tetralogy of Fallot produces 4 cardiovascular alterations.
• Narrow right (rt) ventricular outflow forms pulmonary stenosis.
• Large VSD exists in tetralogy of Fallot.
• An overriding aorta that arises directly above the septal defect is formed.
• The rt ventricular hypertrophy occurs due to higher pressure on the right side.

Persistent Truncus Arteriosus

• Results when the conotruncal ridges fail to form.
• Resulting in no division of the outflow tract
• The pulmonary artery arises some distance above the origin of the undivided tuncus
• VSD is present.
• Undivided truncus overrides both ventricles

Transposition of Great Vessels

• Transposition of great vessels: occurs when the conotruncal septum fails follow its normal spiral course.
• The aorta originates from the right ventricle in transposition of the great vessels.
• The pulmonary artery originates from the left.

Development of the Trachea, Bronchi, and Lungs

• The lower respiratory organs (larynx, trachea, bronchi, lungs) begin to form during the fourth week.
• The respiratory primordium appears as a median outgrowth.
• It outgrows from the ventral wall of the foregut, referred to as the laryngotracheal groove.

Respiratory Diverticulum

• The laryngotracheal groove evaginates into the respiratory diverticulum by the end of the 4th week.
• This diverticulum elongates and is invested with splanchnic mesenchyme.
• The respiratory bud is enlarges from the distal end in globular format.

Tracheoesophageal Septum Formation

• Longitudinal tracheaoesophageal ridges separates foramen
• Fusion of the tracheal esophageal ridges then result in formation of tracheal esophageal septum.
• This septum splits into part-laryngotracheal tube (ventrally) and dorsal part-esophaus

Trachea and Bronchial Buds

• After septum formation the laryngotracheal tube separates from the foregut.
• Trachea and the bronchial (lung) buds are then fromed.

Development of the Trachea

• The endodermal lining of the laryngotracheal tube differentiates into the epithelium and glands of the trachea and the cells of the pulmonary epithelium.
• The mesenchymal surrounding froms the CT, muscle and cartilages

Development of the Bronchi

• The respiratory (resp) bud that developed at the caudal end of laryngotracheal tube during the 4th week divides into two outpouchings which form; the pry bronchial buds.
• These endodermal buds grow laterally into the pericardioperitoneal canals, the primordia of the pleural cavities
• Splanchnic mesenchyme surrounding, differentiate into bronchi and their ramifications.
• Enlarged bronchiole buds are formed from 5th week onwards producing; the main or principal bronchi. -The main bronchi subdivides into lobar bronchi.
• From the 7th: Segmental bronchi from, the primordia of the bronchopulmonary segments is produced from segmental bronchiole surrounded my mesenchyme

Lung Tissue Development

• By 24 weeks respiratory bronchioles have developed.
• The lungs get a layer of visceral pleura from the splanchnic mesoderm.
• Somatic mesoderm layer covers from inside by a parietal pleura, both are separated by intraplerual space filled with a fluid;
• Intraplerual space filled with a fluid.

Periods of Lung Maturation During the Fetal

• Embryonic period
• Pseudoglandular period
• Canalicular period
• Terminal sac period
• Alveolar period

1.Embryonic Period

• It occurs between 4th and 6th week of gestation
• The tracheobronchial buds are formed from subdivion, to the level of the segmentals

2. Pseudoglandular Period

• This period begins between 6-16 weeks
• The terminal bronchiole are resembled to an exocrine glands during this period.
• Branching has continued to form terminal bronchioles.
• No respiratory bronchioles or alveoli are present.
• By 17 weeks all major elements of the lung are formed, except those involved with gas exchange.

3. Canalicular Phase

• This period of Lung Morphogenesis happens from 16 to 26 weeks
• Terminal bronchioles divide into two or more respiratory bronchioles
• These bronchioles divide into 3 to 6 alveolar ducts -These ducts end in terminal sacs (primitive alveoli)
• The tissues become more vascular as capillaries develop during period.

4. Terminal Sac Period

• Lasts from week 26 through birth
• Many more terminal sacs (primitive alveoli) develops and their epithelium becomes very thin.
• Capillaries bulge into developing alveoli.
• The intimate contact between epithelial and endothelial cells establishes the blood-air barrier.

5. Type II Pneumocytes

• Type I alveolar/pneumocyte cells are important for gas exchange
• Pulmonary surfactant is secreted from Type 2 alveolar cells surfactant in terminal cells

Alveolar Period of Development

• Analogous Structures: Form during 32 and up
• Terminal sac clusters: Elicited from respiratory bronchiols and terminate there = Alveolar maturity: Continues after birth by up to 95%

Tracheoesophogeal fistula

• Lower esophaus an trachea: Are both abnormally connected.
• Division impairment: By the tracheoesophogeal septum.

Pulmonary Agenesis

• Is only compatible when the agenesis is unilateral.
• Uncommon; usually occurs at the base of the left lung; is non functional

Diaphragm Development

• The septum transversum is a mass of mesodermal tissue in the cranial region of the embryo.
• Septum transversum is at the level of 3,4 and 5th cerival somites

Events leading formation of diapharm

At the 4th week, - During movement of ventro caudally, forming a thick incomplete parition, between throax & abdomen At 6th week, this expansion - Fusion with mesentery o esophaus: and body parts in the wall Innervated by somites 3,4 & 5

Innervation of the Diaphragm

• The diaphragm is innervated by myoblasts from all the somites 3,4,5 which help movement of
• The diaphragm is innervated by nerve fibers from the cervix of the nerves of spinal sections 3,4,5
• 2 motor & sensory supplies are all derived from the phrnic

Diaphragmatic Defects

• Herniation of congenitals on the diaphragm -- Eventration of the diaphragms to create room for the heart
• Large opening in esophageal to push materials

The cardiovascular system is the first major system to develop in an embryo. Early heart development during the 3rd week is essential to deliver oxygen and nutrients via uteroplacental circulation. Angioblastic cords form endocardial heart tubes that fuse into a single tubular heart, initiating heartbeats around day 22-23.