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The cardiac and smooth muscles are derived from the splanchnic mesoderm. The musculature of the body is derived from the mesoderm as described below:

• Somatic mesoderm: Muscles of limbs and trunk
• Branchial mesoderm: Muscles of head and neck
• Splanchnic mesoderm: Cardiac and smooth muscles

However, there are certain exceptions and some muscles are ectoderm-derived. These include

• Musculature of the iris (spincter and dilator pupillae)
• Erector pili muscles in the skin
• Myoepithelial cells of ducts of sweat glands

The heart develops from angioblastic tissue in the splanchnopleuric mesoderm.

The heart develops from two endothelial heart tubes. These heart tubes are derived from the splanchnic layer of the lateral plate mesoderm also known as splanchnopleuric mesoderm.

Progenitor heart cells form a horseshoe-shaped cluster of cells cranial to the neural fold. This location from which the heart originates is known as the cardiogenic area.

The image below shows location of cardiogenic area during embryological development.

The trabeculated part of the right ventricle is formed from the proximal 1/3rd of the bulbus cordis.

The heart tube begins to bend by day 23. This bending creates the cardiac loop. The fate of various structures formed as a result of this bending is as follows:

Given below are the heart tube and their derivatives:

Part of the heart tube

Bulbus cordisProximal 1/3Mi ddle 1/3 (conus cordis)Distal 1/3 (truncus arteriosus)

Primitive ventricle Primitive atrium

Derivative structure in the he art

Trabeculated part of Rt.ventr icleOutflow tract of Rt. and L t.ventriclesRoot and proxima l portions of aorta and pulmo nary artery

Trabeculated part of the Lt.v entricle

The common atrium that spli ts to Rt. and Lt.atrium

The sinus venosus does not receive blood from the subcardinal veins. They are found in relation to the urogenital ridges and mainly drain the developing kidney.

Sinus venosus is the caudal-most part of the tubular heart and has right and left horns. Each horn of sinus venosus has 3 branches:

• Vitelline vein receives blood from the yolk sac
• Umbilical vein receives blood from the placenta
• Common cardinal vein receives blood from the body wall

The coronary sinus develops from the left horn of the sinus venosus. Hence, its developmental failure will lead to defects of the coronary sinus.

Initially, the communication between the sinus venosus and the atrium is wide. Later, the entrance of the sinus venosus shifts to the right side due to the establishment of left-to-right shunts of blood.

The fate of sinus venosus and its branches near the heart is as follows:

• Right horn of sinus venosus gets incorporated into the right atrium
• Right umbilical vein: obliterates
• Right common cardinal vein: Becomes part of the superior venacava
• Right vitelline vein: Becomes the terminal part of inferior venacava
• Left horn of sinus venosus reduces in size and becomes the coronary sinus
• Left umbilical vein: obliterates
• Left common cardinal vein: reduces the size and becomes the oblique vein of the left atrium
• Left vitelline vein: obliterates

Persistent left-sided superior vena cava occurs due to failure of regression of the left anterior cardinal vein. It may drain either into the coronary sinus or the left atrium.

The formation of the vena cava system begins with the formation of anastomoses between the right and left venous channels. Formation of the major veins takes place as follows:

• The proximal part of the right anterior cardinal vein along with the right common cardinal vein form the superior vena cava
• The distal part of the right anterior cardinal vein between the junction with the subclavian vein and the junction with the transverse anastomosis becomes the right brachiocephalic vein
• The distal part of the left anterior cardinal vein and the transverse anastomosis becomes the left brachiocephalic vein
• The most cranial part of both the anterior cardinal veins above its junction with subclavian vein become the internal jugular vein

The posterior cardinal vein contributes to the development of the right common iliac vein. The common iliac veins develop as follows:

• The right common iliac vein is derived from the most caudal portion of the right posterior cardinal vein.
• The left common iliac vein develops from the anastomosis between the sacrocardinal veins.

The foramen secundum (ostium secundum) is present at the upper part of the septum primum.

During fetal life, oxygenated blood reaches the right atrium from the placenta. This blood then reaches the left atrium through the gap between the atria. The initial gap is the foramen primum formed at the lower end of the septum primum.

Before the foramen primum is closed, another path has to be created for the flow of blood. This gap is the foramen secundum at the upper part of the septum primum.

The limbus fossa ovalis (also known as annulus ovalis) represents the remnant of the lower free edge of the septum secundum.

Fossa ovalis is an oval-shaped depression in the septal wall of the right atrium just above the opening of inferior vena cava. It marks the location of embryonic foramen ovale.

• The floor of fossa ovalis is the remnant of septum primum.
• The limbus of fossa ovalis is the prominent upper margin of the fossa ovalis and a remnant of the septum secundum.

The left 2nd arch artery does not contribute towards the formation of the arch of the aorta. Arch of the aorta develops from the following:

• Ventral part of the aortic sac
• Left horn of the aortic sac
• Left 4th arch artery

The proximal part of the right subclavian artery is derived from the right 4th aortic arch.

The distal segment of the right subclavian artery is derived from the right dorsal aorta and the 7th intersegmental artery.

The 4th aortic arch persists on both sides and forms different structures on either side:

• Left side: Part of the arch of the aorta between the left common carotid and the left subclavian arteries
• Right side: Proximal segment of the right subclavian artery

The heart starts to beat in the 4th week of development.

The appearance of the laryngotracheal (respiratory) diverticulum is regarded as the first sign of respiratory system development. It appears in the ventral wall of the pharyngeal part of the

foregut.

The respiratory system is derived from 2 sources:

• Endoderm of the foregut: The lining epithelium of the larynx, trachea, bronchi, bronchioles, and alveoli are derived from the endoderm.
• Splanchnopleuric intraembryonic mesoderm: The cartilages, muscles, and connective tissue of the respiratory tract are derived from the splanchnopleuric intraembryonic mesoderm surrounding the foregut.

The image given below shows the development of the respiratory diverticulum.

The larynx and trachea develop from the laryngotracheal tube.

The laryngotracheal (respiratory) diverticulum develops from the ventral wall of the pharyngeal part of the foregut.

The free caudal end of the respiratory diverticulum elongates downwards to enter the thorax, where it bifurcates to form the right and left bronchial buds or lung buds, which in turn further develops to form the bronchi and the lung parenchyma.

The part of the diverticulum cranial to the bifurcation is known as laryngotracheal tube, which further develops to form the larynx and trachea.

The most common type of tracheoesophageal fistula (TEF) is esophageal atresia with distal TEF (also known as type C).

It occurs due to the defective formation of the tracheoesophageal septum. There is an abnormal connection (fistula) between the esophagus and the trachea. These defects occur in approximately 1/3000 births. The condition is commonly associated with esophageal atresia.

TEF can be grossly classified into 5 types, types A to E, as shown in the image below:

The given clinical scenario is suggestive of a congenital diaphragmatic hernia. It most commonly occurs due to failure of fusion of pleuroperitoneal folds during the development of the diaphragm.

The diaphragm develops from 4 mesodermal sources as follows:

• Septum transversum becomes the anterior and medial part of the diaphragm including the central tendon
• Pleuroperitoneal membranes (paired structures) become the dorsolateral parts of the diaphragm
• Mesoderm of body wall from somites of cervical segments three to five (C3-5) contribute to the peripheral muscular part
• Mesentery of the esophagus forms the crura of the diaphragm

Congenital diaphragmatic hernia results due to failure of various structures forming diaphragm to fuse. It occurs in 1/2000 births. The defect allows abdominal viscera to enter the pleural cavity. It is most frequently caused by the failure of one or both of the pleuroperitoneal membranes to fuse. Posterolateral defects (Bochdalek’s hernia) are the most common (85–90).

The celiac trunk supplies the foregut derivatives of the digestive system. Arterial supply of the gut:

• Foregut: Celiac trunk
• Midgut: Superior mesenteric artery
• Hindgut: Inferior mesenteric artery

The cecal bud arises as a diverticulum in the post-arterial segment of the midgut loop.

It enlarges and gives rise to the cecum and the appendix. The proximal part of the cecal bud grows rapidly to form the cecum. The distal part remains narrow and forms the appendix.

The midgut loop consists of:

• Cranial limb - It forms
• Jejunum
• Upper part of the ileum
• Caudal limb - It forms
• Lower part of the ileum
• Cecum
• Appendix
• Ascending colon
• Proximal two-thirds of the transverse colon

The midgut normally undergoes a 270° counterclockwise rotation around the axis formed by the superior mesenteric artery.

The midgut forms a U-shaped loop (midgut loop) that herniates through the primitive umbilical ring into the extra-embryonic celom in the umbilicus. During the development of the intestines, the midgut loop rotates a total of 270° counter-clockwise around the superior mesenteric artery.

• Around 90° rotation of the midgut occurs when returning from the physiological hernia in the umbilicus
• The first part of the intestinal loop to retract from the umbilicus is the proximal portion of the jejunum
• The last part of the intestinal loop to retract from the umbilicus is the cecal bud
• The remaining 180° rotation occurs when the intestinal loops are in the abdominal cavity

The persistence of the vitellointestinal duct gives rise to the Meckel diverticulum.

The primitive gut is connected to the definitive yolk sac through a narrow stalk-like communication known as the vitellointestinal duct. In about 2 of the population, a small section of the vitellointestinal duct persists to form an outpouching of the ileum called the Meckel diverticulum or ileal diverticulum.

Abnormalities arising due to incomplete obliteration of the vitellointestinal duct are given below:

The sigmoid colon does not develop from the endodermal cloaca.

The part of the hindgut that is present caudal to the attachment of the allantoic diverticulum is called the cloaca. Between the allantois and hindgut, a wedge of mesoderm, called the urorectal septum, grows. The urorectal septum divides the endodermal cloaca into two parts:

• The primitive urogenital sinus in the anterior part develops into the urinary bladder, urethra, and lower part of the vagina (in females)
• The primitive rectum in the posterior part develops into the lower part of the rectum and upper part of the anal canal.

The tip of the urorectal septum forms the perineal body.

Note: The rectum develops from both the hindgut (proximal part) and cloaca (distal part).

Incomplete separation of cloaca by the urorectal septum causes anal agenesis.

When the urorectal septum is formed, the cloacal membrane gets divided into urogenital membrane anteriorly and anal membrane posteriorly. The ectoderm in the region of the future anal opening is called proctodeum. The proctodeum invaginates and creates the anal pit. The anal membrane now separates the hindgut from the anal pit.

Formation of the anal canal takes place as follows:

• The upper part of the anal canal is formed by the endodermal hindgut and the lower part by the ectodermal anal pit.
• The anal membrane in between breaks around the 9th week and the anal canal becomes continuous.
• The junction between the endodermal and the ectodermal region of the anal canal is marked by the pectinate line.