Abstract
The primitive gut is formed by incorporation of the portion of the endoderm lining the yolk sac during cephalocaudal and lateral folding of the embryo around the fourth week of the embryonic period.
1 The Gastrointestinal System
The primitive gut is formed by incorporation of the portion of the endoderm lining the yolk sac during cephalocaudal and lateral folding of the embryo around the fourth week of the embryonic period.
As the gut tube develops, the endoderm proliferates rapidly and temporarily occludes the lumen of the tube around the fifth week. At around the seventh week, recanalisation of the tube starts due to apoptosis of the endoderm, such that by the ninth week, the lumen is open again.
The primitive gut is divided into three main parts:
◦ The foregut extends from the caudal end of the pharyngeal tube to the liver outgrowth.
◦ Derivatives of the foregut:
— oesophagus
— trachea and lung bud
— stomach
— duodenum proximal to the entrance of the common bile duct
— liver, biliary apparatus and pancreas
◦ Blood supply of the foregut:
— branches of coeliac trunk
◦ The midgut develops from the part of the primitive duct from the liver bud to the junction of the right two-thirds of the transverse colon to the left one-third of the transverse colon in adults.
◦ Derivatives of the midgut:
— duodenum distal to the opening of the common bile duct
— small intestine
— caecum and appendix
— ascending colon and two-thirds of transverse colon
◦ Blood supply:
— branches of superior mesenteric artery
◦ The hindgut develops from the part of the primitive duct from the left third of the transverse colon to the cloacal membrane.
◦ Derivatives of the hindgut:
— distal third of the transverse colon
— descending colon
— sigmoid colon
— rectum
— upper anal canal
◦ Blood supply:
— branches of the inferior mesenteric artery
1.1 Physiological Herniation of the Gut
Due to the differential growth of the cranial part of the midgut as compared to the caudal part, and development of various other organs in the abdominal cavity, the midgut herniates through the umbilical opening into the umbilical cord at around the sixth week of gestation.
At around the tenth week, when the abdominal cavity has sufficiently enlarged, organs begin to return in a specific manner.
During this process, the midgut rotates a total of 270 degrees counterclockwise (90 degrees during herniation and 180 degrees during return).
The plane of rotation is through the superior mesenteric artery.
1.2 Clinical Embryology
◦ A Meckel’s diverticulum occurs due to the persistence of a small portion of the vitelline duct.
◦ It occurs in 2–4% of the population.
◦ The vitelline duct sometimes communicates with the anterior abdominal wall, giving rise to a vitelline duct fistula. This can lead to faecal discharge through the umbilicus.
◦ An omphalocele is a condition that occurs due to the failure of a reduction of physiological herniation during the embryonic period.
◦ The herniated gut is covered by a translucent membrane comprising of peritoneum and amniotic membrane.
◦ The umbilical cord is attached to the top of the omphalocele.
◦ The content of the sac is bowel. There is no evidence of other organs in the hernia sac.
◦ It is associated with chromosomal abnormalities 50% of the time.
◦ Cardiac abnormalities are associated in 50% of cases, and 40% have associated neural tube defects.
◦ Prognosis depends on associated abnormalities.
◦ Gastroschisis occurs due to a developmental defect in the anterior abdominal wall.
◦ The umbilical cord is attached to one side of the defect, usually on the right side.
◦ The herniated sac may contain other abdominal organs like the liver and gall bladder.
◦ It is rarely associated with chromosomal abnormalities or any other abnormality.
2 The Cardiovascular System
2.1 Formation of the Heart Tube
This process begins by the sixteenth day of embryonic life.
The heart progenitor cells, which originate from epiblasts, migrate through the primitive streak, forming the primary heart field (which gives rise to part of the atria, left ventricle and part of the right ventricle) and secondary heart field (which gives rise to part of the right ventricle, part of the atria, conus cordis and truncus arteriosus).
Some of the heart progenitor cells develop into endothelial cells, forming an endothelial tube and others form myoblasts, which surround the tube.
With the folding of the body walls, two loops of the tube fuse, forming a single heart tube.
During the fourth week, this heart tube folds on itself (cardiac looping) and the heart achieves its normal position with the atria posterior and ventricles in a more anterior position.
The smooth wall portion of the right atrium is formed by incorporation of the sinus venosus, whereas the smooth wall of the left atrium is formed by the incorporation of the root of the pulmonary vein.
The ventricles of the heart are formed by the bulbus cordis and the primitive ventricles.
The ascending aorta and pulmonary trunk are developed from the truncus arteriosus, whereas venous drainage of the heart arises from the sinus venosum and sinus venarum. See Figure 12.1.
Figure 12.1 Development of the heart. Used with permission from https://commons.wikimedia.org/wiki/File:2037_Embryonic_Development_of_Heart.jpg
2.2 Formation of the Atrial Septum
The atrial septum formation occurs between the fourth and sixth week.
This starts with formation of the septum primum, which is a thick crescent-shaped membranous structure. It grows from the roof of the atrium towards the endocardial cushion, leaving a gap called the ostium primum.
With further development of the septum primum, the ostium primum closes, but at the same time cells in the ostium primum undergo programmed cell death, forming the ostium secundum.
A thick septum develops to the right side of the septum primum, which acts like a flap narrowing the septum, leaving only a small gap known as the foramen ovale, which remains patent until birth.
2.3 Formation of the Ventricular Septum
The ventricular septum consists of a muscular part, which develops from the floor of the ventricle and grows towards the endocardial cushions and membranous part, which arises from the endocardial cushions and the aortopulmonary septum.
The ventricular septum contains the atrioventricular conducting bundle.
2.4 Development of the Aortic Arches
The first arch disappears (leaving the maxillary artery).
The second arch disappears (leaving the stapedial artery).
The third arch gives rise to the common carotid artery and the first part of the internal carotid artery. The external carotid artery arises as a direct branch from the third arch.
The fourth arch on the left side gives rise to the portion of the aortic arch from the left common carotid to the left subclavian arteries, and on the right side it gives rise to the right subclavian artery (the proximal portion).
The fifth arch disappears.
The sixth arch on the left side gives rise to the left pulmonary artery and ductus arteriosus, and on the right side gives rise to the right pulmonary artery. The distal part of the right side disappears, and the distal part of the left side forms the ductus arteriosus.
3 Fetal Circulation
The main source of oxygenated blood to the fetus is the placenta.
This oxygenated blood from the placenta is carried through the umbilical vein to the fetus.
A greater portion of this blood passes through the ductus venosus to the inferior vena cava (IVC) and a smaller portion passes through the substance of the liver to the IVC.
This blood is carried to the right atrium by the IVC. Most of this passes through the foramen ovale into the left atrium and the rest of it gets mixed up with the deoxygenated blood returning through the superior vena cava to the right atrium, which subsequently passes to the right ventricle.
The deoxygenated blood from the right ventricle enters the pulmonary trunk, the greater part of which is shunted by the ductus arteriosus into the aorta. Only a small portion of blood that reaches the lungs passes to the left atrium.
The left atrium receives oxygenated blood from the right atrium via the foramen ovale and a small amount of deoxygenated blood from the lung.
This oxygenated blood passes into the left ventricle and then into the aorta. Some of this passes into the carotid and subclavian arteries. The rest of the blood mixes with the deoxygenated blood from the ductus arteriosus.
The deoxygenated blood is then carried by the aorta to the placenta via the umbilical arteries.
3.1 Changes in Circulation at Birth
The umbilical arteries contract immediately after birth, which prevents loss of fetal blood into the placenta.
When the infant breathes for the first time, resistance of the pulmonary vasculature drops, leading to an increase in the pressure in the left atrium as compared to the right atrium, which leads to closure of the foramen ovale. Additionally, increase in oxygen concentration in the blood leads to a decrease in prostaglandin, causing closure of the ductus arteriosus.
Due to these closures, blood is prevented from bypassing the pulmonary circulation and the newborn’s blood is oxygenated in the newly operational lung.
4 The Respiratory System
At around the fourth week, the respiratory diverticulum appears (lung bud), which develops as an outgrowth of the ventral wall of the foregut.
With the formation of the tracheoesophageal septum, the foregut is divided into the oesophagus dorsally and the trachea and lung bud ventrally.
Development of the larynx:
◦ Laryngeal epithelium is developed from endoderm.
◦ Laryngeal muscles and cartilages are developed from the fourth and sixth pharyngeal arches.
Development of the trachea and bronchial tree:
◦ The lung bud gives rise to the trachea and the two main bronchi. The right main bronchus further develops into three secondary bronchi and three lobes, and the left main bronchus develops into two secondary bronchi and two lobes.
◦ By the seventh week, 10 tertiary bronchi are formed on the right side and eight on the left side.
◦ The tracheal epithelium and glands develop from the endoderm and tracheal connective tissue. Smooth muscles and cartilages are developed from the splanchnic mesoderm.
◦ The terminal bronchioles further divide into respiratory bronchioles and primitive alveolar sacs. This process starts at the fifth week.
◦ The development of lung tissue is divided into the following four stages:
1. Pseudoglandular stage. This stage lasts from the fifth to the sixteenth week. During this stage, the tertiary bronchi continue branching and form terminal bronchioles. No respiratory bronchioles or alveoli are present.
2. Canalicular period stage. This stage lasts from the sixteenth to the twenty-sixth week. During this stage, each terminal bronchiole divides into two or more respiratory bronchioles, which further divide into three to six alveolar ducts.
3. Terminal sac period. This stage lasts from the twenty-sixth week to birth. During this stage, primitive alveoli and capillaries establish close contact.
4. Alveolar period. This stage lasts from the eighth month to childhood. During this stage, mature alveoli develop and they have well established epithelial and endothelial contacts.
– Type 1 alveolar cells lining the alveolar cavity are responsible for gas exchange. Type 2 alveolar cells are responsible for the secretion of surfactant.
4.1 Clinical Embryology
Tracheoesophageal fistula (TEF):
This abnormality develops due to a defect in the formation of the tracheoesophageal septum.
TEF occurs in around 1 in 3000 births.
Ninety percent of the time, there is formation of a blind oesophageal pouch from the upper portion of the oesophagus, and the lower portion forms a fistula with the trachea.
Isolated oesophageal atresia and H type of tracheoesophageal fistula is present in 4% of cases, whereas other variations form 1% each.
Associated cardiac anomalies occur in 33% of cases.
Sometimes TEF occurs as a component of VACTERL association (vertebral anomalies, anal atresia, cardiac defects, TEF, oesophageal atresia, renal anomalies and limb defects).
5 The Genitourinary System
5.1 Development of the Urogenital System
The urogenital system arises from the intermediate mesoderm, which forms a urogenital ridge on either side of the aorta and the primitive urogenital sinus, which is part of the cloaca.
5.2 Intermediate Mesoderm
The intermediate mesoderm forms a bulge on the posterior abdominal wall lateral to the attachment of the dorsal mesentery of the gut. This bulge is known as the urogenital ridge or nephrogenic cord. Its surface is covered by the coelomic epithelium lining the peritoneal cavity. The urogenital ridge forms:
excretory tubules: development of the kidneys
nephric duct: later becomes the mesonephric duct
paramesonephric duct is formed lateral to the nephric duct
gonads (testis/ovary): develop from the coelomic epithelium lining the medial side of the nephrogenic cord
5.2.1 Cloaca
The terminal part of the hindgut ends in the cloaca, which is an endoderm-lined chamber that contacts the surface ectoderm at the cloacal membrane and communicates with the allantois. The allantois is a membranous sac, which extends into the umbilicus alongside the vitelline duct.
The cloaca is then divided by the urorectal septum:
The dorsal (inferior) portion develops into the rectum and anal canal.
The ventral (superior) portion develops into the bladder and urogenital sinus.
The urogenital sinus gives rise to the bladder and lower urogenital tracts (prostatic and penile urethrae in males; urethra and lower vagina in women). See Figure 12.2.
6 Development of the Kidneys
The urogenital ridge develops into three sets of tubular nephric structures:
1. Pronephros, which mostly regress
◦ mesonephric tubules: these tubules carry out some kidney function at first, but many of the tubules then regress
◦ mesonephric duct (Wolffian duct): persists and opens to the cloaca at the tail of the embryo
The adult kidneys arise from two sources:
The metanephric blastema: a condensation of nearby renogenic intermediate mesoderm from the lowest part of the nephrogenic cord. It forms glomerular capillaries, proximal convoluted tubules, the loops of Henle and distal convoluted tubules.
The ureteric bud develops from an outgrowth of the caudal mesonephric duct, and forms the collecting tubules and ducts, minor and major calyces and the ureters.
6.1 Ascent of the Kidneys
The kidneys initially form near the tail of the embryo. Growth of the embryo in length causes the kidneys to ‘ascend’ to their final position in the lumbar region. Malformations related to the ascent of the kidneys include:
pelvic kidney: one or both kidneys stay in the pelvis
horseshoe kidney: two developing kidneys fuse ventrally into a single, horseshoe shape, which gets trapped in the abdomen by the inferior mesenteric artery
supernumerary arteries: there can often be more than one renal artery per kidney, which is often asymptomatic but can sometimes cause hydronephrosis
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