Gastrointestinal System Development Study Notes

The gastrointestinal tract (GIT) originates during gastrulation from the endoderm of the trilaminar embryo during the third week of development in humans.

It extends from the buccopharyngeal membrane to the cloacal membrane.

All three germ layers contribute to the tract and associated organs.

The primitive gut gives rise to the respiratory system, accessory digestive glands, and urinary bladder.

STRUCTURE OF THE GIT IN EMBRYONIC DEVELOPMENT

  • Buccopharyngeal membrane: Marks the future mouth.

  • Cloacal membrane: Marking the future anus.

  • The respiratory diverticulum, stomach, heart, liver bud, vitelline duct, duodenum, midgut, allantois, and hindgut develop from these initial structures.

EARLY STAGES OF GASTROINTESTINAL DEVELOPMENT

At around 4 weeks:

  • Pharynx forms from the foregut.

  • Distinct regions form: i) Foregut, ii) Midgut, iii) Hindgut.

  • The midgut develops as a pocket from the yolk sac, remaining connected via the vitelline duct.

FOLDING OF THE EMBRYO

The developing endoderm initially connects to the yolk sac. Cranio-caudal and lateral folding bring the endoderm inwards, forming the gut tube.

  • Anterior and posterior intestinal portals are created through this folding, designating foregut and hindgut.

  • The oropharyngeal and cloacal membranes play a significant role in mouth and anus formation, respectively, with minimal mesoderm present for later breakdown.

SUBDIVISIONS OF THE GUT

Formation occurs through "nipping off" from the yolk sac due to head/tail and transverse folding:

  • Foregut: Runs from buccopharyngeal membrane to the cranial intestinal portal.

  • Midgut: Communicates with the yolk sac via the vitello-intestinal duct.

  • Hindgut: Extends from caudal intestinal portal to the cloacal membrane.

DEVELOPMENT AT ONE MONTH

By the end of the first month:

  • Stomach bulge is visible.

  • Dorsal pancreas begins to bud.

  • Midgut connection to the yolk sac reduces to a yolk stalk and a thin vitelline duct.

ORAL CAVITY FORMATION

The oral cavity forms through the breakdown of the buccopharyngeal membrane, creating a connection to amniotic fluid, essential for fetal swallowing.

The breakdown marks the emergence of the pharynx within the pharyngeal arches.

HINDGUT DEVELOPMENT AND STRUCTURES

From the hindgut, the allantois forms and later exits alongside other umbilical cord structures.

The gut wall differentiates from:

  • Endoderm: Epithelial components including glandular epithelium.

  • Mesoderm: Connective tissue and musculature.

FOREGUT DERIVATIVES

The foregut's anterior part becomes the pharynx, leading to the following derivatives:

  • Esophagus

  • Stomach

  • Cranial part of duodenum

  • Liver

  • Pancreas

  • Respiratory system

HISTOGENESIS OF THE GUT

The esophagus retains a simple tubular structure while further differentiation occurs in other components.

GUT-ASSOCIATED ORGAN FORMATION

Organ buds such as the thyroid, lung, liver, and pancreas arise from the endoderm.

The midgut's opening to the yolk sac narrows progressively.

MESODERMAL CONTRIBUTIONS

During lateral folding, mesoderm is integrated into the gut wall, forming:

  • Splanchnic (visceral) mesoderm: Forms the gut wall.

  • Somatic (parietal) mesoderm: Lines the body cavity.

STOMACH DEVELOPMENT

Early development: The stomach appears as a spindle-shaped expansion at about the 10 mm C.R.L stage, with greater curvature directed dorsally.

Rotation of the stomach: Involves:

  • A 180° clockwise rotation around the long axis.

  • A 90° clockwise rotation around the transverse axis.

These rotations position the greater curvature ventrally and the long axis transversely.

REGIONAL ORGANOGENESIS: STOMACH

Stomach differentiates:

  • The left vagus becomes anterior and the right vagus becomes posterior due to rotations.

  • Growth creates greater and lesser curvatures with the pylorus developing a sphincter which may be affected by signaling factors such as SOX-9 and NKX-2.5.

DEVOLUTION OF OMENTA

Dorsal mesogastrium becomes folded, evolving into the greater omentum encasing the omental bursa.

Ventral mesogastrium connects to the liver and lesser curvature of the stomach.

BLOOD SUPPLY OF THE GUT

Three unpaired branches of the abdominal aorta supply the gut:

  • Celiac artery: Supplies derivatives of the foregut.

  • Cranial mesenteric artery: Supplies derivatives of the midgut.

  • Caudal mesenteric artery: Supplies derivatives of the hindgut.

DEVELOPMENTAL ANOMALIES OF THE GIT

Common anomalies include:

  • Pyloric stenosis: Increased musculature surrounding the pylorus, leading to lumen narrowing.

  • Counter-rotation of the gut: Opposes normal direction of rotation.

  • Meckel’s diverticulum: A diverticulum along the small intestine due to persistence of the vitelline duct.

Other anomalies:

  • Omphalocoele: Herniation of abdominal organs through umbilical wall.

  • Imperforate anus: Malformation affecting normal bowel movement.

  • Hirschprung’s disease: Absence of ganglia leads to functional obstruction in the colon.

DEVELOPMENT OF THE LIVER

Hepatic bud formation: Initially grows from ventral aspect of foregut into the ventral mesogastrium.

Liver development: Solid hepatic cords invade mesoderm, becoming surrounded by connective tissue. Significant early liver size due to hematopoietic function.

DEVELOPMENT OF THE PANCREAS

Pancreas forms from dorsal and ventral pancreatic buds originating from the foregut.

Dorsal and ventral buds fuse to form the exocrine pancreas, with duct systems also merging.

FORMATION OF THE RUMINANT STOMACH

The ruminant stomach consists of four compartments: the rumen, reticulum, omasum, and abomasum. The formation starts from the foregut and involves complex folding and modifications over time. The rumen and reticulum are formed as sac-like expansions of the foregut, developing first and allowing for fermentation of fibrous plant material.

The omasum develops as a further extension from the rumen and reticulum, playing a critical role in water absorption. The abomasum, often referred to as the ‘true stomach’, develops later from the distal part of the foregut, getting differentiated from the rest of the gut to function similarly to the monogastric stomach.

Overall, the adaptation of these stomach compartments facilitates an efficient digestive system suited for the herbivorous diet of ruminants.

DEVELOPMENT OF THE SPLEEN

The spleen develops from mesodermal tissue in the dorsal mesogastrium, arising as a mesenchymal condensation between the layers of the dorsal mesentery. During embryonic development, it is initially connected to the stomach by the lienorenal ligament and to the diaphragm above it.

As development progresses, the spleen becomes involved in hematopoiesis (blood cell formation) in the early stages of life, and later plays a critical role in the immune response and blood filtration for the postnatal animal.

The mesoderm’s contribution to the spleen development is crucial for its eventual multifunctional role in both the circulatory and immune systems.