Development of Intestine and Anterior Abdominal Wall

Introduction

• The alimentary canal develops from the primitive gut, an endoderm-lined tube formed during early embryonic development.
• During formation of the head fold and tail fold, a portion of the definitive yolk sac becomes incorporated into the embryo. This incorporated segment forms the primitive gut.
• The primitive gut extends from the buccopharyngeal membrane at the cranial end to the cloacal membrane at the caudal end. In both membranes, ectoderm and endoderm are in direct contact without an intervening mesodermal layer.
• The developing primitive gut communicates with:
  • The yolk sac through the vitellointestinal (vitelline) duct.
  • The allantoic diverticulum, which arises from the caudal part of the gut.
• Based on its location and connections, the primitive gut is divided into three regions:
  • Foregut – the cranial segment located within the head fold.
  • Midgut – the segment that remains temporarily connected to the yolk sac through the vitellointestinal duct.
  • Hindgut – the caudal segment situated within the tail fold.
• The vitellointestinal duct normally degenerates and disappears by the fifth week of development.
• The anterior (cranial) intestinal portal connects the foregut with the midgut, while the posterior (caudal) intestinal portal connects the midgut with the hindgut.
• The buccopharyngeal membrane separates the stomodeum (primitive oral cavity) from the foregut, whereas the cloacal membrane separates the hindgut from the proctodeum.
• The gut tube is suspended from the body wall by mesenteries, initially consisting of ventral and dorsal mesenteries.
• The arterial supply reflects the embryological divisions of the gut:
  • Celiac trunk supplies the foregut.
  • Superior mesenteric artery supplies the midgut.
  • Inferior mesenteric artery supplies the hindgut.
• In early development, the allantois opens into the caudal part of the hindgut called the cloaca. The urorectal septum subsequently divides the cloaca into a ventral primitive urogenital sinus and a dorsal primitive anorectal canal.
• Fusion of the urorectal septum with the cloacal membrane subdivides it into a ventral urogenital membrane and a dorsal anal membrane.

Development Of Oesophagus

The oesophagus develops from the segment of the foregut located between the pharynx and the stomach.

Derivation of Components

• Epithelium is derived from the endoderm of the foregut.
• Muscle layers and connective tissue originate from the surrounding splanchnopleuric mesoderm.

Stages of Development

• In the lower region of the primitive pharynx, a laryngotracheal groove appears. This groove later forms the tracheobronchial (respiratory) diverticulum.
• A tracheoesophageal septum develops and divides the foregut segment below the diverticulum into two parts: a ventral trachea and a dorsal oesophagus.
• Initially the oesophagus is short, but it lengthens as the neck develops and the diaphragm, lungs, and heart descend during embryonic growth.
• The enteric nerve plexuses—Auerbach’s (myenteric) plexus and Meissner’s (submucosal) plexus—are derived from neural crest cells.

Muscular Arrangement

The muscular composition of the oesophagus varies along its length:

• The upper one-third contains striated muscle.
• The middle one-third contains a mixture of striated and smooth muscle.
• The lower one-third consists predominantly of smooth muscle.

Development Of Stomach

Stages of Development

• The stomach begins to develop during the 4th–5th week as a fusiform dilation of the foregut located distal to the oesophagus.
• Initially, the tubular foregut enlarges to form a spindle-shaped sac with distinct ventral and dorsal borders.
• The dorsal border grows more rapidly than the ventral border, producing the greater curvature of the stomach. In contrast, the slower-growing ventral border forms the lesser curvature.
• As development progresses, the stomach acquires its characteristic features, including anterior and posterior surfaces, the greater and lesser curvatures, and the fundus.
• The ventral border of the developing stomach is attached to the septum transversum through the ventral mesogastrium, while the dorsal border is connected to the posterior body wall by the dorsal mesogastrium.
• Several organs arise within these mesenteries. The spleen, celiac trunk, and dorsal pancreatic bud develop within the dorsal mesogastrium, whereas the liver, gallbladder, and ventral pancreatic bud develop within the ventral mesogastrium.

Rotation of the Stomach

The developing stomach undergoes two important rotations that determine its final anatomical position and relationships.

• First rotation (longitudinal axis):
  The stomach rotates approximately 90° clockwise around its longitudinal axis. As a result, the left surface of the primitive foregut becomes the anterior surface of the adult stomach, while the right surface becomes the posterior surface. Consequently, the left vagus nerve supplies the anterior surface and the right vagus nerve supplies the posterior surface.
• Second rotation (anteroposterior axis):
  A further rotation occurs around the transverse (anteroposterior) axis. During this movement, the caudal end of the stomach shifts to the right and upward, forming the pyloric part, while the cranial end moves to the left, contributing to the final orientation of the stomach.

Changes in Mesenteries of Stomach

• The liver develops within the ventral mesogastrium during early embryonic life. As the liver enlarges, it divides the ventral mesogastrium into two main parts:
  • Falciform ligament – extends between the liver and the anterior abdominal wall.
  • Lesser omentum – connects the liver to the lesser curvature of the stomach and the proximal duodenum.
• The ventral mesogastrium also gives rise to the coronary ligament and triangular ligaments of the liver.
• The spleen develops within the dorsal mesogastrium. Its growth subdivides the dorsal mesogastrium into:
  • Gastrosplenic ligament – located between the fundus and greater curvature of the stomach and the spleen; it contains the short gastric vessels.
  • Lienorenal (splenorenal) ligament – extending between the spleen and the posterior abdominal wall, and containing the splenic vessels.
• The remaining part of the dorsal mesogastrium attached to the greater curvature elongates to form the greater omentum.
• A peritoneal recess called the lesser sac (omental bursa) develops behind the stomach, lesser omentum, and greater omentum.

Histogenesis of the Stomach

• The epithelial lining and gastric glands originate from the endoderm of the foregut.
• The muscular layers and connective tissue arise from the surrounding splanchnopleuric mesoderm.

Development Of Duodenum

Stages of Development

The duodenum develops from two embryological regions of the primitive gut.

• The foregut forms the segment of the duodenum proximal to the opening of the bile duct.
• The midgut gives rise to the portion distal to the bile duct opening.

During development, rotation of the stomach shifts the duodenum toward the right side of the abdominal cavity. Initially, the duodenum is suspended from the posterior abdominal wall by a dorsal mesentery known as the mesoduodenum. As development proceeds, most of this mesentery undergoes zygosis (fusion with the posterior abdominal wall), causing the duodenum to become secondarily retroperitoneal. Only a small proximal segment near the pylorus remains intraperitoneal and mobile.

Development of the Lumen

Rapid proliferation of endodermal epithelial cells temporarily obliterates the duodenal lumen around the 8th week of intrauterine life. The lumen subsequently reopens through recanalization, which is usually completed by the third month of fetal development.

Blood Supply

The dual embryological origin explains the arterial supply of the duodenum:

• The foregut-derived portion receives branches from the coeliac trunk.
• The midgut-derived portion is supplied by branches of the superior mesenteric artery.

Development Of Midgut

The primitive midgut extends from the cranial intestinal portal to the caudal intestinal portal. It remains attached to the posterior abdominal wall by a dorsal mesentery. During early development, the midgut communicates with the yolk sac through the vitelline duct (also called the vitellointestinal duct or yolk stalk). Its arterial supply is provided by the superior mesenteric artery.

The superior mesenteric artery divides the midgut into two functional segments:

• Prearterial (proximal) segment
• Postarterial (distal) segment

Derivatives of the prearterial segment:

• Distal part of the duodenum
• Jejunum
• Most of the ileum

Derivatives of the postarterial segment:

• Terminal part of the ileum
• Caecum
• Appendix
• Ascending colon
• Proximal two-thirds of the transverse colon.

Rotation of Gut

The midgut undergoes a total 270° counterclockwise rotation around the axis of the superior mesenteric artery during development. This rotation helps accommodate the rapidly growing midgut loop within the relatively small abdominal cavity.

Stages of Rotation

• The superior mesenteric artery divides the midgut into prearterial (proximal) and postarterial (distal) segments. Before rotation, the prearterial segment lies cranial to the artery in the midline, while the postarterial segment lies caudal to it. Rotation occurs in two stages: an initial 90° rotation followed by an additional 180° rotation.

1. First 90° Rotation

• During early development, the midgut rotates 90° counterclockwise around the superior mesenteric artery (viewed from the ventral side). This movement positions the prearterial segment on the right side and the postarterial segment on the left side. This stage is completed by the early eighth week and facilitates the gradual return of the herniated midgut loop into the abdominal cavity.

2. Remaining 180° Rotation

• After the midgut returns to the abdomen, it undergoes an additional 180° counterclockwise rotation. During this stage, the prearterial segment elongates to form the coils of the jejunum and ileum. These intestinal loops become positioned posterior to the superior mesenteric artery, while the artery lies anterior to the duodenum.
• Meanwhile, the postarterial segment, which gives rise to the caecum and appendix, shifts toward the right side of the abdomen. The caecal bud enlarges and descends to the right iliac fossa, reaching its final adult position. As a result of these rotational changes, the transverse colon passes anterior to the superior mesenteric artery.

Fixation of Gut

• Initially, all segments of the midgut are attached to the posterior abdominal wall by a dorsal mesentery. After the intestinal rotation is completed, certain portions of this mesentery undergo zygosis (fusion with the posterior abdominal wall). As a result, the duodenum, ascending colon, descending colon, and rectum become secondarily retroperitoneal, being covered by peritoneum only on their anterior surface.
• The remaining unfused mesentery persists and forms the mesentery of the small intestine, the transverse mesocolon, the sigmoid mesocolon, and the mesoappendix that supports the appendix.

Development Of Caecum And Appendix

• The caecum and vermiform appendix develop from a caecal bud, which appears as a dilation of the postarterial segment of the midgut loop. This bud becomes visible around the sixth week of intrauterine life.
• The proximal part of the caecal bud enlarges and differentiates to form the caecum, whereas the distal narrow portion persists as the vermiform appendix.
• During growth, the caecal bud forms two sacculations. The right saccule enlarges more rapidly than the left, causing the base of the appendix to shift toward the left side near the ileocaecal junction. This differential growth determines the final position of the appendiceal opening in the caecum.

Development Of Hindgut

The hindgut forms the terminal portion of the primitive gut and gives rise to several components of the large intestine. Its derivatives include:

1. Distal one-third of the transverse colon
2. Descending colon
3. Sigmoid colon
4. Rectum
5. Upper part of the anal canal

• These structures develop from the caudal segment of the embryonic gut tube and contribute to the formation of the distal gastrointestinal tract.
• Development of Urorectal Septum
• The distal part of the hindgut that communicates with the allantois forms a common cavity called the cloaca. During development, a mesodermal partition known as the urorectal septum grows downward and divides the cloaca into two parts: a ventral primitive urogenital sinus and a dorsal primitive rectum.
• The urorectal septum also separates the cloacal membrane into an anterior urogenital membrane and a posterior anal membrane. The cloacal membrane normally ruptures around the seventh week of intrauterine life, establishing external openings. The anal membrane is located at the upper end of the proctodeum.
• The primitive urogenital sinus later forms the urinary bladder and urethra, while the primitive rectum develops into the rectum and the upper part of the anal canal.
• Development of the urorectal septum involves two main components:
  1. The Tourneux fold, a vertical midline fold that grows caudally between the rectum and the urogenital sinus.
  2. The Rathke folds, which arise from the lateral walls of the cloaca and contribute to formation of the septum.

Development Of Anal Canal

The anal canal develops from two embryological sources:

1. The endodermal cloaca (distal part of the primitive rectum) forms the upper part of the anal canal, located above the pectinate line.
2. The ectodermal anal pit, known as the proctodeum, forms the lower part of the anal canalbelow the pectinate line.

Stages of Development

• The proctodeum appears as a depression of the surface ectoderm. At its base lies the anal membrane, which separates the developing anal canal from the exterior.
• Around the ninth week of intrauterine life, the anal membrane perforates, establishing communication between the hindgut and the external environment. The former site of this membrane later corresponds to the pectinate line of the anal canal.
• Thus, the pectinate line represents the junction between the endoderm-derived upper anal canal and the ectoderm-derived lower anal canal. Because of this dual origin, the arterial supply, lymphatic drainage, and nerve supply differ above and below the pectinate line.

Table 13.1: Development of anal canal

Feature	Above the Pectinate Line (Upper Anal Canal)	Below the Pectinate Line (Lower Anal Canal)
Embryonic origin	Endoderm	Ectoderm
Developmental source	Derived from the endodermal cloaca forming the primitive rectum	Derived from the proctodeum (anal pit)
Arterial supply	Superior rectal artery (branch of inferior mesenteric artery)	Inferior rectal artery (branch of internal pudendal artery)
Venous drainage	Superior rectal vein → portal system via inferior mesenteric vein	Inferior rectal vein → systemic circulation → inferior vena cava
Nerve supply	Autonomic (visceral); pain poorly localized	Somatic via inferior rectal nerve; pain well localized

Development Of Anterior Abdominal Wall

• The anterior abdominal wall consists of several layers, including the skin (epidermis and dermis), fascia, muscle layers, and the parietal peritoneum, which encloses the peritoneal cavity.

Stages of Development

• Initially, the embryo is trilaminar, consisting of three germ layers:
  1. Ectoderm
  2. Mesoderm—subdivided into paraxial, intermediate, and lateral plate mesoderm
  3. Endoderm
• The intraembryonic coelom divides the lateral plate mesoderm into two layers: the somatopleuric mesoderm and the splanchnopleuric mesoderm.
• During the fourth week, the embryo undergoes body folding, which includes the head fold, tail fold, and two lateral folds. These folds converge at the umbilical region on the ventral surface.
• As a result of this folding:
  1. A portion of the intraembryonic coelom becomes the peritoneal cavity.
  2. The somatopleuric layers from both sides fuse in the midline to form the connective tissue and muscular components of the anterior abdominal wall.
  3. The ectoderm develops into the epidermis covering the abdominal wall.
• A small unfused ventral region remains at the umbilicus, allowing passage of the umbilical vessels and forming the umbilical cord.

Derivatives Of Gut

The derivatives of the gut tube are listed in the Table 13.2.

Table 13.2: Derivatives of gut

Embryonic Gut Region	Major Derivatives
Foregut	Forms the pharynx, oesophagus, stomach, and the proximal duodenum up to the major duodenal papilla. Gives rise to pharyngeal pouches and their derivatives. Also produces the liver, gallbladder, biliary apparatus, and pancreas. The respiratory tract develops from a ventral foregut diverticulum. Contributes to part of the floor of mouth and the tongue.
Midgut	Develops into the distal duodenum, jejunum, ileum, caecum, appendix, ascending colon, and the proximal two-thirds of the transverse colon.
Hindgut	Forms the distal one-third of the transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal. Endoderm of this region also contributes to the epithelial lining of the urinary bladder and urethra through the cloaca.

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