Gastrointestinal System Development

• The primitive gut forms as a result of the developing embryo undertaking two folds: one in the cranial-caudal axis and the second in the lateral plane.
• At the end of four weeks both the cranial and caudal ends remain closed by membranes.
• Bucco-pharyngeal membranes become the future mouth.
• Cloacal membranes become the future anus.

Gut Division and Fate

• The gut divides into the foregut, midgut, and hindgut.
• Foregut becomes the oesophagus, stomach, proximal half of duodenum, liver, and pancreas.
• Midgut becomes the distal half of duodenum, jejunum, ileum, caecum, ascending, and 2/3 transverse colon.
• Hindgut becomes the 1/3 transverse, descending, sigmoid colon, and rectum.
• Mesenteries hold the primitive gut in position, these are tissues derived from the mesodermal layer.

Mesenteries

• Mesentery is a fold of membrane continuous with both the peritoneal layers, with abdominal organs suggested to develop from within it.
• Mesentery is either dorsal or ventral depending on its relative position to the gut tube.
• The mesentery suspends abdominal organs.
• The mesentery serves as the pathway for blood, innervation, and lymphatics to reach the abdominal organs.
• Ventral mesentery degenerates during development except for the region developing from the foregut.

Gut Rotation

• Gut rotation is a predictable phenomenon where gut loops occur due to differences in the growth rate of the dorsal mesentery
• Gut rotation shifts the stomach to the left and the liver to the right
• The innervation of structures shifts from midline to lateral, or dorsal to ventral.
• The portion of the mesentery effectively attaches abdominal organs to the posterior abdominal wall.
• Examples of mesentery include greater omentum and the mesentery of the small and large intestine.
• Ventral mesentery is only retained in the foregut region, with the falciform ligament and the lesser omentum as examples.
• The lesser omentum links the lesser curvature of the stomach to the back of the liver.
• The lesser and greater sacs are the result of organ rotation.
• The lesser sac is the space behind the stomach, while the greater sac is the rest of the peritoneal cavity.
• The lesser and greater sacs communicate via the epiploic foramen, also known as foramen of Winslow.

The Alimentary System

• The alimentary system is related to: ingestion, secretion of digestive fluids, mixing and propulsion, digestion (break down), absorption and defecation.

Basic Histology

• Serous is the outer membrane, enclosing body cavities, secreting lubricating fluid, and comprised of an outer epithelial layer and inner connective tissue with vessels.
• Muscular layer is longitudinal or circular, responsible for movement of lumen contents.
• Mucosal is the inner layer, lining internal organs or cavities exposed to external environment, secreting mucus, and involved in absorption.

GI Tract Divisions

• Oral cavity contains accessory organs (teeth, salivary glands, tongue and palate).
• Teeth assist in mechanical digestion.
• Palate is the roof of mouth; soft palate blocks nasal cavity during swallowing.
• Tongue performs taste and assists in mixing and swallowing.
• Salivary glands secrete saliva (dissolve foods).
• Pharynx directs food to oesophagus and blocks entry to trachea.
• Oesophagus a muscular tube about 25cm long.
• Oesophagus lies posterior to trachea
• pharynx bordesr the uppper oesophagus

Stomach

• Stomach also known as 'Gaster'
• Stomach is the most dilated part of alimentary canal
• Stomach lies bordered between the oesophagus and the small intestine
• Stomach is J-shaped and located in the abdomen below the diaphragm and under the heart.
• The stomach has 2 openings, 2 curvatures, and 2 surfaces.
• The role of the stomach is mechanical (muscle) and chemical (gastric juices) digestion.
• Gross anatomy of the stomach is comprised of: cardial orifice (link to the oesophagus), pyloric orifice (link to the small intestine), lesser curvature (on the superior border), greater curvature (on the inferior border), and pyloric and cardial sphincters
• The wall of stomach contains circular, longitudinal, and oblique layered muscle.

Small Intestine

• The small intestine is described as a convoluted tube.
• The small intestine extends from the pylorus of the stomach to the ileocaecal junction.
• The small intestine is 5 metres in length.
• The small intestine is divided into 3 main regions consisting of the duodenum, jejunum, and ileum.
• The small intestine is where digestion and absorption take place.

Duodenum

• The term 'Duodenum' means '12 fingers', i.e. about 20-25 cm.
• The duodenum lies in the retroperitoneal space.
• The duodenum can be described as a C shaped tube, with 4 parts: superior, descending, horizontal, ascending.
• The duct from the pancreas and gall bladder open in the duodenum and brings in bile and pancreatic juices.
• The lower edge ends at the duodenal – jejunal flexure.

Jejunum and Ileum

• The term 'Jejunus' means 'fasting' (as it was seen empty in death).
• The jejunum and ileum extend for at least 3 m, from the duodenal - jejunal flexure to the ileocaecal junction.
• The role of the jejunum is of absorption, provided by its large surface area, further increased by circular folds + villi + microvilli.
• The jejunum's motility is possible because It is attached only on one edge

Large Intestine

• Large intestine is the terminal division of the GI tract, from end of ileum to anus with length of about 1.5m.
• The term 'large' refers to a greater cross section compared to the small intestine.
• The large intestine can be described as M shaped, with 4 parts being the ascending, transverse, descending, and sigmoid sections.
• Both the ascending and descending sections are retroperitoneal.
• The role of the large intestine is to reabsorb water, form faeces, and to expel these (defecating).
• This process is controlled by sphincters in the rectum and anus, which close the outside world except when defecating.

Arteries of the Gut

• The superior mesenteric artery supplies the region that develops from the embryonic midgut, i.e. the distal part duodenum to proximal 2/3 of the transverse colon.
• The superior mesenteric artery's main branches are the jejunal and ileal branches, middile colic, right colic and ileocaecal.
• The inferior mesenteric artery supplies the region that develops from the embryonic hindgut.
• The inferior mesenteric artery supplies the distal 1/3 of the transverse colon to halfway down the anal canal.
• Main branches of inferior mesenteric artery are the left colic, sigmoid and superior rectal.