INTRODUCTION:

          The primitive gut is formed as a result of the head-tail and lateral folds of the embryo and the resulting incorporation of the dorsal aspect of the yolk sac into the intra-embryonic coelum. The gut of a 4 week old embryo is closed at the cranial and caudal ends by the bucco-pharyngeal and cloacal membranes, respectively. The endodermal lining of the primitive gut (indicated in yellow in figure 1) gives rise to most of the epithelium and gland primordia found in the digestive system (hepatic diverticulum, pancreatic buds) and also to other primordia such as the lung buds and thyroid diverticulum, which will not be discussed here. The epithelium at the most cranial (lips) and most caudal (anal pit) ends of the gut are of ectodermal origin. This difference in embryologic origin can be readily appreciated upon gross anatomical examination of the endoderm-ectoderm junctions at the lips and anus. The other tissues forming the gut (e.g. connective tissue, muscle) are derived from splanchnic mesenchyme that surrounds the primordial gut. The gut is also intimately associated with a structure of mesodermal origin called the mesentery (indicated in blue in figure 1) that plays a fundamental role in both gut development and the normal function of the adult digestive system.

GENERAL PLAN OF THE GASTROINTESTINAL SYSTEM:

          The primitive gut is commonly divided into three parts: foregut, midgut and hindgut (see fig.1 and table 1). The foregut endoderm gives rise to the oesophagus (marked E in fig.1), stomach(S), proximal half of the duodenum (PD), liver and pancreas. The midgut endoderm gives rise to the distal half of the duodenum(DD), jejunum(J), ileum(I), cecum(C), appendix, ascending colon and the right 3/4 of the transverse colon. The hindgut endoderm develops into the left 3/4 of the transverse colon, the descending colon, sigmoid colon and the rectum(R) down to the ano-rectal line (the endoderm-ectoderm junction).

 

DIVISION	ARTERY	VEIN	LYMPHATICS	SYMPATHETIC	PARASYMPATHETIC
FOREGUT: Oesophagus Stomach Proximal half of duodenum (up to common bile duct (CBD)) Liver Pancreas	CELIAC ARETERY	PORTAL VEIN Spleenic vein Gastric vein	CELIAC NODES	CELIAC GANGLIA	VAGUS
MIDGUT: Distal half of duodenum (from CBD) Jejunum Ileum Appendix Cecum Ascending colon Right 3/4 of transverse colon	SUPERIOR MESENTERIC ARTERY	SUPERIOR MESENTERIC VEIN	SUPERIOR MESENTERIC NODES	SUPERIOR MESENTERIC GANGLIA	VAGUS
HINDGUT: Left 1/4 of transverse colon Descending colon All of rectum down to ano-rectal line	INFERIOR MESENTERIC ARTERY	INFERIOR MESENTERIC VEIN	INFERIOR MESENTERIC NODES	GANGLIA   HYPOGASTRIC PLEXUS	PELVIC SPLANCHNIC NERVES

          The autonomic innervation of the primitive gut reaches the gut parenchyma via the following route: a neuronal axon exits the spinal cord via the ventral root and travels along a mixed spinal nerve. It exits the spinal nerve via the white ramus (at the level of the sympathetic trunk) and passes through a paravertebral ganglion (a.k.a. sympathetic ganglion), without synapsing to any cell body in the ganglion. It then travels via a splanchnic nerve to reach a pre-vertebral ganglion (a.k.a. pre-aortic ganglion) where it synapses with ganglion cell bodies. Axons from this ganlion cells travel through a nerve to synapse with the target tissues in the gut.

THE MESENTERY:

          The primitive gut is enveloped by a mesentery that has a dorsal and a ventral aspect. The ventral mesentery degenerates during development with the exception of the foregut ventral mesentery, which develops into specialized structures (see foregut embryology). The dorsal mesentery has different names according to its position along the gut: mesoesophagus, mesogastrium, mesoduodenum, mesentery proper (distal duodenum, jejunum and ileum), mesocolon and mesorectum.
          The dorsal mesentery is formed by a double layer of mesothelium that suspends the gut from the dorsal wall of the foregut to the hindgut (see fig. 2). A layer of mesothelium lines the whole coelomic cavity (future peritoneal cavity) forming the parietal peritoneum, that lines the somatopleure (body wall), and the visceral peritoneum, that lines the splanchnopleure (gut wall composed of mucosa, submucosa and two muscle layers)(see fig.3).

          The cross-section through the foregut in figure 2 (section FG; fig.3) reveals that the involution of the ventral messentery of the foregut is prevented by the formation of the septum transversum. The septum transversum will contribute to the formation of the diaphragm and to mesenchyme in the foregut.
          Some of the mesenchyme in the mesogastrium agglomerates and develops into the spleenic primordium (see fig.1). This primordium is primed by Hematopoietic Stem Cells that induce the spleen's early hematopoietic function in the fetus. The area of the mesogastrium marked with the letter A in fig.1 corresponds to the mesogastrium that is found between the spleenic primordium and the primitive stomach. This mesogastrium develops in the adult into a ligament called the Gastro-spleenic Ligament. The mesogastrium indicated by the letter B in fig.1 is found between the spleen and the kidney in the adult and is called Lieno-Renal Ligament. The mesogastrium marked by the letter C in fig.1 forms a structure called Omental Bursa that in the adult develops into the Greater Omentum.
          Two parts of the mesentery proper form two important ligaments in the adult gut, namely, the Ligament of Treiz and the Phrenico-Colic Ligament. The Ligament of Treiz (a.k.a. Superior Retention Band or Suspensory Ligament of the Duodenum) is found at the duodeno-jejunal junction (see fig.1). The Phrenico-Colic Ligament (a.k.a. Inferior Retention Band) is found between the transverse and the descending colon at the spleenic flexture of the colon.

          As mentioned above, all the blood vessels, nerves and lymphatics reach the gut via the dorsal mesentery. Of special interest is the Neural Crest Cell (NCC) origin of the autonomic innervation of the gut parenchyma. Of the sympathetic innervation, the paravertebral and pre-aortic ganglia are derived from NCC. The parasympathetic innervation travels via the vagus and innervates postganglionic neurons named Auerbach's Plexus and Meissner's Plexus that are found within the gut wall and are derived from NCC (see fig.3). Auerbach's plexuses are found within the smooth muscle layers of the gut wall and are readily seen in histological sections.
          Another cell type thought to be derived from NCC are the Interstitial Cells of Cajal. These cells are heavily innervated and are thought to act as gut pacemakers.

          A section through the midgut (fig.4) reveals the internalized dorsal aspect of the yolk sac within the intraembryonic coelum. Both intraembryonic and extraembryonic coelum are formed during the lateral folding of the embryonic disc and are found within the umbilical space. In the adult, a remnant of the yolk sac can be found in 1-2% of people as an appendix called Meckel's Diverticulum. This diverticulum looks like a second appendix and can be surgically removed by mistake in appendicitis.

          A section through the hindgut (fig. 5) reveals the dorsal mesentery supporting the gut and enabling its movement within the peritoneal cavity. The ventral mesentery of the hindgut, as well as that of the midgut, involutes after embryonic folding (see also fig. 2).

          It is important to understand that the gut, in its primitive stage, is intraperitoneal. It's suspended in the peritoneal cavity by the dorsal mesentery which enables it to move left and right from the midline. As the gut increases in size and length during development, the dorsal mesentery also undergoes differential growth. During gut fixation, some parts of the gut eventually become attached to the posterior body wall and become secondarily retroperitoneal (as opposed to primarily retroperitoneal organs such as the kidney that develop retroperitoneally and stay that way). The gut segments that become secondarily retroperitoneal are the duodenum, the ascending colon and the descending colon.

 

                                                                                                                                                Continue to Foregut Embryology »

                                       Hugo Viladevall M.D., C.M. 2004
                                       © 2001 Molson Medical Informatics Project

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