|A personal message from Dr Mark Hill (May 2020)
| I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!
||This lecture introduces the early development of the Gastrointestinal Tract (acronym GIT).
Note that the oral cavity and pharynx will be covered in detail in the later Lecture and Practical on head and face development.
Historic drawing of the developing gastrointestinal tract (Kollman)
- Understanding of germ layer contributions
- Understanding of the folding
- Understanding of three main embryonic divisions
- Understanding of associated organ (liver, pancreas, spleen) development
- Brief understanding of mechanical changes (rotations)
- Brief understanding of gastrointestinal tract abnormalities
||Hill, M.A. (2020). UNSW Embryology (20th ed.) Retrieved September 23, 2020, from https://embryology.med.unsw.edu.au
||Moore, K.L. & Persuad, T.V.N. (2011). The Developing Human: clinically oriented embryology (8th ed.). Philadelphia: Saunders.
|Gastrointestinal Tract Movies
(Gestational age GA 5 weeks)
Trilaminar embryo 3 germ layers.
In week 3 the term "gastrulation" means "gut formation" and is the generation of the 3 germ layers.
- Endoderm - epithelium and associated glands, organs
- Mesoderm (splanchnic) - mesentry, connective tissues, smooth muscle, blood vessels, organs
- Ectoderm (neural crest) - enteric nervous system
Both endoderm and mesoderm will contribute to associated organs.
|Folding of the embryonic disc then occurs ventrally around the notochord, which forms a rod-like region running rostro-caudally in the midline.
In relation to the notochord:
- Laterally (either side of the notochord) lies mesoderm.
- Rostrally (above the notochord end) lies the buccopharyngeal membrane, above this again is the mesoderm region forming the heart.
- Caudally (below the notochord end) lies the primitive streak (where gastrulation occurred), below this again is the cloacal membrane.
- Dorsally (above the notochord) lies the neural tube then ectoderm.
- Ventrally (beneath the notochord) lies the mesoderm then endoderm.
The ventral endoderm (shown yellow) has grown to line a space called the yolk sac. Folding of the embryonic disc "pinches off" part of this yolk sac forming the first primitive gastrointestinal tract.
(Gestational age GA 6 weeks) Carnegie stage 11
|Embryo (stage 11 ventral view)
||Embryo (midline section)
- The mesoderm initially undergoes segmentation to form paraxial, intermediate mesoderm and lateral plate mesoderm.
- Paraxial mesoderm segments into somites and lateral plate mesoderm divides into somatic and splanchnic mesoderm.
- The space forming between them is the coelomic cavity, that will form the 3 major body cavities (pericardial, pleural, peritoneal)
- Most of the gastrointestinal tract will eventually lie within the peritoneal cavity.
- Mesoderm and Ectoderm Cartoons
Paraxial and Lateral Plate
(only the righhand side is shown, lefthand side would be identical)
Endoderm and splanchnic mesoderm at the level of the transverse septum (week 4)
- Stage 11 - hepatic diverticulum development
- Stage 12 - cell differentiation, septum transversum (mesoderm) forming liver stroma, hepatic diverticulum (endoderm) forming hepatic trabeculae
- Stage 13 - epithelial cord proliferation enmeshing stromal capillaries
- The liver initially occupies the entire anterior body.
- All blood vessels enter the liver (placental, vitelline; these also form the liver sinusoids) and then leave to enter the heart.
- During week 4 at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen.
- The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach.
- A second rotation (of 90 degrees) occurs on the longitudinal axis establishing the adult orientation of the stomach.
(GA 7 weeks)
- Beginning at week 5 endoderm in the GIT wall proliferates
- By week 6 totally blocking (occluding)
- over the next two weeks this tissue degenerates reforming a hollow gut tube.
- By the end of week 8 the GIT endoderm tube is a tube once more.
- The process is called recanalization (hollow, then solid, then hollow again)
- Abnormalities in this process can lead to abnormalities such as atresia, stenosis or duplications.
- Ventral mesentery lost except at level of stomach and liver.
- contributing the lesser omentum and falciform ligament.
- Dorsal mesentery forms the adult structure along the length of the tract and allows blood vessel, lymph and neural connection.
- At the level of the stomach the dorsal mesogastrium extends as a fold forming the greater omentum
- continues to grow and extend down into the peritoneal cavity and eventually lies anterior to the small intestines.
- This fold of mesentery will also fuse to form a single sheet.
- Mesoderm within the dorsal mesogastrium (week 5) form a long strip of cells adjacent to the forming stomach above the developing pancreas.
- Vascular and immune organ, no direct GIT function.
Foregut - thyroid
Week 8 - 10
(GA 10-12 weeks)
- neural crest migration into the wall forms enteric nervous system (peristalsis, secretion)
- midgut grows in length as a loop extending ventrally, returning as hindgut
- connected by dorsal mesentery
- rotates to form adult anatomical position (abnormalities of rotation)
- continued body growth "engulfs" the intestine by about week 11.
Normal intestinal rotation (note these are gestational age GA weeks)
Cloacal membrane (Week 4, Stage 12)
- Initially the cloaca forms a common urinary, genital, GIT space
- This is divided by formation of a septum into anterior urinary and dorsal rectal (superior Tourneux fold; lateral Rathke folds)
- hindgut - distal third transverse colon, descending and sigmoid colon, rectum.
- anal pit - distal third of anorectal canal (ectodermal)
Gastrointestinal Tract Divisions
|During the 4th week the 3 distinct portions (fore-, mid- and hind-gut) extend the length of the embryo and will contribute different components of the GIT. These 3 divisions are also later defined by the vascular (artery) supply to each of theses divisions.
- Foregut - celiac artery (Adult: pharynx, esophagus, stomach, upper duodenum, respiratory tract, liver, gallbladder pancreas)
- Midgut - superior mesenteric artery (Adult: lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, half transverse colon)
- Hindgut - inferior mesenteric artery (Adult: half transverse colon, descending colon, rectum, superior part anal canal)
Gastrointestinal Tract Blood Supply
|Small Intestine length (mm)
||Liver Growth (weight grams)
||1 to 124 grams (birth)
- Differentiates to form the hepatic diverticulum and hepatic primordium, generates the gall bladder then divides into right and left hepatic (liver) buds.
- Hepatic Buds - form hepatocytes, produce bile from week 13 (forms meconium of newborn)
- Left Hepatic Bud - left lobe, quadrate, caudate (both q and c anatomically Left) caudate lobe of human liver consists of 3 anatomical parts: Spiegel's lobe, caudate process, and paracaval portion.
- Right Hepatic Bud - right lobe
- Bile duct - 3 connecting stalks (cystic duct, hepatic ducts) which fuse.
- Early liver also involved in blood formation, after the yolk sac and blood islands acting as a primary site.
- Pancreatic buds - endoderm, covered in splanchnic mesoderm
- Pancreatic bud formation – duodenal level endoderm, splanchnic mesoderm forms dorsal and ventral mesentery, dorsal bud (larger, first), ventral bud (smaller, later)
- Duodenum growth/rotation – brings ventral and dorsal buds together, fusion of buds, exocrine function (postnatal function)
- Pancreatic duct – ventral bud duct and distal part of dorsal bud
- Pancreatic islets - endocrine function (week 10 onwards)
- Mesoderm within the dorsal mesogastrium form a long strip of cells adjacent to the forming stomach above the developing pancreas.
- The spleen is located on the left side of the abdomen and has a role initially in blood and then immune system development.
- The spleen's haematopoietic function (blood cell formation) is lost with embryo development and lymphoid precursor cells migrate into the developing organ.
- Vascularization of the spleen arises initially by branches from the dorsal aorta.
Spleen week 8 stage 22 embryo
Gastrointestinal Tract Abnormalities
|USA Selected Abnormalities (CDC National estimates for 21 selected major birth defects 2004–2006)
||Cases per Births (1 in ...)
||Estimated Annual Number of Cases
|spina bifida without anencephaly
|patent ductus arteriosus/common truncus
|transposition of the great vessels
|Tetralogy of Fallot
|atrial septal defects/ventricular septal defects
|hypoplastic left heart
|cleft palate without cleft lip
|cleft lip with and without cleft palate
|Esophageal atresia/tracheoesophageal fistula
|Rectal and large intestinal atresia/stenosis
|Reduction deformity, upper limbs
|Reduction deformity, lower limbs
|Trisomy 21 (Down syndrome)
- Links: Human Abnormal Development | CDC Birth Defects - Data & Statistics | USA Statistics | Victoria 2004 | USA 2006 | Europe 2010
|There are several types of abnormalities that impact upon the continuity of the gastrointestinal tract lumen.
- Interuption of the lumen (esophageal atresia, duodenal atresia, extrahepatic biliary atresia, anorectal atresia)
- Narrowing of the lumen (duodenal stenosis, pyloric stenosis)
- Incomplete recanalization resulting in parallel lumens, this is really a specialized form of stenosis.
- This abnormality is a very common (incidence of 1–2% in the general population) and results from improper closure and absorption of the vitelline duct during early development.
- vitelline duct (omphalomesenteric duct, yolk stalk) is a transient developmental duct that connects the yolk to the primitive GIT.
|Presents clinically in symptomatic malrotation as:
- Neonates - bilious vomiting and bloody stools.
- Newborn - bilious vomiting and failure to thrive.
- Infants - recurrent abdominal pain, intestinal obstruction, malabsorption/diarrhea, peritonitis/septic shock, solid food intolerance, common bile duct obstruction, abdominal distention, and failure to thrive.
Ladd's Bands - are a series of bands crossing the duodenum which can cause duodenal obstruction.
- Links: Intestinal Malrotation
|(intestinal aganglionosis, Hirschsprung's disease, aganglionic colon, megacolon, congenital aganglionic megacolon, congenital megacolon)
- A condition caused by the lack of enteric nervous system (neural ganglia) in the intestinal tract responsible for gastric motility (peristalsis).
- Neural crest cells
- migrate initially into the cranial end of the GIT.
- migrate during embryonic development caudally down the GIT.
- Aganglionosis typically at the anal end of GIT.
- increased severity as it extends cranially.
|Gastroschisis (omphalocele, paraomphalocele, laparoschisis, abdominoschisis, abdominal hernia) is a congenital abdominal wall defect which results in herniation of fetal abdominal viscera (intestines and/or organs) into the amniotic cavity.
Incidence of gastroschisis has been reported at 1.66/10,000, occuring more frequently in young mothers (less than 20 years old).
By definition, it is a body wall defect, not a gastrointestinal tract defect, which in turn impacts upon GIT development.
This indirect developmental effect (one system impacting upon another) occurs in several other systems.
- Omphalocele - appears similar to gastroschisis, herniation of the bowel, liver and other organs into the intact umbilical cord, the tissues being covered by membranes unless the latter are ruptured.
Final Thoughts- After Birth
Remember that the GIT does not function until after birth consider:
Links: Gastrointestinal Tract - Abnormalities
- allantois - An extraembryonic membrane, endoderm in origin extension from the early hindgut, then cloaca into the connecting stalk of placental animals, connected to the superior end of developing bladder. In reptiles and birds, acts as a reservoir for wastes and mediates gas exchange. In mammals is associated/incorporated with connecting stalk/placental cord fetal-maternal interface.
- amnion - An extraembryonic membrane]ectoderm and extraembryonic mesoderm in origin and forms the innermost fetal membrane, produces amniotic fluid. This fluid-filled sac initially lies above the trilaminar embryonic disc and with embryoic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membane led to the description of reptiles, bird, and mammals as amniotes.
- amniotic fluid - The fluid that fills amniotic cavity totally encloses and cushions the embryo. Amniotic fluid enters both the gastrointestinal and respiratory tract following rupture of the buccopharyngeal membrane. The late fetus swallows amniotic fluid.
- buccal - (Latin, bucca = cheek) A term used to relate to the mouth (oral cavity).
- buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract (GIT). This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomodeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane.
- cloacal membrane - Forms the external lower membrane limit (caudal end) of the early gastrointestinal tract (GIT). This membrane is formed during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane breaks down to form the initial "anal opening" of the gastrointestinal tract.
- coelom - Term used to describe a space. There are extraembryonic and intraembryonic coeloms that form during vertebrate development. The single intraembryonic coelom will form the 3 major body cavities: pleural, pericardial and peritoneal.
- foregut - The first of the three part/division (foregut - midgut - hindgut) of the early forming gastrointestinal tract. The foregut runs from the buccopharyngeal membrane to the midgut and forms all the tract (esophagus and stomach) from the oral cavity to beneath the stomach. In addition, a ventral bifurcation of the foregut will also form the respiratory tract epithelium.
- gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers ([E#endoderm|endoderm]/mesoderm/ectoderm) have just formed.
- gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo ([E#endoderm endoderm]/mesoderm/ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes.
- hindgut - The last of the three part/division foregut - midgut - hindgut) of the early forming gastrointestinal tract. The hindgut forms all the tract from the distral transverse colon to the cloacal membrane and extends into the connecting stalk (placental cord) as the allantois. In addition, a ventral of the hindgut will also form the urinary tract (bladder, urethra) epithelium.
- intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom.
- neuralation - The general term used to describe the early formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate, then neural groove, then neural tube. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. In humans, early neuralation begins in week 3 and continues through week 4.
- neural crest - region of cells at the edge of the neural plate that migrates throughout the embryo and contributes to many different tissues. In the gastrointestinal tract it contributes mainly the enteric nervous system within the wall of the gut responsible for peristalsis and secretion.
- pharynx - uppermost end of gastrointestinal and respiratory tract, in the embryo beginning at the buccopharyngeal membrane and forms a major arched cavity within the phrayngeal arches.
- somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/90 minutes until on average 44 pairs eventually form.
- splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall.
- stomodeum - (stomadeum, stomatodeum) A ventral surface depression on the early embryo head surrounding the buccopharyngeal membrane, which lies at the floor of this depression. This surface depression lies between the maxillary and mandibular components of the first pharyngeal arch.
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Cite this page: Hill, M.A. (2020, September 23) Embryology BGD Lecture - Gastrointestinal System Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGD_Lecture_-_Gastrointestinal_System_Development
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- © Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
- ↑ <pubmed>20549505</pubmed>| PMC2908440
- ↑ <pubmed>718292</pubmed>