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Early GIT structure |
Later GIT structure |
(Images: modified from Gray's Anatomy) |
We have now reached Week 5 of development. Start by looking briefly at the overview of the Carnegie stage 13/14 embryo GIT from one end to the other.
Then work through the listed specific serial sections of the embryo identifying the GIT features. Alternatively step through the serial sections yourself identifying the tract, its associated mesentries, organs and spaces.
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Bifurcation of the pharynx into anterior respiratory and posterior oesophagous.
The stomach forming beneath the lung buds and adjacent to the developing liver.
Below the stomach the GIT has a large dorsal mesogastrium and finer ventral mesogastrium. Associated with the tract is the large portal blood vessel derived from the vitelline circulation.
At the bottom curvature of the embryo the mesentry association with the GIT shows extensive vitelline vessels running out through the umbilicus. The hindgut can then be seen, ending at the common urogenital sinus, the cloaca.
More? see Section Details
Also, if you are having difficulties understanding the 3D relationships, then this link will show you the Carnegie stage 13 embryo relationship of upper GIT to the developing respiratory system and the 3D reconstruction animations of serial images of the Stage 13/14 Embryo: Gastrointestinal Tract (More? 3D Model Movies)
The stomach initially appears at this stage (5 weeks) as a dilatation of the GIT in the foregut, which over the next 2 weeks will continue to expand to a fusiform structure and differential growth will it rotate in both the longitudinal and the horizontal planes.
Differential growth of the ventral and dorsal stomach walls leads to establishing a lesser and greater curvature.
(Animation based upon images from Human Embryology, Larsen)
Key: Yellow - endoderm of stomach. Red - mesentry (ventral at front and dorsal at back). The dorsal mesogastrium will form the greater omentum and the ventral mesogastrium will form the lesser omentum. Blue - vagus nerve branches (left and right). A 90 degree rotation (during week 7) brings the left vagus anteriorly (to the front) and carries the right vagus posteriorly (to the back).
These combined rotations position the stomach in its adult orientation and movement of the mesenteries also moves the developing liver to the right and generates the greater omentum and lesser sac (see animations below).
Lesser Sac Development
(Animation based upon images from Human Embryology, Larsen)
Key: Yellow - endoderm of stomach. Orange - liver developing in ventral mesogastrium. Red - spleen developing in dorsal mesogastrium.
Development of Lesser Sac and Greater Omentum
(Animation based upon images from Human Embryology, Larsen)
Below are listed a "slice by slice" detailed description of GIT related structures visible in each section some of which will be explained in the Practical by the demonstrator. Clicking on the section number will open the full image.
B1,B2: Pharynx. Crest in ventral floor of pharynx formed by fusion of 3rd pharyngeal arches = hypopharyngeal eminence (precursor of root of tongue). Rathke's pouch = rudimentary adenohypophysis.
B3: Rudimentary thyroid ventral to aortic sac (also seen in B2, ventral to the hypopharyngeal eminence).
B4: Caudal pharynx compressed dorsoventrally.
B7: Glottis drawn off from pharyngeal foregut. Nasal placodes. Pulmonary arteries.
C1,C2: Beginning of trachea and oesophagus with dense mesenchyme. R. nasal pit.
C3,C4: Common cardinal vein in the posterior wall of the intraembryonic coelom - the pleuropericardial folds which contribute later to the formation of the pleura and pericardium. L. nasal pit. In C4, junction of R common cardinal vein with dorsal wall of sinus venosus.
C5: Smaller oesophagus, expanding trachea. Note ventral anchoring of attachment site is at the most cranial extension of the septum transversum. Note also that this attachment now divides the intraembryonic coelom around the trachea into two canals, the L and R pleuro (pericardio-peritoneal) canals. (Canals are lined by coelomic mesothelium and are continuous with whole I-E coelom - they will be referred to hereafter simply as coelomic canals). Note the pleuroperitoneal fold on the medial side of the R common cardinal vein - this fold will form part of the diaphragm.
C5,C6: Lateral extension of pulmonary mesenchyme is moulded to shape of coelomic canals. Oesophagus lumen obliterated (common site of oesophageal atresia and/or tracheo-oesophageal fistula). Prominent R pleuroperitoneal fold.
C7: Note dorsal extent of coelomic canals. Oesophagus lumen reappears caudal to bifurcation. Distinct R (smaller on L) pleuroperitoneal fold below the common cardinal vein.
Dl: Oesophagus/stomach junction. Coelomic canals.
D2: Ovoid stomach with developing space of the lesser sac on R. Dorsal and ventral attachments of the mesenchyme are now known as dorsal and ventral mesogastria. Coelomic canals.
D3: Rotation of stomach (seen from above) to R side. Note change in outline of coelomic canals due to presence of liver. Lesser sac. Note thick mesothelium lining the coelom along Ledge of stomach, the primordium of the spleen and greater omentum along greater curvature. Liver embedded in septum transversum (ventral border of septum transversum contributes to diaphragm).
G7: Caudal pharynx (extending laterally, ventral to dorsal aorta - cf B4). Stomach, mesentery.
G6: Narrow oesophagus. Tracheal bifurcation dorsal to sinus venosus.
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Finally, below are listed some links to Embryo Images by Drs. Kathleen K. Sulik and Peter R. Bream Jr. which has a series of scanning electron microscope images related to GIT development.
If you have some time before we go on to the next page in the Practical, look at the 3 dimensional embryo shape and the cavities associated with early GIT development.
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Link to next page in this GIT Practical - Late Embryo Looks at the appearance of the GIT at Carnegie stage 22, a late embryonic stage. |
Use the alphabetical list below to find definitions of terms that are new to you or use the Google search window to search UNSW Embryology site.
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
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. (More? Placenta | Week 2 Notes | Gastrointestinal Tract Notes)
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. (More? Placenta Notes | Week 2 Notes)
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). (More? GIT Notes)
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 (stomadeum) 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. (More? GIT Notes | Face Notes | Head Notes)
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. (More? buccopharyngeal membrane | GIT Notes)
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. (More? Coelom Notes)
extrahepatic bile ducts - (EHBDs) Gastrointestinal term used to describe the liver and gall bladder hepatic, cystic, and common bile ducts. (More? GIT Notes - Liver | GIT Notes - Gall Bladder)
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. (More? Gastrointestinal Tract - Stomach | Gastrointestinal Tract Notes | Respiratory Notes)
fusiform - (Latin, fusus = spindle-shaped) widest near the middle and tapering in toward both ends, (back in the days of smoking) often described as "cigar-shaped". For example, used to describe the shape of the early developing stomach or the shape of smooth muscle cells. (More? GIT Notes)
gall bladder - The septum transversum differentiates to form the hepatic diverticulum and the hepatic primordium, these two structures together will go on to form different components of the mature liver and gall bladder. In the adult, the gall bladder is a site of bile salt storage and concentration, to then be released into the small intestine where they act to solubilize dietary lipids by their detergent effect. Bile salts are a cholesterol derivative (breakdown product). (More? Gastrointestinal Tract - Gall Bladder | Liver Notes | Gastrointestinal Tract Notes)
glottis - (Greek, = larynx) the boundary between pharynx to the larynx and consists of the vocal folds and their associated intervening space. (More? Gastrointestinal Tract Notes | Respiratory Notes)
greater omentum - Splanchnic mesoderm peritoneal fold extending from the greater curvature of the stomach and hanging ventrally down "like an apron" in the peritoneal cavity over the small intestine. It forms initially in the embryo and fetus as a loop of the dorsal mesentery, which later fuses to form a single sheet attached to the posterior body wall. The lesser omentum is a smaller ventral peritoneal fold extending from lesser curvature of the stomach to liver. (More? GIT Notes)
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. (More? Gastrointestinal Tract Notes | Urogenital Notes)
hypopharyngeal eminence - (hypobranchial eminence) An early embryonic structure in the developing head. A narrow midline mesodermal (mesenchymal) exension lying within the floor curve of the developing pharynx. Fusion of 3rd pharyngeal arches and precursor of root of tongue. Early developing thyroid cells also migrate into this structure as cords of cells. (More? Head Notes | Endocrine Development - Thyroid)
lesser omentum - The stomach has 2 peritoneal folds attached to each of the stomach curvatures. The lesser omentum is the peritoneal fold extending from lesser curvature of the stomach to liver. The greater omentum is the peritoneal fold extending from the greater curvature of the stomach to the colon, and hanging down over the small intestine. (More? GIT Notes)
mesentery - The tissue fold attaching gastrointestinal tract to posterior abdominal wall in which blood vessels, lymph and nerves run. Developmentally derived from lateral plate mesoderm forming splanchnic mesoderm which then forms the posterior mesogastrium. (More? mesogastrium | GIT Notes)
meso - A prefix term used to show an association with the mesentery (e.g. mesogastrium, mesodudenum, mesocolon)
mesogastrium - The developmental term for the splanchnic mesoderm forming early mesenteries (dorsal and ventral) that support the developing gastrointestinal tract. The majority of the ventral mesentery is developmentally lost at the level of the midgut and the dorsal mesentery remains in the adult, through which blood vessels, nerves and lymph connects to the gastrointestinal wall. Note that specific visceral organs also develop within each mesogastrium. (More? GIT Notes)
midgut - The middle of the three part/division (foregut - midgut - hindgut) of the early forming gastrointestinal tract. The midgut is initially connected on the ventral embryo surface to the external yolk sac by a yolk stalk, a narrow tubular connection. The midgut forms all the tract from beneath the stomach (duodenum, small intestine and large intestine) to the distral transverse colon. The midgut develops as an external loop "herniated" ventrally, until early fetal growth of the body wall recaptures this external loop, which also undergoes a rotation about the superior mesenteric artery to establish the adult anatomical position. (More? Gastrointestinal Tract - Intestine | Gastrointestinal Tract Notes)
neural crest - A cell region at edge of neural plate, then atop the neural folds, that remains outside and initially dorsal to the neural tube when it forms. These paired dorsal lateral streaks of cells migrate throughout the embryo and can differentiate into many different cell types (= pluripotential). Those that remain on the dorsal neural tube form the sensory spinal ganglia (DRG), those that migrate ventrally form the sympatheitic ganglia. Neural crest cells also migrate into the somites and regions throught the entire embryo. (More? Neural Crest Notes)
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. (More? Head Notes | GIT Notes | Carnegie stage 13 pharynx)
pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity. (More? Coelom Notes)
septum transversum - (transverse septum) A mesodermal region in the early embryo. Identified externally as the junctional site between amnion and yolk sacs, and internally (within the embryo) lying directly beneath the heart and at the foregut/midgut junction. This ventro-dorsal "plate" of mesoderm contributes several structures including: the central tendon of diaphragm and some of the liver. The transverse septum has an important structural role in early embryonic development and is pierced by the gastrointestinal tract. (More? Gastrointestinal Tract Notes | Liver Notes | Respiratory Development - Diaphragm)
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. (More? Week 3 | Gastrointestinal Tract Notes | Heart Notes )
spleen - The spleen develops within the gastrointestinal tract dorsal mesogastrium mesenchyme. With folding it is located on the left side of the abdomen and has a role initially in blood (haematopoisis, blood cell formation) and later immune system development. The spleen's haematopoietic function is lost with fetal development and lymphoid precursor cells migrate into the developing organ. (More? Spleen Notes | Gastrointestinal Tract Notes)
stomach - Gastrointestinal tract (GIT) foregut organ that has a major function in digestion. In humans, during week 4 initially as a dilatation of the foregut lying behind the heart. Differential growth of the ventral and dorsal walls establishes the greater curvature of the stomach and second rotation (of 90 degrees) occurs on the longitudinal axis establishing the adult anatomical orientation of the stomach. (More? Gastrointestinal Tract - Stomach | Gastrointestinal Tract Notes)
transverse septum - (septum transversum) see septum transversum a mesodermal region in the early embryo. (More? Gastrointestinal Tract Notes | Liver Notes | Respiratory Development - Diaphragm)
vagus - (Latin, vagus = wandering) cranial nerve X (CN X) A mixed nerve that leaves the head and neck to innervate gastrointestinal tract (pharynx, esophagus, stomach) respiratory tract (larynx, lungs), cardiac (heart) and abdominal viscera. This mixed nerve has sensory, motor and autonomic functions of viscera (glands, digestion, heart rate).
vitelline - (Greek, vitelline = yolk) | entry to be edited | refers to the yolk sac and is used to describe the blood vessels associated with the yolk sac (vitelline arteries and vitelline veins). (More? Heart Notes)
vitelline duct - (yolk stalk) Is a narrow endodermal channel between the yolk sac and the developing mid-gut. An abnormality associated with this duct failing to regress is called Meckel's diverticulum. (More? Gastrointestinal Tract Notes | Gastrointestinal Tract Abnormalities)
yolk stalk - Tube-like extension of the yolk sac which lies outside the embryo with the midgut inside the embryo. The endodermal lining is continuous with the endoderm of the gastrointestinal tract. (More? Week 2 Notes | Gastrointestinal Tract Notes | Cardiovascular System - Blood)
On this third Practical page are a number of images of the whole week 5 (stage 13/14) embryo and sections through this embryo to see the internal structure and cavities that have now developed.
There are also animations to help with the concept of folding, these can be played back and forth by dragging the control at the bottom of the movie.
The embryo at this stage is curved into a tight "C" shape, so in the head and tail region the embryo is cut twice by these relatively horizontal cross-sections. This also means that as we step down through the body of the embryo, where visible, the righthand second cut is moving in the opposite direction.
These notes and linked materials have been prepared for Educational purposes only.
Next Mark Hill Practical:
Sexual Differentiation covers early stages of male female development differences.
Movie of Human Embryo Growth (this shows a human embryo growing, all images are to scale)
Movie of Mouse Embryo Growth (this shows a mouse embryo growing)
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