We have now reached late embryonic development. Start by looking briefly the process of how the definitive GIT tube is formed and then at the overview of the Carnegie stage 22 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. Note you should also be comparing the GIT appearance with the earlier embryonic (13/14) Carnegie stage.
Observe:By the end of week 8 the GIT endoderm tube is a tube once more. Beginning at week 5 endoderm in the GIT wall proliferates to the extent of totally blocking (occluding) by week 6, over the next two weeks this tissue degenerates reforming a hollow gut tube. The process is called recanalization (hollow, then solid, then hollow again), abnormalities in this process can lead to duplications or stenosis.
(Animation based upon images from Human Embryology, Larsen)
In week 9 (early fetal) the endoderm of this now hollow tube differentiates into the mucosal epithelium.
Splanchnic mesoderm will form the submucosa connective tissue and smooth muscle (circular and longitudinal) layers.
Neural crest cells migrate into this tissue and will form the nerve plexus innervation.
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| If you are having difficulties understanding the 3D relationships, then this link will show you the 3D reconstruction animations of serial images of the Stage 22 Embryo: Gastrointestinal Tract (More? 3D Model Movies) |
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. The sections start at the level of the liver, as the upper parts (head, neck) of the GIT will be covered in your Head Development practical. Clicking on the section number will open the full image.
E6: Liver. Ductus venosus. Cardio-oesophageal junction (cf. E5). Inferior vena cava.
E7: Stomach body, with mucosa, submucosa and muscularis externa. Lesser sac. Lesser omentum. Pyloroduodenal junction. Folded duodenal mucosa. Inferior vena cava. Portal vein. Hepatic ducts. Gallbladder.
F1: Stomach body. Spleen. Pyloric canal. Duodenum. Portal vein. Pancreas. Small intestine loop (jejunum) cut tangentially, ventral to liver.
F2,F3: Stomach, spleen. Superior mesenteric artery. Superior mesenteric vein crossing cranial to body of pancreas. Tail of pancreas. Duodenum. Small intestinal loop herniating from abdominal cavity into the coelom of the umbilical cord (remnant of extra-embryonic coelom).
F4: Greater curvature of stomach (tangential section). Lesser sac. Greater omentum. Duodenal/jejunal junction. Note colon (small lumen, darkly-staining wall) and its mesocolon. Note the sections of small and large intestine within the umbilical cord coelom and their mesenteries. Note the thickened jelly to one side of the umbilical cord, containing umbilical vein and R umbilical artery.
F5: Lesser sac. Greater omentum. Duodenum. Jejunum (cut twice with mesentery in between). Colon and mesocolon.
F6: Both umbilical arteries now inside abdominal cavity with urachus between them. Greater omentum and lesser sac. Jejunum with mesentery. Colon with mesocolon. Three layers of abdominal muscles.
F7: Umbilical cord containing umbilical arteries and small dark allantois. Umbilical cord
coelom containing mainly, small intestinal loops with their mesentery. In abdominal cavity:
colon with mesocolon, jejunum. Greater omentum and lesser sac.
G1,G2: Umbilical cord and coelom containing small intestine loops. Knees. Bladder with
umbilical arteries either side. Colon and mesocolon. Jejunum (G1 only).
G3: Bladder. Rectum. Umbilical arteries arising from common iliac arteries.
G4: Rectum.
G5: Recto-anal junction with rectovesical pouch of peritoneal cavity.
G6-G7: Anal canal with triangular lumen.
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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 and structures associated with late embryonic GIT development.
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Link to next page in this GIT Practical - Fetal Looks at the appearance of the GIT at the Fetal 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 |
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)
duodenum - (Latin, duodenarius = twelve, due to approximate length of 12 finger-widths) First part of small intestine lying between the end of the stomach (pyloric valve) and the jejunum. (More? Gastrointestinal Tract Notes)
enteric nervous system - (ENS) Gastrointestinal tract neural network located within the tract wall that locally controls and coordinates intestinal functions (motility, epithelial secretion and blood flow) derived mainly from the neural crest. (More? Neural Crest 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)
herniated - The discription of the process of protrusion of an organ through a weak spot in the surrounding tissue. In normal development, herniated midgut, describes the gastrointestinal tract growth outside the abdominal wall prior to body wall growth. In abnormal development, abnormal protrusion of organs in the diaphragm, abdominal or groin areas (hiatal hernias or inguinal hernias). Occurs normally in the development of the gastrointestinal tract when the midgut is initially herniated at the umbilicus during embryonic development.
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)
ileum - Third part of small intestine lying between the jejunum and large intestine. (More? Gastrointestinal Tract Notes)
islets of Langerhans - A small cell clusters spread throughout the exocrine pancreas with an endocrine (hormonal) function. (More? Endocrine Development - Pancreas)
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)
meso - A prefix term used to show an association with the mesentery (e.g. mesogastrium, mesodudenum, mesocolon)
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)
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)
omphalomesenteric duct - (vitelline duct) A connection through the umbilicus between the yolk sac to the primitive gut that disappears at 8 to 9 weeks of gestation. Many developmental abnormalities are associated with failure of the vitelline duct to resorb, most commonly Meckel's diverticulum. (More? GIT Abnormalities | GIT Movies | Pubmed omphalomesenteric duct malformations)
pancreas - The gastrointestinal tract associated organ with both exocrine (pH change and digestive enzyme secretion) and endocrine (hormone secretion) functions. In humans, the pancreas develops at the foregut/midgut junction (the septum transversum) and initially form connected to the gastrointestinal tract as two pancreatic buds (dorsal and ventral endoderm) which later fuse to form a single organ. The pancreas exocrine function (alkylate acidic stomach contents and amylase protein digestion) begins mainly fter birth. The endocrine function (alpha cell - glucagon, delta cell - somatostatin, beta cell - insulin) can be measured from 10 to 15 weeks onward. (More? Gastrointestinal Tract - Pancreas | Endocrine Development - Pancreas)
peritoneal cavity - The anatomical body cavity in which the lower body organs lie: intestines, liver, bladder, uterus, ovary. The peritoneal cavity forms initially from two separate regions of the early intraembryonic coelom (formed in the lateral plate mesoderm), which with embryo folding, fuse to form a single cavity. Note the single intraembryonic coelom forms all three major body cavities: pericardial, pleural, peritoneal. (More? Coelomic Cavity Notes | Gastrointestinal Tract Notes)
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)
stenosis - Term used to describe an abnormal narrowing, usually in relation to a tube. For example, blood vessel, gastrointestinal tract or respiratory tract. (More? Gastrointestinal Tract Abnormalities)
urorectal septum - (URS) The structure which develops to separate the cloaca (common urogenital sinus) into an anterior urinary part and a posterior rectal part. (More? Urogenital Notes)
urorectal septum malformation - The abnormalities associated with the urorectal septum (URS) and urogenital organs due to developmental abnormality. (More? Urogenital Abnormalities | Urogenital Notes)
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 fourth Practical page are a number of images of the whole week 8 (stage 22) 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 GIT calalization, these can be played back and forth by dragging the control at the bottom of the movie.
The embryo at this stage is now straighter and sections near the pelvis show the embryo with his legs forward and bent at the knee.
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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