UNSW Embryology

 ANAT2341 - Embryology - Lecture 9

© Dr Mark Hill (2009)

Acknowledgements

Introduction

Stage 11 Embryo - the developing foregut, beginning at the buccopharyngeal membrane and running behind the developing heart to the transverse septum.

Lecture Notes under Development (notice removed when completed) This lecture is an introduction to the events in Gastrointestinal Tract (GIT) development. If you are interested in further reading, I have also included below links to more detailed textbooks with further information and images. Please note this additional information is not necessarily examinable, but may be useful if you have not previously studied biology.

Looping cartoon overview of endoderm (yellow) folding to form the gastrointestinal tract, yolk stalk and yolk sac.

Page Links: Introduction | Lecture Slides | Lectopia Audio | Lecture Objectives | UNSW Embryology | Comments | Online Books | Internet Links | Next | Glossary | Terms |

Lecture Slides

These links are to PDF versions of the Lecture slides: 1 slide/page viewing 51 pages | 4 slide/page printing 13 pages | 6 slide/page printing 9 pages

Lectopia Audio link to the audio recording of Lecture 09 Gastrointestinal Tract

The online text Online Books and UNSW Embryology links listed on this page worth reviewing for this lecture.

Lecture Objectives

• Understanding of germ layer contributions to the gastrointestinal tract
• Understanding of three main GIT embryonic divisions
• Understanding of associated organ development (liver, pancreas, spleen)
• Brief understanding of mechanical changes (rotations) during GIT development
• Brief understanding of gastrointestinal abnormalities

Lecture Summary

The following text is extracted and modified from lecture slides and should be used as a "trigger" to remind you of key concepts.

• Lecture Overview Gastrointestinal Tract
  • Trilaminar embryo contribution
    • Endoderm, splanchnic mesoderm, neural crest
  • Yolk sac, fore-, mid-, hind-gut
  • Membranes, oral, cloacal
  • Folding, rotation, herniation
  • Mesentery, blood supply
  • Tract
    • Esophagus, stomach, small intestine (ileum), large intestine (colon)
  • Associated organs
    • Liver, gall bladder, pancreas, spleen
• Textbook References
  • Human Embryology (2nd ed.) Larson, Ch 9 p229-260
  • The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud, Ch 12 p271-302
  • Before We Are Born (5th ed.) Moore and Persaud, Ch13 p255-287
  • Essentials of Human Embryology Larson, Ch 9 p123-146
  • Human Embryology Fitzgerald and Fitzgerald, Ch19,20 p119-123
• Online References
  • UNSW Embryology
    • GIT Notes http://embryology.med.unsw.edu.au/Notes/git.htm
    • GIT Lecture http://embryology.med.unsw.edu.au/Science/ANAT2341lecture09.htm
    • GIT Laboratory (tomorrow) http://embryology.med.unsw.edu.au/Science/ANAT2341lab05.htm
  • Embryo Images - GIT Development http://www.med.unc.edu/embryo_images/unitdigest/digest_htms/digesttoc.htm
  • Developmental Biology 6th ed. Gilbert, Scott F. http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowTOC&rid=dbio.TOC&depth=2
• Background Reading
  • Endoderm Development
  • Mesoderm Development
    • Lateral plate mesoderm
    • Splanchnic mesoderm
    • Intraembryonic coelom
  • Head Development
    • Formation of pharynx
  • Neural Crest
    • Nerve plexus
• GIT System
• Trilaminar Contribution
• Overview - simple tube converted into a complex muscular, glandular and duct network with associated organs
• Endoderm - epithelium of tract, glands
• Mesoderm - Splanchnic - muscular wall, connective tissue
• Ectoderm - Neural Crest - Muscular wall neural plexus
• Gastrulation
• Endoderm Layer
• Early cells migrate adjacent to epiblast line Primary Yolk sac to form Yolk Sac
• Mesoderm Layer
• Later cells migrate intraembryonic mesoderm migrate laterally, rostrally, caudally continues until week 4
• ECM adhesive pathways
• Membranes- Ectoderm/Endoderm
• Rostrocaudal axis ectoderm and endoderm without mesoderm form ends of gut tube
• Buccopharyngeal membrane Rostral (head) breaks down to form oral cavity
• Cloacal membrane Caudal (tail) breaks down to form anus, urinary and genital openings
• Germ Cell Layers
• Ectoderm - nervous system, epidermis epithelia
• Mesoderm - muscle, skeleton, connective tissue
• Endoderm - gastrointestinal tract epithelia, respiratory tract, endocrine
• Movie: Embryo Folding
• Movie: Endoderm Development
• Embryonic Endoderm
• Forms the linings of two tracts (tubes)
• Digestive tract
• Buds from this tube form the liver, gallbladder, and pancreas
• Respiratory tract
  
• outgrowth of the digestive tract
  
• bifurcates into two lungs
  
• Pharynx
  
• common chamber in anterior region
  
• outpockets generate
  
• tonsils, thyroid, thymus, parathyroid glands
  
• Movie: Yolk Sac / Endoderm
  
• GIT- Early Parts 4th week- 3 parts
• Foregut
  
• Midgut
  
• Hindgut
  
• 2 specialized regions - buccopharyngeal membrane. cloacal membrane
  
• apposition of ectoderm with endoderm
  
• cloaca not just GIT
  
• Early GIT
  
• Oral Cavity (mouth)
  
• formed following breakdown of buccopharyngeal membrane
  
• oropharyngeal, oral
  
• formed mainly by pharynx lying within pharyngeal arches
  
• opening of GIT means that it contains amniotic fluid
  
• also swallowed later in development
  
• Endoderm / Pharynx
  
• Visceral Mesoderm Patterning
  
• Foregut Bifurcation
  
• Movie: GIT Canalization
  
• week 5 - endoderm in GIT wall proliferates
  
• week 6- totally blocked (occluding)
  
• over next two weeks tissue degenerates reforming a hollow gut tube
  
• week 8- GIT endoderm a tube again
  
• Process is called recanalization
  
• hollow, then solid, then hollow again
  
• Abnormalities
  
• lead to duplications or stenosis
  
• Liver
  
• Arises at embryonic junction
  
• externally is where ectoderm of amnion meets endoderm of yolk sac
  
• internally is where foregut meets midgut
  
• Septum transversum
  
• mesenchymal structure of transverse septum provides a support within which both blood vessels and liver begin to form
  
• Early Liver Development
  
• Liver Bud differentiates to form: hepatic diverticulum, hepatic primordium
  
• generates gall bladder then divides into right and left hepatic buds
  
• 3 connecting stalks
  
• cystic duct
  
• hepatic ducts fuse to form bile duct
  
• Hepatic Bud
  
• Left Hepatic Bud
  
• left lobe
  
• quadrate and caudate
  
• both q and c anatomically Left
  
• Right Hepatic Bud
  
• right lobe
  
• Haemopoiesis
  
• Embryonic liver involved in blood formation
• after yolk sac acting as primary site
• Liver Structural Origins
  
• Hepatic Buds
  
• hepatocytes
  
• produce bile from week 13
  
• meconium of newborn
  
• Vitelline Veins
  
• sinusoids
  
• Mesenchyme
  
• connective tissue
  
• Kupffer cells
  
• Stomach Development
  
• During week 4
  
  • tube begins to dilate
    
  • forming an enlarged lumen in the tube
    
• Dorsal border grows more rapidly than ventral
  
  • establishes greater curvature of stomach
    
• Second rotation (90 degrees) occurs on longitudinal axis
  
• Movie: Stomach Development
  
• Movie: Lesser Sac Development
  
• Embryonic Mesogastrium
  
• Pancreas Development
  
• arises from 2 sources
  
• the hepatic diverticulum (ventral)
  
• the duodenum (dorsal)
  
• differentiates to establish specific cells for endocrine and exocrine function
  • MH - Pancreas endocrine function covered in Endocrine Development Lecture
    
• Pancreas Development
  
• Pancreas Epithelial Migration
  
• Exocrine/Endocrine Differentiation
  
• Direct interactions
  
  • Stimulation- solid lines with arrows
    
  • Inhibition- blunt lines
    
• Indirect interactions
  
  • dashed lines
    
• Spleen Development
  
• spleen arises in week 5
  
• within dorsal mesogastrium as proliferating mesenchyme
  
• Hemopoietic function
  
  • Cells arise from yolk sac wall
    
  • spleen generates red and white cells in 2nd trimester
    
• Midgut Herniation
  
• Much of midgut is herniated at umbilicus
  
• external to abdomen through development
  
• until week 11
  
• rotation of midgut occurs to place elongating GIT in correct abdominal position with its associated mesentry
  
• Dorsomesogastrium
  
• Midgut Herniation
  
• Movie: GIT Rotation
  
• Movie: Cloaca
  
• Common urogenital sinus
  
• Vitelline Blood Vessels
  
• cover entire surface of yolk sac
  
• connect to embryo through yolk stalk
  
• Arteries
  
• arises from dorsal aorta
  
• contribute to adult GIT arteries
  
• Veins
  
• empty into sinus venosus
  
• contribute to the adult portal system
  
• GIT Blood Supply
  
• Foregut - celiac artery
  
• Midgut - superior mesenteric artery
  
• Hindgut - inferior mesenteric artery
  
• GIT Mesentries
  
• common dorsal mesentry
  
• generate the mesentries of the GIT
  
• GIT Mesentries
  
• ventral mesentry
  
• contributes to lesser omentum and falciform ligament
  
• Stage 13/14 GIT Overview
  
• Additional Slide:
  
• Embryo, Yolk Sac and Placenta
  
• Additional Slide:
  
• Endoderm / Pharynx
  
• Additional Slide: Gray’s Anatomy

UNSW Embryology

The following links are to UNSW Embryology additional resources that provide further background information on the Lecture topics. Note that not all information found on these additional links is considered examinable and the lecture slides and laboratory classes should be used as your initial guide for course theory content. Links: Gastrointestinal Tract Notes | GIT Overview | GIT Abnormalities | GIT Stage 13/14 | GIT Stage 22 | GIT Selected stage 22 | GIT Histology | GIT Liver | GIT Gall Bladder | GIT Spleen | GIT Pancreas | GIT Stomach | GIT GIT Folding | GIT Intestine | GIT Molecular | GIT Tongue | GIT Taste | GIT Postnatal | GIT Milk | GIT WWW Links Timeline - Embryonic Week 3 | Carnegie Stages | ANAT2341 Lab 05 The search window below allows a search within the UNSW Embryology website.

Coordinator Comments

Dr Mark Hill

Folding of the trilaminar embryonic disc generates an embryo structure resembling a hollow tube or cylinder lined by endoderm and having all three layers forming its wall. This endoderm/splanchnic mesoderm is the early gastrointestinal tract (GIT), pinched off from the yolk sac, which now lies ventrally outside the embryo. The yolk sac remains connected, by the yolk stalk, to the GIT at the level of the midgut. The GIT system is a single continuous tube with connected organs, that will have components contributed from all three germ cell layers. Early GIT and respiratory development are closely linked and respiratory development will be covered in the next lecture. These lecture pages are being updated for the current course, so it is worth coming back again later to see if any changes have occurred. Please let me know by email of broken links or content that is not clearly covered in this supporting online material.

Online Books

Developmental Biology 6th ed. Gilbert, Scott F. Sunderland (MA): Sinauer Associates, Inc.; c2000.

Below are listed links that relate to this Lecture from the textbook "Developmental Biology" which is available free online. You can either click the provided links or do your own search using the search link.

Endoderm | Figure 15.26. Formation of the human digestive system | Figure 15.27. Endodermal development of a human embryo | Figure 15.28. Regional specification of the visceral mesoderm | Figure 15.31. Partitioning of the foregut | Figure 15.33. extraembryonic membranes of the chick | The Specification of Liver and Pancreas

Search Developmental Biology

Eurekah Bioscience Collection Chapters taken from the Eurekah Bioscience database. Eurekah.com and Landes Bioscience ; c2003 Origins of the Enteric Nervous System (ENS)

Molecular Biology of the Cell 4th ed. Alberts, Bruce; Bray, Dennis; Lewis, Julian; Raff, Martin; Roberts, Keith; Watson, James D. New York and London: Garland Publishing; c2002.

Below are listed links that relate to this Lecture from the textbook "Molecular Biology of the Cell" which is available free online. You can either click the provided links or do your own search using the search link.

Search Molecular Biology of the Cell

Internet Links

Course Homepage | Course Calendar | Course Handout 2008 - PDF Document (8 pages, 192Kb) | Textbooks | Journals

Online References

Blue Histology - Nervous Tissue |

NCBI Bookshelf | Acrobat Reader

Next

Lecture 10 - Respiratory Development Wed 13:00 - 14:00 Australian School Business 119 (K-E12-119)

Glossary

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

Terms

atresia - (Greek, a = without + tresis = perforation) Term used for anatomical closing or absence of a cavity or opening that should exist. Used as an antomical, pathological and clinical term: esophageal atresia, biliary atresia, duodenal atresia, jejunal atresia, choanal atresia, urethral atresia, bronchial atresia.

biliary cells - The liver epithelial cell formed from hepatoblast differentiation (hepatoblasts form from endoderm). (More? Gastrointestinal Tract - Liver)

buccal (Latin, bucca = cheek) term used to relate to the mouth (oral cavity). (More? GIT Notes)

buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) Forms the external upper membrane limit (cranial 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 "oral opening" of the gastrointestinal tract. (see also cloacal membrane)

(More? GIT 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)

enteroendocrine cells - Endocrine cells found within the epithelium of the gastrointestinal tract. (More? Endocrine Notes)

extrahepatic bile ducts - (EHBDs) is used to describe the hepatic, cystic, and common bile ducts. (More? 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)

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)

greater omentum - The greater omentum is the peritoneal fold extending from the greater curvature of the stomach and hanging down "like an apron" ventrally 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)

hepatic - (Greek, hepato = liver) relates to the liver and its associated structures. (More? Gastrointestinal Tract - Liver)

hepatic duct - the liver excretory duct, joins with gall bladder cystic duct to form the common bile duct. (More? Gastrointestinal Tract - Liver)

hepatoblast - The undifferentiated liver progenitor cell formed initially from endoderm, which willlater form both hepatocytes and biliary cells. (More? Gastrointestinal Tract - Liver)

hepatocyte - The functional liver cell formed from hepatoblast differentiation (hepatoblasts form from endoderm). (More? Gastrointestinal Tract - Liver)

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)

interstitial cells of Cajal - (ICC) Neural cells located in the gastrointestinal tract (enteric nervous system) located within the smooth muscle wall (tunica muscularis) that act as electrical pacemakers to coordinate muscular slow wave contraction propagation. (More? Neural Crest Notes)

insulin - A protein hormone, produced by specialized cells of the pancreas, that regulates glucose uptake; a signal for the absorptive state; promotes the synthesis of glycogen and inhibits its breakdown. (More? Endocrine Development - Pancreas)

intestinal immune system - consists of a system of functional regions and cells including: Peyer's patches, isolated lymphoid follicles, cryptopatches and mesenteric lymph nodes.

lesser omentum - The smaller of two splanchnic mesoderm peritoneal folds (lesser/greater), the lesser extends from lesser curvature of the stomach to liver. The greater omentum extends from the greater curvature of the stomach and hanging down "like an apron" ventrally over the small intestine. (More? GIT Notes)

meconium - The gastrointestinal contents that accumulate in the intestines during the fetal period. This material is a mixture of liver and glandular secretions, amniotic fluid, and cellular debris. Meconium is also used to describe the first postnatal rectal discharge from the neonate. Fetal stress in the third trimester or at parturition can lead to premature meconium discharge into the amniotic fluid, ingestion by the fetus and damage to respiratory function. Damage to placental vessels meconium myonecrosis may also occur. (More? Birth)

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 - 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). Neural crest cells also migrate into the mesoderm to form the enteric nervous system of the gut. (More? Neural Crest Notes)

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)

parenchymal - (parenchyma) Histological term used to describe the functional cells of an organ, tissue or structure. The term is often paired with stromal (stroma), which describes the supportive cells within an organ, tissue or structure.

peritoneal cavity - 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)

pleuroperitoneal membrane - An early embryonic membrane that forms inferiorly at the septum transversum to separate peritoneal cavity from pleural cavity.

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 from the splitting of the lateral plate mesoderm. This mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The same lateral plate mesoderm lying above the buccopharygeal membrane will form the heart. The cavity in the lateral plate mesoderm (intraembryonic coelom) will form the three major body cavities including the peritoneal cavity of the gut. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with ectoderm and the body wall.

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

stenosis - Term used to describe an abnormal narrowing, usually in relation to a tube. For example, in a blood vessel or in the gastrointestinal tract. (More? Gastrointestinal Tract Abnormalities)

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)

stomadeum - (stomadeum) 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. (More? Gastrointestinal Tract Notes | Head Notes)

stromal - (stroma) Histological term used to describe supportive cells within an organ, tissue or structure. The term is often paired with parenchymal, which describes the functional cells of an organ, tissue or structure.

transverse septum

(septum transversum) see septum transversum a mesodermal region in the early embryo.

References

These references are background reviews and research article readings and is not examinable.

Articles

• Steptoe PC, et al.

Some Recent Reviews

Articles

UNSW Embryology ISBN: 978 0 7334 2609 4

UNSW CRICOS Provider Code No. 00098G