ANAT2341 Lab 5 - Trilaminar Embryo

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Lab 5: Introduction | Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Online Assessment


Begin by very briefly covering the first 3 weeks of development. Our story of GIT development begins in the third week with the formation of the 3 germ cell layers, one layer the endoderm will form the lining of the entire gastrointestinal tract and also contibute the respiratory tract and other organs.

Note that the description below, and throughout the practical, has been substantially simplified.

Week 3

Gastrulation/Neuralation: The inner cell mass forms a flat sheet of cells and cells migrate through a specific region of the sheet (primitive streak) turning the single layer into first 2 then 3 layers (trilaminar embryo).


The next process to follow is the folding of the embryonic disc which will form the "tube" of the GIT. Forming the ends of this tube are the 2 membranes which form the upper and lower limits of the GIT.

Endoderm cartoon.jpg

Note that in addition to gastrulation, neuralation (forming the early neural tube that makes the nervous system) and somitogenesis (segmentation of the mesoderm forming the axial skeleton) are the other major processes occuring in week 3 to 4.

Folding of the embryonic disc 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 primative GIT.

File:Endoderm 002 icon.jpg</wikiflv> Week3_folding icon.jpg</wikiflv>
Quicktime | Flash
Quicktime | Flash

Week 4

Carnegie stage 10, 23 day, 5-11 somite pairs

Stage10 sem10.jpg

Stage10 sem1.jpg


During the process of gastrulation the embryonic disc formed 3 layers, except in 2 specific membrane regions where ectoderm and endoderm have no mesoderm between them: buccopharyngeal membrane and cloacal membrane. These will form the upper and lower extend of the GIT.

Buccopharyngeal membrane

also called mouth or oral membrane

Carnegie stage 10 (21 days, 4-5 somite pairs, ventral sem)

Stage10 sem4.jpg

Carnegie stage 11 (25 days, 20 somite pairs)

Cloacal membrane - not clearly visible in the above section

Splanchnic Mesoderm

Mesoderm cartoon4.gif

The cartoon above is a section through the trunk of the trilaminar embryo showing the further development of the 3 layers and the space (coelom) that forms in the mesoderm (only the righthand side is shown, lefthand side would be identical).

Within the embryonic disc lateral plate mesoderm a space (coelom) forms, it lies within the embryo and so is called the intraembryonic coelom. This single "horseshoe-shaped" space will form the 3 major body cavities: pericardial (around the heart), pleural (around the lungs) and peritoneal (around the GIT and visceral organs).

The mesoderm adjacent to the endoderm is now called the splanchnic mesoderm which forms the connective tissue and muscular wall of the GIT.

Note intraembryonic coelomic cavity communicates with extraembryonic coelom (space outside the embryo) through portals (holes) initially on lateral margin of embryonic disc.

Lab 5: Introduction | Trilaminar Embryo | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | Online Assessment

ANAT2341 Course Timetable  
Week (Mon) Lecture 1 (Mon 1-2pm) Lecture 2 (Tue 3-4pm) Practical (Fri 1-3pm)
Week 2 (1 Aug) Introduction Fertilization Lab 1
Week 3 (8 Aug) Week 1 and 2 Week 3 Lab 2
Week 4 (15 Aug) Mesoderm Ectoderm Lab 3
Week 5 (22 Aug) Early Vascular Placenta Lab 4
Week 6 (29 Aug) Gastrointestinal Respiratory Lab 5
Week 7 (5 Sep) Head Neural Crest Lab 6
Week 8 (12 Sep) Musculoskeletal Limb Development Lab 7
Week 9 (19 Sep) Renal Genital Lab 8
Mid-semester break
Week 10 (3 Oct) Public Holiday Stem Cells Lab 9
Week 11 (10 Oct) Integumentary Endocrine Lab 10
Week 12 (17 Oct) Heart Sensory Lab 11
Week 13 (24 Oct) Fetal Birth and Revision Lab 12

ANAT2341 2016: Moodle page | ECHO360 | Textbooks | Students 2016 | Projects 2016

ANAT2341Lectures - Textbook chapters  
Lecture (Timetable) Textbook - The Developing Human Textbook - Larsen's Human Embryology
Embryology Introduction Introduction to the Developing Human
Fertilization First Week of Human Development Gametogenesis, Fertilization, and First Week
Week 1 and 2 Second Week of Human Development Second Week: Becoming Bilaminar and Fully Implanting
Week 3 Third Week of Human Development Third Week: Becoming Trilaminar and Establishing Body Axes
Mesoderm Fourth to Eighth Weeks of Human Development Fourth Week: Forming the Embryo
Ectoderm Nervous System Development of the Central Nervous System
Early Vascular Cardiovascular System Development of the Vasculature
Placenta Placenta and Fetal Membranes Development of the Vasculature
Endoderm - GIT Alimentary System Development of the Gastrointestinal Tract
Respiratory Respiratory System Development of the Respiratory System and Body Cavities
Head Pharyngeal Apparatus, Face, and Neck Development of the Pharyngeal Apparatus and Face
Neural Crest Nervous System Development of the Peripheral Nervous System
Musculoskeletal Muscular System Development of the Musculoskeletal System
Limb Development of Limbs Development of the Limbs
Renal Urogenital System Development of the Urinary System
Genital Urogenital System Development of the Urinary System
Stem Cells
Integumentary Integumentary System Development of the Skin and Its Derivatives
Endocrine Covered through various chapters (see also alternate text), read head and neck, neural crest and renal chapters.
Endocrinology Textbook - Chapter Titles  
Nussey S. and Whitehead S. Endocrinology: An Integrated Approach (2001) Oxford: BIOS Scientific Publishers; ISBN-10: 1-85996-252-1.

Full Table of Contents

Heart Cardiovascular System Development of the Heart
Sensory Development of Eyes and Ears Development of the Eyes
Fetal Fetal Period Fetal Development and the Fetus as Patient
Birth and Revision
Additional Textbook Content - The following concepts also form part of the theory material covered throughout the course.
  1. Principles and Mechanisms of Morphogenesis and Dysmorphogenesis
  2. Common Signaling Pathways Used During Development
  3. Human Birth Defect

Glossary Links

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 | Numbers | Symbols | Term Link

Cite this page: Hill, M.A. (2021, May 15) Embryology ANAT2341 Lab 5 - Trilaminar Embryo. Retrieved from

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G