ANAT2341 Lab 5 - Trilaminar Embryo

Introduction

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).

Folding

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.

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.

<html5media height="540" width="390">File:Week3_folding.mp4</html5media> Click Here to play on mobile device

This animation shows folding of the embryonic disc beginning week 3 of development. Embryonic disc (midline section) shown to the left and early placenta to the right. The embryonic disc dorsal (ectoderm) top and ventral (endoderm) bottom. Cranial end of disc to the left and caudal end of disc to the right. Note also the early cardiac region shown at the cranial end of disc and the allantois at the caudal end of disc extending into the connecting stalk. Folding of the embryonic disc brings the cardiac region and allantois ventrally brings the buccopharyngeal membrane, cloacal membrane and connecting stalk ventrally encloses the embryo in amniotic cavity narrows the connection between yolk sac and midgut of the gastrointestinal tract to a channel, the yolk stalk or vitelline duct blue - epiblast layer of amniotic cavity yellow - endoderm layer of the yolk sac and gastrointestinal tract red - outside embryo extraembryonic mesoderm and inside embryo mesoderm

<html5media height="340" width="300">File:Endoderm 003.mp4</html5media> Click Here to play on mobile device

This animation shows the early development of endoderm forming the gastrointestinal tract, yolk sac and allantois. The movie starts approximately week 3 and continues through week 4.

Week 4

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

Membranes

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)

Carnegie stage 11 (25 days, 20 somite pairs)

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  Low power ventral view of the Buccopharyngeal Membrane

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  Higher power ventrolateral view of the Buccopharyngeal Membrane

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  Close up view of the degenerating Buccopharyngeal Membrane

Cloacal membrane - not clearly visible in the above section

Splanchnic Mesoderm

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.

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. Chapter 1. Principles of endocrinology Chapter 2. The endocrine pancreas Chapter 3. The thyroid gland Chapter 4. The adrenal gland Chapter 5. The parathyroid glands and vitamin D Chapter 6. The gonad Chapter 7. The pituitary gland Chapter 8. Cardiovascular and renal endocrinology 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. Principles and Mechanisms of Morphogenesis and Dysmorphogenesis Common Signaling Pathways Used During Development Human Birth Defect

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Cite this page: Hill, M.A. (2024, April 23) Embryology ANAT2341 Lab 5 - Trilaminar Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/ANAT2341_Lab_5_-_Trilaminar_Embryo

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G