ANAT2341 Lab 5 - Early Embryo

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

We have now reached the end of Week 4 and beginning of 5 of development. Start by looking briefly at the overview of the Carnegie stage 13 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.

Tract Development

The early developing gastrointestinal tract The later developing gastrointestinal tract
Early Embryo Later Embryo

Carnegie Stage 13 (Week 4-5)



Stage13 bf1.jpg Stage13 sem1.jpg
The individual serial slices have also been incorporated into a 3D model of this embryo.
Stage 13 - Gastrointestinal Tract
Section Name Description
Stage 13 image 057.jpg

Stage 13 image 058.jpg




Head arches cartoon.jpg Note how in this region it is arched over and cut twice in this section.

The righthand side towards the buccopharyngeal membrane, the lefthand side descending into the embryo body. Stage13 B2 excerpt.gif

Central region is the floor of pharynx formed by fusion of 3rd pharyngeal arches = hypopharyngeal eminence (precursor of root of tongue).

Rathke's pouch forming the rudimentary adenohypophysis (anterior pituitary).

Stage 13 image 059.jpg B3L Laryngeal tracheal groove - beginning of ventral compression, at 90 degrees to the lateral plane of the pharynx above this point.

Rudimentary thyroid ventral to aortic sac (also seen in B2, ventral to the hypopharyngeal eminence).

Stage 13 image 060.jpg B4L Laryngeal tracheal groove - Caudal pharynx compressed dorsoventrally.

Note that it lies between the aortic sac (ventrally) directly above the heart and the paired vessels of arch artery 6 and the dorsal aortas. The pale staining region behind these blood vessels is where the vertebral column will form.

Stage 13 image 061.jpg B5L Laryngopharynx - now compressed dorsoventrally between the paired arch artery 6 vessels.
Stage 13 image 063.jpg B7L Glottis - initial separation of the oesophagus (dorsal) from the trachea (ventral).

Note that this is occurring at the level of the heart atria.

Nasal placodes. Pulmonary arteries.

Stage 13 image 064.jpg C1L Gastrointestinal tract oesophagus (dorsal) is now separate from the respiratory trachea (ventral).
Stage 13 image 065.jpg C2L Oesophagus and trachea both surrounded by dense mesenchyme.

Right nasal pit.

Stage 13 image 066.jpg

Stage 13 image 067.jpg



Oesophagus and trachea both surrounded by dense mesenchyme.

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.

In C4, junction of right common cardinal vein with dorsal wall of sinus venosus.

Left nasal pit.

Stage 13 image 068.jpg C5L Smaller oesophagus and expanding trachea, this is also the upper region of the lung buds.

The ventral anchoring of attachment site is at the most cranial extension of the septum transversum. This attachment now divides the intraembryonic coelom around the trachea into two canals, the left and right pleuro (pericardio-peritoneal) canals.

Canals are lined by coelomic mesothelium and are continuous with whole intraembryonic coelom (they will be referred to hereafter simply as coelomic canals).

The pleuroperitoneal fold on the medial side of the right common cardinal vein will form part of the diaphragm.

Stage 13 image 069.jpg C6L Trachea expanded and beginning to bifurcate to the major bronchial branches for each lung.

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.

Stage 13 image 070.jpg C7L Trachea bifurcated to the major bronchial branches for each lung.

Note dorsal extent of coelomic canals.

Oesophagus lumen reappears caudal to bifurcation.

Distinct R (smaller on L) pleuroperitoneal fold below the common cardinal vein.

Stage 13 image 071.jpg D1L Oesophagus/stomach junction.

Right lung bud bronchus still visible, left bronchus ends above this section. Note the oesophagus now lies in the midline between the 2 bronchi.

Coelomic canals.

Stage 13 image 072.jpg D2L 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.

Stage 13 image 073.jpg D3L Rotation of stomach (seen from above) to right side.

Note change in outline of coelomic canals due to presence of liver.

Lesser sac. Note thick mesothelium lining the coelom along left edge 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).

Stage 13 image 074.jpg D4L Rotation of stomach (seen from above) to right side.

Ventral mesogastrium - Stomach is attached ventrally to the liver. (note the position of the ductus venosus)

Dorsal mesogastrium - within this structure the spleen will begin to form and later the greater omentum.

Peritoneal spaces - identify greater and lesser sac.

Stage 13 image 076.jpg D6L Pyloric region of stomach.

Ventral mesogastrium - Stomach is closely attached ventrally to the liver.

Dorsal mesogastrium - within this structure the spleen will begin to form and later the greater omentum.

Peritoneal spaces - identify greater and lesser sac.

Stage 13 image 081.jpg E4L Midgut.

Region close to the umbilicus. Note the close associated portal vein and the paired placental (umbilical) veins.

Stage 13 image 085.jpg F1L Midgut.

Looping out of body wall ventrally (cut tangentially).

Also note the righthand side hindgut region.

Stage 13 image 098.jpg G7L Caudal pharynx (extending laterally, ventral to dorsal aorta - cf B4). Stomach, mesentery
Stage 13 image 097.jpg G6L Narrow oesophagus. Tracheal bifurcation dorsal to sinus venosus.

Peritoneal Cavity

  • Fusion of the two separate intra-embryonic coelom spaces.
  • Intestinal loss of ventral attachment, except at the level of the stomach, dorsal attachment becomes true mesentery.

Stomach Development

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.

<html5media height="520" width="490">File:Stomach rotation 01.mp4</html5media>

Click Here to play on mobile device

Stomach rotation 01 icon.jpg
This animation shows a general overview of stomach growth and its rotation during development. (animation does not show second rotation)

  1. week 4 - forget inferior to transverse septum proliferates (expands), forms fusiform structure (Latin, spindle).
  2. week 5 - dorsal wall grows rapidly (greater curvature), bends tube ventrally (90 degrees)
  3. week 7 and 8 - dorsal mesentery growth, rotates stomach laterally (90 degrees) greater curvature to left, lesser curvature to right

Colour Code

  • red - mesenteries ventral (front) dorsal (back)
    • dorsal mesentery associated with stomach greater curvature
  • blue - left and right vagal nerve branches.

Lesser Sac Development

<html5media height="540" width="390">File:Lesser sac 01.mp4</html5media>

Click Here to play on mobile device

Lesser sac 01 icon.jpg
This animation shows the development of the lesser sac associated with the rotation of the stomach and growth of the liver.

This cross-sectional view of the abdomen viewed from above, with dorsal (back) top and ventral (front) bottom of animation.

Later the retroperitoneal position of the developing kidneys is also shown either side of the dorsal (thoracic) aorta.


  • spleen in mesentery
  • stomach endoderm of gastrointestinal tract
  • liver
  • mesentery

Respiratory Development

Historic drawings of embryonic lung development

Bailey287.jpg Bailey288.jpg Bailey289.jpg
Human embryo (CRL 4.3 mm) Human embryo (CRL 8.5 mm) Human embryo (CRL 10.5 mm)

See Adult Respiratory Tract

Respiratory system overview (stage 13)

Week 4 - laryngotracheal groove forms on floor foregut.

Week 5 - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)

Week 6 - descent of heart and lungs into thorax. Pleuroperitoneal foramen closes.

Week 7 - enlargement of liver stops descent of heart and lungs.

Pseudoglandular Stage

  • week 5 - 17
  • tubular branching of the human lung airways continues
  • by 2 months all segmental bronchi are present.
  • lungs have appearance of a glandlike structure.
  • stage is critical for the formation of all conducting airways.
  • lined with tall columnar epithelium, the more distal structures are lined with cuboidal epithelium.

Lung development stage13-22.jpg

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

Gastrointestinal Tract Movies  
Mesoderm 001 icon.jpg
 ‎‎Week 3 Mesoderm
Page | Play
Week3 folding icon.jpg
 ‎‎Week 3
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Amnion 001 icon.jpg
 ‎‎Amniotic Cavity
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Endoderm 002 icon.jpg
Page | Play
Stomach rotation 01 icon.jpg
 ‎‎Stomach Rotation
Page | Play
Gastrointestinal tract growth 01 icon.jpg
 ‎‎Tract Growth
Page | Play
Greater omentum 001 icon.jpg
 ‎‎Greater Omentum
Page | Play
Lesser sac 01 icon.jpg
 ‎‎Lesser sac
Page | Play
Urogenital septum 001 icon.jpg
 ‎‎Urogenital Septum
Page | Play
 ‎‎GIT Stage 13
Page | Play
 ‎‎GIT Stage 22
Page | Play
Stage23 MRI S04 icon.jpg
 ‎‎Sagittal GIT
Page | Play
 ‎‎GIT Motility
Page | Play
Gastroschisis 01.jpg
Page | Play
Omphalocele 01 icon.jpg
Page | Play
Stage 13 (week 5) Stage 22 (week 8) Stage 23 (week 8) GIT Abnormalities Ultrasound


  • 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.
  • 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.
  • 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).
  • 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 (stomodeum) 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.
  • 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.
  • 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.
  • 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.
  • gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers ([E#endoderm|endoderm]/mesoderm/ectoderm) have just formed.
  • gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo ([E#endoderm endoderm]/mesoderm/ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes.
  • 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.
  • intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom.
  • neuralation - The general term used to describe the early formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate, then neural groove, then neural tube. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. In humans, early neuralation begins in week 3 and continues through week 4.
  • neural crest - region of cells at the edge of the neural plate that migrates throughout the embryo and contributes to many different tissues. In the gastrointestinal tract it contributes mainly the enteric nervous system within the wall of the gut responsible for peristalsis and secretion.
  • 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.
  • somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/4 hours (mouse 1 pair/90 min) until on average 44 pairs eventually form.
  • 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.
  • stomodeum - (stomadeum, stomatodeum) 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.

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

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Cite this page: Hill, M.A. (2024, April 24) Embryology ANAT2341 Lab 5 - Early Embryo. Retrieved from

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