BGDA Practical 7 - Week 3

From Embryology
Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8


Key events of human development during the third week (week 3) following fertilization clinical GA week 5.

Note that during this time the conceptus cells not contributing to the embryo are contributing to placental membranes and the early placenta. This page describes the mechanical events and changes occurring in each of the 3 germ layers (some concepts will also be covered in later weeks).


Through week 3 the process of gastrulation continues, as cells to migrate through the primitive streak contributing to mesoderm.

As the embryonic disc grows overall in size, the primitive streak appears to become more caudal as it does not increase in size.

Stage7 primitive streak labelled.jpg Trilaminar embryo.jpg


Endoderm, mesoderm and ectoderm layers. There are two major folding processes that take place during this time.

  1. Folding of the whole embryonic disc ventrally, separates the endoderm to form the epithelial lining of the gut. Folding of the embryonic disc occurs ventrally around the notochord, which forms a rod-like region running rostro-caudally in the midline.
  2. Folding of the ecoderm will form a neural groove, then closing to form a neural tube, separating the neural ectoderm from the embryo surface ectoderm.

Mesoderm Segmentation

Different regions of mesoderm form early intermediate structures.

  1. Cardiac development - forming the simple heart tube within splanchnic mesoderm.
  2. somitogenesis - when part of the mesoderm layer segments commences during week 3 to form balls of mesoderm called somites. The later migration of cells forms the mesoderm germ layer. An embryonic connective tissue (mesenchyme) which forms nearly all the connective tissues of the body (the head is different). Somitogenesis is when part of this layer segments during week 3 to form balls of mesoderm called somites, note that the majority of somites form during week 4.
  3. intra-embryonic coelom - 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 central portion of the embryonic disc forms the neural plate, the edge of this plate forms neural crest and outside of this again will contribute the epitheium of the skin. (this will be covered in more detail week 4).

Neural Plate Neural Groove
Mesoderm-cartoon1.jpg [[File::Mesoderm-cartoon2.jpg|300px]]

Stage7-sem2.jpg Stage8 SEM1.jpg Stage9 sem4c.jpg
Carnegie stage 7 Carnegie stage 8 Carnegie stage 9


Week3 folding icon.jpg
 ‎‎Week 3
Page | Play
Amnion 001 icon.jpg
 ‎‎Amniotic Cavity
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The major folding processes that take place during this time, 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.

Endoderm cartoon.jpg


Mesoderm means the "middle layer" and it is from this layer that nearly all the bodies connective tissues are derived. In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised. Humans are vertebrates, with a "backbone", and the first mesoderm structure we will see form after the notochord will be somites.

Facts: Week 4, 22 - 23 days, 2 - 3.5 mm, Somite Number 4 - 12

View: This is a dorsal view of the human embryo, the amniotic membrane has been removed. Top embryo is an early stage 10, bottom is late stage 10.

Mesoderm Development

  1. epiblast -> mesoderm + axial mesoderm (notochord)
  2. lateral plate + paraxial mesoderm + axial mesoderm
  3. lateral plate + intermediate mesoderm + somites (body), paraxial mesoderm (head) + axial mesoderm
  4. somatic mesoderm + intraembryonic coelom + splanchnic mesoderm + intermediate mesoderm + somites (body), paraxial mesoderm (head) + axial mesoderm
Mesoderm cartoon.gif

Axial Mesoderm

Notochord secreting sonic hedgehog
Stage 7 notochord

The notochord

  1. mechanical role in embryonic disc folding
  2. molecular role in patterning surrounding tissues
Notochord 01 icon.jpg
 ‎‎Week 3 Notochord
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Notochord 02 icon.jpg
 ‎‎Week 3 Notochord
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Adult - contributes to the nucleus pulposus of the intervertebral disc

Paraxial Mesoderm

Stage 7 paraxial mesoderm
  • differentiates rostro-caudally (head to tail)
  • remains unsegmented in the head region.
  • segments in the body region to form pairs of somites along the length of the embryo.

Adult - contributes vertebral column (vertebra and IVD), dermis of the skin, skeletal muscle of body and limbs

Intermediate Mesoderm

Stage 7 intermediate mesoderm
  • named by position (between paraxial and lateral plate)
  • differentiates rostro-caudally (head to tail)
  • forms 3 sets of "kidneys" in sequence
  1. pronephros
  2. mesonephros
  3. metanephros

Adult - metanephros forms the kidney

Lateral Plate Mesoderm

Stage 7 lateral plate
  • a "horseshoe shaped" space forms in the middle
  • somatic mesoderm - closest to ectoderm, forms connective tissue of the body wall and the skeletal elements of the appendicular skeleton and sternum.
  • space - forms the 3 body cavities (pericardial, pleural, peritoneal)
  • splanchnic mesoderm - closest to endoderm, forms heart and blood vessels; connective tissue and smooth muscle of the gastrointestinal tract

Adult - body and limb connective tissues, gastrointestinal tract (connective tissues, muscle, organs), heart

Heart Development

Early Development of Heart Tube.jpg Splanchnic mesoderm lying above the notochord (prechordal splanchnic mesoderm) forms a pair of simple tubes, that will fuse to form the primordia of the heart tube.

More: Primitive Heart Tube

Somite Development

Stage11 sem100.jpg

Embryo (Carnegie stage 11) SEM

Somite initially forms 2 main components

  • ventromedial- sclerotome forms vertebral body and intervertebral disc
  • dorsolateral - dermomyotome forms dermis and skeletal muscle
Somite cartoon1.png Somite cartoon2.png Somite cartoon3.png Somite cartoon4.png
paraxial mesoderm early somite sclerotome and dermomyotome dermatome and myotome
  Sclerotome   Dermatome
  • sclerotome later becomes subdivided
    • rostral and caudal halves separated laterally by von Ebner's fissure
  • half somites contribute to a single vertebral level body
  • other half intervertebral disc
  • therefore final vertebral segmentation “shifts”
  • connective tissue underlying epidermis
  • begins as a dorsal thickening
  • spreads throughout the body
  • Body - epaxial and hypaxial muscles
  • Limbs - flexor and extensor muscles


Vertabra 003 icon.jpg
  • sclerotome from somites at the same segmental level contribute the right and left half of each vertebral and intervertebral element
  • sclerotome later becomes subdivided into rostral and caudal halves (separated by von Ebner's fissure)
  • half somites contribute to a single vertebral level body
  • other half intervertebral disc
  • therefore final vertebral segmentation “shifts”


epaxial and hypaxial muscles
Somite 001 icon.jpg
Forms 2 muscle groups in body and limbs Development of the sclerotome and myotome components of the somite.


  • connective tissue underlying epidermis
  • begins as a dorsal thickening
  • spreads throughout the body

Note - Dermatome is the term also used clinically postnatally to describe the region of skin supplied by a single spinal nerve.

Week 2 and 3 Movies

Week 2

Week2 001 icon.jpg
 ‎‎Week 2 - Implant
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Chorion 001 icon.jpg
 ‎‎Week 2 - Bilaminar
Page | Play

Week 3

Mesoderm 001 icon.jpg
 ‎‎Week 3 Mesoderm
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Notochord 01 icon.jpg
 ‎‎Week 3 Notochord
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Notochord 02 icon.jpg
 ‎‎Week 3 Notochord
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 ‎‎Nodal Cilia
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Week3 folding icon.jpg
 ‎‎Week 3
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Amnion 001 icon.jpg
 ‎‎Amniotic Cavity
Page | Play
Quail HH stage 2 fibronectin movement.jpg
 ‎‎Gastrulation ECM
Page | Play

Week 3 Interactive Component

Attempt the Quiz - Week 3  

Here are a few simple Quiz questions that relate to Week 3 (GA week 5) from the lecture and practical.

See your Quiz Result - Answer all the questions, then click "submit" to complete. The page will reload and you can then reopen this table to see your result and feedback.


1 During embryonic disc folding in week 3 only the ectoderm layer folds ventrally left, right and above and below the notochord.


2 Which of the following is the most correct statement about the primitive streak:

  extends from the primitive node to the parachordal plate
  is a primary inducer during organogenesis
  is a site of migration of epiblast cells to form the mesoderm
  persists as the parachordal plate
  all of the above are correct

3 The migration of intra-embryonic mesoderm occurs between the ectoderm and endoderm to give rise to:

 the buccopharyngeal membrane
 the cloacal membrane
 the allantois
 the somites
 all of the above

4 The buccopharyngeal membrane is made up of:

 embryonic ectoderm, mesoderm and endoderm
 endoderm fused with mesoderm
 ectoderm fused with mesoderm
 ectoderm fused with endoderm
 the cephalic end of notochordal process

5 The intraembryonic coelom finally forms

  pericardial, pleural and peritoneal cavity.
  single pericardial cavity and two pleural and two peritoneal cavities.
  neural tube lumen, gastrointestinal tract lumen and neural crest.
  the paired dorsal aortas running the embryo.

6 The amniotic cavity appears initially as a slit-like space within the:

  exocoelomic (Heuser’s) membrane
  extra-embryonic mesoderm
  connecting stalk

7 Which of the following processes are necessary for the early development of the human embryo?

  cell proliferation
  cell migration
  cell fusion
  differential growth
  all of the above

Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8

Practical 6: Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.


Week 3 - Human Embryo Stages and Events (GA week 5) 
Embryo Week: Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9
Stage 7
Gastrulation primitive node (Hensen's node, primitive knot) The small circular region located at the cranial end of the primitive streak, where gastrulation occurs, and is a controller of this process. The process establishes the 3 germ cell layers: Endoderm, Mesoderm and Ectoderm. The second role is to act as an initial generator of the left-right (L-R) body axis.
Stage 8
Stage8 human.jpg
Neural - neurogenesis, neural groove and folds are first seen
Stage8 SEM1.jpg
Stage 9
Stage9 bf1c.jpg
Musculoskeletal - somitogenesis, first somites form and continue to be added in sequence caudally (1 - 3 somite pairs).

Neural - the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open

Neural Crest - mesencephalic neural crest is visible[1]

  Heart - cardiogenesis, week 3 begins as paired heart tubes.
Note - the day timing of stages is only approximate, system names link to first page of that specific system, and events are based upon the literature cited below.
  1. <pubmed>17848161</pubmed>


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Cite this page: Hill, M.A. (2024, April 16) Embryology BGDA Practical 7 - Week 3. Retrieved from

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