2011 Lab 3 - Week 3

From Embryology
2011 Lab 3: Introduction | Week 3 | Week 4 | Abnormalities | Online Assessment | Group Project


There are two major folding processes that take place during this time.

  1. 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.
  2. 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.

Amnion 001 icon.jpg Week3 folding icon.jpg

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 Endoderm 002 icon.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

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

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
  • space - forms the 3 body cavities (pericardial, pleural, peritoneal)
  • splanchnic mesoderm - closest to endoderm

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

Somite Development

stage 11 Embryo

Somite initially forms 2 main components

  • ventromedial- sclerotome forms vertebral body and intervertebral disc
  • dorsolateral - dermomyotome forms dermis and skeletal muscle


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

Forms 2 muscle groups in body and limbs

Somite 001 icon.jpg 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

Week2 001 icon.jpg Mesoderm 001 icon.jpg Chorion 001 icon.jpg Amnion 001 icon.jpg Week3 folding icon.jpg
Implantation Mesoderm Chorionic Cavity Amniotic Cavity Week 3

Embryo Stages and Events

Day Stage Event
Stage 7
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 second role is to act as an initial generator of the left-right (L-R) body axis.
Stage 8
Stage8 human.jpg
Neural System Development neurogenesis, neural groove and folds are first seen
Stage8 SEM1.jpg
Stage 9 Stage9 bf1c.jpg Musculoskeletal System Development somitogenesis - first somites form and continue to be added in sequence caudally

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

  Cardiovascular System Development cardiogenesis - week 3 begins as paired heart tubes.

2011 Lab 3: Introduction | Week 3 | Week 4 | Abnormalities | Online Assessment | Group Project

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. (2024, April 16) Embryology 2011 Lab 3 - Week 3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2011_Lab_3_-_Week_3

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