2009 Lecture 5

From Embryology

Mesoderm Development

Carnegie stage 9 showing somite formation
Carnegie stage 9 scanning electron microscope image showing somite formation


We have seen the following processes during early human development so far: fertilization and blastocyst development in the first week, implantation in the second week, early placentation and bilaminar to trilaminar in the third week. In the third to fourth week we will now follow the development of the trilaminar embryo as each layer begins to differentiate into the primordia of different tissues within the embryo. From this point onward the lectures will not be in a strict timeline format as we will have to follow each layer (ectoderm, mesoderm, endoderm) forward through its early development, and then jump back to discuss the next layer.

This lecture will look at mesoderm development and formation of the body cavities.

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.

Coelom, meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The intraembryonic coelom is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities (pericardial, pleural, peritoneal).


  • Understanding of events during the third week of development
  • Understanding the process of notochord formation
  • Understanding the process of early somite development
  • Understanding the process of body cavity formation
  • Understanding the future fate of mesoderm components
  • Brief understanding of early heart formation

UNSW Embryology Links

Lecture Summary

The following text is extracted and modified from 2008 lecture slides and should be used as a "trigger" to remind you of key concepts.

Mesoderm cartoon1.gif Mesoderm cartoon2.gif

Mesoderm cartoon3.gif Mesoderm cartoon4.gif

Notochord (Axial mesoderm)


Stage 7 mesoderm
    • generated from epiblast cells migrating through the primitive streak
    • epiblast cells expressing fibroblast growth factor (FGF2)
    • forms a layer between ectoderm and endoderm with notochord down midline
    • present before neural tube formation
  • divides initially into 3 components
  • Paraxial mesoderm - somites - musculoskeletal structures
  • Intermediate mesoderm - kidney
  • Lateral plate mesoderm - body wall structures

Mesoderm cartoon1.gif


  • Embryonic connective tissue, describes the cell morphology (Histology is not epithelial organization)
    • epithelial to mesenchymal transitions
    • mesenchymal to epithelial transitions

Paraxial Mesoderm

Mesoderm cartoon2.gif

    • lies adjacent to notochord
    • Forms 2 components
      • Head - unsegmented paraxial mesoderm
      • Body - segmented paraxial mesoderm
    • Generates trunk muscles, skeleton, dermis of skin, blood vessels, connective tissue
  • Segmented Paraxial Mesoderm
    • segments called somites
    • first pair of somites (day 20)
    • segmentation imposes a pattern on
    • nerves, vasculature, vertebra....
    • somites appear in ordered sequence cranial to caudal
    • appearance so regular used to stage the embryo
    • Hamburger & Hamilton 1951- chicken
  • thought to be generated by a "clock" (1 pair every 90 minutes)
  • neural tube begins to close at 4th somite level
  • 44 pairs of somites

Intermediate Mesoderm

Mesoderm cartoon2.gif

  • lies between paraxial and lateral mesoderm
  • generates urogenital system
    • Wolffian duct, kidney
    • MH - covered in Kidney Development Lecture/Laboratory

Lateral Plate Development

Mesoderm cartoon3.gif

The intraembryonic coelom divides the lateral plate into 2 portions

  • Somatic mesoderm- closest to ectoderm
    • body wall osteogenic, chrondrogenic and fibrogenic
    • except ribs and scapula
  • Splanchnic mesoderm - closest to endoderm
    • heart and smooth muscle of GIT and blood vessels

Intraembryonic Coelom

Mesoderm cartoon4.gif

  • small spaces (vacuoles) begin appearing within the lateral plate mesoderm
  • small spaces enlarge forming a single cavity within the lateral plate mesoderm
    • divides into 2 parts at about day 18-19
  • this cavity is called the Intraembryonic Coelom
    • coelom is a general term for a "cavity" and can lie within the embryo (intraembryonic) and outside the embryo (extraembryonic)
    • later anatomical spaces within the embryo and fetus can also be described as coeloms
  • later when the embryonic disc folds the intraembryonic coelom in the lateral plate mesoderm will later form all 3 major body cavities
    • Pericardial
    • Pleural
    • Peritoneal

Somite Formation

Somite cartoon1.png Somite cartoon2.png Somite cartoon3.png Somite cartoon4.png Somite cartoon5.png

Mesoderm Movies

  • ball forms through epithelialization and interactions (cell-cell, cell-extracellular matrix, ECM) fibronectin, laminin
  • has 2 populations of cells - peripheral columnar and central mesenchymal
  • early somite has cavity- somitocoel, cavity is lost during growth
  • somite enclosed by ECM connected to nearby tissues

Somite Specification

  • Different segmental level somites have to generate different segmental body structures?
  • somite has to form different tissues?
  • Somite Differentiation
  • Compartmentalization accompanied by altered patterns of expression of Pax genes within the somite

Somite initially forms 2 main components

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

Somite Axial Specification

  • rostro-caudal axis appears regulated by Pax/Hox expression, family of DNA binding transcription factors
  • Movie: Somite Development


  • 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"


  • Myoblast determining transcription factor MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed
    • basic Helix Loop Helix
    • form myotome
  • Myotome component of Somite
    • epaxial myotome (dorsomedial quarter) forms the dorsal epimere (erector spinae)
    • hypaxial myotome (dorsolateral quarter) forms the ventral hypomere, 3 primary muscle layers which are different at neck, thorax and abdomen
Stage 14 Embryo showing somites and limb buds (Week 5)

  • Chick Embryo Mesoderm
  • Body Musculature - Myotome derivatives-mouse embryo
  • Lateral Plate Mesoderm
  • Limb Musculature
  • Dermomyotome- Muscle (MyoD)
  • MyoD Pax 3
  • Somite Differentiation
    • migrating neural crest cells enter cranial half, will form dorsal root ganglia (DRG)
    • sclerotome bulges ventro-medially towards notochord, then surround and engulf notochord
      • not movement of sclerotome, growth of surrounding tissues
      • notochord forming nucleus pulposus of IVD


  • lateral myotome edge migrates at level of limbs
  • upper limb first then lower
  • mixes with somatic mesoderm
  • dermotome continues to contribute cells to myotome

Muscle Development Abnormalities

  • Duchenne Muscular Dystrophy
    • Embryonic muscle development normal and changes occur postnatally
    • X-linked dystrophy, large gene encoding cytoskeletal protein - Dystrophin
    • progressive wasting of muscle, die late teens
  • Becker Muscular Dystrophy, milder form, adult onset

Axial Segmentation - Somite Specification Signals

Chicken Model - Somite formation

  • Somitomere to Somite
  • Chicken Stages -regular appearance of somites allowed early experimenters to accurately stage the embryo
  • Chicken
    • Advantages - accessible, easy to manipulate, limb grafts/removal, chimeras, developmental processes
    • taxon-Gallus gallus
    • develops and hatches in 20-21 days
    • Fertilized eggs easily maintained in humidified incubators
    • Embryo Staged growth
    • Series of Embryonic Chicken Growth
    • Hamburger & Hamilton J. Morphology, 88 49 - 92, 1951 [1]
  • Zebrafish, Xenopus
  • Paraxial Segmentation
  • Stage 13/14 Embryo

Take the Quiz

1 Mesenchyme refers to the middle layer of the trilaminar embryo


2 The intraembryonic coelom forms within :

lateral plate
neural tube
intermediate mesoderm

3 All paraxial mesoderm segments into somites.


4 Somites are developmental structures that contribute the following adult structures :

vertebra, notochord, dermis, skeletal muscle
vertebra, intervertebral discs, dermis, skeletal muscle
kidney, body wall connective tissue, sensory ganglia
kidney, gastrointestinal tract smooth muscle, mesentry



  • The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Mesoderm Ch15,16: p405-423, 426-430 Body Cavities Ch9: p174-184
  • Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Mesoderm Ch11 p311-339 Body Cavities Ch6 p127-146

Additional Textbooks

  • Before We Are Born (5th ed.) Moore and Persaud Ch16,17: p379-397, 399-405
  • Essentials of Human Embryology Larson Ch11 p207-228
  • Human Embryology Fitzgerald and Fitzgerald Body Cavities Ch5 p29-32, Ch7 p47,48
  • Human Embryology and Developmental Biology ?Carlson Ch9,10: p173-193, 209-222 Body Cavities Ch5 p29-32, Ch7 p47,48

Online Textbooks



Mesoderm 001 icon.jpg Somite 001 icon.jpg Vertabra 003 icon.jpg
Mesoderm Somite Structures Vertebra

UNSW Embryology Links

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

Course Content 2009

Embryology Introduction | Cell Division/Fertilization | Cell Division/Fertilization | Week 1&2 Development | Week 3 Development | Lab 2 | Mesoderm Development | Ectoderm, Early Neural, Neural Crest | Lab 3 | Early Vascular Development | Placenta | Lab 4 | Endoderm, Early Gastrointestinal | Respiratory Development | Lab 5 | Head Development | Neural Crest Development | Lab 6 | Musculoskeletal Development | Limb Development | Lab 7 | Kidney | Genital | Lab 8 | Sensory - Ear | Integumentary | Lab 9 | Sensory - Eye | Endocrine | Lab 10 | Late Vascular Development | Fetal | Lab 11 | Birth, Postnatal | Revision | Lab 12 | Lecture Audio | Course Timetable

Cite this page: Hill, M.A. (2024, April 16) Embryology 2009 Lecture 5. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2009_Lecture_5

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