Lecture - Mesoderm Development

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Mesoderm formation

Having now reached week 3 in development we will now begin to look separately at the 3 transient germ layers (ectoderm, mesoderm and endoderm) formed by the process of gastrulation. Beginning with the mesoderm layer, the middle embryonic connective tissue (mesenchyme) layer. Transient in terms of temporary structures that will become something else later in development.

Mesoderm initially forms a multilayered cellular layer separating ectoderm and endoderm, mesoderm also lies outside the embryo as extra-embryonic mesoderm (covered in placenta lecture). Embryonic mesoderm will form most of the adult connective tissues and muscle.

Towards the end of week 3 this layer begins to "partition" into different transient components based upon their location within the layer and the signals the cells are receiving. This partitioning process can be either in terms of cell differentiation or structural. This lecture will describe these initial regions and the tissues they will eventually form. Note that later lectures (muscle, skeleton, limb, integumentary and heart) will revisit these tissues later in development.


  • Understanding of events during the third week of development
  • Understanding the process of early somite development
  • Understanding the process of body cavity formation
  • Brief understanding of the future fate of mesoderm components
  • Brief understanding of early heart formation
Presomitic mesoderm movie 3 icon.jpg
 ‎‎Presomite Mesod
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These are mesoderm cells migrating from the primitive stria.

Lecture Resources

File:ANAT2341 Lecture 7 - Beverdam - Mesoderm Developmentv2.pdf

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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Week3 folding icon.jpg
 ‎‎Week 3
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Vertebra 003 icon.jpg
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Somite 001 icon.jpg
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Somitogenesis 01 icon.jpg
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Mesoderm migration movie 1 icon.jpg
 ‎‎Mesoderm Move
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Presomitic mesoderm movie 3 icon.jpg
 ‎‎Presomite Mesod
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UNSW Embryology logo
Hill, M.A. (2018). UNSW Embryology (18th ed.) Retrieved June 20, 2018, from https://embryology.med.unsw.edu.au
Textbook cover Larsen's human embryology 5th edn.
Moore, K.L., Persaud, T.V.N. & Torchia, M.G. (2015). The developing human: clinically oriented embryology (10th ed.). Philadelphia: Saunders.
The following chapter links only work with a UNSW connection.
Textbook cover Larsen's human embryology 5th edn.
Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen's human embryology (5th ed.). New York; Edinburgh: Churchill Livingstone.
The following chapter links only work with a UNSW connection.
Recent Research  
Some recent papers that relate to mesoderm development.

Sandeep Kumar, Thomas J Cunningham, Gregg Duester Nuclearreceptor corepressors Ncor1and Ncor2(Smrt) are required for retinoic acid-dependent repression of Fgf8 during somitogenesis. Dev. Biol.: 2016; PubMed 27506116

Taijiro Yabe, Kazuyuki Hoshijima, Takashi Yamamoto, Shinji Takada Mesp quadruple zebrafish mutant reveals different roles of mesp genes in somite segmentation between mouse and zebrafish. Development: 2016; PubMed 27385009

Wouter Masselink, Nicholas J Cole, Fruzsina Fenyes, Silke Berger, Carmen Sonntag, Alasdair Wood, Phong D Nguyen, Naomi Cohen, Franziska Knopf, Gilbert Weidinger, Thomas E Hall, Peter D Currie A somitic contribution to the apical ectodermal ridge is essential for fin formation. Nature: 2016; PubMed 27437584

Take the Mesoderm Quiz.

Notochord (Axial mesoderm)


Stage 7 mesoderm
The trilaminar embryo
  • 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 - urogenital (kidney and genital)
  • Lateral plate mesoderm - body wall, body cavities, cardiovascular and GIT structures

Mesoderm Development

The four images below beginning at week 3 show cross-sections of the trilaminar embryo and the sequence of mesoderm development.

Mesoderm-cartoon1.jpg Mesoderm-cartoon2.jpg
Mesoderm-cartoon3.jpg Mesoderm-cartoon4.jpg

Mesoderm Overview

Trilaminar embryo.jpg Stage11 sem100.jpg
Week 3

Trilaminar embryo

Compare this week 3 trilaminar embryo with the week 4 embryo.

  • Mesenchyme - embryonic connective tissue, describes the cell morphology (developmental transitions: epithelial to mesenchymal, mesenchymal to epithelial)

(Note - 2 these images are not to scale)

Week 4

Scanning electron micrograph of a cross-section of a human embryo at week 4 (stage 11).

Note the mesoderm structures now present and their relative position and size within the embryo.

Compare the mesoderm structures to those formed by ectoderm (neural tube and epidermis) and endoderm (epithelia of developing gastrointestinal tract).

Human Embryo Week 4 (Carnegie stage 10) - transverse section
Stage10 K12202-01.jpg Stage10 K12202-02.jpg

Paraxial Mesoderm

Hamburger & Hamilton Stage 10 (33 hours)
Presomitic mesoderm migration (chicken)

  • lies adjacent to axial mesoderm (notochord) and 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 - transient embryonic structures.
  • 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

Model for Sprouty4 and FGF in mesoderm segmentation.jpg

Model for Sprouty4 and FGF in mouse mesoderm segmentation

Somite Formation

Carnegie stage 9 scanning electron microscope image showing somite formation
Carnegie stage 13 somitocoel
Carnegie stage 13 sclerotome
Presomitic mesoderm movie 3 icon.jpg
 ‎‎Presomite Mesod
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Somitogenesis 01 icon.jpg
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Stage10 bf6.jpg
Somite cartoon1.png Somite cartoon2.png
Somite cartoon3.png Somite cartoon4.png
Somite cartoon5.png Stage11 sem100.jpg
Stage11 sem13.jpg

Week 4 Carnegie stage 11

  • 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

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

Somite initially forms 2 main components

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


Human embryo (Carnegie stage 13) caudal trunk
  • 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"


  • later divides into dorsal dermatome and ventral myotome
    • This topic of muscle and skeleton development will be covered in 2 later lectures Musculoskeletal Development and Limb Development)
  • lateral myotome edge migrates at level of limbs
  • upper limb first then lower
  • mixes with somatic mesoderm
  • dermotome continues to contribute cells to 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)


  • 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
    • from 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

Intermediate Mesoderm

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

Lateral Plate Development

lateral plate
  • lying at the surrounding edge of he embryonic disc
  • a cavity begins in this week to form within the mesoderm itself


Intraembryonic Coelom

  • small spaces (vacuoles) begin appearing within the lateral plate mesoderm
  • enlarge forming a single cavity within the lateral plate mesoderm
    • divides lateral plate mesoderm into 2 parts at about day 18-19
  • this cavity is called the Intraembryonic Coelom
  • when the embryonic disc folds the intraembryonic coelom will form all 3 major body cavities:
  1. Pericardial
  2. Pleural
  3. Peritoneal

Coelom is a general term for a "cavity" and can lie within the embryo (intraembryonic) and outside the embryo (extra embryonic). Later anatomical spaces within the embryo and fetus can also be described as coeloms.


Somatic Mesoderm

The intraembryonic coelom divides the lateral plate into 2 portions
  • closest to ectoderm
  • body wall osteogenic, chrondrogenic and fibrogenic
  • except ribs and scapula
Lateral plate somatic mesoderm cartoon.jpg

Lateral plate somatic mesoderm[1]

Splanchnic Mesoderm

  • lies closest to endoderm
  • prechordal splanchnic mesoderm - cardiac mesoderm
  • splanchnic mesoderm - smooth muscle of gastrointestinal tract (GIT) and blood vessels


Stage 9 Dorsal Stage 9 Ventral
Carnegie Stages: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | About Stages | Timeline


Days (approx)
(not to scale)
15 - 17
(week 3)
gastrulation, notochordal process
17 - 19
1.0 - 1.5
primitive pit, notochordal canal
19 - 21
1.5 - 2.5
Somite Number 1 - 3 neural folds, cardiac primordium, head fold
22 - 23
(week 4)
2 - 3.5
Somite Number 4 - 12 neural fold fuses
23 - 26
2.5 - 4.5
Somite Number 13 - 20 rostral neuropore closes
26 - 30
3 - 5
Somite Number 21 - 29 caudal neuropore closes
28 - 32
(week 5)
4 - 6
Somite Number 30 leg buds, lens placode, pharyngeal arches
Stage 13/14 shown in serial embryo sections series of Embryology Program

Stage14 sem1.jpg

Stage 14

Links: Somitogenesis
  1. Sheng G. (2015). The developmental basis of mesenchymal stem/stromal cells (MSCs). BMC Dev. Biol. , 15, 44. PMID: 26589542 DOI.

ANAT2341: Timetable 2018 | Moodle

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Cite this page: Hill, M.A. (2018, June 20) Embryology Lecture - Mesoderm Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Mesoderm_Development

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© Dr Mark Hill 2018, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G