Lecture - Mesoderm Development

From Embryology

Objectives

Mesoderm formation
  • 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
Podcast icon.jpg Lectopia Lecture Audio

Lecture 5 Audio

References

The Developing Human: Clinically oriented embryology

The Developing Human, 8th edn.jpg Citation: The developing human : clinically oriented embryology 8th ed. Moore, Keith L; Persaud, T V N; Torchia, Mark G Philadelphia, PA : Saunders/Elsevier, c2008.

Larsen's human embryology

Larsen's human embryology 4th edn.jpg Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009.

UNSW Embryology

Logo.png Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.

Notochord (Axial mesoderm)


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

Mesoderm Development

The trilaminar embryo

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

Mesoderm cartoon1.gif Mesoderm cartoon2.gif

Mesoderm cartoon3.gif Mesoderm cartoon4.gif


Mesenchyme

  • Embryonic connective tissue, describes the cell morphology
    • epithelial to mesenchymal transitions
    • mesenchymal to epithelial transitions

Paraxial Mesoderm

Hamburger & Hamilton Stage 10 (33 hours)
  • lies adjacent to 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
    • 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
Mesoderm cartoon2.gif

Somite Formation

Carnegie stage 9 scanning electron microscope image showing somite formation
Carnegie stage 13 somitocoel
Carnegie stage 13 sclerotome
Somitogenesis 01 icon.jpg
Early somite induction signals in the mouse

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


  • 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


Sclerotome

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"

Dermomyotome

  • 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

  • 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)

Muscle

  • 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
Mesoderm cartoon2.gif

Lateral Plate Development

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

Mesoderm cartoon2.gif Stage 7 lateral plate

Intraembryonic Coelom

Mesoderm cartoon3.gif

  • 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

Mesoderm cartoon3.gif

The intraembryonic coelom divides the lateral plate into 2 portions

  • closest to ectoderm
  • body wall osteogenic, chrondrogenic and fibrogenic
  • except ribs and scapula

Splanchnic Mesoderm

  • closest to endoderm
  • heart, smooth muscle of gastrointestinal tract (GIT) and blood vessels
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

Somitogenesis

Stage
Days (approx)
Size
(mm)
Images
(not to scale)
Events
7
15 - 17
(week 3)
0.4
Link=Carnegie_stage_7
gastrulation, notochordal process
8
17 - 19
1.0 - 1.5
Link=Carnegie_stage_8
primitive pit, notochordal canal
9
19 - 21
1.5 - 2.5
Link=Carnegie_stage_9
Somite Number 1 - 3 neural folds, cardiac primordium, head fold
10
22 - 23
(week 4)
2 - 3.5
Link=Carnegie_stage_10
Somite Number 4 - 12 neural fold fuses
11
23 - 26
2.5 - 4.5
Link=Carnegie_stage_11
Somite Number 13 - 20 rostral neuropore closes
12
26 - 30
3 - 5
Link=Carnegie_stage_12
Somite Number 21 - 29 caudal neuropore closes
13
28 - 32
(week 5)
4 - 6
Link=Carnegie_stage_13
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

Glossary Links

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Cite this page: Hill, M.A. (2020, November 25) 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 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G