Lecture - Mesoderm Development

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

Notochord (Axial mesoderm)

Stage 7 embryonic disc
Stage 7 primitive-streak-node
Stage 7 cloacal-oral-membranes
Stage 7 notochord

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

Stage 7 paraxial mesoderm
Stage 7 intermediate mesoderm
Stage 7 lateral plate

Paraxial mesoderm - somites - musculoskeletal structures
Intermediate mesoderm - kidney
Lateral plate mesoderm - body wall structures

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.

Mesenchyme

Embryonic connective tissue, describes the cell morphology (Histology is not epithelial organization)
- 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

Somite Formation

Carnegie stage 9 scanning electron microscope image showing somite formation

Early somite induction signals in the mouse

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

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

later divides into dorsal dermatome and ventral myotome
- (MH - 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

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

Axial Segmentation - Somite Specification Signals

Intermediate Mesoderm

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

Lateral Plate Development

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
small spaces 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
- 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
when the embryonic disc folds the intraembryonic coelom will form all 3 major body cavities:

Pericardial
Pleural
Peritoneal

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

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

Co-ordinator Note

Dr Mark Hill

ANAT2341 Embryology S2 2011

Lecture content is made available on the Monday of the associated lecture week.
Some guest lecturers will provide additional handouts or lecture notes.
For background information see Embryonic Development and Systems Notes
2011 Timetable | 2011 Students

--Mark Hill 06:47, 20 July 2011 (EST)

Course Content 2011

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Cite this page: Hill, M.A. (2024, June 2) Embryology Lecture - Mesoderm Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Mesoderm_Development

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