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Learning Objectives
Development Of Mesoderm, Paraxial Mesoderm And
ScleroMyotome And Formation Of Cartilages
At the end of the presentation, the student should be
able to :
1. Define Epiblast.
2. Term Hypoblast.
3. Describe Chorionic cavity.
4. Let know the formation of mesoderm.
5. Tell the differentiation of Trilaminar Germ Disc.
6. Set apart the germ layer derivatives.
7. Describe Somitogenesis.
8. Tell the development of Skeletomuscular system.
9. Explain Myogenesis.
10. Describe the development of cartilage.
LECTURE OUTLINE
DEVELOPMENT OF MESODERM, PARAXIAL
MESODERM AND
SCLEROMYOTOME AND FORMATION OF
CARTILAGES
1.
Formation Of Epiblast And Hypoblast :
By the 8th day, the inner cell mass differentiates into
two layers of cells.
Epiblast (Columnar)
Bilaminar Germ Disc
Hypoblast (Cuboidal)
Epiblast: (Primary Ectoderm).
The epiblast, whilst referred to as the primary ectoderm,
differentiates to form all three layers of the trilaminar
embryonic disc in a process called gastrulation. It lies above the
hypoblast.
Hypoblast: (Primary Endoderm).
It lies beneath the epiblast and consists of small cuboidal
cells. Extraembryonic endoderm (including Yolk sac) is derived
from hypoblast.
Hypoblast→ Heuser’s Membrane →Primary Yolk Sac.
Extra-embryonic Mesoderm →
Extra-embryonic Cavity (chorionic cavity)
--visceral layer.
--parietal layer.
Secondary yolk sac: yolk sac.
Body Stalk (Connecting Stalk):
Formed by extra-embryonic mesoderm.
Formation Of Mesoderm:
In the early of the 3rd week.
--Primitive Streak:
Cells of epiblast proliferate to form a
longitudinal arranged cell cord.
---primitive streak
---primitive node (primitive knot)
---primitive pit (blastopore)
---Mesoderm: Intra-embryonic mesoderm.
---Endoderm: Hypoblast cells are replaced by epiblast cells.
---Ectoderm: Epiblast changed the name into ectoderm.
By the end of the 3rd week:
Trilaminar Germ Disc:
endoderm + mesoderm + ectoderm.
---Head Process→ Notochordal Tube → Notochord
---Buccopharyngeal membrane.
---Cloacal membrane.
3.Differentiation Of Trilaminar Germ Disc:
4th –8th weeks.
---Differentiation:
Some cells which are primordial and immature
differentiate into different cells which have specific structure
and function.
---Induction: Some tissues effect the differentiation, and
determine the differentiating orientation of another tissue.
(2) Differentiation of Mesoderm:
17th day.
---Paraxial Mesoderm:
Somite: 20th days, 3 pairs/per day, 42-44 pairs by the end of
5th weeks.
-Sclerotome: →bone, cartilage.
-Dermatome: → dermis and hypodermis.
-Myotome: → skeletal muscle.
---Intermediate Mesoderm:
→ Kidney and reproductive system.
---Lateral Mesoderm:
Intra-embryonic coelom: →body cavity.
Parietal Or Somatic Mesoderm: →muscle, CT, parietal layer of
pleura, peritoneum and pericardium.
Visceral Or Splanchnic Mesoderm: →muscle, CT of digestive
tract, visceral layer of pleura, peritoneum and pericardium.
Somitogenesis (1):
 Process of segmentation development of axial system 
vertebrae, muscles and innervations.
 Somites form from paraxial mesoderm in anterior-posterior
gradient, begins at neurulation.
 Two parallel columns of mesodermal cells form along the
longitudinal axis, on each side of the notochord and neural
tube.
 Transverse fissures form in the Columns  forming
somitomeres in cranial-caudal direction.
 First seven pairs of cranial somitomeres form head
mesenchyme  migrate, form masticatory and facial
muscles.
 Mechanisms of compaction  laminin, collagen and
fibronectin increases cell-to-cell adhesions and gap
junctions.
Somitogenesis(2):
1. The periodicity of somite formation.
--Formation of somites depends upon ‘clock and wave’mechanism.
--The ‘clock signal’ is Notch and Wnt genes in rostral  caudal
direction.
--A pair formed/90mins. Avian embryos aged by number of
somites.
--Variable in mammals. Paired somites synchronised.
--Clock set in presomitic mesoderm.
2. Three morphological regions in Mitosis.
Ventromedial  sclerotome 
Chondrocytes  form axial skeleton.
--Syndetome arising within sclerotome form tendons.
 Dorsolateral layer  dermatome, form dermis of
skin.
 Ventral layer  myotome; form axial and
appendicular muscles.
Mechanism of Somitogenesis (3):
1.
Periodicity 
Somites bud off in anterior – posterior
direction (Notch and Wnt).
2. Fissure formation 
Somitomeres, compaction,
regulated by ephrin tyrosine kinase.
3, Epithelialisation 
Mesenchymal cells form
epithelial.
--Synthesize extracellular matrix organising protein;
fibronectin and N-cadherin adhesion protein  rearrange
outer cells of each somite into epithelium.
--Fibronectin regulated by transcription factors
(Paraxis and Mesp2).
4. Specification
 Form specific structures, specified
according location and expression of Hox gene determined
early in somitogenesis of somite depend upon location.
5. Differentiation 
Committed to specific cell lineage
within each region. E.g. dermatome  dermis, primaxial
muscles (close to neural tube), abaxial muscles (farthest from
neural tube).
Development of the skeletomuscular system.
 Sclerotome formation induced by Sonic hedgehog gene
secreted by notochord and floor plate of neural tube.
 Sclerotome expresses transcription factors, Pax-1 and Pax9 induces mitosis  sclerotomal cells  mesenchymal cells
 cartilage  ossifies into bone.
 Dorsal sclerotome influenced by Fgf8 secreted by myotome
secretes scleraxis  tendon.
 Myotome  axial muscles.
Myogenesis:
 Paracrine factors instruct myotomal cells to become
muscles by inducing the synthesis of MyoD 
migration to sites.
A. Determination of myotome cells by Paracrine
factors.
B. Committed myoblasts divide, induced by fibroblast
growth factors (FGF).
C. Cell alignment under influence of cell adhesion
molecules.
D. Myoblasts stop dividing and fuse to form myotubes.
E. Maturation of myotubes.
F. Stem muscle fibres form, begin contraction.
Cartilage development A:
An avascular mesenchymal membrane consists of stem
cells, imbedded in a delicate collagen matrix, that make
contact with each other via cytoplasmic processes.
Cartilage development B:
Mesenchymal cells begin to differentiate into
protochondral cells and divide to produce chondroblasts.
Cartilage development C:
The center fills with chondroblasts as the top and
bottom edges continue to differentiate.
Cartilage development D:
Stem cells remain on the edges.
Chondroblasts in the center produce cartilage matrix and
continue to divide so that the cartilage grows by
interstitial growth.
Cartilage development E:
Stem cells at the edges produce fibroblasts that make
the fibrous Perichondrium and add new chondroblasts to
the edge by appositional growth.
Chondroblasts in the center continue to divide, butas the
matrix solidifies, they don’t separate.
Cartilage development F:
As the cartilage matures, stem cells in the chondrogenic
Perichondrium continue to generate new chondroblasts
slowly by appositional growth, and chondrocytes in the
center become relatively dormant.