Paraxial and intermediate mesoderm
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Transcript of Paraxial and intermediate mesoderm
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Paraxial and
Intermediate
mesoderm
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Paraxial Mesoderm
• Forms the back of the embryo, along
the spine
• Somites - a series of dorsal paired
segments occurring along the
notochord in vertebrate embryos.
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Somite Derivatives
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Somitogenesis
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Periodicity
Somites are formed by a “clock and wave” mechanism:
Oscillating signal (clock) is provided by Notch and Wnt
pathway
Rostral to caudal gradient provides a moving “wave” of an FGF
With each wave of gene expression, another somite is formed
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Separation
Ephrin tyrosine kinase
receptors (Eph A4) and
their ligands (Ephrin B2)
are able to elicit cell- cell
repulsion between
posterior somite and
migrating neural crest
cells.
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Epithelialization
• A formed somite is
surrounded by
epithelial cells.
• Synthesis of
extracellular matrix
proteins, Fibronectin
and N-cadherin.
• Paraxis gene encodes
for the transcription of
the ECM proteins.
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Specification
• Each somite forms a specific type of
vertebrae
• Tissue specified by:
– notochord (Shh)
– neural tube floor plate (Shh)
– neural tube roof plate (Wnts)
– lateral plate mesoderm (Fgfs)
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Axial specification
Somites are specified according to the Hox genes they
express
Thoracic somite
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Differentiation
• Sclerotome - cartilage of vertebrae and part of rib
• Dermamyotome - remaining portion of the somitecontains precursors for:
• Dermatome – dermis (mesenchymal connective tissue of the skin)
• Myotome – muscle
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• Primaxial (epaxial) myotome – intercostal muscles of the ribs; deep muscles of the back
• Abaxial (hypaxial) myotome – body wall, limbs, tongue
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Determination of Somites
Sclerotome – Shh (high) from the notochord and neural tube floor plate
- Sclerotome cells secrete Pax1 (transcription factor) cartilage/ vertebrae formation
Dermatome – neurotrophin3 (NT3) and Wnt1 from neural roof plate, forms dermis
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Myotome: Abaxial - Wnt1, Wnt3a from the neural tube
Primaxial - Shh (low), Wnt (epidermis) + BMP4 (lateral plate mesoderm)
Notochord : degenerates through apoptosis (remnants remain as nucleus pulposus)
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Myogenesis
• Specification and differentiation of
myogenic BHLH (basic helix-loop-helix)
proteins: MyoD and Myf5
• Muscle cells come from primaxial and
abaxial lineages in the somite
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Osteogenesis
Bones
– The skeleton is generated from:
• Somites→ vertebrae
• Lateral plate mesoderm → limb bones
• Neural crest cells → brachial arch, craniofacial
bones
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• Intramembranous Ossification
– The direct conversion of mesenchymal tissue into bone
– This process occurs primarily in the bones of the skull
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• BMP activate Cbfa1,
a transcription factor
that transforms
mesenchymal cells to
osteoblasts
• Absence of Cbfa1 is
the elimination of
ossification
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• Endochondral Ossification
– Mesenchymal cells differentiate into cartilage,
and this cartilage is later replaced by bone.
– Bone formation characteristic of the
vertebrae, limbs, and ribs.
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• Blood vessels invade the cartilage model; hypertrophic chondrocytes die; replaced by osteoblasts
• ECM mineralizes • New bone material added
peripherally from the internal surface of the periosteum
• Osteoclasts (lateral plate mesoderm) hollow internal region bone marrow cavity
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Intermediate Mesoderm
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Kidney
• Nephron – functional unit, contains over
10,000 cells with at least 12 different cell
types
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1. Pronephros
- pronephric duct arises in
the intermediate mesoderm
- duct cells migrate caudally
- anterior of the duct induced
the adjacent mesenchyme
to form pronephric tubules
- pronephros forms
functioning kidney of fish
larvae and amphibian
larvae
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as the pronephric tubules degenerate, middle portion of nephric duct induces a new set of kidney tubules in the adjacent mesenchyme:
2. mesonephros
- as more tubules are induced caudally, anterior tubules begin to regress via apoptosis
- mesonephros function as one of the main sources of the hematopoietic stem cells
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3. metanephros - permanent kidney of
amniotes
- metanephrogenic mesenchyme is
committed and forms in the posterior
regions of the intermediate mesoderm
- induces the formation of a branch from
each of the paired nephric ducts
- ureteric buds eventually separate form the
nephric duct; become collecting ducts and
ureters - take urine to the bladder
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