Paraxial and

Intermediate

mesoderm

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.

Somite Derivatives

Somitogenesis

50 – chicks

65 – mice

500 – snakes

Periodicity

Somites are formed by the:

Notch pathway – induces hairy1 expression

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.

Epithelialization

• Synthesis of

extracellular matrix

proteins, Fibronectin

and N-cadherin will

form tight junctions and

generate their own

basal laminae

• Paraxis gene encodes

for the transcription of

the ECM proteins.

Axial specification

– Each somite forms a specific type of vertebrae• Thoracic vertebrae: ribs

• Cervical vertebrae: neck

– Somites are specified according to the Hox genes they express

Thoracic somite

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

• Primaxial (epaxial) myotome – intercostal muscles of the ribs; deep muscles of the back

• Abaxial (hypaxial) myotome – body wall, limbs, tongue

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

Myotome:

• Abaxial - Wnt1, Wnt3a from the neural tube, forms body wall, limbs, tongue

• Primaxial - Shh (low), Wnt (epidermis) + BMP4 (lateral plate mesoderm),

forms muscle of the back

Notochord : degenerates through apoptosis (remnants remain as nucleus pulposus)

Myogenesis

• Muscle formation

• Specification and differentiation of

myogenic BHLH (basic helix-loop-helix)

proteins

Myogenesis

Muscle cell fusion

Osteogenesis

Bones

– The skeleton is generated from:

• Somites→ vertebrae

• Lateral plate mesoderm → limb bones

• Neural crest cells → brachial arch, craniofacial

bones

• Intramembranous Ossification– The direct conversion of mesenchymal tissue into bone

– This process occurs primarily in the bones of the skull

• BMP activate Cbfa1,

a transcription factor

that transforms

mesenchymal cells to

osteoblasts

• Absence of Cbfa1 is

the elimination of

ossification

• Endochondral Ossification

– Mesenchymal cells differentiate into cartilage,

and this cartilage is later replaced by bone.

– Bone formation characteristic of the

vertebrae, limbs, and ribs.

Collagen X

and fibronectin

Osteoclasts

Intermediate Mesoderm

Kidney

• Generates the urogenital system, the

kidneys and gonads

• Nephron – functional unit, contains over

10,000 cells with at least 12 different cell

types

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

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

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 that take urine to the bladder

WT1 → secrete GNDF and HGF → uteric bud formation