Paraxial and intermediate mesoderm form important structures. Paraxial mesoderm forms somites through a "clock and wave" mechanism, with each somite giving rise to vertebrae, muscle, and dermis. Somites are specified by surrounding tissues to develop appropriately. Intermediate mesoderm forms the kidney, with the pronephros, mesonephros, and metanephros appearing sequentially in vertebrate development. The metanephros becomes the permanent kidney in amniotes.

1. Paraxial and
Intermediate

3. 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.

4. Somite Derivatives

5. Somitogenesis

6. 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

7. 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.

8. 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.

9. 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)

10. Axial specification
Somites are specified according to the Hox genes they
express
Thoracic somite

11. Differentiation
• Sclerotome - cartilage of vertebrae and part of rib
• Dermamyotome - remaining portion of the somite
contains precursors for:
• Dermatome – dermis (mesenchymal connective tissue of the
skin)

12. • Primaxial (epaxial) myotome – intercostal muscles of
the ribs; deep muscles of the back
• Abaxial (hypaxial) myotome – body wall, limbs,
tongue

13. 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

14. 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)

15. 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

18. Osteogenesis
Bones
– The skeleton is generated from:
• Somites→ vertebrae
• Lateral plate mesoderm → limb bones
• Neural crest cells → brachial arch, craniofacial
bones

19. • Intramembranous Ossification
– The direct conversion of mesenchymal tissue into bone
– This process occurs primarily in the bones of the skull

20. • BMP activate Cbfa1,
a transcription factor
that transforms
mesenchymal cells to
osteoblasts
• Absence of Cbfa1 is
the elimination of
ossification

21. • Endochondral Ossification
– Mesenchymal cells differentiate into cartilage,
and this cartilage is later replaced by bone.
– Bone formation characteristic of the
vertebrae, limbs, and ribs.

23. • 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

25. Intermediate Mesoderm

26. Kidney
• Nephron – functional unit, contains over
10,000 cells with at least 12 different cell
types

27. 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

28. 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

29. 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