Developmental Biology

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
50 – chicks
65 – mice
500 – snakes

6. Periodicity
Somites are formed by the:
 Notch pathway – induces hairy1 expression

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
• 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.

9. 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

10. 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)

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

12. 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

13. 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)

14. Myogenesis
• Muscle formation
• Specification and differentiation of
myogenic BHLH (basic helix-loop-helix)
proteins

15. Myogenesis

16. Muscle cell fusion

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

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

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

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

21. Collagen X
and fibronectin
Osteoclasts

23. Intermediate Mesoderm

24. Kidney
• Generates the urogenital system, the
kidneys and gonads
• Nephron – functional unit, contains over
10,000 cells with at least 12 different cell
types

25. 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

26. 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

27. 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

28. WT1 → secrete GNDF and HGF → uteric bud formation