Paraxial and intermediate mesoderm

A ppt presentation for Developmental biology with the Paraxial and Intermediate Mesoderm

1. Paraxial and Intermediate mesoderm

2. 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.

3. Somite Derivatives

4. Somitogenesis somitomeres Paraxial Mesoderm ( presomitic mesoderm) somites

5. 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

6. 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.

7. 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.

8. 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 )

9. Axial specification 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) Myotome – muscle

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 Primaxial - Shh (low), Wnt (epidermis) + BMP4 (lateral plate mesoderm) Notochord : degenerates through apoptosis (remnants remain as nucleus pulposus )

14. 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

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

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

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

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

19. 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

20. Intermediate Mesoderm

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

22. 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

23. 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

24. 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