Organogenesis(1). Somitogenesis and derivatives of somites

1. Organogenesis(1). Somitogenesis and derivatives of somites M.A.Kai-Kai

2. Learning Objectives • Understand process of somitogenesissegmental pattern of somitomeresand somites along the neural tube. • Review the adult derivatives of each of the threemorphological subdivisions of the somitesdermatome, sclerotome and myotome. • Understanding of patterns of osteogenesisfrom the sclerotome. • Understanding the process of myogenesis. • Examples of congenital malformations in development of the skeleton.

3. FORMATION OF THE MAMMALIAN GASTRULA - 7 Different types and locations of mesoderm are a result of different routes of migration through and under the primitive streak Dorsal view of embryonic disc Rostral EMBRYONIC 3 1 Chordamesoderm DISK 2a 2 Paraxial mesoderm 5 1 (a) Segmented (b) Unsegmented Head mesoderm HENSEN'S 3 NODE 4 Embryonic 4 lateral mesoderm PRIMTIVE Extra-embryonic STREAK 5 lateral mesoderm 6 6 Intermediate mesoderm 2b Caudal

4. ADULT DERIVATIVES OF THE TYPES OF MESODERM MESODERM TYPE INTERMEDIATE STRUCTURE MESODERM DERIVATIVE Head Head muscle, skull cartilage Chorda- Notochord Limb muscles Axial skeleton Paraxial Somites Trunk muscles ICM EPIBLAST MESODERM Dermis Parts of kidney and reproductive tract Intermediate Limb skeleton Heart Lateral Body cavity dividers Blood cells Amnion, Chorion Yolk sac, Allantois

5. Somitogenesis(1) • Process of segmentationdevelopment of axial systemvertebrae, muscles and innervation • Somitesform from paraxial mesoderm in anterior-posterior gradient, begins at neurulation. • Two parallel columns of mesodermal cells form along the longitudinal axis, on each side of the notochord and neural tube. • Transverse fissures form in the columnsforming somitomeres in cranial-caudal direction. • First sevenpairs of cranial somitomeres form head mesenchymemigrate, form masticatory and facial muscles. • Somite8-46(rabbit) become segmented into block-like somites. Number of pairs constant for each species. • Mechanisms of compactionlaminin, collagen and fibronectin increases cell-to-cell adhesions and gap junctions.

6. Mechanism of Somitogenesis(2) The somites are cubic blocks of paraxial mesoderm lying on either side of the neural tube • Periodicitysomites bud off in anterior – posterior direction(Notch and Wntgene). 2.Fissure formationsomitomeres, compact 3,Epithelialisationmesenchymal cells form epithelial(Paraxis) --synthesise extracellular matrix --fibronectin and N-cadherin adhesion protein rearrange outer cells of each somite into epithelium 4.Specificationform specific structures,according location and expression of Hox gene determined early in somitogenesis. --Ventromedialsclerotome axial skeleton and tendons --Dorsolaterallayerdermatome, form dermis of skin. --Ventral layermyotome; form axial and appendicular muscles. 1&2 4&5 3

7. Vertebra/axial skeleton and tendon Dermis Dermatome Myotome Muscles of back Sclerotome Shoulder muscle Limb muscle Muscles of body wall SOMITOGENESIS -3 Migration from differentiated regions of the somite give rise to dermis, musculature and axial skeleton SOMITE 9 OF 12 SOMITE CHICK (33 h) Neural tube Pronephric tubule (see later) Splanchnopleure Aorta Somatopleure 5.Differentiationcommitted to specific cell lineage within each region.e.g. myotomemuscles of back(close to neural tube), abaxial muscles of body wall(farthest from neural tube).

8. DERIVATIVES OF SOMITES(4) Somites Paraxial mesodermSomitogenesis Ventromedial region Dermomyotome Sclerotome Ventral region Dorsolateral region Myotome Chondrocytes Dermatome Skeletogenic cells Myogenic cells Cartilage Dermis and supporting tissues Myoblasts Tendon Ossification into bone Red arrows show Regulation by genes Axial and appendicular Skeletal muscles

9. Osteogenesis:The development of bones(1) Calcified organic matrix Mesoderm Mesenchymal cells Osteoprogenitor cells Osteoblasts: bone forming cells Osteocytes: mature bone cells Haversian Canals: surround blood vessels & nerve cells Osteoclasts: a cell that breaks down bone

10. Skeletal System(2) • The skeletal system consists mainly of boneand cartilageprovides supporting framework for muscle • Bone is specialised connective composed ofcells, organic matrix and inorganic matrix. • Bone is formed by process of oestogenesis • Cell typesosteoblasts, osteocytes and osteoclasts participate in oestogenesis. • The organic matrix consists of type I collagen and amorphous ground substance containing proteoglycans forms about one-third of bone mass. • Two-thirds of bone is mineralised matrix of calcium phosphate in form of hydroxyapatite crystals. • Bone has range of physical properties giving high degree of flexibility, undergoes continual replacement and remodelling.

11. Somite Regulation of development of the musculoskeletal system(3). Wnt Shh + (Notochord & neural Plate) - BMP Myotome (muscle) • Myotomeinduced by Wnt genes • Sclerotome formation is regulated by fibroblast growth factors(Fgf)from myotome and sonic hedgehog(Shh) gene secreted by notochord and neural tube • Shh inhibits bone morphogenetic protein(BMP) gene in part that forms tendon. • Soxstimulates formation of cartilage, but inhibits scleraxisgene(sclerotome forms tendon). • Cartilage cells ossifies into bone. Sclerotome Fgf Sox Scleraxis Cartilage Bone Red arrows show Regulation by genes Tendon

12. Osteogenesis:The development of bones(4) • Two major methods of osteogenesis: 1.Intramembranous ossification, no cartilagenous stage Mesenchymal(neural crest) condense to form osteoblastform osteoid matrixcacified/osteocytes(e.g flat bones of the skull. 2. Endochondral ossification. Mesenchymal cellscartilage ossification into bone(e.g.long bones).

13. (5) Fig.2

14. Osteogenesis: Enchondral ossification(6) Fig.3 The sclerotome cell can become a chondrocyte characterised by Sox9 or an osteocyte (osterix transcription factor). Chrondrocyte secrete inhibitor factor that repress the bone pathway. Pre-osteoblast & Osteoblast

15. MYOGENESIS(1) • Myotomal cells express transcription factor ;myogenic proteinscells migrate to sites and induces muscle differentiation(A). • Cells proliferate and form committed progenitor myoblasts(B). • Committed myoblasts divide, induced by fibroblast growth factors. • Cell alignment under influence of cell adhesion molecules(C). • Myoblasts fuse to form myotubes (D). • And maturation of myotubes(E). • Stem muscle fibres form, begin contraction(F) (Gilbert 2006) Fig.5

16. MYOGENESIS(2) • Group into cranial-caudal axis. • Dorsal of transverse processes of vertebraeform epaxial muscles. • Ventral of transverse processes form hypaxial muscles and muscles of body wall • Extrinsic and intrinsic muscles of limbs are derived from myotome • Some skeletal of limbs, cardiac and smooth muscles derived from mesoderm Epaxial muscles Cervical vertebra D Transverse process V Hypaxial muscles

17. Malformations(1) Bone • Osteogenetic defectsinherited characterised by extreme fragility of bones, long bones prone to fracture • Vertebral defects --spinal bifidaocculta/block vertebrae fusion of 2 or more adjacent vertebrae and hemivertebrae. --hemivertebraeonly one half develops, condition confined to thoracolumbar region results from failure of sclerotome differentiation on one side of developing vertebral Body --cervical vertebraeabnormal segmentation of caudal occipital and cervical sclerotomes result in atlantoaxial malformations --abnormal curvatureof vertebral column; Lordosisabnormal ventral curvature Kyphosisabnormal dorsal curvature. --short-spined dogs results from compaction and fusion of thoracic and lumbar vertebrae --Stenosis of vertebral foramen constricts the spinal cord and neurological defects. • Rib defectsassociated with abnormalities of vertebral column or sternum • Sternal defects --.incomplete fusion of paired sternal bbones during morphogenesis --associated with ectopic heart. • Limb defects

18. Malformation of Limb Development(2) • Autonomyof development produces many abnormalities. • Limb reductionsinvolve loss of specific parts,e.g. • Ameliacomplete absence of limb • Ectromeliapartial or complete absence of parts, e.g.carpal ectromelia. 3.Micromelialimb reduced in size. • Limb duplications. • Polydactylyextra digits • Whole or partial limbs • Limb and joint deformities. • Arthrogryposiscrooked limb, heredity in animals. • Deficiencyin gene expression e.g.Hox and BMP. Polydactyly

20. Summary • Somitogenesis is the process of segmentation of the paraxial mesoderm in anterior-posterior gradient, begins at neurulation. • The first seven pairs of cranial somitomeres form head mesenchyme,migrate to specific regions and form masticatory and facial muscles. • Somite 8-46(rabbit) become segmented into block-like somites. Number of pairs constant for each species. • Each somite has three morphological regions.Ventromedialsclerotome chondrocytesform axial skeleton and syndetome from within sclerotome form tendons. The dorsolateral layerdermatome, form dermis of skin.Ventral layermyotome; form axial and appendicular muscles. • The molecular mechanisms regulating osteogenesis and myogenesis are transcription factors, growth factors,the Hox gene, Shh,BMP, and Wnt genes. • Few examples of common congenital malformations in development of the axial skeleton and in limb development

21. References • Carlson, B. M., Foundations of Embryology (6th.Edition) 1996. McGraw-Hill inc. London. Page 393 - 424 • Gilbert, S.F., Developmental Biology (8th. Edition) 2006. Sinauer Associates Inc. Sunderland, Massachuetts. USA. Page 505 -527 • McGeady, T.A., Quinn, P.J., Fitzpatrick, E.S., & Rayan, M.T., (2006). Veterinary Embryology. Page 184 -203 • Noden, D.M., DeLaHunta, A., The Embryology of Domestic Animals. 1985, Williams & Wilkins. London. Page196 -206