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

Learning Objectives   Understand process of somitogenesis  segmental pattern of somitomeres and somites along the neural tube.   Review the adult derivatives of each of the three morphological subdivisions of the somites  dermatome, sclerotome and myotome.   Understanding of patterns of osteogenesis from the sclerotome.   Understanding the process of myogenesis.   Examples of congenital malformations in development of the skeleton.

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

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

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

Somitogenesis(1)   Process of segmentation  development of axial system  vertebrae, muscles and innervation   Somites form 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 seven pairs of cranial somitomeres form head mesenchyme  migrate, form masticatory and facial muscles.   Somite 8-46(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.

1. Periodicity  somites bud off in anterior – posterior direction(Notch and Wnt gene). 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 --Dorsolateral layer  dermatome, form dermis of skin. --Ventral layer  myotome; form axial and appendicular muscles. The somites are cubic blocks of paraxial mesoderm lying on either side of the neural tube Mechanism of Somitogenesis(2) 1&2 3 4&5

Sclerotome Myotome Dermatome SOMITE 9 OF 12 SOMITE CHICK (33 h) Aorta Pronephric tubule (see later) Splanchnopleure Somatopleure Neural tube SOMITOGENESIS -3 Migration from differentiated regions of the somite give rise to dermis, musculature and axial skeleton Vertebra/axial skeleton and tendon Muscles of back Shoulder muscle Limb muscle Muscles of body wall Dermis 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). (dorsolateral) (Ventromedial) (Ventral)

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

Skeletal System(2)   The skeletal system consists mainly of bone and 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.

Regulation of development of the musculoskeletal system(3).   Myotome induced 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.   Sox stimulates formation of cartilage, but inhibits scleraxis gene(sclerotome forms tendon).   Cartilage cells ossifies into bone. Myotome (muscle) Fgf Sclerotome Cartilage Scleraxis Tendon Shh + - BMP Somite Wnt Sox Red arrows show Regulation by genes Bone (Notochord & neural Plate)

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).

Fig.2 (5)

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 Fig.3 Osteogenesis: Enchondral ossification(6)

  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) MYOGENESIS(1) Fig.5

Malformations(1) Bone   Osteogenetic defects  inherited characterised by extreme fragility of bones, long bones prone to fracture   Vertebral defects --spinal bifida occulta/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 curvature of 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

Malformation of Limb Development(2)   Autonomy of development produces many abnormalities.   Limb reductions  involve loss of specific parts,e.g. 1. Amelia  complete absence of limb 2. Ectromelia  partial or complete absence of parts, e.g.carpal ectromelia. 3.Micromelia  limb reduced in size.   Limb duplications. 1. Polydactyly  extra digits 2. Whole or partial limbs   Limb and joint deformities. Arthrogryposis  crooked limb, heredity in animals.   Deficiency in gene expression e.g.Hox and BMP. Polydactyly

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

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