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Bio 127 - Section III ______Organogenesis

______Paraxial and Intermediate Mesoderm Gilbert 9e – Chapter 11 ______

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Organogenesis Encompasses : ______

______• The Emergence of the Ectoderm • Neural Crest Cells and Axonal Specificity ______• Paraxial and Intermediate Mesoderm • Lateral Plate Mesoderm ______• Endoderm ______

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Student Learning Objectives ______

1. You should understand that the mesoderm forms all of the organs ______between the ectodermal wall and the endodermal tissues.

2. You should understand that the paraxial mesoderm forms structures at the back of the embryo surrounding the spinal cord, including the ______somites and their derivative cartilage, bone, muscle and dermis.

3. You should understand that the intermediate mesoderm forms the structures of the urogenital tract, including the kidneys, gonads, ______ductwork and the adrenal cortex.

4. You should understand that the mesoderm helps both the ectoderm and the endoderm form their own tissues. ______

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______The mesoderm forms during gastrulation and neurulation, same as ectoderm ______

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Major lineages of the mesoderm ______Somite Terminology: sclerotome: vertebral and rib cartilage myotome: muscles of back, rib cage, abdomen dermamyotome: dermal cells, limb muscle ______syndetome: most dorsal, tendons arthrotome: most central, vertebral joints/discs, proximal ribs “unnamed”: most posterior, dorsal aorta and intervertebral arteries ______

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Paraxial mesoderm is made up of head mesoderm and somites ______

We’ll look closely at the somites.... ______

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The head mesoderm forms the ______muscles and connective tissues of the head and eyes. It even forms under the direction of different transcription factors and suffers different disease states. ______

______Somitogenesis ______

______1. Establishment of periodicity 2. Fissure formation (separation) ______3. Epithelialization 4. Specification ______5. Differentiation ______

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Somitogenesis: Periodicity ______

______Every 90 minutes in chick Periodic (less exact in mice) formation of somites Total of 50 in chick ______is inherant 65 in mice to the cells 500 in snakes of the Notch and Wnt signals ______mesoderm oscillate like a clock

FGF signals sweep rostral-to-caudal in wave ______

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Delta-Notch are expressed at presumptive boundaries ______

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______Delta-Notch dictates WHERE a somite ______will form ______

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______Notch controls the wavelike expression of hairy1 ______

Where Notch is expressed Hairy-1 stays on long-term ______

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The posterior edge is the edge that signals separation ______

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Fissure Formation: Separation from unsegmented mesoderm ______

______The FGF wavefront sets up an oscillation in Wnt and Notch signaling as it passes ______

______Notch expression gives final position of Hairy-1 ______Hairy-1 causes Ephrin expression which repels neighbors (remember how Ephrin repelled motor axons here also) ______

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Epithelialization of somites ______

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That same posterior ______edge starts mesenchymal to epithelial ______transition

- N-cadherin ______- rho family - actin change ______

______Specification of paraxial mesoderm ______

occurs early due to Hox expression.... ______

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transplants form what they would have in original position ______

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Determination and differentiation in somites ______

______• All of the cells of the somite are competent to form all of the derivative cell types ______– cartilage, bone, muscle, tendons, dermis, vascular cells, meninges ______• Their fate depends on their position near the neural tube, notochord, epidermis and ______intermediate mesoderm ______

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Determination and differentiation in somites ______

First step is notochord induction of sclerotome ______

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______Epithelial to mesenchymal transition causes them to migrate to form vertebral cartilage, leaves dermamyotome epithelium ______

______Determination and differentiation in somites ______

The second step is the segregation of dermamyotome ______

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______Central and bilateral myotome surrounds dermatome ______

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Determination and differentiation in somites ______

Dermatome forms back dermis, brown fat ______

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- Primaxial myotome forms back and intercostal muscles ______- Abaxial myotome forms abdominal muscle, tongue, limbs - Central myotome proliferates madly and makes most cells ______

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Figure 11.12 Primaxial and abaxial domains of vertebrate mesoderm (Part 2) ______

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______Mechanisms of Tissue Formation from Somites ______

• Myogenesis: Muscle Formation ______

• Osteogenesis: Bone Formation ______

• Vascular Replacement in the Dorsal Aorta ______

• The Syndetome: Tendon Formation ______

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Myogenesis: Muscle Formation ______

• The paraxial, abaxial and central somite ______

• Cells in the center give rise to satellite cells ______– maybe stem cells, maybe committed progenitors – remain viable for the life of the organism – exit cell cycle upon injury and differentiate to muscle ______

• Classic skeletal muscle differentiation – paracrine signals induce MyoD, Myf-5 ______– TFs for muscle genes and for themselves! ______

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Myogenesis: Muscle Formation ______

Adult muscle cells (myotubes) are large and multinucleated ______

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muscle satellite cells don’t express MyoD until injury ______

______Myogenesis: Muscle Formation ______

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Myogenesis: Muscle Formation ______

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In culture it doesn’t ______matter what species you place together they will fuse. ______

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Osteogenesis: Bone Formation ______

• Four different sources of bone: ______– Somites form the axial skeleton – Lateral plate mesoderm form the limb skeleton – Cranial neural crest forms the bones of face and head ______– Mesodermal mesenchyme in patella, periosteum

• Two different processes: ______– Endochondrial ossification in the first two – Intramembraneous ossification in the second two ______

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______Osteogenesis: Bone Formation ______

Endochondrial literally means “within cartilage” ______bone model ______- vertebrae hypertrophic -ribs chondrocytes leave cell cycle, -pelvis enlarge, calcify -limbs their ECM and ______then apoptose ______Shh ______

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Osteogenesis: Bone Formation ______

The step-wise progression continues out away from the center – “growth plates” ______

______The calcified The center is ECM plus Ihh remodeled by cause bone cells osteoclasts from (osteoblasts) to ______the blood to differentiate from form marrow somite progenitors ______Bone growth ceases when the secondary center finishes up ______

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Osteogenesis: Bone Formation ______

No calcium Normal bone formation ______

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______Endochondrial Ossification of Vertebrae ______

1. Sclerotome ______mesenchyme are attracted 2. As motor axons by notochord extend toward and neural tube muscles they go ______secretions through sclerotome and split them rostral-to-caudal ______

The caudal end of one then recombines with the rostral ______end of the next to form the bone model and then bone ______

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Vascular Replacement in the Dorsal Aorta ______

Blood vessels are a single layer of endothelium surrounded by multiple layers of smooth muscle ______

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______The dorsal (or descending) aorta forms a primary model by vasculogenesis and then both the endothelium and smooth muscle are replaced by somite. (the same thing happens to the ascending aorta by neural crest cells!) ______

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The Syndetome: Tendon Formation ______

Tendon joins bone to muscle. The last row of sclerotome is induced by the overlying myotome to differentiate into those connectors. ______

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______Formation of the Kidneys from Intermediate Mesoderm ______

• The adult kidney is very complex ______– A single nephron has 10,000 cells, 12 cell types – Each is positioned exactly for its job relative to others ______

• The embryo increasingly needs to filter blood ______– IM mesoderm 1st forms organizer, the pronephric duct – This tissue then induces three stages of kidney ______– The first two are transitory, the third persists ______

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General scheme of development in the vertebrate kidney ______

Pronephros is functional in ______fish, amphibians, not in mammals, then degenerates ______

Mesonephros is Nephric duct functional in some is the primitive mammals, including ______organizer: humans, degenerates Wolffian Duct in females, forms epididymous and vas deferens . ______

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Metanephros formed by reciprocal induction with Wolffian Duct ______

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Intermediate mesoderm ______mesenchyme develops into kidney, while.... ______

______Figure 11.26 Kidney induction observed in vitro ______

....the Wolffian Duct matures into the collecting duct ______

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