Chapter 9: EARLY DEVELOPMENT OF VERTEBRATES: BIRDS AND MAMMALS

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ChipzThatLeo
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266783
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Chapter 9: EARLY DEVELOPMENT OF VERTEBRATES: BIRDS AND MAMMALS
Updated:
2014-03-19 16:01:14
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Human Developmental
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Bird Development
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  1. Amniote egg amd human fetus have the same features
    • yolk sac
    • chorin
    • all antois
    • Amnion
    • (Extra embryonic membrane)
  2. Avian Reproduction - Eggs
    Favorite organism for embryological studies

    Internal fertilization in Oviduct, then albumin and shell are secreted

    • Eggs are Telolecithal with a small blastodisc (actual embryo)
    • Cleavage: Formation of Epiblast and Hypoblast in Blastoderm

    Gastrulation: Primitive stalk




    Meroblastic-Incomplete. ex: Teloecithal and Centrolecithal eggs
  3. Cleavage
    • Discoidal meroblastic cleavage – occurs only in blastodisc
    • Further divisions result in a single-layered balstoderm –   basal region is in contact with yolk
    • No cleavage divisions in the yolk

  4. Therefore equatorial and vertical division:
    • 5 to 6 layered
    • Blastoderms held together by tight junctions (adhesion by E-cadherin).
    • Subgerminal space: between the outer layer and the yolk. Several layers of cells
  5. Formation of Chick Blastoderm


    Epiblast – top single layer of blastoderm --> forms embryo

    Cells delaminate and migrate individually into subgerminal cavity to form poly-invagination island (primary hypoblast)






    A sheet of posterior margin of blastoderm (Koller’s sickle) migrate anteriorly, join poly-invagination islands forming secondary hypoblast --> extra-embryonic endoderm



  6. Primitive Streak
    • Primitive streak is the principle structure in avian and mammalian gastrulation – structure through which all endodermal and mesodermal cells must pass

    • A.3-4 hrs after fertilization
    • B.7-8 hrs after fertilization


    • 1. Thickening of epiblast at the posterior region (just anterior to Kollers sickle)
    •   – caused by ingression of endodermal precursors from epiblast into blastocoel and
    •   -  migration of cells from the lateral region of posterior epiblast


    Further migration elongates primitive streak towards future head region
  7. Primitive Streak


    • •Primitive streak defines anterio-posterior axis
    • •Migrating cells enter dorsal side and move towards its ventral side
    • •Separates left from right side of embryo
    • •Further convergence results in depression called primitive groove
  8. Migration through Primitive Streak
    • 1. Cells involute over edge of groove and pit and ingress into blastocoel
    •   2. Once in blastocoel, cells flatten and migrate
    •   3. In chick, the cells migrate individually after undergoing epithelial-to-mesenchymal cell transformation
    •   - Scatter factor (190 kd protein), secreted by the   cells as they enter the streak – possibly down   regulate expression of E-cadherin (adhesion   protein) à loose mesenchymal cells
  9. Hensen’s Node
    • C. 15-16 hrs
    • D. 16-19 hrs


    • Anterior end of primitive streak thickens to form primitive knot (Hensen’s node)
    • Center of the node has a funnel-shaped depression – primitive pit – functional equivalent of dorsal lip of blastopore
  10. Migration through Hensen’s node
    • 1. First batch of cells to enter Hensen’s node --> migrate anterior--> pharyngial endoderm of foregut
    •   2. Second batch of cells --> migrate anterior (not too far) ventrally, remain between endoderm and epiblast --> head mesenchme/ prechordal plate mesoderm
    • These early-ingressing cells form Head process anterior to Hensen’s node
  11. Formation of Presumptive Head and Notochord
    • 3. Next cells migrating through Hensen’s node --> form chordamesoderm (notochord) cells --> extend up to presumptive midbrain and meet precordal plate
    •   4. Cells migrate inwardly through lateral part of primitive streak --> separate into two layers
    •   - deeper layer --> endoderm   
    •   - upper layer --> mesoderm
    • By about 22 hrs, presumptive endodermal cells are inside and mesodermal cells are migrating to their respective positions
  12. Regression of Primitive streak and Hensen’s node
    • Regression of primitive streak pushes the Hensen’s node downwards (posterior)
    • Formation of notochord:
    •   - anterior portion of notochord from the ingressed cells through Hensen’s node
    •   - posterior portion of notochord (downwards from somite 17) from mesodermal tissue ingressed through primitive streak


    Hensen’s node regresses to its most posterior portion --> ANUS
  13. Chick Gastrulation
    Head start: While posterior portion is undergoing gastrulation, cells at  anterior end begin to form organs. Anterior end is more advanced than posterior end for the next several days
  14. The Chick “Organizer"
    • Koller’s sickle cells consists of:
    •   - Hensen’s node (Dark green cells)
    •   – Primitive streak (Red cells)
    • Thus equivalent to amphibian ‘organizer”
    • Hensen’s node is equivalent to amphibian dorsal lip of blastopore
  15. Epiboly of Ectoderm
    • Ectodermal precursor cells proliferate --> migrate to surround yolk by epiboly through underside of vitelline envelope
    •   - cells in the outer edge of growing ectodermal cells have  long filopodia
    •   -takes about 4 days to complete
  16. Axis formation
    • Dorso-vental axis (back-belly): created by pH gradient (axis can be reversed by manipulating pH)
    •   - albumin, pH 9.5; relatively –ve membrane potential --> DORSAL SIDE
    •   - submarginal space, pH 6.5; relatively  +ve membrane potential --> VENTAL SIDE


    • Anterio-posterior axis: aided by gravity
    •   - developing ovum spins 10-20 revolutions per hr
    •   --> shifts lighter particles towards one side of blastoderm --> posterior end
    •   --> Opposite is anterior end
  17. Right-Left axis
    • Left side - Heart and spleen
    • Right side – Liver
    • Regulated by Asymmetric gene expression
    • Cilia in Hansen’s node direct the flow of fluid carrying expression of specific genes
    • Eg – Shh; FGF8 and TGFb family
  18. Left Axis


    • LEFT SIDE of Hensen’s node:
    • – expresses sonic hedgehog (Shh) and Lefty-1 genes

    -Lefty-1  blocks FGF8 expression

    -Shh activates expression of Cerebus gene --> blocks BMP --> no block of Nodal and Lefty1 – repression of Snail and activation of Pitx2 -->  activate organ primordia --> left side structures
  19. Right Axis


    • RIGHT SIDE of Hensen’s node:
    • – expression of activin and its receptor --> activates FGF8 protein
    • -FGF8 blocks Caronte (car) gene
    • Normal activation BMP --> repression of Nodal and Lefty 2
    • --> expression of Snail and repression of Pitx2
    • --> activation of morphogenic factors of right side

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