Chapter 9: EARLY DEVELOPMENT OF VERTEBRATES: MAMMALS

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ChipzThatLeo
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Chapter 9: EARLY DEVELOPMENT OF VERTEBRATES: MAMMALS
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2014-03-25 15:00:00
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Mammals
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Human Development
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  1. Membranes of the Amniote Egg:
    Reptiles, Birds, and Mammals
  2. Chick Development
    Blastoderm

    • Sub-germinal
    • space

    • Gastrulation
    • through Primitive streak

    • Formation
    • of Hansen’s node
  3. Mammalian reproduction
    • Smallest
    • eggs/zygotes

    •    – 100
    • mm in diameter

    • Both
    • fertilization and development are INTERNAL

    • Oocyte
    • released from the ovary and swept into oviduct à Fertilization
    • in the ampulla

    • Day 1
    • – completion of meiosis of egg (after the sperm entry) and first cleavage of
    • zygote

    • Cilia
    • pushes embryo into uterus – first cleavage occurs along the journey
  4. Early Mammalian Development


    • 1. Divisions are very slow (12-14 hrs
    • apart)

    • 2.
    • Rotational cleavage:

      First cleavage – normal meridional

    •   Second cleavage – one of the two blastomeres
    • divide meriodinally and
    • the other divide equatorially

    • 3.
    • Asynchronous divisions – cell divisions are not synchronized – no exponential
    • increase in the number of blastomeres

    • 4.
    • Mammalian genome is activated during early cleavage (mouse switches to zygotic
    • control by 2-cell stage)
  5. Early
    Mouse Development - Compaction
    5. Compaction, unique for mammals – at 8 cell stage cells maximize their contact by tight junctions (E-cadherin) --> 16 cell morula

    Morula:Small internal cells --> inner cell mass (ICM) --> embryo, yolk sac and amnionLarge external cells --> trophoblast --> chorion (embryonic portion of placenta)

    Chorion – nourishment to developing fetus, regulates immune response
  6. ICM
    • 64
    • cell stage – only 13 cells form ICM, rest are trophoblasts

    • Differentiation: Trophoblast
    • cells get separated from ICM and form a layer – required for adhesion of embryo
    • to uterus

      ICM supports trophoblast division by secreting FGF4

    • Blastomers upto 8
    • cell stage– totipotent



    • Trophoblast
    • cells express

    •   Eomesodermin -
    • T-Box transcription factor

      - maintains trophoblast layer

    •   CDX2
    • - Homeodomain-
    • transcription factor,

    •   - downregulates Oct4
    • and Nonag  (characteristic of ICM)



    • ICM –
    • expresses pluripotency
    • genes

    •   - Oct4 –
    • blocks convertion to trophoblasts

    •   - Nonag
    • – prevents ICM from becoming hypoblasts

    •   - Sat3 (phosphorylated) –
    • required for self-renewal of ICM
  7. Core Transcriptional Circuitry for the Pluripotency of ES Cells
  8. Timing of human monozygotic twinning
    with relation to extra-embryonic
    membranes
  9. Production of Chimeric Mice
  10. Cellular Derivatives
  11. Blastocoel
    Cavitation:

    •   Plasma membrane of trophoblast
    • contain Na pump (Na/K ATPase) facing blastocoel --> pumps Na ions into blastocoel --> osmotic gradient --> water -->
    • expansion

    •   - ICM is positioned on one side
    •   - Trophoblasts form
    • a ring  around ICM
  12. Escape from Zona Pellucida
    • Zona pellucida
    • prevents blastocyst to adhere to oviduct

    •   (if the adhesion happens – ectopic or tubal
    • pregnancy --> harmful hemorrhage)

    • Hatching of Blastocyst: When
    • blastocyst reaches uterus – trophoblast
    • membrane secretes strypsin
    • (trypsin-like protease) – lyses a hole -->
    • hatching
  13. Implantation of Human Embryo
  14. Adhesion to Endometrium
    • Blastocyst
    • comes in contact with uterine wall, uterine epithelium (endometrium) catches on
    • to the wall with the help of mucopolysaccharides and
    • proteoglycans

    • Hypoblast
    • cells contain integrins --> help in binding to extracellular matrix
    • (ECM) (particularly collagen
  15. Implantation
    • Once in contact with ECM --> trophoblasts
    • secrete another set of proteases (collagenase, stromelysin and plasmogen
    • activator) --> digest ECM à blastocyst bury itself into uterine wall
  16. Placenta
    • Mammalian
    • embryo gets its nutrients from the mother – requires restructuring of maternal
    • anatomy and complement changes in developing fetus tissues.

    • Chorion –
    • fetal portion of placenta comes from trophoblast
    • cells supplemented with mesodermal cells from ICM.

    • Decidua –
    • maternal portion of placenta.
  17. Derivatives of Trophoblast cells
    Cytotrophoblasts - original layer of mono-nucleated trophoblast --> adhere to endometrium --> reach to uterine blood vessels by proteolytic activity --> remodel blood vessels so that uterine blood bathes fetal blood vessels

    Syncytiotrophoblasts - some of the trophoblasts undergo only nuclear divisions (without cytokinesis) --> multinucleated cells --> form villi --> ingress into uterine tissue by digesting uterine wall --> uterus sends blood vessels
  18. Segregation
    of Epiblast and Hypoblast
    Epiblast --> gives rise to embryonic tissue and amniotic ectoderm

    Hypoblast layer delaminates from ICM --> lines blastocoel cavity --> extra-embryonic endoderm --> yolk sac

    Amniotic cavity: forms in the center by inner epiblasts
  19. Embryonic Orientation


    • Primitive
    • streak establishes Embryonic orientation:

    • Anterior-Posterior -
    • primitive streak (Hansen’s node) migrates 
    • towards posterior end

    • Right-Left -
    • Position of primitive streak, and

    • Dorsal-Ventral -
    • primitive streak occurs on dorsal side
  20. Gastrulation
    • Begins at the posterior end of the embryo
    • by node formation (Hensen’s node)

    • Precursors for mesoderm and endoderm
    • migrate through primitive streak

    • Epiblast
    • looses E-cadherin, detaches from neighbors --> migrates through primitive streak as individual cells --> gives rise to endoderm and mesoderm

    • Cells
    • migrate from the epiblast to
    • the middle layer between it and the hypoblast --> forming
    • intra-embryonic mesenchyme (loose embryonic connective tissue)

    • Mesenchymal
    • cells migrate from primitive streak cranially

    • 1)Cardiogenic
    • mesenchyme (most cranial point),

    • 2)Notochordal
    • process (also in the cranial direction)

    • 3)Lateral
    • mesenchyme (to the lateral edges where it meets with extra-embryonic
    • mesenchyme)
  21. Fate of the Germ Layers


    ECTODERM

    • •SURFACE
    • ECTODERM -  epidermis, and other  external structures

    • •NEUROECTODERM
    • -  central and peripheral nervous system,
    • neural crest cells and derivatives  



    • MESODERM-  Cardiovascular
    • system, reproductive/excretory organs, smooth and striated muscle, connective
    • tissues, vessels, skeleton



    • ENDODERM -
    • Epithelial lining of respiratory and GI tract

  22. Neural tube and Notochord
    • Presumptive
    • neural tube cells are small and ciliated.

    •   -present in the mid line – arranged like a
    • rostrum

      - In fold dorsally to form neural tube

    • Presumptive
    • notochord cells at the base give rise to notochord
  23. Extraembryonic membranes
    • Extraembryonic
    • Endoderm: some
    • of the hypoblast cells divide --> form
    • a membrane along inside of cytotrophoblast
    • cells --> enclose exocoelomic cavity or primitive yolk sac (former
    • blastocyst cavity)

    • Extraembryonic
    • Mesoderm:
    • produced either from epiblast or cytotrophooblast
    • cells

    • Secondary
    • Yolk Sac:
    • cells from hypoblast form new cavity within primitive yolk sac --> develops into secondary yolk sac (true
    • yolk sac)
  24. Hox Gene hypothesis
    • Hox
    • genes:
    • Expression of Hox
    • genes specify A-P axis

    • Evolutionary
    • significance: Hox
    • genes are homologous to homeotic gene complex of fruit fly, same pattern of
    • expression

    • Different
    • regions of the body from head to tail are characterized by different
    • concentrations of Hox gene
    • expression
  25. Dorsal – Ventral Axis
    • Dorsal
    • axis comes from ICM cells that are in contact with trophoblast –
    • Embryonic side of developing blastocyst

    • Hypoblast
    • is formed on the side of ICM that is exposed to blastocyst fluid --> becomes Ventral side – Abembryonic side
    • of the developing blastocyst

    • As the development proceeds – the notochord maintains dorsal-ventral polarity by
    • inducing specific genes
  26. Left – Right Axis
    • Asymmetrical
    • mammalian body:

    • Heart
    • forms at the midline of the embryo -->
    • moves to the left side of chest cavity, and loops to right

    • Spleen
    • – found solely on the left side

    • Liver
    • – major lobes of liver are on the right side

    • Large
    • Intestine – loops right to left

    • Lung
    • – Right lung has more lobes than left lung
    • Ciliary
    • movement of cells of the node determines Left-Right Axis – cilia causes flow of
    • fluids in yolk sac cavity from right to left

    • Possibly
    • activates inversion
    • of embryonic turning (inv) gene
    • --> inv gene
    • products --> activate Nodal and Lefty2 --> switches on the Pitx2  and block Snail --> left side structures

    • Non-stimulation
    • of Nodal and Left2 --> Snail is ON à right side structures

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