week 4

• What is the source of all germ layers (and therefore all tissues and organs in the embryo)?

  The epiblast, through gastrulation
• What is the notochord?

  Flexible rodlike structure

  
  

  In vertebrates, the main longitudinal structural elements

  
  

  It serves as the axial skeleton of the embryo until other elements form (e.g vertebrae) form.

  
  

  Gives structure in early development

  
  

  In later vertebrate development, it becomes part of the vertebral column, cushioning it so bone does not grind on bone.
• Does the notochord last past development?

  In vertebrates - no. It is only a transient structure.

  
  

  In lower types of chordates and in primitive fish, the notochord persists into adulthood.
• Where do notochords come from?

  Mesoderm
• How do notochords development?

  Generated from chordamesoderm

  
  

  Chordamesoderm sits immediately beneath the developing CNS

  
  

  It develops a thick etracellular sheath and vacuole - producing osmotic pressure
• Major roles of notochord

  - Patterning the neural tube

  - Reinforcement and maintenance of events during gastrulation (e.g. left right asymmetry)
• Where is the notochord positioned?

  Centrally in embryo - with respect to DV and LR axes
• Patterning the neural tube

  Notochord signals the formation of the floor plate - mainly via the Hedgehog pathway
• Day 19-21 of human embryonic development

  The chordal process

  
  

  Primitive streak is lengthening out as the embryo also lengthens
• Day 20 of human ed


  Chordal tissue forms chordal plate by bonding with the endoderm - just for ONE day.

  
  

  It then separates
• Day 22 - 25 of human ed


  Chordal tissue breads away from the endoderm and the ectoderm

  
  

  It does this to form the notochord
• Primary neuralation


  Beginning stages of the nervous system
• Induction of neural plate

  As the notochord develops, it exerts an inductive effect on the ectoderm to specify it.

  
  

  These cells become neuroectoderm and form neural plate

  
  

  Day 19
• What happens to the ectoderm?


  Half will become nervous tissue

  
  

  The rest will become epidermis
• Paraxial mesoderm


  Forms in to cylinder shaped segments and elongates

  
  

  As primitive streak regresses, the paraxial mesoderm buds off into somitomeres (and then somites)

  
  

  What happens to the somites depends on its location on the axis

  
• Specialisation of mesoderm


  Neural tube in middle

  
  

  Next to it paraxial mesoderm

  Then intermediate mesoderm

  
  

  Thin extremities of int. mesoderm form the lateral plate mesoderm
• Somitomeres


  small bulges in the paraxial mesoderm

  
  

  Will compact into discrete bodies - somites
• Clock and wavefront model


  Formation of somites, the wave of developmental signals causes the periodic formation of new somites
• Somites


  Transitional embryonic structures, responsible for the segmental organisation of the embryo
• Somites contain precursor cells for

  Sclerotome

  Myotome

  Dermatome
• Sclerotome


  Axial skeleton

  
  

  Will form vertebrae and rib cartilage
• Myotome


  Precursor cells for muscle

  
  

  The striated musculature of the neck, trunk and extremeties
• Dermatome


  The subcutaneous tissue and skin
• How conserved is somitogenesis?

  It is highly conserved

  
  

  It is a precise program of segmentation from cranial to caudal direction

  
  

  Very tightly regulated
• Regulation of somitogenesis


  Somites are specified along AP axis by Hox gene patterning
• Notch signalling


  Determines the placement of somite formation
• What drives the budding off in somitogenesis?


  The waves of signalling
• How tight is the regulation of somitogenesis?

  It is so tightly controlled can use this to work out what day of development they are.

  
  

  Counting the somites defines the embryonic age - so if you don't know when the mating occurred or if the development has been a bit slower.
• How quickly do somites form in humans?

  From Day 25, 3-4 somites are formed per day

  
  

  42-44 somite pairs are formed but some are only transient so we end up with 35-37
• Intermediate mesoderm fate


  Goes on to form the gonads and the kidneys
• Lateral plate mesoderm fate

  Somatic

  Splanchnic

  Extra-embryonic
• Somatic mesoderm


  Forms the lateral and ventral walls of the embryo
• Splanchnic mesoderm


  Involved in formation of the wall of the digestive tract
• Cavity that is surrounded by lateral plate mesoderm


  Intraembryonic coelom (cavity)
• Three main components of the cytoskeleton

  Microtubules

  Microfilaments (actin filaments)

  Intermediate filaments
• Role of the cytoskeleton

  - Shape

  - Support

  - Movement (of the cell, organelles)
• Function of the cytoskeleton

  Migration of cells

  Movement of organelles

  Cilia and microvili movements

  Cell contraction

  Endocytosis

  Cell shape maintenance
• Endocytosis


  process by which a cell takes material into the cell by infolding of the cell membrane
• What is the cytoskeleton composed of in non dividing ell?

  - Actin supporting plasma membrane and span the whole cell

  - microtubules throughout cytoplasm

  - intermediate filament support microtubules
• Microtubules


  Strongest and largest component

  Resist compression

  Most rigid because of their tubular conformation

  
  

  Composed of alpha- and beta-tubulin that forms a long hollow cylinder (heterodimer)