Lateral Mesoderm and Endoderm

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					Organogenesis Part 2

  V. Lateral Plate Mesoderm
  VI. Endoderm
  VII. Development of the Tetrapod Limb
  VIII. Sex Determination
V. Lateral Plate Mesoderm

                            paraxial mesoderm

                                            intermediate mesoderm

                lateral plate mesoderm
Lateral Plate Mesoderm
       - Somatopleure: somatic mesoderm plus ectoderm
       - Splanchnopleure: splanchnic mesoderm plus endoderm
       - Coelom: body cavity forms between them
Lateral Plate Mesoderm

  • The Coelom:
    – eventually left and right cavities fuse into one

    – runs from neck to anus in vertebrates

    – portioned off by folds of somatic mesoderm
       • pleural cavity: surrounds the thorax and lungs
       • pericardial cavity: surrounds the heart
       • peritoneal cavity: surrounds the abdominal organs
Figure 12.1 Mesodermal development in frog and chick embryos (Part 3)
Heart Development

  • The heart is the first organ to function in
    the embryo and the circulatory system is
    the first functional system.
    – heartàarteriesàcapillariesàveinsàheart

  • Before the embryo can get very big it must
    switch from nutrient diffusion to active
    nutrient transport
Heart Development

          Anatomical Stages:   •   Tube Formation
                               •   Looping
                               •   Chamber Formation



                                   human timeline
Heart Development: Tube Formation

   presumptive heart cells are specified       outflow forming cells (red)
   but not determined in the epiblast          migrate in first, inflow second

 near node

                                “the heart field”
Heart Development: Tube Formation
  The cardiogenic mesoderm migrates out of the mesodermal layer
  towards the endoderm to form endocardial tubes on either side.

                                                    At the same time
                                                    the endoderm is
                                                    folding inward
Heart Development: Tube Formation
 The endoderm continues folding inward until it forms its own tube,
 which drags the two endocardial primordia close to each other.

                                               The endocardial tubes
                                               are surrounded by
                                               myocardial progenitors

                                              When the endocardial
                                              tubes get close enough,
                                              they fuse together
Heart Development: Tube Formation

                              If you mess with
                              endoderm migration
                              or signaling, you end
                              up with two hearts
Heart Development: Tube Formation
  • Heart Tube Cell Biology
    – Splanchnic mesoderm cells express cadherins and form an
      epithelial sheet for their inward migration - MET

    – The presumptive endocardial cells undergo EMT to migrate
      away from the sheet and another MET to form tubes

    – The cells in the original mesodermal sheet form the myocardium

    – The myocardial epithelium fuses first and the two endocardial
      tubes exist together inside for a while before fusing

    – Both the rostral end (outflow) and caudal end (inflow) remain as
      unfused double tubes

    – The heart beat starts spontaneously as myocardial cells express
      the sodium-calcium pump - before fusion is even complete
Heart Development: Looping and Chamber Formation

                                   left-right asymmetry is
                                   due to Nodal and Pitx2


  posterior        right         left

                 Looping requires: cytoskeletal rearrangement
                                   extracellular matrix remodeling
                                   asymmetric cell division
Heart Development: Looping and Chamber Formation

                   valve formation
                                     heart valves keep
                                     the blood from
                                     flowing back into
                                     the chamber it was
                                     just ejected from

                                     The septa separate
                                     the two atria and
                                     the two ventricles
Heart Development: Looping and Chamber Formation

                   The truncus arteriosis, or outflow tract,
                   also becomes septated allowing one great
                   artery to flow from right ventricle to lungs
                   and the other from left ventricle to the body.
Heart Development: Looping and Chamber Formation

  • The tricuspid valve is between the right
    atrium and right ventricle.
  • The pulmonary or pulmonic valve is
    between the right ventricle and the
    pulmonary artery.
  • The mitral valve is between the left atrium
    and left ventricle.
  • The aortic valve is between the left
    ventricle and the aorta.
Heart Development: Looping and Chamber Formation

           Steps: 1. Endocardial cushions form and fuse
                  2. Septa grow towards cushion
                  3. Valves form from myocardium

    In utero, the foramen ovale allows right left shunting of blood
Embryonic circulatory systems

                                All of the blood must
                                circulate outside of the
                                embryo for oxygenation
Redirection of human blood flow at birth
Blood Vessel Development

 • The vessels form independently of the heart

 • They form for embryonic needs as much as adult
    –   Must get nutrition before there is a GI tract
    –   Must circulate oxygen before there are lungs
    –   Must excrete waste before there are kidneys
    –   They do these through links to extraembryonic
Blood Vessel Development

 • The vessels are constrained by evolution
    – Mammals still extend vessels to empty yolk sac
    – Birds and mammals also build six aortic arches as if we
      had gills, eventually settling on a single arch

 • The vessels adapt to the laws of fluid dynamics
    –   Large vessels move fluid with low resistance
    –   Diffusion requires small volumes and slow flow
    –   Highly organized size variance controls volume
    –   And superbranching smaller vessels control speed
Embryonic circulatory systems
Blood Vessel Development
Blood Vessel Development

                           is the de novo
                           of mesoderm
                           into endothelium

                           It is followed by
                           the endothelium
                           recruiting smooth
                           muscle cell coat
Blood Vessel Development

                           Starts in the
                           mesoderm as
                           well as in the
                           large embryonic
                           blood vessels
Blood Vessel Development

                           is the growth
                           and remodeling
                           of the 1st vessels
                           in response to
                           blood flow and
                           recruitment signals
Blood Vessel Development
Blood Vessel Development
                       Secondary Vasculogenesis

                       1. PEO forms from splanchnic
                       mesoderm overlying the liver
                       2. PEO contacts the ventricle
                       and migrates as epicardium

                       3. Subset of epicardial cells
                       delaminate towards myocardium

                       4. These undergo MET to
                       form coronary endothelium

                       5. Coronary arteries then plug
                       into the aorta where nerves are
Blood Vessel Development
Blood Vessel Development

                           It is a common
                           phenomenon for
                           arteries and nerves
                           to form together

                           Less so for veins....
Blood Vessel Development

 • Lymphatic drainage forms from jugular vein

   – Sprouts as lymphatic sacs by angiogenesis

   – Continues to form secondary drainage system

   – Major conduit for immune cells
Where do the hematopoietic stem cells of the adult bone marrow come from?

                                              Splanchnic mesoderm of
                                              (AGM) region in embryo

                                                       from sclerotome

                                                       from many sites
Wherever they come from....

Development of the Endoderm

 • The Digestive Tube
    –   Anterior endoderm forms anterior intestinal portal
    –   Posterior endoderm forms posterior intestinal portal
    –   Midgut goes through expansion and contraction to yolk
    –   Each end has ectodermal cap, then forms an entrance

 • The Derivatives
    –   4 pharyngeal pouches form head and neck structures
    –   Floor between 4th pair buds out to form respiratory tube
    –   Gut tube forms esophagus, stomach, SI, LI, rectum
    –   Gut tube buds out to form liver, gall bladder, pancreas
Development of the Endoderm

Development of the Endoderm

  The cranial neural crest cells migrate
  through this endoderm and contribute
  component structures around them
Development of the Endoderm

     Localized Wnt/B-Catenin and retinoic acid cause budding
Development of the Endoderm
                              Normal-time birth is
                              signaled from the lungs
Development of the Endoderm

  specification of the
  gastrointestinal tract
Development of the Endoderm

                              Reciprocal Induction
                                specification of
                                both endoderm
                                 and mesoderm
Development of the Endoderm

                              Mesoderm also
                              induces liver bud
Development of the Endoderm
Development of the Endoderm
The Extraembryonic Membranes

  • Adaptation for development on dry land
    – As the body starts to develop epithelium
      expands to isolate embryo within them

  • Four sets of extraembryonic membranes
    – Somatopleure forms amnion and chorion
    – Splanchnopleure forms yolk sac and allantois
The Extraembryonic Membranes

          Somatopleure forms amnion and chorion
          Splanchnopleure forms yolk sac and allantois
The Extraembryonic Membranes
 The amnion folds up to cover the embryo and keep it from drying out

                                             The cells of the
                                             amnion secrete water
The Extraembryonic Membranes

 The chorion surrounds the entire embryo and controls gas exchange

                                 In birds and reptiles it lines shell
                                 In mammals it forms the placenta
The Extraembryonic Membranes
 The yolk sac expands to surround yolk (even if you don’t have any)
The Extraembryonic Membranes

    The allantoic membrane creates a space for waste storage

                                              Bird and reptile
                                              eggs gotta’ have it

                                               We don’t use it for
                                               waste but it contributes
                                               to our umbilical cord

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