Chapter 13: Lateral plate mesoderm

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Chapter 13: Lateral plate mesoderm
2014-04-17 07:12:41

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  1. Lateral plate Mesoderm
    • Lateral plate mesoderm: splits horizontally into 2 regions 
    • 1. Somatic Mesoderm: dorsally – underlies ectoderm 
    • 2. Splanchnic Mesoderm: ventrally

    Space between these layers becomes coelom 

    Coelom is divided into 3 cavities  - Plural  - Pericardial  - Peritoneal
  2. Development of Circulatory system and Heart
    Circulatory system is the first functional system.

    Heart is the first functional organ.

    Heart arises from two regions of splanchnic mesoderm (one on either side of body) --> each interacts with adjacent tissue --> becomes functional heart

    • Time-line for heart development in Humans: 
    • Day 21 – primitive heart tube 
    • Day 22 – heart begins to beat 
    • Day 23 – heart begins to fold 
    • Day 28 – folding is complete 
    • Day 63 – semilunar valves are complete
  3. Cardiogenic Mesoderm

    Presumptive heart cells originate in early primitive streak, near the Hensen’s node.

    While migrating through primitive streak they encounter cardiomyogenic signals (+) and inhibitory signals (-) for cardiomyogenesis.

    Resulting in Horseshoe shaped Cardiomyogenic mesoderm
  4. Specification of cardiomygenic mesoderm
    Cardiomygenic mesoderm gives rise to:

    • - Cells that form endocardium
    • (inner lining of heart) --> continuous with blood vessels

    - Cell that form myocardium (muscles of heart)  

    - Arterial myocytes – outer anterior portion of heart (atria)  

    - Ventricular myocytes – outer posterior  portion of heart  (ventricles and Purkinje fibers)
  5. Folding of Splanchnic Mesoderm

    As neurulation proceeds --> foregut is formed by inward folding of splanchnic mesoderm

    Endocardial primordia --> cardiac tubes on either side of the gut

    Folding brings two cardiac tubes together à mycardia unite into single tube (29 hrs in chick; 3 weeks in human)
  6. Cardia Bifida
    Bilateral origin of heart can be demonstrated by surgically cutting ventral midline of the primordial tubes --> formation of separate hearts on either side of the body
  7. Heart formation – Joining of two tubes
    • Two endocardial tubes fuse along the embryonic midline.

    Heart starts to beat on 22nd day in Humans (while paired primordia are still fusing). Pacemaker is sinus venosis)

    However, circulation  starts around 30th day

    The single tubular heart  develops many  constrictions outlining  future structures.
  8. Embryonic Heart tube
    • Blood flows through the tube from posterior to anterior.

    Tube is divided into 4 sections:

    • Truncus arteriosus (aortic arch) --> will form ascending aorta and pulmonary trunk   Ventricle  
    • Atrium 
    • Sinus venosus –  receives blood from  vitelline veinsà will  form superior and  inferior vena cavae
  9. Heart formation – Looping
    • Developing heart forms constrictions --> bulbus cordis, ventricle and atria.

    Primitive atrium is still paired and connected caudally to the paired sinus venosus.

    Heart tube bends  ventrally, caudally and  slightly to the right

    During looping --> anterior-posterior embryonic polarity changes to right-left polarity seen in adult

    Paired sinus venosus extend laterally --> sinus horns.

    Paired atria form a common chamber and move into the pericardial sac.

    Partitioning of atrium from ventricle is due to the secretion of cardiac jelly --> form endocardial cushion
  10. Factors involved in Looping
    Looping is dependent on left-right patterning proteins – Nodel and Lefty2

    - Nkx2 regulates Hand1 and Hand2 transcription factors

    • Hand 1 --> future left ventricle.
    • Hand 2 --> future right ventricle
  11. Formationof Heart Chambers

    Atrial and Ventricular septa grow towards endocardial cushion – 33 days in Human.

    Growth of primary and secondary atrial septa direct the flow of blood – 3 months in Human
  12. Embryonic Circulation - Chick
    While the heart is still looping, blood circulates through the vessels that are already differentiated 

    (Red arrows - general direction of blood flow).

    Vitelline arteries bring the blood to yolk sac and vitelline veins return blood.

    Blood leaves heart  --> aortic arches --> dorsal aortae --> vitelline arteries --> yolk sac --> picks up nutrients and O2 --> vitelline vein --> sinus venosus --> heart
  13. Embryoniccirculation - Human
    • Mammalian embryo gets food and oxygen from placenta.
    • Umbilical vein --> food and O2 from placenta to embryo

    Umbilical artery --> waste products from embryo to placenta.
  14. Mixing of blood
    • High O2 blood in umbilical vein is gradually decreased  due to mixing with low O2 blood from: 

    1. Liver – with blood from portal system 

    2. Inferior vena cava – with blood from lower extremities

    3. Right atrium – with blood from head 

    • 4. Left atrium – with blood from
    • lungs --> at the entrance of ductus arteriosus into aorta
  15. Fetal Circulation
    Ductus arteriosus diverts blood from pulmonary artery into descending aorta --> placenta.

    Since blood does not return from pulmonary vein, blood leaves right atrium --> foramen ovale --> left atrium --> left ventricle
  16. Changes at Birth
    When first breath is drawn --> mechanical  pressure of air in lungs --> blood pressure in  the left side of heart increases --> snap closure of septum over foramen ovale

    Decrease in prostaglandins in newborn --> contraction of muscles surrounding ductus arteriosus.

    Switching of respiratory circulation from placenta to lungs

    Separation of pulmonary and systemic circulations
  17. The Hemoglobin Molecule
    The amount of oxygen bound to hemoglobin depends on the PO2 of plasma
  18. Dissolution Curves of Hemoglobin

    Wherever the dissociation curve has a steep slope, even a slight change in PO2 causes hemoglobin to load or unload a substantial amounts of O2.

    Hemoglobin can release O2 reserve to tissues with high metabolism, even at lower PO2
  19. FetalHb
    Fetal Hb consists of a2-g2, instead of a2-b2 found in adult Hb

    fHb has higher affinity to O2 compared to aHb

    At low O2 environment present at placenta, adult Hb gives away the O2, whereas fHb binds to O2.

    Complete switching from fHb to aHb takes place about 6 months after birth
  20. 2,3 Biphosphoglycerate
    2,3-Bisphosphoglycerate (or 2,3-DPG) is present in RBC

    -It binds with greater affinity to deoxygenated hemoglobin than it does to oxygenated hemoglobin

    -In bonding to partially deoxygenated hemoglobin it allosterically up-regulates the release of the remaining oxygen molecules bound to the hemoglobin, thus enhancing the ability of RBCs to release oxygen near tissues that need it most.

    -Interestingly, fetal hemoglobin (fHb) exhibits a low affinity for 2,3-BPG, resulting in a higher binding affinity for oxygen.
  21. Formation of Blood Vessels
    Rather than sprouting from heart, blood vessels form independently --> link up to heart afterwards

    Physiological constraint – growing embryo needs food   supply and elimination of wastes

    Evolutionary constraints – due to similarity of   embryological evolutionary pattern --> some vessels develop despite their utility

    Eg:   1. Vitelline artery and vein in mammals   2. Aortic arches in higher vertebrates

    Physical constraint – effective way of fluid movement   is through large vessels, however diffusion will be low

    Fluid from larger diameter --> smaller diameter --> change in velocity

    Larger vessels --> many smaller vessels

    Murray’s law (Law of continuity): cube of the radius of parent vessel = cube of the radii of smaller vessels (Cecil Murray, 1926)
  22. Formation of aortic arches
    Similarity in aortic arches between Chick and Human

    Paired and numbered (Cranial to Caudal)

    Truncus arteriosus pumps blood --> arches either side of foregut à dorsal aorta
  23. Fate of aortic arches - Human embryo
    • In Human aortic arches either degenerate or transform into
    • Aches 1 and 2 – from minor arteries in head  Arch 3 – forms carotid artery 
    • Arch 4 – forms a portion of aorta on left and   subclavian on right 
    • Arch 5 – missing or underdeveloped 
    • Arch 6 – pulmonary artery on both sides
  24. Formation of Blood Vessels and Cells
    Hemangioblasts – common precursors for cells forming blood vessels and blood cells

    Lateral plate mesoderm --> hemangioblasts --> aggregate into blood islands

          Inner cells --> hematopoietic stem cells --> blood cells

          outer cells --> angioblasts --> blood vessels
  25. Vasculogenesis and Angiogenesis
    Vasculogenesis – de novo creation of blood vessels from lateral plate mesoderm --> hemangioblasts

    First phase: hemangioblasts --> aggregate into blood islands (inner and outer cells)nouter cells à angioblasts à blood vessels

    Second phase: angioblasts --> differentiate into endothelial cells --> form the lining of blood vessels

    Third phase: endothelial cells form tubes and   connect to form primary capillary plexus

    Angiogenesis – formation of distinct tissue specific network of capillaries
  26. Heart defects – Persistent truncus arteriosus

    1 in 10,000 live births

    Pulmonary trunk and aorta do not separate --> ventricular septal defect

    Body and lungs receive deoxygenenated blood

    Untreated infants die within 2 years

    Surgical treatment involves repair of ventricular septa and implantation of shunt between right ventricle and pulmonary arteries
  27. Heart defects–Transposition of great vessels
    5 in 10,000 live births

    Left ventricle empties into pulmonary circulation (pulmonary trunk)

    Right ventricle empties into systemic circulation (aorta)

    Not immediately fatal, but leading cause of death in  infants under 1     year of age
  28. Heart defects – Tetralogy of Fallot
    10 in 10,000 live births – major heart disorder – several types of malformations

    1. Pulmonary Stenosis – pulmonary trunk pinches close --> stopping blood flown

    2. Ventricular Septal-defect

    3. Rightward displacement of aorta (overriding aorta) normal over left  ventricle

    4. Ductus arteriosus –Failure of closure after  birth
  29. Endoderm
    Endoderm forms lining of digestive and respiratory tubes.

    Respiratory tube is the out-growth of digestive tube

    Pharynx, the common chamber

    Oral end is initially blocked  by ectoderm –  oral plate, or  stomodeum.

  30. Folding of Endoderm
    • Early in the 4th week, the primitive gut is an endoderm-lined tube.

    The foregut is connected to the midgut and extends cranially behind the heart.

    The hindgut extends caudally from the midgut.

    Midgut region is connected to the yolk sac by means of the vitelline duct or yolk stalk.

    Endoderm pinches in forward --> midgut

    Cranial end – foregut

    Caudal end - hindgut

    In 22nd day human embryo stomodeum breaks --> oral opening of digestive tube
  31. Derivatives of Digestive Tube
    Posterior to pharynx, digestive tube constricts --> esophagus, stomach, small intestine and large intestine

    Endodermal cells form the linings

    Mesodermal cells surround and form muscles for peristaltic movements
  32. Formation of Pituitary Gland
    Pituitary gland: Floor of diencephalon --> infundibulum --> neural pituitary

    Roof of oral endoderm --> Rathke’s pouch --> glandular pituitary
  33. Fate of Pharyngeal pouches
    • 1st pair – auditory cavities -->
    • middle ear and eustachian tubes

    2nd pair – walls of tonsils

    • 3rd pair – thymus and inferior
    • parathyroid gland

    4th pair – superior parathyroid

    Small central diverticulum --> thyroid

    Pharyngeal floor --> respiratory diverticulum 

  34. Formation of Intestine
    • The midgut elongates rapidly and extends
    • beyond the body wall in the umbilical cord (physiological umbilical herniation)

    Growth of Intestine results in the formation of loops in abdominal cavity

    • Hindgut --> part of the large intestine, rectum
    • and anal canal

    Urinary bladder is continuous with the hindgut in the cloacal region

  35. Separation of Urinary and Intestinal tracts

    Caudal end of intestine meets overlying ectoderm (Hensen’s node)

    • The urinary and intestinal tracts
    • are separated as the cloaca becomes partitioned by the urorectal septum
  36. Formation of digestive glands
    • Liver, gall bladder, ventral and dorsal pancreatic buds extend from Gut tube as
    • diverticula. 

    • Position of the ventral pancreatic bud as well as the duct system shifts, allowing
    • subsequent fusion of the two pancreatic buds below the stomach.

         - Ventral bud --> uncinate process, inferior part

    • - Dorsal bud --> remainder of the pancreas
  37. Respiratory tube
    • Respiratory tubes branches from lower
    • portion of pharynx as laryngotreacheal groove

    Anteriorly the groove becomes trachea

    Ventrally, the groove splits into paired bronchi --> branches into broncheoles.

    Alveoli are differentiated after 28 weeks
  38. Extraembryonic Membranes
    • Somatopleural membranes
    • – formed by combination of ectoderm and mesoderm

    Amnion – prevents desiccation

    • Chorion – gas exchange, forms fetal
    • component of placenta

    Splanchnopleural membranes – formed by combination of endoderm and mesoderm

    Yolk sac  - nutrient supply

    • nAllantois – waste removal and
    • calcium transport