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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
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
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
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)
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)
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
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.
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
- Sinus venosus – receives blood from vitelline veinsà will form superior and inferior vena cavae
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
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
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
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
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.
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
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
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
The Hemoglobin Molecule
The amount of oxygen bound to hemoglobin depends on the PO2 of plasma
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
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
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.
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)
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
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
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
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
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
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
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
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.
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
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
Formation of Pituitary Gland
Pituitary gland: Floor of diencephalon --> infundibulum --> neural pituitary
Roof of oral endoderm --> Rathke’s pouch --> glandular pituitary
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
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
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
Formation of digestive glands
- Liver, gall bladder, ventral and dorsal pancreatic buds extend from Gut tube as
- 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
- 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
- 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