1. C E N The endoderm folds to become the gut tube and the body sides
form to enclose body cavities S So Sp
8 day mouse, transverse
N Neural tube
S Somite
So Lateral mesoderm (somatopleure)
Sp Lateral mesoderm (splanchnopleure)
E Gut tube E
9 day mouse, transverse
A Split aorta
G Gut tube
So Somatopleure, parietal mesoderm*
Sp Splanchnopleure, visceral mesoderm*
C Cavity C
* Collectively a serous membrane around the body cavity A G So Sp

2. The pleuropericardial cavity divides
and the cavities are lined by parietal and visceral surfaces
Aorta
Oesophagus
Lungs
Peripleural
cavity
Heart
Pericardial
cavity
Parietal surface of pleural and pericardial cavities
Form a single serous membrane sac around the cavity
Visceral surface of pleural and pericardial cavities

3. The peritoneal cavity is subdivided laterally at the rostral end
but is a single cavity at the caudal end
Aorta
Pancreas
Dorsal mesentery
Stomach
Small intestine
Liver
Ventral mesentery
Parietal surface of peritoneal cavity
Form a single serous membrane sac around the cavity
Visceral surface of peritoneal cavity

4. DEVELOPMENT OF THE HEART AND GREAT BLOOD VESSELS
LEARNING OUTCOMES
1. explain the early development of the heart from splanchnic mesoderm ahead of the neural
plate which is then folded beneath the pharynx of the head fold.
2. outline the fusion of the endocardial tubes to form the simple linear heart with atrium, ventricle
and valvular flaps pumping blood into the aortic arches.
3. define the three circulatory arcs of the heart supplying the body tissues, the yolk sac (vitelline)
and the allantois and describe their functions
4. describe the role of the yolk sac splanchnopleure in early haematopoiesis
5. Understand the developmental process by which the aortic arches and truncus arteriosis are
adapted to give the aortic and pulmonary trunks and the carotids
6. Show how septum formation in the primitive linear heart allows separate pumping of blood into
the aorta and the pulmonary trunk
7. describe the congenital abnormalities of septal defects, patent ductus arteriosus, and
persistent aortic arch

5. FORMATION OF THE MAMMALIAN GASTRULA - 9
ICM
EPIBLAST
NOTOCHORD
CHORDA-
HEAD
LATERAL
PARAXIAL
SOMITES
AXIAL SKELETON
TRUNK MUSCLES
LIMB MUSCLES
INTERMEDIATE
YOLK SAC
ALLANTOIS
AMNION
CHORION
MESODERM
TYPE
DERMIS
PARTS OF KIDNEY
AND REPRODUCTIVE TRACT
HEAD MUSCLES, SKULL,
CARTILAGE
DERIVATIVE
STRUCTURE
HEART
BODY CAVITY DIVIDERS
LIMB SKELETON
BLOOD CELLS

6. THE MAP OF ORIGINS * epithelial part only of organ Hair, nails
Sweat glands
Anal
canal
Extraembryonic endoderm of yolk sac and allantois
INNER CELL
MASS
EPIBLAST
MESODERM
ECTODERM
ENDODERM
Lungs*
Oral
epithelium
Mammaryglands
Olfactory
epithelium
Thyroid*
Pharynx
Gut
tube*
Surface
epithelium
Teeth
enamel
Stomodeal
epithelium
Pharyngeal
pouches
Liver*
Anterior
pituitary
Allantois*
Pancreas*
Neural
tube
Head
mesoderm
Bladder*
Middle
Ear*
Paraxial
mesoderm
Lateral
mesoderm
Skull
Branchial
cartilage
Brain
Cranial motor nerves
Parathyroid*
Spinal cord
Intermediate
mesoderm
Extraembryonic mesoderm of yolk sac and allantois
Splanchnic mesoderm
Blood
cells
Vascular endothelium
Heart
Connective tissue,
smooth muscle
of viscera and blood vessels
Notochord
Tonsils*
Sclerotome
Eye
Spinal motor nerves
Myotome
Kidney and reproductive tracts
Somatic mesoderm
Parietal pleura, pericardium, peritoneum
Neural
crest
Primary
germ cells
Axial
skeleton
Dermatome
Extraembryonic mesoderm of amnion and chorion
Gonads*
Dermis
Visceral pleura
Visceral peritoneum
Mesenteries
Cranial sensory
nerves
Trunk
Muscles
Teeth
dentine
Appendicular muscles
Adrenal
medulla
Sympathetic
ganglia
Spinal sensory nerves
Melanocytes
THE MAP
OF ORIGINS

7. THE EARLY DEVELOPMENT OF THE HEART - 1
The cardiac tube folds under the gut tube……
THE EARLY DEVELOPMENT OF THE HEART - 1 A
VITELLINE VEINS
GUT TUBE
CARDIAC TUBE B
The heart is a U-shaped tube at this stage and the forming blood vessels are initially unconnected
DORSAL AORTA
ENDODERM
PERICARDIAL CAVITY
CARDIAC PRIMORDIUM
(SPLANCHNIC MESODERM)
1. The cardiac primordia are established in the early gastrula as regions of splanchnic mesoderm ahead of the neural plate. As a result of the head fold, this region ends up beneath the pharynx.
2. Initially two ventrolateral tubes fuse to give a midline structure composed of an endocardium (inner lining) and a myocardium (muscular wall).
The cardiac primordia are established in the early gastrula as regions of splanchnic mesoderm ahead of the embryo itself. As a result of the head fold, this region ends up beneath the pharynx.

8. ……and connects bilaterally with the dorsal aorta via the aortic arches
FUSED DORSAL AORTA 1 ST
AORTIC ARCH (R)
VENTRICLE
ATRIUM
VENOUS RETURN FROM
ORAL PLATE
CARDINAL VEINS,
VITELLINE VEIN AND
ALLANTOIC (UMBILICAL)
VEIN
3. This tube then develops the atrial and ventricle regions with valvular flaps to prevent back flow so that the heart can function as a simple peristaltic pump.
4. The dorsal aorta form independently and then grow to meet the ventral output from the heart in the aortic arches
The sides of the U-tube then fuse to produce the atrial and ventricle regions with valvular flaps to prevent back flow so that the heart can function as a simple peristaltic pump.
The dorsal aorta form independently and then grow to meet the ventral output from the heart in the aortic arches

9. This pattern of mammalian development
is a good example of recapitulation
5. The diagram shows 6 aortic arches but these do not fully form in mammals (in particular 1 and 2 and 5) and 1 and 2 are regressing while the later arches are forming
6. See McGeady, chapter 15, Noden p214, Patten p618, Latshaw p 185
The diagram shows 6 aortic arches but, in mammals, 1 and 2 are regressing while the later arches are forming and arches 5 never form

10. REMINDER:
The branchial arches and clefts and the juxtaposed pharyngeal pouches are a recapitulation of the respiratory anatomy of fish
1. Certain cranial nerves innervate the branchial arches and the adult derivatives of these.
2. Between the branchial arches are the branchial clefts which align with diverticula of the pharynx called the pharyngeal pouches.
3. In fish, there are 6 pharyngeal pouches and associated branchial cleft s and these become continuous to form the external gill arches.
4. In mammals and birds this fusion does not take place and only the first 3 or 4 branchial arches, clefts and pharnygeal pouches are prominent.

11. The heart twists so that the atrium is rostral to the ventricle
Mouse, 9 day, frontal
Mouse, 10 day, frontal
Mouse, 8 day, sagittal
Mouse, 8 day, frontal
The heart twists so that the
atrium is rostral to the ventricle
The heart folds under the pharynx
Mouse, 9 day, side

12. The embryonic circulation has three circulatory arcs through which blood is pumped by a simple linear heart
Aortic arches
Dorsal aorta
Cardinal veins
Mesonephros
Chorio-allantoic
placenta
Allantoic artery
Allantoic vein
Yolk sac
Vitelline vein
Vitelline artery
Deoxygenated blood
Mixed blood
Oxygenated blood

13. 1. There are three circulatory arcs supplied by the heart.
THE CIRCULATORY ARCS OF THE EMBRYONIC BLOOD SUPPLY 1.
BODY CIRCULATION
TRANSPORT OF O2
/FOOD MATERIALS TO TISSUES
TRANSPORT OF WASTE MATERIALS AWAY 2.
VITELLINE CIRCULATION
CARRIES MOBILISED FOOD MATERIALS FROM THE YOLK SAC
LOST FUNCTION IN MAMMALS BECAUSE SAC EMPTY
CARRIES FIRST BLOOD CELLS FROM YOLK SAC
SPLANCHNOPLEURE 3.
ALLANTOIC CIRCULATION
IN MAMMALS TAKES OVER THE FUNCTIONS OF THE
VITELLINE ARC IN BIRDS
SUPPLIES FOOD MATERIALS FROM MATERNAL
CIRCULATION
RETAINS AVIAN FUNCTION OF REMOVAL OF WASTE
AND GAS EXCHANGE
1. There are three circulatory arcs supplied by the heart.
2. The body circulation provides the developing tissues with oxygen and food materials and removes waste products.
3. The vitelline circulation carries mobilised materials from the yolk sac in birds. In mammals the vitelline circulation persists even though the yolk sac itself is empty and this circulation allows blood cells produced in the yolk sac splanchnopleure to become part of the general circulation. In some mammals such as the horse, the vitelline circulation assumes a more prominent role in persisting for some time as the supply to the chorio-vitelline placenta
4. The allantoic circulation in mammals assumes the function of the vitelline arc in birds in that it brings food materials from the maternal circulation to the embryo. But it also retains the avian functions of removal of waste and gas exchange.
5. The aortic arches are a recapitulation of an evolutionary phase when the gill arches carried out the essential function of oxygenating the blood.
6. Their function in mammalian and avian embryos where the adult forms have replaced their gills by lungs is simply to connect the ventral heart output with the dorsal aorta.
7. However, the aortic arches undergo major changes during development to contribute to the adult arteries close to the heart.
8. In the hindgut region of the embryo, the dorsal aorta feeds into the mesonephros for the removal of urea from the blood. The blood from the mesonephros is collected in the cardinal vein

14. Haematopoiesis begins in the splanchnopleure of the yolk sac before transferring to the embryo itself later in development
MESENCHYME IN
SPLANCHNOPLEURE
OF YOLK SAC
CELL CLUSTERS
ENDOTHELIAL CELLS
HAEMATOPOIETIC
CELLS
FORMATION OF BLOOD
VESSEL
AGGREGATION OF FURTHER
MESENCHYME TO FORM
MUSCULAR AND CONNECTIVE
TISSUE WALL
1. The first blood cells arise in the splanchnic mesoderm on the endodermal wall of the yolk sac.
2. Cells in the outer zone of these blood islands become vascular endothelium and enclose haematopoietic stem cells.
3. The function of haematopoiesis is transferred during foetal development from yolk sac to liver to bone marrow.

15. See Latshaw p185; Noden p213, 214, Patten p618, McGeady Ch 10, p15
The simple tubular heart twists to prepare for septum formation and the creation of a four-chambered organ. The aortic arches are selectively modifed to give rise to the great arteries
THE HEART AND THE AORTIC ARCHES - FORMATION OF THE GREAT BLOOD VESSELS
After birth venous return is from
vena cava (blue arrows) and
pulmonary veins (red arrows) LA RA LV RV
CAROTIDS (from L and RIII)
AORTA
(from LIV)
DUCTUS
ARTERIOSUS
(LVI to LIV)
SEPTA
RIGHT
SUBCLAVIAN
(from RIV)
PULMONARY
TRUNK
(from LVI) A V I II
III IV VI
VENOUS RETURN V A
III IV VI TA
1. The diagram shows 6 aortic arches but this does not occur at the one time except in fish. Also in mammals. the 5th aortic arch does not form
2. The twisting of the heart tube leads to the atrium behind and slightly rostral to the ventricle. This sets the scene for septum formation which divides the heart vertically to separate the left and right sides and horizontally to separate atria from ventricles
3. To coincide with these change, the truncus arteriosus divides within itself into a pulmonary artery leading from the right ventricle and the aorta itself leading from the left ventricle
4. The connection between the dorsal roots of the 3rd and 4th aortic arches are lost and the 3rd arch persiste as the brachiocephalic trunk which divides into the external and internal carotid arteries and the right and left subclavian arteries
5. The key changes are
(a) the regression of the right 4th aortic arch and the development of the left 4th into the dorsal aorta
(b) the regression of the right 6th aortic arch and the development of the left 6th into the pulmonary trunk dividing into the left and right pulmonary arteries but remaining connected via the ductus arteriosus to the aorta (this is the original 4th to 6th aortic arch connection in the dorsal root). The ductus arteriosus closes at birth in response to the changing blood pressures and oxygenation.
See Noden p237, 248, 248; Patten p629, 640;
See Latshaw p185; Noden p213, 214, Patten p618, McGeady Ch 10, p15
NOTES:
1. View from ventral surface
2. RA - Right atrium, LA - Left atrium, RV - right ventricle,
LV - left ventricle, TA = truncus arteriosus

16. The separation between atria and between ventricles and between atria and ventricles occurs by means of septum formation
The septa of the heart form simultaneously
Remarkably the heart continues to function through these remodelling changes

17. Mouse, 10 days, frontal section
section of truncus arteriosus
Cushions form within the truncus arteriosus and will fuse to form the aortico-pulmonary septum separating the aortic and pulmonary flows A
Blood from the atrium passes to the ventricle by means of a channel. The beginnings of interatrial septum formation can be seen (A)

18. There is a split between deoxygenated blood returning from the rostral end of the foetus and oxygenated blood returning from the placenta. This spit is achieved by directed flow through the foramen ovale
FOETAL CIRCULATION 25
Brachycephalic vessels 14 To 19
lungs DA
From lungs FO 25 19 25 14
Liver 30 22
Trunk
Placenta
Hindlimb

19. CHANGES IN THE CIRCULATION AT BIRTH
Contraction of allantoic artery and veins to force placental blood
into main circulation. Rupture of umbilical cord
Contraction of Ductus arteriosus and closure of Foramen ovale so that right side blood is directed to lungs
The changes at birth are as follows
Umbilical (allantoic) arteries contract. Allantoic veins contract forcing blood into circulation (may be 30% total blood). Cord ruptures.
Ductus arteriosus contracts so that all right side blood goes to the lungs and all left side blood to the body
Prior to birth most blood coming through the caudal vena cava (which is relatively oxygen-rich because of placental exchange), is directed through the foramen ovale and hence via the dorsal aorta to prefernential cranial offshoots (the carotids). The foramen ovale closes at birth and all venous return is then directed to the right ventricle and hence the lungs, while the only blood entering the left atrium is oxygen-rich blood from the lungs

20. DEFECTIVE SEPTUM FORMATION
INTER-VENTRICULAR SEPTAL DEFECT
(Tetralogy of Fallot is variation on this)
There is a relatively high incidence of developmental abnormalities at birth (about 1% in dogs) and this may be because of the relatively complex remodelling that occurs.
Inter-atrial septal defects. Failure of closure of foram ovale leading to mixing of blood in the two atria
Interventricular septal defects. Blood passes from left ventricle to right leading to increased pulmonary pressure
Tetralogy of Fallot. Defective aortic/pulmonary septum formation associated with a ventricular septal defect. Blood from right ventricle enters aorta leading to right ventricular hypertrophy to compensate
INTER-ATRIAL SEPTAL DEFECT
(persistent Foramen ovale)

21. P E R S I S T E N C E O F A O R T I C A R C H E S A N D V A S C U L AG A N O M A L I E S P E R S I T N G H A O C V i g h t 4 L e f 6 s o p a u
In patent Ductus arteriosus, blood is forced from dorsal aorta into pulmonary artery and sometimes back into right ventricle. Cardiac output is increased to maintain body circulation
Persistent right aortic arch 4 forms the arch of the aorta while the left arch 6 forms a patent ductus arteriosus with a persistnet segment of the left dorsal aorta
Other defects
Pulmonary stenosis is a narrowing of the pulmonary artery impeding blood flow to the lungs
Aortic stenosis is a narrowing of the aorta leading to left ventricular hypertrophy
Ectopic heart is a result of interference with the normal descent into the thoracic cavity. In cattle, an ectopic heart in the neck can be compatible with a normal life
Congenital venous shunts include persistence of the normal foetal liver shunt which takes a proportion of blood by-passing the liver. If this persists after birth, there is a build-up of toxic wastes in the blood P A T E N T D U C T U S A R T E R I O S U S

22. REFERENCES Carlson BM (2003) Patten's Foundations of Embryology
Noden DM, de Lahunta (1985) A Embryology of domestic animals
McGeady TA, Quinn PJ, Fitzpatrick ES, Ryan MT (2006) Veterinary embryology
University of North Carolina web site: