Download Embryology of the Body Cavities and Diaphragm (and some heart) As

Embryology of the Body Cavities and Diaphragm (and some heart)







As neural tube forms, two other simultaneous processes
are taking place
o Cranio-caudal folding rolls the ends of the embryo
under the ventral side, putting the heart in the right
place
o Lateral folding brings the edges of the embryo
“down” and under the embryo
These two processes form the walls of the developing
embryo and form the gut tube by trapping part of the yolk
sac in the embryo
Gut tube is suspended in the hollow body cavity by 2
mesenteries (bilayered folds of ectodermal tissue that will
carry neurovasculature to GI tract)
Cardiac field forms at anterior end of embryo
o Around day 18, progenitor heart cells (via splanchnic lateral plate mesoderm) on the
cranial end of the embryo comprise a region known as the cardiogenic area (heart field)
 PHCs arranged into tubes (endocardial tubes)
 As a result of lateral folding of the embryo, the endocardial tubes meet along
the midline
 Tubes fuse and through apoptosis, form a single tube
 As a result of cranial-caudal folding, cardiac fields thorax
o Cells differentiate and begin to beat in a coordinated fashion even without any blood to
pump
o Connected to the amnion by a band of tissue called septum transversum (part of
primordial diaphragm)
By the end of the fourth week (day 27), the lateral body wall folds meet in the midline and fuse
to close the ventral body wall
o Formed primitive cardiac tube and dorsal aortae
 Fluid can pump through
o As the cranial end of the embryo folds under, it brings the cardiac tube ventrally, tucking
it under the embryo caudal to the developing head
 This folding brings the septum transversum caudally and places it caudal to the
developing heart (primordium of diaphragm).. will form central tendon of
diaphragm
 When developing brain is folded ventrally, it brings the oropharyngeal
membrane to ventral surface
Extraembryonic coelom forms from the chorionic space
Intraembryonic coelom forms from the splanchnic and somatic layers of mesoderm
o These communicate with each other until the lateral folds fuse in the ventral midline
 When intraembryonic coelom begins to close off, forms 3 body cavities
 Pleural cavity: thoracic; contains lungs
o Formed by the pericardialperitoneal canals


Pericardial cavity: thoracic; contains heart
 Peritoneal: abdominal; contains GI structures
All form from single tube formed by folding of embryo
Mesenteries



The coelom is lined with epithelium derived from the somatic mesoderm
o This will become the parietal pleura/pericardium/peritoneum
The organs in the coelom are covered with epithelium derived from the splanchnic mesoderm
o This will become the visceral pleura/pericardium/peritoneum
Where the peritoneum reflects from the body wall to the gut tube, it form bilayered
mesenteries
o These are important for innervation and vasculature
Germ Layer Derivatives



Endoderm: epithelium of internal organs and GI tract
Ectoderm- epidermis, cutaneous glands, internal ear, lens, CNS, cells of PNS, etc.
Mesoderm
o Lateral- CT, heart, blood, spleen, etc.
o Intermediate- UG system, gonads
o Paraxial- mm. of the head, striated muscle, skeleton, dermis
Separating the Heart from the Lungs



Growth of bronchial buds into the pericardioperitoneal canals induces formation of 2 ridges in
lateral wall of canal
Superior ridge forms pleuroperitoneal folds, which will form fibrous pericardium and separate
the heart from the lungs
Septum transversum is joined by the development of the lower ridge in the
pericardialperitoneal canals, the pleuroperitoneal folds
o Together with the dorsal mesentery of the esophagus, these form the primordial
diaphragm
o Esophageal mesentery forms the crura of the diaphragm
 Rest of diaphragm mostly muscle derived from ingrowth of the abdominal wall
mm
o Innervation of the diaphragm arises early, when septum transversum lies near cervical
somites
 Hence why the phrenic nn. Arise from cervical nn. 3-5
 Periphery of diaphragm innervated by intercostal nn.
o 4 SEPARATE EMBRYONIC TISSUES TO COMPLETE DIAPHRAGM



Failure of fusion or function of any of these structures that separates body cavities results in the
possibility of herniation of organs into an inappropriate compartment
Defect in diaphragm formation
o Intestine physically in thorax
Eventration- properly formed diaphragm but innervation of muscles are absent
o Intestine pushes upward but not in thoracic cavity
Heart Embryology
Positioning and Formation of the Heart Tube

Splanchnic lateral plate mesoderm (remember, this also gives rise to PHCs) differentiates into:
o Myoblasts, which give rise to myocardium
 Conducting system is formed from ‘unique’ myoblasts
o Mesothelium, which gives rise to the viscera pericardium (epicardium)
Differentiation of the Heart Tube




Bulbus Cordis
o Conus Cordis (CC; inferior)
 Incorporated into the upper parts of the
ventricles (conus arteriosis (RV) and aortic
vestibule (LV))
o Truncus arteriosus (TA; superior)
 Forms the proximal parts of the aorta and
pulmonary artery
Primary ventricle gives rise to the trabeculated part of the
ventricles (trabeculae carneae)
Primitive atrium gives rise to trabeculated part of the atria
(pectinate mm.)
Sinus venosus is incorporated into the atria and forms the
smooth regions of the atria, as well as the coronary sinus and
IVC
broken line
delineates the 2
parts of bulbis
cordis
Cardiac Loop


During 4th week, bulbus cordis moves caudally, ventrally and to the right
Sinus venosus, primitive atrium and primitive ventricle move dorsally, cranially and to the left
Atrial Septum Formation


Begins to form at end of 4th week
Process begins when a thin, flexible septum, the septum primum, grows into the primitive
atrium
o An opening forms (the foramen primum)
o Before the foramen primum is completely closed by growth of the septum primum, cells
in the upper part of the septum primum undergo apoptosis, forming the foramen
secundum
 Foramen secundum MUST start to form before foramen primum diminishes


Another septum (more rigid), septum secundum, forms cranially and to the right of the septum
primum
o Incomplete growth of the septum secundum results in the formation of the foramen
ovale
Septum PRIMUM now acts as a valve of the foramen ovale, allowing oxygenated blood to move
from RALA (right to left shunt)
Closure of the Foramen Ovale (3rd opening on 2nd septum)


Very shortly after birth, septum primum and septum secundum form a seal, thus closing the
foramen ovale
o Remaining depression, fossa ovalis, is visible on the interatrial septum when looking into
the RA
Atrial Septal Defect
o Persistent foramen ovale
o Two types: primum and secundum
 Secundum most common
 Results from excessive apoptosis in the septum primum
 Usually no other associated deficits
Ventricular Septum Formation



Formed by 2 mechanisms (2 parts of the septum): membranous and muscular
o Muscular part forms when the myocardium near the apex grows upward
o Growth downward of the endocardial cushion forms the membranous part
 Endocardial cushion forms from the migration of neural crest cells
Usually defect in membrane not muscle
Ventricular Septal Defect
o If in membrane, usually other malformations since neural crest is responsible for
forming the membranous portion
Formation of the Great Vessels- IVC

Many embryonic veins degrade but the right horn of the sinus venosus persists as the coronary
sinus and IVC
Formation of Great Vessels- Aorta and Pulmonary Trunk





TA cranial continuation of the bulbus cordis
Pair of bulbar ridges, derived from neural crest, arise from opposite sides of the bulbus cordis
and TA
o Through a process known as aorticopulmonary septation, the bulbar ridges grow inward
toward one another and fuse in the midline to form the aorticopulmonary septum
 Results in formation of aorta and pulmonary trunk
The ridges are spirally oriented, thus the relative positions of the aorta and pulmonary trunk are
also spirally arranged (they cross)
Aorticopulmonary septum extends down to the membranous IV septum
Tetralogy of Fallot
o Due to unequal division of aorticopulmonary septum
o 4 anatomical abnormalities: narrowing of pulmonary trunk, overriding aorta,
membranous ventricular septal defect, right ventricular hypertrophy
o Blood moving though VSD increases pressure in the RV, causing hypertrophy
o Mixing of blood results in cyanosis