Download embryo 13, 171-185

Embryology Ch13 pp171-178
1) Formation of the Cardiac Septa (overview)
a) Formed between the 27th and 37th days
b) Embryo grows from approximately 5mm to 16-17mm
c) Formation occurs in two ways
i) Endocardial cushions have a single tissue mass grow and from septa
(1) Mass grows from both sides until they meet and fuse
(2) One mass grows until it reaches the other side.
ii) Narrow strip of tissue on the wall does not grow while both sides undergo rapid
expansion (does not involve cushions)
2) Septum Formation in the Common Atrium
a) Sickle-shaped crest grows from roof of common atrium into lumen called the septum
primum
i) Space between septum primum and endocardial cushions is the ostium primum
b) Superior and inferior endocardial cushions grow along edge of septum primum and close
ostium primum
i) Cell death in upper septum primum forms ostium secundum (allows blood flow from
right to left primitive atrium
c) Right atrium expands due to sinus horn and causes the septum secundum to form.
i) Septum secundum forms in a crescent shape and begins to overlap ostium secundum
which forms the oval foramen.
ii) Upper part of septum primum disappears and the remaining part is the valve of the oval
foramen
(1) This allows blood to flow right to left. After birth, lung circulation increases
pressure in left atrium and presses the oval foramen against the septum secundum
until they fuse.
(a) In 20% of cases, fusion is incomplete resulting in a condition called probe
patency of the oval foramen (doesn't allow intracardiac shunting of blood)
d) Further Differentiation of the Atria
i) Both atria enlarge by incorporation of the sinus horn's.
ii) The pulmonary vein is incorporated into the left atrial wall until the four branches are
inserted and forms the smooth-walled part of the adult atrium
iii) The original embryonic left atrium is represented by the trabeculated atrial appendage
while the smooth-walled part is from the pulmonary veins
iv) The original right atrium becomes the trabeculated right atrial appendage
v) The smooth-walled sinus venarum originates from the right horn of the sinus venosus
3) Septum Formation in the Atrioventricular Canal
a) Atrioventricular endocardial cushions appear at the anterior and posterior borders of the
atrioventricular canal
b) The bulbo (cono) ventricular flange separates the atrioventricular canal from the bulbus
cordis (primitive right ventricle)
i) End of 5th week the posterior flange terminates allowing the canal to access both left
and right primitive ventricles
c) Lateral atrioventricular cushions appear on the right and left borders of the canal while the
anterior and posterior cushions fuse and separate the canal right and left atrioventricular
orifices
4) Atrioventricular Valves
a) After formation each atrioventricular orivice is surrounded by mesenchymal tissue
b) Bloodstream hollows out the tissue
c) Muscular chords hold onto the dense mesenchymal tissue
d) The muscular chords degenerate and are replaced by dense connective tissue called
chordae tendineae and anchor the valves to the papillary muscles.
e) Left forms a bicuspid valve and the right forms a tricuspid valve
5) Clinical Correlates Heart Defects (probably want to read because there is a lot of compact
information)
a) Largest category of human birth defects (in 1% of live births)
b) Higher rate in stillborns and babies with chromosomal abnormalities
c) Usually caused by a combination of genetic and environmental influences (multifactorial
causes)
d) Examples of teratogens include rubella virus, thalidomide, RA (Accutane), alcohol, and
insulin-dependent diabetes
e) Heart defects are heterogeneous in origin and difficult to classify epidemiologically
f) We are tracing genes that are responsible for many defects and they tend to be related to
other known abnormalities
g) Ventricular inversion is when the left and right ventricles become switched. Sometimes
known as L-transposition of the great arteries.
h) Atrial septal defect (ASD) is a congential heart abnormality that is more common in female
infants.
i) Ostium secundum is the most significant defect and is characterized by a large opening
between the left and right atria.
i) The most serious abnormality is a complete absence of the atrial septum called common
atrium or car triloculare biventriculare
j) Premature closure of the foramen leads to hypertrophy of the right atrium and ventricle
while the left side is underdeveloped
k) A persistent atrioventricular canal is when the atrioventricular cushions fail to fuse
l) Ostium primum defect is when the endocardial cushions partially fuse
m) Tricuspid atresia is no right atrioventricular orifice (also fusion or absence of tricuspid
valves)
n) Ebstein anomaly is when the tricuspid valve is displaced toward the apex of the right
ventricle resulting in hypertrophy of the right atrium and small right ventricle.
Embryo CH 13 p. 179-185
Vocabulary
A. Bulbus cordis - Segment of the primitive heart tube apparent at the end of the third week.
Its inferior end will form much of the trabeculated right ventricle, while its superior end will
first form an outflow channel called the conotruncus:
B. Conotruncus
C. Conus cordis - This segment is remodeled to form outflow regions of the definitive left and
right ventricles.
D. Truncus arteriosus - This segment is divided into ascending aorta and pulmonary trunk
E.
F. Septum Formation in the Truncus Arteriosus and Conus Cordis
G. TRUNCUS ARTERIOSUS
H. A septum must be formed to separate the flow between pulmonary artery and aorta
I.
Occurs in week 5 of development
J.
Ridges (right superior truncus swelling and left inferior truncus swelling) appear in
the truncus
K.
These ridges both grow downwards and to opposing sides toward aortic sac in a
spiral fashion
L.
They then fuse to form the aorticopulmonary septum which separates the aortic
and pulmonary channels
M. CONUS CORDIS
N. Separation of the outflow tracts of right and left ventricles occur here in concert with the
aorticopulmonary septum development
O. Swellings occur here as in the TA
P. They grow toward each other and up to meet with the septum formed in the TA
Q. The inferior fusion of the swellings within the conus cordis divides this area into
anterolateral and posteromedial sections
R. Right ventrical outflow tract is the anterolateral portion
S. Left ventrical outflow tract is the posteromedial portion
T. NEURAL CREST CELLS
U. Contributes to septum formation in TA and CC
V. Control cell production and lengthening of the outflow tract region by the Secondary Heart
Field
W. Problems with these can disrupt signaling to SHF, as well as cause conotruncal formation
abnormalities
X.
Y. Septum Formation in the Ventricles
Z. Happening at same time as septal formation in the conotruncus
AA.
Myocardium and trabeculae of primitive ventricles expand until the medial walls
merge to form the muscular interventricular septum by end of 7 week.
th
BB.
The interventricular foramen closes to form the membranous part of
interventricular septum
CC.
Growth of the septum in the conotruncus meets with outgrowth of the anterior
endocardial cushion to close the foramen
DD.
SEMILUNAR VALVES
EE.Valve formation occurs with the growth of the truncus swellings
FF.
Begin as tubercles on main truncus swellings (x2)—one to become aortic, the other
pulmonary
GG.
Third tubercle appears opposite fused truncus swellings
HH.
Tubercles hollow out upper surface to form the valves
II.
Neural crest cells may be involved
JJ. Clinical Correlates
What it is
Caused by
Effects
Frequency
Tetralogy of
Fallot
anterior
displacement of
conotruncal septum
abnormal migration
of neural crest cells
9.6/10,000
Persistent
truncus
arteriosus
no division of
outflow tract
conotruncal ridges
fail to form
Transposition of
great vessels
aorta arising from
right ventricle
pulmonary artery
arising from left
ventricle
outflow tract
abnormalities along
with other stuff
conotruncal septum
running straight
down instead of
spiraling
narrow right ventricular outflow
region
defect of interventricular septum
aorta directly above septal defect
hypertrophy of right ventricle
wall
pulmonary artery arises far from
undivided truncus
defective interventricular septum
undivided truncus receives blood
from both ventricles
defect in membranous part of
interventricular septum
open ductus arteriosus
facial defects, thymic hypoplasia,
parathyroid dysfunction,
persistent truncus arteriosus,
tetralogy of Fallot, craniofacial
malformation
not stated
DiGeorge
sequence
Valvular stenosis
Valvular stenosis
of pulmonary
artery
fusion of semilunar
valves in either
aorta or pulmonary
artery
trunk of pulmonary
artery is narrow or
atretic
Aortic valvular
stenosis
fusion of thickened
valves
Aortic valvular
atresia
complete fusion of
semilunar aortic
valves
22qI I deletion
(abnormal neural
crest development)
0.8/10,000
4.8/10,000
34/10,000
patent oval foramen is only outlet
for blood from right side of heart
ductus arteriosus is only access
route to pulmonary circulation
aorta, left ventricle, and left
atrium are underdeveloped
open ductus arteriosus can
accompany
Ectopic cordis
heart lies on surface
of chest
failure of ventral
body wall to close