Week 3 - ANSWER prior: heart consists of pair of tubes on either side of midline; 2 distinct layer
of cells; inner, thinner layer forms endocardium and outer, thicker layer forms myocardium; 21-
23 days: heart tubes --> midline folding process --> single endocardial heart tube that connects
with AO arch system --> cephalic end and sinus venosus at caudal end
Sinus Venosus (week 3) - ANSWER formed by union of 3 veins: common or cardinal, vitelline,
and umbilical; ultimately forms the SVC, IVC, pulmonary veins, coronary sinus, oblique vein of
LV, and posterior portions of RA and LA
Primitive Atrium (week 3) - ANSWER will divide into RA and LA upon development of IAS
Atrioventricular Sulcus (week 3) - ANSWER Divides the primitive atrium and primitive ventricle
Primitive Ventricle (week 3) - ANSWER originates superior to primitive atrium; finds proper
position inferior to primitive atrium; ultimately forms apical portion of LV; will contribute to the
inlet portion of RV
Interventricular Sulcus (week 3) - ANSWER Divides the primitive ventricle and bulbus cordis
Bulbus Cordis (week 3) - ANSWER 3 parts: proximal portion = forms apical portion of RV, mid
portion "conus cordis" = forms RVOT and LVOT, distal portion "truncus arteriosus" = forms aorta
and main pulmonary artery
Aortic Sac (week 3) - ANSWER forms 5 pairs of aortic arches; left 4th arch = definitive AO arch,
distal left 6th arch = ductus arteriosus; additional arches form proximal portions of = right and
left PA, innominate artery, right subclavian artery, and common carotid arteries; some AO
arches are obliterated/disappear by birth
, Week 4 - ANSWER bilateral heart tubes completely fused into single tube; muscular contractions
occur and HEART BEGINS TO BEAT; initially HR is 60-70 bpm but by birth is ~130-150 bpm
Bulboventricular Loop (week 4) - ANSWER heart grows rapidly forcing tube to fold in on itself
forming S-shaped loop. Normally folds to right "d-looping" and finds correct anatomical position
Septum Primum (week 4) - ANSWER apprears from the dorsal wall of primitive atrium and
grows toward the endocardial cushions
*"ostium primum" remains
Sinoatrial Orifice (week 4) - ANSWER sinus venosus and primitive atrium communicate via this;
centrally located; over time, shifts to right side of primitive atrium so that it communicates
solely with the RA
Endocardial Cushions (week 4) - ANSWER appear and serve as primitive valves within the
atrioventricular canal and provides communication between primitive atrium and primitive
ventricles; a common orifice; will eventually separate the canal into left and r ight sided orifices
that develop into MV and TV
Ventricular Septum (week 4) - ANSWER appears as a small ridge on the floor of the primitive
ventricle and the ventricles begin to expand
Truncus Arteriosus (week 4) - ANSWER the one great vessel leaving the heart; situated on the
roof of the primitive atrium; delivers all necessary blood to aortic sac; AO and main PA will
evolve from this
Ostium Secundum (week 5) - ANSWER septum primum reaches the endocardial cushions -->
closure of ostium primum; perforations develop within the septum primum; ostium secundum
results from these perforations in the superior portion of septum primum
of cells; inner, thinner layer forms endocardium and outer, thicker layer forms myocardium; 21-
23 days: heart tubes --> midline folding process --> single endocardial heart tube that connects
with AO arch system --> cephalic end and sinus venosus at caudal end
Sinus Venosus (week 3) - ANSWER formed by union of 3 veins: common or cardinal, vitelline,
and umbilical; ultimately forms the SVC, IVC, pulmonary veins, coronary sinus, oblique vein of
LV, and posterior portions of RA and LA
Primitive Atrium (week 3) - ANSWER will divide into RA and LA upon development of IAS
Atrioventricular Sulcus (week 3) - ANSWER Divides the primitive atrium and primitive ventricle
Primitive Ventricle (week 3) - ANSWER originates superior to primitive atrium; finds proper
position inferior to primitive atrium; ultimately forms apical portion of LV; will contribute to the
inlet portion of RV
Interventricular Sulcus (week 3) - ANSWER Divides the primitive ventricle and bulbus cordis
Bulbus Cordis (week 3) - ANSWER 3 parts: proximal portion = forms apical portion of RV, mid
portion "conus cordis" = forms RVOT and LVOT, distal portion "truncus arteriosus" = forms aorta
and main pulmonary artery
Aortic Sac (week 3) - ANSWER forms 5 pairs of aortic arches; left 4th arch = definitive AO arch,
distal left 6th arch = ductus arteriosus; additional arches form proximal portions of = right and
left PA, innominate artery, right subclavian artery, and common carotid arteries; some AO
arches are obliterated/disappear by birth
, Week 4 - ANSWER bilateral heart tubes completely fused into single tube; muscular contractions
occur and HEART BEGINS TO BEAT; initially HR is 60-70 bpm but by birth is ~130-150 bpm
Bulboventricular Loop (week 4) - ANSWER heart grows rapidly forcing tube to fold in on itself
forming S-shaped loop. Normally folds to right "d-looping" and finds correct anatomical position
Septum Primum (week 4) - ANSWER apprears from the dorsal wall of primitive atrium and
grows toward the endocardial cushions
*"ostium primum" remains
Sinoatrial Orifice (week 4) - ANSWER sinus venosus and primitive atrium communicate via this;
centrally located; over time, shifts to right side of primitive atrium so that it communicates
solely with the RA
Endocardial Cushions (week 4) - ANSWER appear and serve as primitive valves within the
atrioventricular canal and provides communication between primitive atrium and primitive
ventricles; a common orifice; will eventually separate the canal into left and r ight sided orifices
that develop into MV and TV
Ventricular Septum (week 4) - ANSWER appears as a small ridge on the floor of the primitive
ventricle and the ventricles begin to expand
Truncus Arteriosus (week 4) - ANSWER the one great vessel leaving the heart; situated on the
roof of the primitive atrium; delivers all necessary blood to aortic sac; AO and main PA will
evolve from this
Ostium Secundum (week 5) - ANSWER septum primum reaches the endocardial cushions -->
closure of ostium primum; perforations develop within the septum primum; ostium secundum
results from these perforations in the superior portion of septum primum
