Heart Tube Formation

Notes

Key Points/Stages ~Days 18-22

Progenitor heart cells arise near the primitive streak on approximately day 16, and migrate through splanchnic layer of lateral plate mesoderm.
Blood islands form in the cardiogenic region.
Blood islands unite to form right and left endocardial tubes.
A single endocardial primitive heart tube forms when lateral embryonic folding facilitates fusion of the paired tubes.
On approximately day 21, myocardial progenitor cells surround the heart tube.
The myocardium forms and secretes cardiac jelly.
Soon, the heart tube will comprise 3 layers: 3 layers: inner endocardium, muscular myocardium, and outer epicardium.
The epicardium arises from the proepicardial organ, which arises from a mass of mesodermal cells at the caudal end of the tube that cover the myocardium.

~Day 18

Key points:

Cardiac progenitor cells migrate/ingress through the streak and craniolaterally through the mesoderm to form blood islands in the cardiogenic region.
These blood islands unite to form the cardiac crescent in the 1st heart field; then second heart field forms.

1st Heart Field gives rise to the left and right atria, left ventricle, and a portion of the right ventricle.
2nd heart field gives rise to the rest of the right ventricle, and the outflow tracts (including the bulbus/conus cordis and the truncus arteriosus).

Blood islands of the heart fields give rise to right and left endocardial tubes.

Anatomical Features:

Amniotic cavity lined by ectoderm.
Yolk sac lined by endoderm.
Mesoderm overlying the amniotic cavity comprises the parietal, aka, somatic mesoderm.
Mesoderm overlying the yolk sac comprises splanchnic, aka, visceral mesoderm; houses myocardial cells.
Intra-embryonic cavity will eventually be divided to form the pleural, pericardial, and peritoneal cavities.

Paired dorsal aortae are present, formed from blood islands that lie close to the midline.

~ Day 22

Key Points:

Days 18-22, lateral embryonic folding facilitates fusion of the endocardial tubes, while cephalic folding brings the heart and pericardial cavity into the thorax.

Anatomical Features:

Amnion (aka, amniotic membrane), which comes to envelop the embryo.
Surface ectoderm is deep to the amnion.
The neural tube has formed between the neural crest, dorsally, and the notochord, ventrally.
Paired dorsal aortae and the foregut are present.
Parietal mesoderm has fused ventrally and contributes to the body wall, deep to the surface ectoderm.
The splanchnic mesoderm covers the ventral surface of the foregut.
Dorsal mesocardium secures the single endocardial tube to the parietal peritoneum; the dorsal mesocardium later gives rise to the transverse sinus.
The pericardial cavity, is the space surrounding the heart tube.
Heart tube features:

Endocardium
Myocardium
Cardiac jelly, an extracellular matrix produced by the myocardium, lies between the two layers of the heart.

Review adult histology

Regions of the heart tube, from caudal to cranial:

Right and left horns of the sinus venosus
Atrium
Ventricle
Bulbus cordis
Truncus arteriosus (which is often referred to as a proximal subdivision of the bulbus cordis)
Aortic sac; the aortic sac gives rise to the aortic arches.