Heart Tube Formation
Sections
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.
Full-Length Text
Here we will learn about formation of the primitive heart tube.
To begin, start a table, and denote the following key points:
Progenitor heart cells arise near the primitive streak on approximately day 16, and migrate through splanchnic layer of lateral plate mesoderm, where they form blood islands in the cardiogenic region.
The blood islands unite to form right and left endocardial tubes.
A single endocardial primitive hear 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.
Let's illustrate key steps of heart tube formation occurring between ~days 18-22.
First, draw the embryo in dorsal view; indicate cranial and caudal orientation.
Show the epiblast pulled back to reveal the mesoderm (recall that the epiblast gives rise to the ectoderm, mesoderm, and endoderm during gastrulation).
Indicate the primitive streak, and, at its cranial 1/3rd, show the cardiac progenitor cells.
The 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.
Write that the 1st heart field ultimately gives rise to the left and right atria, left ventricle, and a portion of the right ventricle.
Show the second heart field, and write that it gives rise to the rest of the right ventricle, and the outflow tracts (including the bulbus/conus cordis and the truncus arteriosus).
As mentioned in the introduction, the blood islands of the heart fields give rise to right and left endocardial tubes;
To see this, let's draw the embryo in cross section.
First, draw the amniotic cavity, which is lined by ectoderm, and, the yolk sac, which is lined by endoderm.
Now, show the mesoderm, and, indicate that the mesoderm overlying the amniotic cavity comprises the parietal, aka, somatic mesoderm.
The mesoderm overlying the yolk sac comprises splanchnic, aka, visceral mesoderm.
Indicate the intra-embryonic cavity, which will eventually be divided to form the pleural, pericardial, and peritoneal cavities.
Now, within the splanchnic mesoderm, indicate myocardial cells; between the mesoderm and endoderm, indicate the paired endocardial tubes.
Dorsally, show the paired aortae, which form from blood islands that lie close to the midline.
Over the next few days, lateral embryonic folding facilitates fusion of the endocardial tubes, while cephalic folding brings the heart and pericardial cavity into the thorax.
To show this, first draw the amnion (aka, amniotic membrane), which comes to envelop the embryo; indicate the surface ectoderm, and show that the neural tube has formed between the neural crest, dorsally, and the notochord, ventrally.
Show the paired dorsal aortae and the foregut.
Bilaterally, show the that the parietal mesoderm has fused and contributes to the body wall, deep to the surface ectoderm.
The splanchnic mesoderm covers the ventral surface of the foregut.
Show that the dorsal mesocardium secures the single endocardial tube to the parietal peritoneum; as we learn elsewhere, the dorsal mesocardium later gives rise to the transverse sinus.
Then, label the pericardial cavity, which is the space surrounding the heart tube, and indicate the features of the heart tube, which now comprises a tube of endocardium surrounded by myocardium. Show that cardiac jelly, an extracellular matrix produced by the myocardium, lies between the two layers of the heart.
Finally, let's draw the heart tube in coronal section.
First, outline the tube;
Then, label the sections in the direction of blood flow, from caudal to cranial:
Right and left horns of the sinus venous,
Atrium,
Ventricle,
Bulbus cordis,
Truncus arteriosus (which is often considered to be a proximal subdivision of the bulbus cordis),
And, finally, the aortic sac; as we'll discuss elsewhere, the aortic sac gives rise to the aortic arches.
Show that the epicardium, which arises from the proepicardium caudally, comes to cover the surface of the heart; it will later give rise to the coronary vessels.