Development of the cardiovascular system

The cardiovascular system starts developing at the beginning of week 3 of intrauterine life.

At that point, the embryo is a flat little pancake made up of two layers: the epiblast on the dorsal, or back, side, and the hypoblast on the ventral, or front, side.

A line called the primitive streak appears on the epiblast back of this two-layered creature.

Cells migrate along the primitive streak during gastrulation, resulting in a three-layered embryo pancake, with each layer containing germ cells that form organs and tissues of the body.

The ventral, or bottom, germ layer is called endoderm, the dorsal, or top, germ layer is called ectoderm, and the layer in between these two is called mesoderm.

The heart derives from a part of the mesoderm called the visceral mesoderm.

Let’s look at this three-week-old creature from above. Mesoderm cells go through the primitive streak and make their way up to the embryo’s head, forming an area that’s called the primary heart field, a horseshoe-shaped area that has two limbs, with one on either side of the future brain.

This region lies on a blanket of endoderm cells that secrete vascular endothelial growth factor, which is called VEGF for short.

VEGF signals the cells in the limbs of the horseshoe to self-organize into two heart tubes.

A primitive pericardial cavity also appears lateral to each endocardial tube.

At its inferior end, each endocardial tube connects to a vitelline vein, which comes from an extraembryonic tissue called the yolk sac and through which blood enters the endocardial tube.

Blood exits each endocardial tube at its superior end through a dorsal aorta, which then continues down the embryo’s back.

During lateral folding, the flat embryo goes from a trilaminar disc to a more cylindrical shape.

The lateral borders of the embryo reach out towards each other and meet anteriorly at the midline, forming a cylindrical shape.

This process makes the two endocardial tubes fuse into one, forming the primitive heart tube.

The left and right vitelline veins also fuse to form the sinus venosus, which is the inflow tract of the heart tube.