Rush Center for Congenital
and Structural Heart Disease




Embryonic folding

The germ disc, early in the third week, has the appearance of a flat oval disc and is composed of two layers:  The epiblast and hypoblast.  The epiblast faces the amniotic cavity and the hypoblast faces the yolk sac.  The primitive groove first appears at about 16 days of development and extends half the length of the embryo.  The primitive groove ends caudally with the primitive pit, which is surrounded by a node.  The primitive groove provides a longitudinal axis landmark and bilateral symmetry.  Structures on the right and left side of the embryo will represent the future right and left side respectively of the individual [361].  The primitive groove serves as a conduit for epiblast cells which detach from the edge of the groove and migrate inwards.  This process is achieved through the cellsí foot like processes, called pseudopodia.  This migration process is called gastrulation, which is a process of widespread migration and reorganization.  Epiblast cells initially migrate towards the hypoblast and replace it to form the endoderm.  After the endoderm is formed cells from the epiblast continue to migrate inwards to infiltrate the space in between the epiblast and the endoderm to form the intraembryonic mesoderm.  After this process is completed the epiblast will be termed the ectoderm [361-363].  (Figure 1)

Figure 1

Formation of the 3 layers of the germ disc:  Cells from the epiblast detach, migrate through the primitive groove to form the endoderm and mesoderm layers.


Epiblast cells detaching from the primitive groove and entering the primitive streak are not committed to a particular line of development.  It is only after their migration into the mesoderm that those destined to become premyocardial cells exhibit such tendency [361].  Initially those undifferentiated cells, also called primary mesenchyme move to a lateral position in the mesoderm and join the lateral plate mesoderm which splits into a splanchnic layer close to the endoderm and a somatic layer close to the ectoderm.  The precardiac cells segregate into the splanchnic mesoderm within an epithelial sheet.  The flat germ disc transforms into a tubular structure during the fourth week of gestation [361- 364].  This is achieved through a process of differential growth causing the embryo to fold in two different dimensions:


1.      Cranio-caudal axis due to the more rapid growth of the neural tube forming the brain at its cephalic end.  Growth in this direction will cause the embryo to become convex in shape (Figure 2).

2.      Lateral folding, causing the two lateral edges of the germ disc to fold forming a tube like structure.

On day 17, first evidence of blood vessel formation is noted in the splanchnopleuric mesoderm of the yolk sac.  Blood islands, which are mesodermal aggregations, develop next to the endoderm.  Blood islands segregate into a core of hemoblasts and a surrounding region of flat endothelial cells (Figure 3a).  The first indication of any cardiovascular development is around day 18-19.  Prior to embryonic folding mesoderm cells in the mesoderm region of the disc embryo form angioblasts which coalesce to form networks of angiogenic cords which invade various structures to form the embryonic vasculature, this process is called vasculogenesis.  Angiogenic cords on either side of the neural crest coalesce to form an agnioblastic plexus in the mesoderm of the germ disc.  The angiogenic plexus develops into capillaries.  The source of blood cells originally comes from the yolk sac, thereafter a sequence of embryonic organs provide stem cells for blood cells (hematopoiesis), such as the liver, spleen, thymus and eventually the bone marrow [365]

Capillaries in at the rostral end of the embryo and on the side of the neural tube join to form a pair of blood vessels on each side of the neural crest (total of 4 blood vessels).  These blood vessels run along the long axis of the germ disc with one pair of blood vessel at the lateral edge of (one on each edge) and the other pair more medially, on either side of the neural tube (Figure 3).  The blood vessels on either side of the neural tube join at their rostral end.

As the embryo folds in its lateral dimension it will cause the lateral edges of the germ disc to approach each other until they meet, causing the embryo to acquire a tubular form [361, 362].  The two outer endocardial tubes will come close to each other in the medially, ventral to the primitive gut and start fusing, cranially to caudally, thus forming a single median tube: The primitive heart tube (Figure 4). [362, 366]


Figure 3

i.  Angiogenic cell clusters occupy a horse-shoe shaped area rostral and around the brain plate. 
ii. These cell clusters are the first evidence of vascular structure in the embryo.





Figure 3

The cell clusters coalesce to form 2 pairs of endocardial tubes on each side of the neural fold.  The lateral tubes come close together as the embryo folds to eventually join and form the single heart tube.  The median endocardial tubes remain on either side of the neural fold to form the dorsal aortae.