Primitive blood circulation

Overview

Primitive blood circulation refers to the earliest functional vascular system established during embryonic development. It is the first system to become operational, ensuring the transport of nutrients, gases, and waste products before the definitive organs develop. The primitive circulation arises during the third week of embryogenesis, concurrent with gastrulation and vasculogenesis in the extraembryonic mesoderm.

Formation of the Primitive Circulatory System

1. Vasculogenesis

Vasculogenesis progress

The process begins with the differentiation of mesodermal cells into angioblasts, the precursors of endothelial cells. These angioblasts aggregate to form blood islands—clusters found primarily in:

• The yolk sac (extraembryonic mesoderm)
• The connecting stalk
• The chorion

Within these blood islands:

• The peripheral cells flatten to form endothelial cells.
• The central cells become primitive hematopoietic cells, the first blood precursors.

Gradually, these endothelial-lined spaces fuse to form a primitive capillary network.

2. Angiogenesis

Once the primitive vessels form, they sprout and remodel through angiogenesis—budding and fusion of endothelial cells to connect the extraembryonic and intraembryonic circulations.

Components of the Primitive Circulation

1. Extra embryonic Circulation

This part supplies the embryo with nutrients and oxygen before the placenta is fully functional.

• Yolk Sac Circulation: Blood vessels develop in the vitelline arteries and veins. The vitelline arteries carry blood from the embryo to the yolk sac, while vitelline veins return oxygenated blood back to the sinus venosus of the heart.
• Umbilical Circulation: The umbilical arteries transport deoxygenated blood from the embryo to the chorionic villi, and the umbilical vein returns oxygenated blood to the embryo.

2. Intra embryonic Circulation

Develops simultaneously with the cardiac tube. The primitive heart tube connects cranially with the aortic arches and caudally with the vitelline and umbilical veins.

The primitive heart and vessels form a closed loop:

1. Blood exits the heart via truncus arteriosus → aortic arches → dorsal aorta.
2. From the dorsal aorta, blood is distributed to the yolk sac and chorion via vitelline and umbilical arteries.
3. Blood returns through vitelline and umbilical veins into the sinus venosus, then back into the primitive heart.

The Primitive Heart

Development

• Originates from splanchnic mesoderm in the cardiogenic region.
• Two endocardial tubes fuse into a single primitive heart tube.
• This tube differentiates into regions: sinus venosus, atrium, ventricle, bulbus cordis, and truncus arteriosus.

Function

By day 22–23 of development, the heart begins to beat and pump blood through the primitive vascular system—marking the start of embryonic circulation.

Transition to Definitive Circulation

Circulatory system during fetal life

As development progresses:

• The placenta replaces the yolk sac as the main site of gas and nutrient exchange.
• The primitive heart undergoes looping and septation, forming right and left sides.
• The aortic arches remodel into definitive arteries (e.g., carotid, subclavian, pulmonary).
• The vitelline and umbilical vessels regress or transform into parts of the portal system and ligaments of the liver.

Clinical Relevance

• Abnormal vasculogenesis can lead to congenital malformations of the heart and major vessels.
• Defective placental circulation in early stages can cause embryonic hypoxia or developmental arrest.
• The study of primitive circulation is crucial for understanding embryonic cardiovascular malformations such as persistent truncus arteriosus or vitelline vein anomalies.

Sources

• Sadler, T. W. Langman’s Medical Embryology, 15th Edition.
• Moore, K. L. The Developing Human: Clinically Oriented Embryology.
• Young, B., & Heath, J. W. Wheater’s Functional Histology.
• Junqueira, L. C. Basic Histology: Text and Atlas.
• Netter, F. Atlas of Human Embryology.