The lymphatic vascular system: secondary or primary?

Area of Science:

• Evolutionary biology
• Vascular biology
• Immunology

Background:

• The prevailing view considers the blood vascular system primary and the lymphatic vascular system secondary.
• Blood vascular diseases are a major cause of mortality, while lymphedema is generally manageable.
• Ontogenetic development shows blood vessels and the heart forming before lymphatic vessels.

Purpose of the Study:

• To investigate the evolutionary and developmental origins of the vascular systems.
• To challenge the established hierarchy of blood versus lymphatic vascular systems.
• To explore the primary functions of early vascular systems in phylogeny and ontogeny.

Main Methods:

• Comparative analysis of vascular system development across different species (insects, molluscs, arthropods, vertebrates).
• Examination of gene expression patterns (e.g., flt-4/VEGFR-3) during vascular development.
• Review of the functional roles of circulatory systems in defense and nutrient transport.

Main Results:

• Evidence suggests the earliest vascular systems possessed lymphatic functions.
• Macrophages, crucial for defense, emerge before erythrocytes, circulating in early lymphohematic systems.
• Early vertebrate blood vessels exhibit lymphatic characteristics and express lymphatic markers like flt-4.

Conclusions:

• The primary function of early vascular systems appears to be defense and immune cell circulation (lymphatic function).
• Nutritive function became dominant in blood vessels of larger organisms.
• The vertebrate blood vascular system may have evolved from an ancestral lymphatic system, potentially featuring lymph hearts.