What are Rouget cells (pericytes) in capillaries?

What Are Rouget Cells in Capillaries?

Rouget cells are the historical name for pericytes—mural cells that wrap around capillaries and are embedded within the vascular basement membrane, distinct from smooth muscle cells and serving primarily as homeostatic sensors and signaling intermediaries rather than contractile elements. 1, 2

Historical Context and Nomenclature

• Rouget cells were first described in 1873 by French scientist Charles-Marie Benjamin Rouget, who initially characterized them as contractile elements surrounding capillaries 1, 2, 3
• The term "pericyte" was introduced 50 years later by Karl Wilhelm Zimmermann, derived from "peri" (around) and "cyte" (cell), reflecting their anatomical position encircling blood vessels 2, 3
• The historical name "Rouget cell" is now largely obsolete, having been replaced by the term "pericyte" in modern scientific literature 1

Anatomical Location and Structural Characteristics

Brain pericytes are characterized by longitudinal processes that span multiple vessels and surround approximately 85% of intraparenchymal microvasculature 4

Key anatomical features include:

• Located abluminal to endothelial cells (outside the endothelial layer but within the vessel wall) and luminal to parenchymal cells 1
• Completely enclosed within the vascular basement membrane, which is the defining characteristic that distinguishes them from other perivascular cells 2
• Do NOT express smooth muscle actin (αSMA), unlike smooth muscle cells 4

Distinction from Smooth Muscle Cells

A critical modern understanding is that capillary pericytes are NOT contractile cells, contrary to Rouget's original hypothesis 4

The evidence clearly demonstrates:

• Only arteriolar and precapillary smooth muscle cells (SMCs), not capillary pericytes, are contractile and able to modulate vasomotility following sensory stimulation or direct optogenetic activation 4
• SMCs are ring-like cells that surround only about 15% of brain microvessels, robustly express αSMA, and have a transcriptome clearly segregated from pericytes 4
• High-resolution intravital imaging has definitively shown that viable SMCs in precapillary arterioles, but not capillary pericytes, undergo transient hypercontractility leading to vasospasm during ischemia 4

Common Pitfall to Avoid

Many older studies incorrectly attributed contractile function to pericytes based on brain slice preparations and classical "bump-on-a-log" morphology 4. However, markers such as NG2 and PDGFRβ are not specific for distinguishing pericytes from SMCs, and vessel diameter/branch order are unreliable identification features 4. Unambiguous identification requires ultrastructural analysis or semi-thin sections demonstrating location within the vascular basement membrane 2.

Functional Role: Signaling Rather Than Contraction

Pericytes display intermittent calcium (Ca++) transients that do NOT correlate well with changes in vasomotility, unlike SMCs 4

Their actual functions include:

• Pericyte Ca++ transients correlate better with patterns of neuronal activation, suggesting they participate in intercellular signaling mechanisms rather than contractile functions 4
• Pericytes likely act as homeostatic sensors (detecting oxygen, CO2, or glucose levels) or respond to activity-dependent local release of neurotransmitters or K+ ions 4
• Signals may propagate through gap junction channels to more proximal SMCs or via endothelial cells to regulate vasomotility in upstream microvasculature 4
• Pericytes are important for maintenance of homeostasis, blood-brain barrier integrity, angiogenesis, and neovascularization 2

Additional Biological Properties

• Pericytes are thought to be multipotential stem cells with the ability to differentiate along mesenchymal and neurological lineages 1
• They actively communicate with cells of the neurovascular unit and make regulatory adjustments in response to stress stimuli 1
• Pericytes may play immune functions, including attracting innate leukocytes, regulating lymphocyte activation, and contributing to clearance of toxic by-products through direct phagocytic activity 3
• Pericytes are a heterogeneous population with tissue-specific variations in distribution, morphology, molecular marker expression, plasticity, and functions 5, 3