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Small Vessel Pathology ²
  Capillaries: Muscle
  Endoneurial Microvessels: Nerve

Capillaries   Muscle     Normal     Staining Analysis     Ultrastructure     Disorders     Features       C5b-9 Deposits       Enlargement         Differential diagnosis         Pipestem         SRP Myopathy         Thick vessel walls         TS-HDS neuropathy       Loss or Damage         DM-VP         GvHD         Systemic Sclerosis           Minimal myopathy             Ultrastructure           Myopathy + Capillary Δ     Hemosiderin   ELS vessels Nerve   Endoneurial Microvessels     Microvasculopathy       C5b-9 Deposits     HIEM (Nerve)   Endoneurial inflammation     Ipilimumab     Perineuritis   Subperineurial edema

Endomysial Capillary: Normal Lumen: Contains RBC Inner region: Endothelial cell processes Outer region (Wall): Connective tissue, Single, thin, homogneous layer

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ENDOMYSIAL CAPILLARIES

Capillary
  Endothelium
    Cells = E: Lumenal side of vessel wall
  Vessel Wall
    Pericytes = P
      Number: None to Several
      Location: Within capillary wall, outside of endothelial cell processes
      Structure: Cell body & processes often surrounded by basal lamina that extends within capillary wall
    Basal lamina: 1 to 3 Thin layers around outside of capillary
    Collagen fibrils
      Orientations
        Longitudinal: Seen as punctate in cross section, or
        Circumferential: Seen as thin fibrils
      Size: Smaller than endoneurial or endomysial collagen fibrils
      Likely molecular composition: Collagen IV
  Fibroblasts: Processes may surround vessel wall

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Capillary Structure: Components

Capillary wall   Surrounds endothelial cell layer   Structure: May be homogeneous or layered   Contains     Pericytes (P) & Pericyte processes     Connective tissue       Basal Lamina       Collagen fibrils & other molecules: Components         Collagen IV         Laminin         Glycoproteins: Fibronectin, Nidogen-1, Nidogen-2 (Entactin)         Proteoglycans (as HSPG): Perlecan, Agrin   Thickening: Differential diagnosis Fibroblasts   Processes may surround capillary wall   Marker: Vimentin

Endothelial cells (E)   Surround lumen   Stains: Ulex Europaeus Agglutinin I (UEAI); CD31   Other markers: ADGRF5, BTNL9, Glycolipids (VWF; LDL-C R), Endothelin-1, eNOS Pericytes (P) 5   Location: Within basal lamina of vessel wall   Contractile   Frequency vs Endothelial cells: 1:10 to 1:100   Origins     Skeletal muscle: Mesoderm     Brain: Ectoderm-derived neural crest   Markers: PDGRFβ; NG2, VEGF, Desmin, KCNJ8, ABCC9, CD13, mAb(3G5), Gelsolin, Igkv8-30 Smooth muscle   Markers: Smooth muscle actin (SMA); PDGRFβ, CD13, NG2, CD146, Desmin, Gelsolin (Not in skeletal muscle) 4   More prominent in post capillary venules Histiocytes   Markers: CD68, CD163, CD206, LYVE1, F4/80, HAM56, Acid phosphatase

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Muscle Capillaries: Features

Anatomy

• Adult: Normal
  • Numbers: All muscle fibers have adjacent capillaries
  • Orientation: Capillaries usually run along length of muscle fibers
• Child (< 2 years)
  • Numbers: Many muscle fibers have no adjacent capillary

Plasticity

• Increased Capillary Numbers
  • Muscle activity: Exercise; Electrical stimulation
  • Denervation
  • Anti-angiogenic thrombospondin-1 (TSP-1): Deletion
• Reduced Capillary Numbers
  • Disuse: Bedrest; Spaceflight
  • Muscle-specific VEGF-A deletion
  • TSP-1 induction
  • Perifascicular loss: Dermatomyositis with Vascular Pathology

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Endomysial Capillary Features: Stain & Molecular Analysis

Endothelial Cells	Basement Membrane	Immune
H&E: Morphology Ulex: Distribution;   #; Size; Stain intensity ATPase pH 4.3: #+ Alkaline phosphatase: #+ NADH: + or - EM: Tubuloreticular aggregates Other markers: VWF,   BTNL9, ADGRF5	Morphology   H&E   GT/VvG   Ultrastructure Molecular   Decorin/Col IV   PAS: + or - Pericytes   PDGFRβ	Humoral   C5b-9 Cellular   Acid phosphatase   HAM56 Cell involvement   Muscle: MHC I   Capillary: MxA

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Endomysial Capillaries: Normal

UEA I stain

Endomysial capillaries: Normal
  Normal number: Every muscle fiber has at least one adjacent capillary
  Orientation: Run parallel to muscle fiber length; Cut in cross-section

UEA I stain

All muscle fibers have at least one adjacent capillary

UEA I stain

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There are few or no capillaries immediately adjacent to the perimysium

UEA I stain

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Endomysial Capillaries: Reduced numbers

Some muscle fibers have no adjacent capillary

UEA I stain

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Endomysial Capillaries: Partial Denervation

UEA I stain

Endomysial capillaries
  Orientation: Abnormal; May be circumferential around muscle fibers (Above) cut in cross-section
  Number: Increased; Every muscle fiber has several adjacent capillaries

UEA I stain

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Endomysial Capillaries: Dermatomyositis with Vascular pathology (DM-VP)

Endomysial capillaries
  Orientation: Normal
  Number: Reduced or absent Endothelium (Ulex stain) in regions with small muscle fibers (Arrow)
    Endothelium is lost but basal lamina is still present
      & may have C_5b-9 deposition
  Size: Increased in regions neighboring muscle fiber atrophy

UEA I stain

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Endomysial Capillaries: Enlarged

UEA I stain

UEA I stain

Patient history: Cardiac failure & Interstitial lung disease

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Muscle: Endomysial Capillary Vasculopathy

Muscle: C_5b-9 Deposits
  On Endomysial capillaries

Microvasculopathy

Control Muscle

Microvasculopathy: C_5b-9 Deposits on many endomysial capillaries

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Muscle (Endomysial) Capillaries: Ultrastructure

Dermatomyositis (DM-VP)   Damaged   Large   Tubuloreticular profiles Normal: A; B Thick Walls   Homogenous   Multilayered   TS-HDS antibodies Systemic sclerosis

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From: R Schmidt

Endomysial Capillaries: Normal
  Lumen: Patent
  Endothelial cell processes
    Form inner layers of capillary wall
    Cytoplasm often contains small vesicles
  Outer Wall: Surrounds endothelial cells & their processes
    Pericyte processess (White arrow)
    Basal lamina (Black arrow)
      Mostly Single layer
      Thicker than muscle basal lamina (Right & Bottom Right)

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Endomysial Capillary: Normal
  Lumen: Patent; Contains erythrocyte
  Endothelial cell processes: Form inner layer of capillary wall
  Pericyte: Not visible
  Basal lamina (White arrow): Thin
  Nucleus (Large; Upper right): Probably a muscle fiber nucleus

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Endomysial Capillary: Normal
  Lumen: Patent
  Endothelial cell processes: Form inner layer of capillary wall
  Pericyte: Not visible
  Basal lamina (White arrow): Thin
  Nucleus (Large): Neighbors capillary; May be a resident histiocyte

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Endomysial Capillary: Basal lamina thick & homogeneous
  Lumen: Patent
  Endothelial cell processes: Form inner layer of capillary wall; Vesicles in cytoplasm
  Basal lamina: Thick, Homogeneous; Contains a few small cell processes

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Endomysial Capillary: Basal lamina thick & homogeneous
  Lumen: Patent
  Endothelial cell processes: Form inner layer of capillary wall; Vesicles in cytoplasm
  Basal lamina: Thick, Homogeneous; Contains a few small cell processes

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Endomysial Capillary: Basal lamina thick & homogeneous
  Lumen: Patent
  Endothelial cell processes: Form inner layer of capillary wall; Vesicles in cytoplasm
  Basal lamina: Thick, Homogeneous; Contains a few small cell processes

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Endomysial Capillary: Basal lamina thick & homogeneous
  Lumen: Patent
  Endothelial cell processes: Form inner layers of capillary wall
  Basal lamina
    Width: Thick
    Structure: Homogeneous
    Contains a few small cell processes
    Outer edge: Irregular

From Robert E Schmidt

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Endomysial Capillary in Muscle
  Lumen: Contains erythrocyte
  Endothelial cell: contains large nucleus
  Basal lamina: Mildly thick: Outer edge irregular

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Endomysial Capillaries: Thick walls with Multilayered structure
  Lumen: Patent; May contain erythrocyte (Above)
  Endothelial cell processes: Thick; Form inner layers of capillary wall
  Pericyte processes: Present within basal lamina layers
  Basal lamina
    Width: Thick
    Structure: Multilayered; Varied around vessel circumference
    Contains pericyte processes
    Outer edge: Irregular
  Fibrocyte processes (Arrows): Neighbor outer edge of capillary

From: R Schmidt

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Also see: Systemic sclerosis, Endomysial capillaries

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Endomysial Capillary
  Lumen: Present
  Endothelial cell processes: Contain multiple small vacuoles
  Wall: Thick; Homogeneous; Contains 2 or 3 pericytes
  Neighboring macrophage (Below; Arrow)

From: R Schmidt

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NERVE: Endoneurial Microvessels

• Microvessel Properties
  • Orientation
    • Along nerve length
    • Many oblique & transverse anastomoses
  • Density
    • 60 to 100/mm²
    • Some reduction with increased age
  • Sizes
    • Larger than capillaries in other tissues
    • Normal range: 5 to 22 μm; Mean 9 μm;
  • May stain for
    • Alkaline phosphatase
    • UEAI lectin: Endothelial cells
    • Scaffolding proteins: Link endothelial cell membranes to cytoplasmic components
      • ZO-1 (TJP1)
      • CLDN5
• Microvessel Components ³
  • Endothelial cells
    • Morphology
      • Number: ~ 4 to 5 per vessel cross-section
      • Location: Single layer next to vessel lumen
      • Borders
        • Linked by: Tight junctions; Claudins & Adhesion molecules
        • Component of: Blood-Nerve barrier
      • No fenestrae
      • Contain
        • Weibel-Palade bodies: ? Produce Factor VIII-related antigen
        • Pinocytotic vesicles
      • Surrounded by: Pericytes & Basal lamina connective tissue
    • Molecular
      • Vessel specific (Endoneurial & Epineurial)
        • CLDN5
        • ESAM
        • MPDZ
        • CDH6: Adherens junctions; Large axons
      • Specific for Endoneurial Microvessels
        • ABCB1
        • SLC1A1
  • Basement Membrane
    • Composed of: Layers of
      • Collagen
      • Connective tissue
      • Pericyte processes
  • Pericytes
    • Locations: May reside within basal lamina or around microvessel
    • Processes: May neighbor, or surround, peripheral sides of endothelial cells
    • May be: Resident histiocytes
  • Smooth Muscle: Not generally present
  • Blood-Nerve Barrier
• Pathology & Ultrastructure
  • Basement Membrane
    • Thick
      • Homogeneous
      • Layered
    • C_5b-9 deposition
    • Damaged & Rarified
    • Disorganized
  • Endothelium
    • Cells & processes: Increased numbers
    • Permeability: Increased; Fenestration
    • "Closed" vessels: Lumen occluded by endothelial cell processes
  • Surrounding cells

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H&E stain Endoneurial Microvessel (Arrow)   Orientation: Along length of nerve

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Endoneurial Microvessel Walls
  Endothelial cells: Toward lumen
  Pericytes (Arrows): Within, or outside, microvessel wall (Basal lamina)

Toluidine blue stain

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Endoneurial Microvessels: Near Normal
Endoneurial Microvessels
  Lumen: Filled with RBCs
  Endothelial cell with nucleus (Black arrow)
    Neighbored around lumen by Endothelial cell processes
  Pericyte (Gray arrow)

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Endoneurial Microvessel: Surrounded by Pericyte processes Endoneurial Microvessel
  Lumen: Filled with RBC
  Endothelial cell with nucleus
    Neighbored around lumen by other Endothelial cell processes
  Pericytes
    Surround microvessel
  Basal lamina: Surrounds microvessel & pericytes

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Endoneurial Microvessel: Surrounded by Pericyte processes Endoneurial Microvessel
  Lumen: Contains 1 RBC
  Endothelial cells: 2 nuclei are present
    Neighbored around lumen by other Endothelial cell processes
  Pericyte processes
    Surround microvessel

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Endoneurial Microvessels: "Closed"; Lumen is filled by endothelial cell processes Endoneurial Microvessel, "Closed"
  Lumen: None; Region is filled by endothelial cell processes
  Endothelial cells: 1 nucleus is present
    Neighbored in lumen by Endothelial cell processes
  Pericyte processes: Surround endothelial cell processes
  Basal lamina: Few thin layers of connective tissue incompletely surround vessel
  Also see: HIEM

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Endoneurial Microvessel, "Closed"
  Lumen: None; Region is filled by endothelial cell processes
  Endothelial cells: 1 nucleus is present
  Pericyte processes: Surround endothelial cell processes
  Basal lamina: Increased layers of connective tissue surround vessel

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Endoneurial Microvessel, "Closed"; Surrounding cells
  Lumen: None; Region is filled by endothelial cell processes
  Endothelial cells: 2 nuclei are present
  Pericyte processes: Surround endothelial cell processes
  Basal lamina: Few thin layers of connective tissue incompletely surround vessel
  Surounding cells: Increased cellularity aroound endoneurial microvessel

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Endoneurial Microvessel Endothelium: Increased numbers & processes of endothelial cells
  Lumen: Present
  Endothelial cells & processes: Multiple
  Pericyte processes: Surround endothelial cells
  Basal lamina: Layers of connective tissue around vessel moderately increased
  Disease: HIEM

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Endomysial Capillaries & Endoneurial Microvessels: Thick Walls

Endoneurial Microvessel Wall
  Thick
  Basal Lamina: Multiple layers
  Pericyte process (White arrow): Inner region of capillary wall
Endothelial cells: Center of vessel
Lumen: Small
Surrounded by: Fibroblast process (Dark arrow)

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Capillary Walls Thick: Differential diagnosis ¹

• Increased age
• Gastrocnemius muscle: Large endomysial capillaries
• Diabetes
  • Nerve: Multiple basal lamina layers
  • Muscle: Homogenous, thick walls
• Chronic idiopathic axonal neuropathies
• CMT I
• Collagen VI myopathies
• Hypoxia
• M-protein related neuropathy
• TS-HDS antibody-related polyneuropathy: Pathology
• Amyloid: Replaces or damages endoneurial microvessel walls
• HIEM: Damaged basal lamina layers in endoneurial microvessels
• Systemic sclerosis: Endomysial capillaries with Multiple basal lamina layers

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Endoneurial Microvessel: Thick Basal Lamina with Damaged Layers
  Lumen: Present
  Endothelial cells & processes: Present
  Pericyte processes: Surround endothelial cell layer
  Basal lamina
    Connective tissue layers: Increased, Damaged
    Similar to: HIEM
Patient Clinical: No diabetes, Motor-sensory axonal PN
  Similar to: HIEM pathology

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Endoneurial Microvessel: Thick Basal Lamina
  Lumen: Present
  Endothelial cells & processes: Present
  Pericyte processes: Surround endothelial cell layer
  Basal lamina: Increased connective tissue layers
Patient Clinical: No diabetes, Motor-sensory axonal PN

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From: Chunyu Cai Endoneurial microvessel (Diabetes)   Basal lamina: Thick; Multilayered (Arrow)   Endothelial cell: Dark-stained cytoplasm

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Ultrastructure: From Robert E Schmidt Diabetes Endoneurial Microvessel   Basal lamina:     Thick     Contains: Multiple thin layers

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Endoneurial Microvessel: Basal Lamina Fragmentation (HIEM)

Endoneurial Microvessel
    Basal lamina: Rarified & Wide, especially outer layers
  Lumen: Present
  Endothelial cells & processes: Present
  Pericyte processes: Not prominent
  Basal lamina (Arrow)
    Layers of connective tissue around vessel: Moderately increased
    Outer layers of connective tissue: Pale & Disorganized

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Endoneurial Microvessel: Basal Lamina Disorganized (HIEM)

Endoneurial Microvessel
    Basal lamina: Rarified & Wide, especially outer layers
  Lumen: Present
  Endothelial cells & processes: Present
  Pericyte processes: Not prominent
  Basal lamina (Arrow)
    Layers of connective tissue around vessel: Moderately increased
    Outer layers of connective tissue: Pale & Disorganized

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Endoneurial microvessels   Basal lamina: Thick; Homogeneous (Arrows)

Endoneurial microvessel   Basal lamina: Normal (Arrow)

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Endoneurial Microvessels: Walls have varied thickness

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Humoral Immune Endoneurial Microvasculopathy (HIEM) ²⁸

Microvessels   C5b-9 deposition on Microvessels   C5b-9 around Endothelial cells   Endoneurial Microvascular Endothelial cells: Large   Ultrastructure Nerve   Histology   Axon loss     Varied: Toluidine blue     Large > Small

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Associated Clinical Syndrome

• Confluent Asymmetric Neuropathy (HIEM)

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C5b-9 stain HIEM Nerve C5b-9 Stains Most Endoneurial Microvessels

Disease Control Nerve C5b-9 Stains Perineurium & Epineurial vessels but not Endoneurial microvessels C5b-9 stain

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HIEM Nerve
C_5b-9 Stains Endoneurial Microvessels

C5b-9 stain

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C5b-9 stain

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Endoneurial Microvasculopathy   HIEM: C5b-9 stains Endoneurial microvessels C5b-9 stain

Disease Control Nerve   C5b-9 does not stain Endoneurial microvessels C5b-9 stain

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C5b-9 (Green) + UEAI (Red) stain

HIEM: C_5b-9 stain (Green) around Endoneurial Microvessel endothelium (Red)

C5b-9 (Green) + UEAI (Red) stain

C5b-9 (Green) + UEAI (Red) stain

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Toluidine blue stain

Endoneurial Microvessel Pathology
  Vessel size: Normal or Large
  Vessel wall: Thick
  Endothelial cells: Large
  Basal lamina: Multi-layered; Fragmented
See: Ultrastructure

Toluidine blue stain

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HIEM: Endoneurial Microvessel Ultrastructural Pathology

Basal lamina   Fragmented: Common   Disorganized   Also see: Microvessel pathology Closed lumens Endothelial processes increased

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Endoneurial Microvessel
    Basal lamina: Fragmented, Rarified & Wide, especially outer layers
  Lumen: Present
  Endothelial cells & processes: Present
  Pericyte processes: Not prominent
  Basal lamina (Arrow)
    Layers of connective tissue around vessel: Moderately increased
    Outer layers of connective tissue: Pale & Disorganized Endoneurial Microvessel
    Basal lamina: Fragmented, Rarified & Wide, especially outer layers
  Lumen: Present
  Endothelial cells & processes: Present
  Pericyte processes: Not prominent
  Basal lamina (Arrow)
    Layers of connective tissue around vessel: Moderately increased
    Outer layers of connective tissue: Pale & Disorganized

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Endoneurial Microvessel
    Basal lamina: Rarified & Wide, especially outer layers
  Lumen: Absent; Filled by endothelial cell processes
  Endothelial cells & processes: Present
  Pericyte processes: Not prominent
  Basal lamina (Arrow)
    Layers of connective tissue around vessel: Moderately increased
    Outer layers of connective tissue: Pale & Disorganized

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From R Schmidt

HIEM Microvessel
  Endothelial cells: Multiple, Large
  Pericyte: Processes surround microvessel
  Basal lamina: Disorganized around outside of vessel

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From R Schmidt

HIEM Microvessel
  "Closed": No lumen
  Endothelial cells: Multiple, Large
  Pericyte (Arrow): Process partially surrounds microvessel
  Basal lamina: Connective tissue layers surround most of the vessel

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HIEM: Nerve anatomic pathology

H&E stain HIEM nerve: No lymphocytic inflammation

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VvG stain

HIEM: Differential fascicular involvement
  Myelinated axon loss: Varied (Above)
  Endoneurial ATPase stain: Varied (Below)

ATPase pH 4.3 stain

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Toluidine blue stain

HIEM sural nerve
  Moderate loss of large & small myelinated axons
  Regeneration: Clusters of small myelinated axons
  Endoneurial microvessels
    Basal lamina: Thick
    Endothelial cells: Large

Toluidine blue stain

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Toluidine blue stain

HIEM sural nerve
  Large & small myelinated axons: Severe loss (Above)
  Small unmyelinated axons: Relatively preserved (Below)

Neurofilament stain

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Control sural nerve
  Large & Small axons are present

Neurofilament stain

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Go to Moderate & large vessel vasculitis
Return to Neuromuscular Home Page
Return to Vasculitis

References
1. Acta Neuropathol 2000;100:445-450
2. Am J Physiol Heart Circ Physiol 2016 Jul 1
3. J Peripher Nerv Syst 2019;24:195-206
4. Sci Rep 2025;15:28874
5. Oxid Med Cell Longev 2010;3:290-303, Sci Rep 2025;15:28874

1/9/2026