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Paul Banaszkiewicz Paul Banaszkiewicz Section Editor, Segment Author
Chris Ghazala Christopher George Ghazala Segment Author

Section editor: Paul Banaszkiewicz

Segment author: Christopher George Ghazala; Paul Banaszkiewicz

Document history :6/07/2019

  • It is important to understand the structure and function of synovium as it is frequently altered in many disease processes such as rheumatoid arthritis.
  • Synovial fluid mechanics may be covered as part of lubrication
  • The internal epithelial lining of a joint capsule is covered by synovium, and secretes synovial fluid to lubricate the joint surfaces and provide nourishment to the articular cartilage and surrounding tissues.
  • The synovium is formed by two to three layers of synovial cells, known as synoviocytes, that merge on the deep layer with loose fibrocollagenous tissue that contains a rich blood supply and numerous cell types including adipocytes, fibroblasts, mast cells and macrophages.
  • Under light microscopy, synoviocytes appear as flat oval cells with dark-staining nuclei. Cells lining the lumen have ovid nuclei and extensive cytoplasmic processes that extend from the nucleus. These surface synoviocytes are known as type B cells and produce synovial fluid; whereas the deeper layer are known as type A cells. Type A cells are phagocytic and type B cells have numerous endoplasmic reticulum for protein synthesis.

BS2SYN 1Harry.png

Figure 1. The arthritic synovial joint is characterised by inflammation and thickening of the SM with influx of lymphocytes and macrophages.

A cells

  • A cells are macrophages.

B cells

  • B cells are exocrine/synthetic cells.
  • Synthesise hyaluronic acid from protein and glycosaminoglycan.

C cells

  • May exist as an intermediate cell type.
  • Arranged into microvilli and villi resulting in a large surface area (knee = 100 m2; whole body = 1000 m2).
  • Control diffusion
  • Ingest debris
  • Secrete immunoglobulins
  • Secrete lysosomal enzymes
  • Secrete hyaluronate
  • Secrete lubricating glycoproteins
  • Reduce friction in joint
  • Synovial cells may transform into chondrocytes – occur at attachment site of the synovial membrane to periphery of articular cartilage.
  • Synovial cells are capable of rapid and complete repair or regeneration.
  • When challenged with an infectious inoculum, synovial membrane undergoes an acute inflammatory response with accumulation of polymorphonuclear leukocytes and monocytes.
  • Complement activation produces chemotactic factors and other mediators of inflammation that enhance the inflammatory process.
  • Lysosomal enzymes attack hyaluronate leading to loss of the diffusion barrier.
  • Inflamed synovium contains large clefts which probably permit the passage of molecules of almost any size.
  • Synovium becomes a “leaky sieve.”
  • If infection is not quelled, synovium will necrose within 24 hours, followed by necrosis of chondrocytes and failure of glycosaminoglycan production.
  • Acute inflammatory response along with cellular disintegration (with release of lysosomal enzymes and proteases) will further injure chondrocytes.
  • Antibiotics can also easily get into the joint.
  • But – lysosomal enzymes (collagenase, protease, galactosidase) damage the joint – therefore joint toilette required.
  • Hydrocortisone blocks the release of lysosomal enzymes.
  • Synovial fluid is an ultrafiltrate of blood plasma (by molecular sieving) plus hyaluronic acid and glycoproteins.
  • Does not contain fibrinogen – thus no clotting.
  • Exhibits non-Newtonianfluid characteristics – viscosity is not constant and inversely depends on shear rate. The relationship between viscosity and shear rate depends on amount of hyaluronate.
  • Nourishes articular cartilage through diffusion.
  • Lubricates via the following mechanisms:
  1. Hydrodynamic (fluid separates the surfaces under load)
  2. Boundary (slippery surfaces)
  3. Weeping (fluid shifted to loaded areas)
  4. Boosted (fluid entrapment)
  • Lubricin (a glycoprotein) is a key lubricating component.

Table 1. Synovial fluid analysis

 

Normal

OA

RA

Septic A

Gout/Pyroph.

Appearance

Clear

Clear

Turbid

Turbid

Clear

WBC/ml

100

1000

30,000 (PMN <75%)

100,000 (PMN >75%)

10,000 (PMN >75%)

Cells

None

Mononuclear

Neutr.

Neutr.

Neutr.

Crystals

No

No

No

No

Yes

Clots

No

No

Yes

Yes

Yes

Viscosity*

Normal

High

Low

Low

Low

Culture

Neg.

Neg.

Neg.

Pos.

Neg.

Glucose

= Serum

=

Low

Low

?

CASE BASED DISCUSSIONS

Synovial fluid

Question: What is synovial fluid?

Synovial fluid is a viscoelastic fluid that fills the cavity between two articulating bones and serves as a lubricating fluid

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Further Reading

  • 1. Lowe JS, Anderson PG. Stevens and Lowe’s human histology, 4th ed. Philadelphia: Elsevier Mosby (2015).
  • 2. O’Connell JX. Pathology of the synovium. Am J Clin Pathol 2000; 114: 773–784.