synovium physiology and changes in rheumatoid arthritis

1. Exploring the Synovium- in health and RA
Dr Koushik Mukherjee
Resident

2. Historical aspects
• The word ‘Synovia’ came from Greek συν- "with" and Latin ovum
"egg" and -ia because it resembles egg white in consistency and
external appearance
• In attributing the gout to coagulation of gluten album (white glue) or
joint fluid, Paracelsus first implicated this fluid in the pathogenesis of
gout.
• Both Paracelsus and his followers, including Severinus credited the
wound surgeons with the origin of the term synovia.

3. “For I declare that foot gout in its
various forms does not produce
such pains unless gluten, which
the surgeons call synovia,
becomes affected. And authors
have been wrong in setting forth
in their sections on podagra that it
is a natural flux with this kind of
pain. This is untrue and a manifest
error, for without gluten there is
no pain in the peripheral
members. Also an unnatural
binding of this sort as a cause of
pain, if it takes away freedom of
motion, surpasses the agonies of
the wheel and the cross.”

4. Definition
• Synovium is the soft tissue (membranous structure) lining the spaces of
diarthrodial joints, tendon sheaths, and bursae intra-articular tendons and
ligaments, as well as fat pads
• Joints include the temporomandibular joint and the facet joints of vertebral
bodies
• Does not cover articular cartilage or meniscal tissue.
• Ensheaths tendons where they pass beneath ligamentous bands and
bursae that cover areas of stress such as the patella and the olecranon

5. The cartilage-
synovium junction

6. Definition
Synovial
membrane
Intima or
Synovial lining
20 to 40 μm
Subintima or
Sublining
5mm
specialized
tissue-resident
macrophages
and fibroblasts
-adipose
tissue
-blood
-lymphatic
vessel
-resident
fibroblasts
-infiltrating
cells
-collagenous
extracellular
matrix (ECM)

7. Synovial membrane- not a true lining
• No well-formed basement membrane separates the intima from the
subintima
• lacks tight junctions, epithelial cells, and a well-formed basement
membrane
• derived from ectoderm, variable in its structure
• At many sites, there is no discrete membrane, especially where
subintima consists of fat pad or fibrous tissue
• Subintima merges with the densely collagenous fibrous joint capsule

8. Embryology
CD55
VCAM-1
UDPGD
hyaluronan
CD44
β1
integrins
-High concentration of hyaluronan:
saturate CD44 and disaggregation
-Low concentration of hyaluronan:
Cell aggregation
development of the joint cavity arises from differential
tissue expansion than through loss of solid elements
- continuous influx model of joint development

9. Structure- types of synovium
• Microscopic anatomy described by Key
Areolar synovium
-A more or less continuous layer of
intimal cells comprising macrophages
and fibroblasts lies two or three deep
on the tissue surface
-beneath these cells are capillaries
-into the tissue, there is a plexus of
small arterioles and venules often
associated with mast cells
Adipose synovium
-occurs as fat pads and within villi
-has a complete intimal cell layer and
a superficial net of capillaries
-intima may lie directly on
adipocytes, but there is usually a
band of collagen-rich substratum
-Villi usually have a central
arteriole and venule,can be avascular
Fibrous synovium
-consisting of fibrous tissue such as
ligament or tendon on which lies an
intermittent layer of cells
-may be indistinguishable from
fibrocartilage, especially in the
annular pads found in finger and toe
joint
-lymphatics-infrequent

10. Structure- Synovial lining cells
• SLC- two types of cells: originally described by Barland and associates
Type A synoviocytes Type B synoviocytes
Comprises 20% of SLC, distribution highly variable Comprises 80% of SLC
conspicuous Golgi apparatus, large vacuoles, and small
vesicles, and they contain little rough endoplasmic reticulum
abundant rough endoplasmic reticulum widely distributed in
the cytoplasm, and the Golgi apparatus, vacuoles, and
vesicles are generally inconspicuous, contain longitudinal
bundles of different-sized filaments
plasma membrane possesses numerous fine extensions,
termed filopodia
Frequent invaginations are seen along the plasma membrane
occasionally cluster at the tips of the synovial villi have prominent cytoplasmic extensions that
extend onto the surface of the synovial lining
Smooth surfaced nucleus large indented/convoluted nucleus relative to the area of the
surrounding cytoplasm

11. Type A SLC

12. Type B SLC

13. Immunohistochemical Profile of Synovial Cells
Type A synoviocytes Type B synoviocytes
CD45 uridine diphosphoglucose dehydrogenase (UDPGD)
monocyte/macrophage receptors
CD163 and CD97
CD44
lysosomal enzymes CD68 CD55
neuron-specific esterase vascular cell adhesion molecule–1
cathepsins B, L, and D cadherin-11
CD14 (co-receptor for the detection of bacterial
lipopolysaccharide and is expressed by circulating
monocytes and monocytes newly recruited to tissue)
PGP.95
FcγRIII (CD16) TLR2
MHC II Clusterin and podoplanin (gp38)
Z39Ig, a complement-related protein
Variable expression of β2 integrin chains CD18, CD11a,
CD11b, and CD11c
CR2 (in culture), CXCL12 and BMP under various conditions

14. Turnover of SLCs
• Proliferation of SLCs in humans is low (labeling index of approximately
0.05% to 0.3% when exposed to 3H thymidine vs 50% for bowel crypt
epithelium)
• Proliferating cells are generally synovial fibroblasts
• little is known about SLC natural life span, recruitment, or mode of
death
• cultured fibroblast-like synoviocytes tend to be resistant to apoptosis

15. Origin of Synovial Lining Cells
• type A SLC population is bone marrow–derived and represents cells
of the mononuclear phagocyte system
• Type B intimal cells represent a resident fibroblast population in the
synovial lining
-little is known about the cells from which they derive and about how
their recruitment is regulated
-mesenchymal cells might differentiate into the synovial lining
fibroblast
-specific transcription factor directing mesenchymal stem cell
differentiation into the synovial fibroblast are yet to be discovered

16. Subintimal Layer
• SLCs are not separated from the underlying subintima by a well
formed basement membrane
-however most components of basement membrane are present in the
extracellular matrix surrounding SLCs
-tenascin X, perlecan (a heparan sulfate proteoglycan), type IV collagen
laminin, and fibrillin-1 are present
-laminin-5 and integrin α3β3γ2 are absent
• The subintima is composed of loose connective tissue of variable
thickness and variable proportions of fibrous/collagenous and
adipose tissue

17. Subintimal Layer
• Under normal healthy conditions, inflammatory cells are virtually
absent from the subintima, apart from a sprinkling of macrophages
and scattered mast cells
• Rich source of mesenchymal stem cells
• The subintima contains types I, III, V, and VI collagen,
glycosaminoglycans, proteoglycans, and extra-cellular matrices,
including tenascin and laminins
• Integrin receptors for collagens, laminin, and vitronectin are absent or
at best weakly expressed by subintimal cells
• receptors for fibronectin (CD49d and CD49e) are detected, and
CD44, the HA receptor, is strongly expressed in most subintimal cells
• β2 integrins are largely limited to perivascular areas

18. Subintimal vasculature
• The vascular supply to the synovium is provided by many small
vessels and is shared in part by the joint capsule, epiphyseal bone,
and other perisynovial structures
• Large synovial artery in deep layers of synovium
microvascular units in the more superficial subsynovial layers
• Precapillary arterioles probably play a major role in controlling
circulation to the lining layer
• The intimal lining is devoid of blood vessels

19. Subintimal Lymphatics and nerve supply
• lymphatic vessels are less common in the fibrous synovium compared with
areolar and adipose variants of human subsynovial tissue
• lymphatics are present in the superficial, intermediate, and deeper layers
of synovial membrane in synovium (superficial-low in healthy synovium)
• The synovium has a rich network of sympathetic and sensory nerves
• Sympathetic- myelinated and terminate close to blood vessels where they
regulate vascular tone
• Sensory: respond to proprioception and pain via large myelinated nerve
fibers and via small (<5 μm) unmyelinated or myelinated fibers with
unmyelinated free nerve ends (nociceptors).

20. Function of synovium : in joint movement
• Joint movement: 4 characteristics required:
-deformability, porosity, nonadherence, and lubrication
• In healthy person synovium is highly deformable to enable movement
between tissues rather than within tissues
• Deformability limits the extent of synovial ischemia-reperfusion injury
during joint motion by maintaining a relatively low intra-articular
pressure
• The synovial microvasculature and the intimal lining are porous to
permit robust diffusion of nutrients to cartilage

21. Function of synovium
• Intimal cells on the synovial surface adhere to underlying cells and
matrix but do not adhere to opposing synovial and cartilage surfaces
-mechanism unknown but might involve the arrangement of cell surface
and tissue matrix molecules, such as collagen, fibronectin, and HA or
due to regular movement of the normal synovial lining
• Lubrication help minimize wear and damage that result from normal
daily activities
• Synovial membrane may also contribute to concentration of
lubricants in synovial fluid because it is a semi-permeable membrane

22. Function of synovium in Lubrication
• Boundary lubrication: the protective effect of particular lubricating
molecules adsorbing to a surface and repelling its opposing interface
- by changing the physicochemical characteristics of a surface
-by reducing articular friction and wear by providing a smooth and
slippery coating
-loaded cartilage also play a role
• Hyaluronic acid maintains SF viscocity and is a viscohydrodynamic
lubricant at low-load interfaces, such as synovium-on-synovium and
synovium-on-cartilage
• Lubricin is responsible for boundary lubrication

23. Camptodactyly–arthropathy–
coxa vara–pericarditis (CACP)
syndrome due to absence of
lubricin

24. Function of synovium: Synovial fluid formation
• Synovial fluid is in part a filtrate of plasma to which additional
components, including HA and lubricin, are added and removed by
the SLCs
• Plasma proteins diffuse according to their size (albumin vs fibrinogen)
• Clearance is unrestricted through lymphatic system
• Egress>> ingress leads to subatmospheric intra articular pressure
• fat-soluble molecules like the respiratory gases, oxygen and carbon
dioxide cross in unrestricted manner

25. Synovial fluid
formation

26. Function of synovium: Chondrocyte nutrition
• Synovium enhance the nutrition of chondrocytes, which reside in
articular cartilage
• Three potential mechanism of nutrient transfer:
-diffusion
-active transport by chondrocytes
-pumping by intermittent compression of cartilage matrix
• cartilage does not survive without synovial fluid contact in vivo

27. RA synovium- change in SLC
• In RA synovium, SLC is 4-10 cells deep (vs normal 1-2 layer)
• MLS and FLS both increased in RA (MLS>FLS)
• MLS synoviocytes tend to accumulate in the more superficial regions of the
intimal lining
• MLS increase in RA due to ingress of new bone marrow–derived precursors
and are highly activated and produce many cytokines
• FLS: local proliferation in response to PDGF, TGF-β, TNF, and IL-1 and
arachidonic acid metabolites as well as migration of pluripotential
mesenchymal stem cells from BM
Mitotic figures in rheumatoid synovium are rare, and thymidine uptake
indicating DNA synthesis occurs in only a small percentage of synovial cells

28. RA synovium- change in FLS
• RA FLS and synovium display abnormalities of choline metabolism
with high expression of choline kinase α
• Glucose metabolism is dysregulated in RA FLS, with a shift toward
glycolysis in RA FLS compared with OA FLS
• As RA evolves, FLS assume a more stimulatory phenotype and can
activate vascular endothelium
• FLS from highly inflamed synovium exhibited a TGF-β gene signature

29. Aggressive Behaviour of RA-FLS
• Tumour like properties:
-RA FLS can proliferate in an anchorage-independent manner
-cultured RA synoviocytes can exhibit defective contact inhibition and
express proto-oncogenes, such as c-Myc
-Increased telomerase activity is present in RA synovium
-overexpression of certain miRNAs or differential DNA methylation is
associated with increased cytokine production or more aggressive
behavior

30. • Matrix invasion:
-altered RA FLS can adhere to and invade into the cartilage matrix (d/t
VCAM-1 and proteases)
- RA FLS can “metastasize” from joint to joint and contribute to the
polyarticular nature of RA
- excessive production of IL-1 and underexpression of IL-10 contribute to the
invasive properties of RA synoviocytes
-RA FLS have prolonged and exaggerated responses to synovial cytokines
-Phosphatases are dysregulated in RA FLS and contribute to pathogenic
fibroblast behavior
Aggressive Behaviour of RA-FLS

31. Intimal lining hyperplasia Immunostaining for macrophages Perivascular T cell aggregate
Histopathologic appearance of rheumatoid arthritis
synovium

32. Change of immunohistochemical profile in RA
• Expression of metalloproteinases, cytokines, adhesion molecules, and
other cell surface molecules is strikingly increased
• PD1 increases in early and established RA
• The ligands for PD-1, PD-L1, and PD-L2 are increased in synovial tissue
on transcriptomic analysis in RA
-protein expression for the ligands is minimal even before the disease
becomes clinically manifest suggesting a homeostasis between PD-1
and its ligands in normal synovium that is lost in inflammation
Cause for Immune check point inhibitor induced arthritis

33. Signaling pathways activated in RA synovium
• NF-κB, JAK/STAT, Notch, hypoxia-inducible factor 1, α subunit (HIF-1α)
• NF-κB activation could facilitate synovial hyperplasia by promoting
proliferation and inhibiting apoptosis of RA FLS
- by countering the cytotoxicity of TNF and Fas ligand
• STAT3 expression in the synovium correlates with synovitis and is activated
by IL-6
• Notch mediates angiogenesis
• Hypoxia induces activation of phospho (p)-STAT3/p-STAT1, NF-κB, and
Notch in synovial cells

34. Role of infiltrating cells in RA synovium
• T cells usually constitute 30% to 50% of cells in RA synovia, and most
are CD4+ (memory cell phenotype)
• In chronic RA, besides CD4+ lymphocytes organizing into lymph node
like structure, CD8+ cells, plasma cells, macrophages and B
lymphocytes can be present
• This inflammatory aggregates lead to higher local RF and ACPA
production, higher CRP elevation and systemic inflammation

35. • Synovial B cells and plasma cells in RA exhibit evidence of antigen-
driven maturation and antibody production with oligoclonality
• DCs can potentially present antigens to T cells in synovial germinal
centers
• Mast cells produce small molecule mediators of inflammation and can
participate in disease initiation by increasing vascular permeability
• Neutrophils are sparse in RA synovium but are usually abundant in
synovial effusions
Role of infiltrating cells in RA synovium

36. SF changes in RA
• RA synovial effusions contain neutrophils and mononuclear cells,
including T lymphocytes and macrophages
• Immune complexes that contain autoantibodies such as RFs or ACPAs
can fix complement, leading to the generation of chemoattractants
• Small molecule mediators of inflammation such as prostaglandins and
leukotrienes are present in RA synovial fluid.
• In inflamed synovium, greater endothelial permeability permits
profuse ingress of all proteins reduction of joint stability

37. • In inflamed synovium, synovial fluid may exhibit low partial pressure
of oxygen, high partial pressure of carbon dioxide, decreased pH, and
increased lactate production
• resultant hypoxia and acidosis can have serious implications for the
synovial microcirculation and chondrocyte metabolism
SF changes in RA