Synovium
- lines the joint cavity ( Diarthroses – freely moveable joints located in the limbs and jaw ) -> covers the fibrous joint capsule and extends to the bone – cartilage interface.
- Not a true lining epithelium ( no basement membrane)
- Healthy synovium also covers intra-articular ligaments and tendons.
Functions :
- Nutrient and metabolite trafficking.
- Cellular barrier.
- Production of synovial fluid.
Synovial membrane
- 3 patterns:areolar , fibrous and fatty
- 2 functional compartments
Functional compartments
- Synovial intima lining superficial layer( 1- 3 cells deep synoviocytes) ; site of production of synovial fluid.
- Subintima containing the neurovascular bundle
Synoviocytes :
- Type A [10-20% ] macrophages – hematopoetic in origin – markers CD 68 , NSE . Less RER ( rough endoplasmic reticulum )
- Type B [80-90%] fibroblasts – mesenchymal interzone origin precursor cell. Increased RER. Marker CD 55 . Increased expression of UDPGD ( uridine diphosphoglucose dehydrogenase ) —> involved in synthesis of hyaluronic acid.
Synovial fluid :
- nutrition for articular cartilage and lubrication.
- Filtrate of plasma from fenestrated endothelium +
- Lubricants : carbohydrate( Hyaluronic acid) ; glycoprotein ( Lubricin)
Synovial fluid :
Normal. :- < 200 cells / mm3 ; < 20 % N
Non inflammatory :- 200 to 2000 cells / mm3 ;<50%N
Inflammatory :-> 2000 cells / mm3 ; variable N
Subintimal nerve supply :
- Sympathetic – regulate vascular tone
- Sensory – Pain :- A delta and C fibres via free nerve endings—> release of neuropeptides causing inflammation ( Substance P , CGRP and VIP ) ; Proprioception A beta fibres
References
Firestein and Kelley’s Textbook of Rheumatology 12 th edition Chapter 2 Authors Douglas J Veale and Gary S Firestein