Histology of Joints

General Description of Synovial Joints

Most common: plane, hinge, condylar

Less common: ball and socket, ellipsoidal, pivot, saddle joints

cavity separates bone, permits movement and can be subdivided by menisci

articular surface: covered by hyaline cartilage
Articular ends of adjacent bones are connected by ligaments and a fibrous capsule lined by a Synovial membrane

shape determines the classification of the joint along with its type of movement.

Blood and lymph Supply

enter at or near line of capsular attachment Þ rich capillary network Þ allows plasma easy access to joint

innervation to one joint can come from several segments and can supply other joints

supplies ligaments, capsule and synovial membrane; contains autonomic and sensory fibers Þ proprioception

Tissues of the Diarthrodial joint

Tissue type	Functions	Anatomy	Histology
Synovial Membrane	Phagocitic (Type A Cell) Secretion of hyaluronic acid, collagen and fibronectin (Type B); production of synovial fluid; thickens during disease	Lines inner surface of capsule but does not cover articular cartilage	Vascularized and innervated, loose irregular CT; 1-2 cells thick; Type: A = macrophage; B = fibroblastic
Synovial Fluid	Joint lubrication, nourishment of cartilage and debris removal	Viscosity due to hyaluronic acid Lubrication due to lipids: DPPC and lubrican	Is a plasma filtrate that is egg white like. Has a few lining cells
Articular Cartilage (hyalin cart Þ type II collagen) rich in proteoglycan	Provide sliding surface, resist compression (proteoglycan = spongy shock absorber), distribute load. papain Þ protein that produces rigidity	Covers surface between adjacent bones (ie. joints, ear, bridge of nose); arthritis = loss of this cartilage	Avascular, lymphatic, and aneural Nutrients diffuse via matrix to it. See histology of Cartilage below
Meniscus (fibrocartilage) (decrease load bearing 400%)	Shock absorption, load sharing and redistribution, proprioception, lubrication, joint stability, Mechanoreception	Half moon shaped fibrous tissue. Covers cartilage but only attaches to Synovium	Cells: fibrochondrocytes Matrix: Type I >> II – complex Avascular inner Zone Vascular outer Zone
Insertions to Bone	To transmit force by attaching flexible, strong, tension-bearing structure to rigid non compliant bone	Functional Types: Joint capsule Ligaments Tendons	Morphological Types: Direct Most fibers in bone Acute to bone Sharp boundary Indirect most fibers in periostium parallel to bone blends
Joint Capsule	Unite articular end of adjacent bones (small fibers inserted in a random pattern)	-lined by synovial membrane -vessels enter at point of attachment	Small fibers inserted in a random pattern
Ligaments (has to resist tension) (complicated motions)	Attach bone to bone, joint stabilizer, Mecanoreception. Small diameter fibers that insert parallel ("collateral") or inter woven pattern (‘cruciate")	Nomenclature based on: Attachment:, function, relation to joint or each other.	Cells: fibroblasts Matrix: collagen I and III with crimp Elastin, proteoglycans, glycoproteins
Tendons (has to resist tension)	Efficiently transmit muscle force to skeleton so has large parallel fibers that insert uniformly Þ unidirectional pull	Nomenclature based on muscle	Cells: Tenocytes = a variety of Fibroblasts Matrix: Collagen Type I (95%), III and IV, elastin, proteoglycan

Histology of Cartilage

Collagen fibers create an arched pattern so that fibers at surface run parallel to shear forces and thus confer resistance to shear force. Note: resistance to compression comes from the proteoglycans which are so loaded with (-) charge they attract H₂O and act like a spongy shock absorber releasing H₂O upon compression and reabsorbing it upon release

Zones of Cartilage over Bone

Tangential Zone: collagen fiber parallel to surface
Transitional Zone : collagen fibers curving
Radial zone: collagen fibers oriented perpendicular to surface
Tidemark
Calcified cartilage layer: transitional zone
Subchondral Plate – cancellous bone

Zones of Direct Insertion

Zone I: tendon or ligament
Zone II: fibrocartilage fibers in original orientation
Zone III: mineralized fibrocartilage – ends at Tidemark
Zone IV: collagen of tissue blends with that of bone

Tendon

like ligament, made up of dense regular connective tissue. Can be either sheath covered or Paratenon covered (Non-sheathed)

Layers of Paratenon-covered tendon from out side to in:

Paratenon: loosely arranged collagen fibers, elastin and vessels surrounding tendon. Vessels enter tendon directly without mesotenon

Epitenon: fine fibrous and synovial like cellular layer adherent to the tendon surface.

Endotenon: surround each fasicle, together with blood vessels and nerves; continuous with epitenon

Fascicle: consists of multiple tendon bundles

Sheath covered tendon is the same except instead of the paratenon layer has continuos with the Epitenon:

Mesotenon: a fold of connective tissue carrying segmental vessels, similar to mesentery, and
Tendon Sheath: a fibrous connective tissue cylinder lined by smooth synovial layer continuos with mesotenon

Insertions: collagen fibers of soft tissues (ligaments, joint capsules, tendons) penetrate underlying bone and embedded within it are called Sharpeys Fibers