Histology of Joints

Question 1

What is a joint?

Answer 1

A joint is an articulation between adjacent bones, they are part of the skeletal system and allow movement. The degree of movement varies between joints and the amount of movement is often inversely related to the stability of the joint.

Question 2

What are the different macroscopic classifications of joints?

Answer 2

Fibrous, cartilaginous and synovial.

Question 3

What are the different classifications of joints according to amount of movement?

Answer 3

Synarthrotic (immovable), amphiarthrotic (slightly moveable) and diathrotic (freely moveable).

Question 4

Fibrous joints

Answer 4

Fibrous joints have little or no movement, and bones are held together by dense connective tissue. The length of the CT ‘ligament’ determines mobility. Fit between adjacent bones also determines mobility.

Question 5

Examples of fibrous joints

Answer 5

(1) Teeth are joined to the jaw by a fibrous tissue called a gomphosis;
(2) Interosseous ligament/membrane (syndesmosis - little movement);
(3) Skull sutures, note skull sutures often ossify with age (synostosis - no movement);
(4) Posterior sacroiliac joint (note anterior SI joint is synovial).

Question 6

Cartilaginous joints

Answer 6

Limited movement and the bones are held together by cartilage. There are two types of cartilage joints, depending on the type of cartilage present:
(1) Synchondrosis which are united by hyaline cartilage, also called a ‘primary cartilaginous joint’; (2) Symphysis which are united by fibrocartilage and are termed ‘secondary cartilaginous joints’.

Question 7

Examples of primary cartilaginous joints

Answer 7

Growth plate, rib 1-sternum.

Question 8

Examples of secondary cartilaginous joints

Answer 8

Intervertebral discs, pubic symphysis

Question 9

Synovial joints

Answer 9

These are the most mobile, however, the degree of movement is variable between different joints. Articular surfaces are covered by articular cartilage (modified hyaline) and the joint is completely enclosed by a capsular ligament lined by synovial membrane. May contain menisci and/or bursae. The capsule is continuous with the periosteum.

Stability in the synovial joints comes from the ligaments, bone fit and surrounding muscles (also menisci if present).

Question 10

Articular cartilage

Answer 10

The articular surfaces of synovial joints are covered by a type of hyaline cartilage called articular cartilage. It is remnant of the cartilage model of the developing bone. Articular cartilage has a more complex histological structure than hyaline cartilage, it also has no perichondrium.

Articular cartilage is avascular and aneural. Nutritional needs are provided through diffusion, synovial fluid and subchondral bone. The edges of the articular cartilage is often well-nourished from highly vascular synovial membrane. Within articular cartilage the collagen bundles are arrange in an ‘arched’ distribution.

Question 11

What are the four zones of articular cartilage?

Answer 11

(1) Superficial (tangenitial) zone: closest to the articular surface, contains pressure resistant elongated flattened chondrocytes and bundles of collagen parallel to the articular surface. Highest concentration of collagen fibres.
(2) Intermediate (transitional) zone: round chondrocytes, less organised, ECM mostly proteoglycans. Collagen fibres moving from parallel to the articular surface to perpendicular to articular surface.
(3) Deep (radial) zone: small round chondrocytes in short columns perpendicular to articular surface, collagen between chondrocytes parallel to the long axis of the bone.
(4) Calcified zone: calcified matrix, small chondrocytes separated from the deep zone by the tidemark, a heavily calcified line.

Replacement chondrocytes form just above the tidemark region and migrate slowly towards the articular surface.

Question 12

Joint capsule

Answer 12

Develops from mesenchyme, outer layer dense connective tissue, highly innervated, highly vascular, inner layer synovial membrane 1-2 layers of cells (epitheloid). The inner cell layer (intima) can be closely adhered to the joint capsule or folded (villus-like) and is not always continuous. Proliferation of fold - multiple villi (pannus) seen in rheumatoid arthritis. Below the intima is the subintima containing varying amounts of loose CT which may contain adipose or fibres and many capillaries

Question 13

Synovium

Answer 13

Synovium is highly vascular, it covers all areas except where subjected to compressive forces. Consists of 1-2 layers of cells called synoviocytes. Type A synoviocytes are macrophage-like synovial cells and type B are fibroblast-like synovial cells. The fibroblast cells produce hyaluronan. Synovium also lines bursae and a few tendon sheaths (wrist and ankle).

Question 14

Menisci

Answer 14

Are composed of fibrocartilage, no perichondrium, and not covered with synovium. Improve bone fit and therefore stability of a joint, act as a shock absorber, aid in weight distribution.

Question 15

Bursa

Answer 15

Is a synovial lined fluid filled sack located around joints or between bones and tendons. It reduces friction between muscles, tendons and bones. May or may not communicate with a joint cavity. If friction occurs, causing inflammation of the bursa the condition is called bursitis.

Question 16

Ligaments

Answer 16

Stabilise joints with attachments from bone to bone located close to the joint. They are mostly composed of dense regular connective tissue

Question 17

Types of ligaments

Answer 17

(1) Capsular ligament: ligament is part of the capsule; (2) extracapsular ligament: ligament is separate and outside the capsule; (3) intracapsular ligament: ligament is separate and inside the joint capsule.

Question 18

How are synovial joints classified?

Answer 18

Classification is based on the shape of the articulating surfaces. The type of movement that occurs at the joint is dependent in part on the shape of the joint surfaces. The degree of movement is dependent on the bone shape, number and size of ligaments, surrounding muscles and the presence of menisci. To a lesser extent, other issues that may affect joint mobility include body type, old age, pain etc.

Question 19

What are the different classifications of synovial joints?

Answer 19

(1) Ball and socket,
(2) saddle,
(3) ellipsoid/condyloid,
(4) pivot,
(5) hinge; and
(6) plane.

Question 20

Ball and socket joint

Answer 20

Multiaxial and in general very mobile e.g. hip and shoulder. Most are very mobile however the bone shape, surrounding ligaments and muscles make the hip far more stable than the shoulder.

Question 21

Saddle joint

Answer 21

Carpometacarpal of the thumb (biaxial), movement in 2 directions.

Question 22

Ellipsoid/condyloid

Answer 22

Metatarsophalangeal (biaxial) movement in 2 direction flexion/extension, adduction/abduction movement in one direction is usually much greater than the other.

Question 23

Pivot joint

Answer 23

Radioulnar, uniaxial, only movement is rotation

Question 24

Hinge joint

Answer 24

Interpharyngeal, uniaxial, flexion/extension

Question 25

Plane joint

Answer 25

Intertarsal, slight movement gliding usually limited but very strong ligamentous attachments, usually uniaxial.

Question 26

Innervation of joint

Answer 26

Hilton’s law: nerves that innervate muscles that act across a joint also innervate the joint. Joint capsule is innervated, articular cartilage not innervated.

Question 27

Synovial fluid

Answer 27

Synovial fluid Is blood ultrafiltrate and secretions from synovial cells. Clear, colourless consistsency of egg white (caused by hyaluronic acid).

Cells of the synovium are permeable to water and small molecules. Creates a thin film of fluid in each joint cavity; lubricates the joint surface and provides some metabolic support to the articular cartilage. Also contains some phagocytic cells to remove microbes and debris from normal wear and tear.

Question 28

What are the functions of articular cartilage?

Answer 28

The function is shock absorption, load distribution and movement