Histology of Joints

Prof. Dr. Bülent MızrakBAU International University-Batumi

JOINTS

Joints may be classified into two main functional groups, synovial and non synovial, both of which may show wide morphological variations.

Synovial joints

In this type of joint, there is extensive movement of the bones upon one another at the articular surfaces. The articular surfaces are maintained in apposition by a fibrous capsule and ligaments and the surfaces are lubricated by synovial fluid. Synovial joints are known as diarthroses. In some diarthroses such as the temporomandibular and knee joints, plates of fibrocartilage may be completely or partially interposed between the articular surfaces but remain unattached to the articular surfaces.

Non-synovial joints

These joints have limited movement, the articulating bones having no free articular surfaces, instead being joined by dense collagenous tissue. This may be of three types:

• Dense fibrous tissue. This forms the sutures between the bones of the skull and permits moulding of the fetal skull during its passage through the birth canal. The sutures are progressively replaced by bone with advancing age. Such fibrous tissue joints are called syndesmoses, and, when replaced by bone, are called synostoses.

• Hyaline cartilage. This type of joint, called a synchondrosis or primary cartilaginous joint, unites the first rib with the sternum and is the only synchondrosis found in the human adult.

• Fibrocartilage. The opposing surfaces of some bones are covered by hyaline cartilage but, instead of a synovial space, are directly connected to each other by a plate of fibrocartilage. Such fibrocartilaginous joints are called symphyses or secondary cartilaginous joints and occur in the pubic symphysis and at the intervertebral discs. The fibrocartilage disc of the pubic symphysis develops a central cavity and the intervertebral discs have a fluidfilled central cavity.

Typical synovial joint

In synovial joints, the articulating bone surfaces are covered by a thick layer of hyaline cartilage (articular cartilage). This provides smooth, low-friction surfaces and also offers a degree of resistance to compressive forces, acting act as shock-absorbers in weight-bearing joints. The joint is enclosed within a fibrocollagenous joint capsule which is lined internally by a specialised secretory cell layer, the synovium. The synovium secretes a small amount of lubricant fluid into the synovial cavity, aiding the smooth articulation of the cartilage-covered bone surfaces.

Excessive movement at the joint is limited by the fibrous joint capsule and by external fibro-elastic ligaments which prevent over-flexion and over-extension. In some joints such as the knee, there are internal ligaments (the cruciate ligaments) which prevent excessive joint movement, particularly excessive twisting rotation. Damage to these ligaments may occur, often during sporting pursuits, and this can lead to instability of the knee joint. Muscles attach to bones via tendons, and these may also play a role in stabilising synovial joints.

Synovial jointH&E, monkey (LP

This micrograph illustrates a typical synovial joint, in this case a distal interphalangeal joint from a monkey. The articular surfaces of the terminal phalanx T and the middle phalanx M are covered by hyaline cartilage C. The joint space is artefactually widened. In vivo, the articular surfaces are maintained in close contact by a fibrous capsule Cp which is inserted into the articulating bones at some distance beyond the articular cartilages. The synovium S is a specialised layer of collagenous tissue which lines the inner aspect of the capsule. Note the extensor tendon E which inserts into the base of terminal phalanx.

Articular cartilageH&E (LP)

This photomicrograph shows the articular cartilage on the surface of the head of the femur of a young adult. It is composed of hyaline cartilage C and is attached to the cortical bone B of the head of the femur. The bluish colour of the cartilage on H&E staining is due to the presence of glycos-aminoglycans in the matrix. It is these, together with the collagen of the matrix, which provides the resistance to compression that is such an important property of hyaline cartilage. Both the glycosaminoglycans and collagen are synthesised and maintained by the chondrocytes Ch. In this young person, the articular cartilage layer is thick and healthy. In older people, the cartilage near the surface undergoes degenerative changes as a result of wear and tear, eventually leading to arthritis.

Arthritis

Osteoarthritis is a degenerative disease of synovial joints due to excessive wear and tear, leading initially to degenerative change in the articular cartilages of both opposing bone ends which participate in the joint. Eventually the cartilage is eroded completely and the cortical bone of one bone end is in frictional contact with the cortical bone of the opposing bone. Both areas of cortical bone undergo refashioning to become thick layers with hard surfaces (eburnation), and continued use of the joint may produce tiny eroded bone fragments which float in the fluid of the joint cavity and eventually become deposited in the synovium of the joint capsule.

Rheumatoid arthritis is a destructive disease of synovial joints in which the synovium lining the joint capsule becomes thickened and heavily infiltrated with lymphocytes and plasma cells. The articular cartilage is destroyed and replaced by fibrovascular tissue (pannus).

There are many other causes of arthritis, including bacterial infection (septic arthritis) and deposition of crystals in the joint (crystal arthropathy, e.g. gout).

Joint capsule and synoviumH&E (LP)

This photomicrograph shows the relationship between the articular surfaces of bone and the joint capsule. The bone end is cortical bone C, covered by a cap of articular cartilage AC. This protrudes into the joint cavity JC. The joint cavity is contained by a dense collagenous fibrous capsule Cp which is lined internally by a layer of synovium S. The synovial lining cells secrete serous fluid which lubricates the articulation of the joint. The collagen fibres of the joint capsule merge with those of the periosteum P over the shaft of the bone.

Synovium(a) H&E (LP) (b) H&E (HP)

The inner surface of the capsule of synovial joints and tendon sheaths is lined by a specialised collagenous tissue, the synovium, which is responsible for the elaboration of the synovial fluid that lubricates the movement of articular surfaces. Depending on the location, the bulk of the synovial tissue may be of loose collagenous type (areolar synovium), of more dense collagenous type (fibrous synovium) or predominantly composed of fat (adipose synovium), as in the case of intra-articular fat pads. As seen in micrograph (a), the surface of the synovium S is thrown up into folds and small villi which may extend for some distance into the joint cavity JC. The synovial tissue contains numerous blood vessels V, lymphatics and nerves.

Micrograph (b) illustrates the free surface of the synovium, which is characterised by a discontinuous layer of cells up to four cells deep. These synovial cells are not connected by junctional complexes and do not rest on a basement embrane. As a result, the synovial surface does not constitute an epithelium. The synovial cells are of mesenchymal origin. The majority are plump, with an extensive Golgi complex and numerous lysosomes, features suggestive of macrophages (type A synoviocytes). The remainder have profuse rough endoplasmic reticulum and represent fibroblasts (type B synoviocytes). Also in this micrograph, note the rich network of capillaries and the thick strands of collagen which would define this as fibrous synovium.

In the normal joint, the synovial fluid is little more than a thin film covering the articular surfaces. In that the articular space is not demarcated from the synovium by an epithelium, the synovial fluid represents a highly specialised fluid form of synovial extracellular matrix rather than a secretion in the usual sense. Its major constituents are hyaluronic acid and associated glycoproteins which are secreted by the type B synoviocytes. Its fluid component is a transudate from the synovial capillaries. This arrangement facilitates the continuous exchange of oxygen, carbon dioxide and metabolites between blood and synovial fluid, which is the major source of metabolic support for articular cartilage. Normal synovial fluid also contains a small number of leucocytes (<100/mL), predominantly monocytes.

The intervertebral joints

• The vertebral bodies are united by symphysial joints, the intervertebral discs, which permit movement between the vertebral bodies while maintaining a union of great strength. The fibrocartilage of each intervertebral disc is arranged in concentric rings, forming the annulus fibrosus. Within the disc, there is a central cavity containing a viscous fluid, the nucleus pulposus, which acts as a shock absorber. The annulus fibrosus is reinforced peripherally by circumferential ligaments. A thick ligament extending down the anterior aspect of the spinal column merges with and further reinforces the annulus fibrosus and a similar but thinner ligament reinforces the posterior aspect.

• The vertebral arches articulate with each other by pairs of synovial joints known as facet or zygapophyseal joints. Strong elastic ligaments connecting the bony processes of the vertebral arches contribute to the stability of the spinal column.

Disc degeneration and prolapse

The intervertebral discs act as shock absorbers, supporting and springing the vertebral column. In bipeds (like humans) they are particularly vulnerable to damage because of the weight they have to support and the rotational and flexional/ extensional forces they are subjected to in daily activities. These can lead to weakening of the annulus fibrosus, which may give way, allowing the soft central nucleus pulposus to extrude (disc prolapse) through into the spaces beneath the ligaments. This leads to soft tissue swelling around the protrusion which may involve the spinal nerve roots emerging from the spinal column. Nerve damage may produce severe pain symptoms in the leg (e.g. sciatica).

Intervertebral discHaematoxylin-von Kossa (LP)

The intervertebral disc lies between the surfaces of adjacent vertebral bodies B and acts as a shock absorber. The disc is composed of an outer compact region of dense fibro-collagenous tissue containing occasional chondrocytes, the annulus fibrosus AF, with a variable thin layer of hyaline cartilage between this and the bone. The annulus fibrosus surrounds a central area of semi-fluid gelatinous matrix material known as the nucleus pulposus NP.

TendonH&E (MP)

Tendons are tough, flexible straps or cords which connect muscles to bone. They are composed of compact linear collagen fibres with the compressed nuclei of inactive fibroblasts (tenocytes) between the collagen bundles. Tendon T is poorly vascularised and heals slowly when damaged. It also contains tiny nerve fibres and tendon stretch receptors. Some tendons have a thin outer layer of synovium S. These tendons run for part of their course through a cylindrical fibrous sheath which is lined internally by synovium. The synovia secrete lubricatory fluid.

Tendon insertions(a) Muscle insertion, H&E (MP) (b) Insertion into bone,

H&E (MP)

Micrograph (a) shows two masses of skeletal muscle M which are inserted into a common tendon T. Within the muscle close to the tendon, some of the muscle fibres show splitting of their ends. In addition, some of the collagen fibres from the tendon

penetrate the muscle to form a complex interdigitation with the split muscle fibres (myotendinous junction), thus increasing the surface area for anchorage and so improving the overall strength of the attachment. Tendons sometimes attach to bone by the tendon collagen fibres intermingling with the collagen of the periosteum, a few of the fibres penetrating into the bone. At other sites, the collagen fibres of the tendon T penetrate directly into the bone B in the form of Sharpey fibres SF. This is illustrated in micrograph (b).